Category Archive Anatomy A – Z

Organization of Motor Neuron Pathways

The motor system is the part of the central nervous system that is involved with the movement.

The midbrain nuclei include four motor tracts that send upper motor neuronal axons down the spinal cord to lower motor neurons. These are the rubrospinal tract, the vestibulospinal tract, the tectospinal tract, and the reticulospinal tract.

The function of lower motor neurons can be divided into two different groups: the lateral corticospinal tract and the anterior cortical spinal tract. The lateral tract contains upper motor neuronal axons that synapse on the dorsal lateral lower motor neurons, which are involved in distal limb control.

The anterior corticospinal tract descends ipsilaterally in the anterior column, where the axons emerge and either synapse on ventromedial lower motor neurons in the ventral horn ipsilaterally or descussate at the anterior white commissure where they synapse on ventromedial lower motor neurons contralaterally.

The ventromedial lower motor neurons control the large, postural muscles of the axial skeleton. These lower motor neurons, unlike those of the dorsal lateral, are located in the ventral horn throughout the spinal cord.

The Role of the Basal Ganglia in Movement

The basal ganglia are responsible for voluntary motor control, procedural learning, and eye movement, as well as cognitive and emotional functions.

Location of the Basal Ganglia

The basal ganglia (or basal nuclei) are a group of nuclei of varied origin in the brains of vertebrates that act as a cohesive functional unit. They are situated at the base of the forebrain and are strongly connected with the cerebral cortex, thalamus, and other brain areas.

The basal ganglia are associated with a variety of functions, including voluntary motor control, procedural learning relating to routine behaviors or habits such as bruxism and eye movements, as well as cognitive and emotional functions.

Action Selection

Currently, popular theories hold that the basal ganglia play a primary role in action selection. Action selection is the decision of which of several possible behaviors to execute at a given time.

Experimental studies show that the basal ganglia exert an inhibitory influence on a number of motor systems, and that a release of this inhibition permits a motor system to become active. The behavior switching that takes place within the basal ganglia is influenced by signals from many parts of the brain, including the prefrontal cortex, which plays a key role in executive functions.

Movement

The greatest source of insight into the functions of the basal ganglia has come from the study of two neurological disorders, Parkinson’s disease and Huntington’s disease. For both of these disorders, the nature of the neural damage is well-understood and can be correlated with the resulting symptoms.

Parkinson’s disease involves the major loss of dopaminergic cells in the substantia nigra. Huntington’s disease involves the massive loss of medium spiny neurons in the striatum.

The symptoms of the two diseases are virtually opposite: Parkinson’s disease is characterized by a gradual loss of the ability to initiate movement, whereas Huntington’s disease is characterized by an inability to prevent parts of the body from moving unintentionally.

It is noteworthy that, although both diseases have cognitive symptoms, especially in their advanced stages, the most salient symptoms relate to the ability to initiate and control movement. Thus, both are classified primarily as movement disorders.

A different movement disorder, called hemiballismus, may result from damage restricted to the subthalamic nucleus. Hemiballismus is characterized by violent and uncontrollable flinging movements of the arms and legs.

Function in Eye Movement

One of the most intensively studied functions of the basal ganglia is their role in controlling eye movements. Eye movement is influenced by an extensive network of brain regions that converge on a midbrain area called the superior colliculus (SC).

The SC is a layered structure whose layers form two-dimensional retinotopic maps of visual space. A bump of neural activity in the deep layers of the SC drives eye movement toward the corresponding point in space.

Motivation

Although the role of the basal ganglia in motor control is clear, there are also many indications that it is involved in the control of behavior in a more fundamental way, at the level of motivation. In Parkinson’s disease, the ability to execute the components of movement is not greatly affected, but motivational factors such as hunger fail to cause movements to be initiated or switched at the proper times.

The immobility of patients with Parkinson’s disease has sometimes been described as a paralysis of the will. These patients have occasionally been observed to show a phenomenon called kinesia paradoxical, in which a person who is an otherwise immobile response to an emergency in a coordinated and energetic way, then lapses back into immobility once the emergency has passed.

The role in the motivation of the limbic part of the basal ganglia—the nucleus accumbens (NA), ventral pallidum, and ventral tegmental area (VTA)—is particularly well established. Thousands of experimental studies combine to demonstrate that the dopaminergic projection from the VTA to the NA plays a central role in the brain’s reward system.

Numerous things that people find rewarding, including addictive drugs, good-tasting food, and sex, have been shown to elicit activation of the VTA dopamine system. Damage to the NA or VTA can produce a state of profound torpor.

Neurotransmitters

In most regions of the brain, the predominant classes of neurons use glutamate as the neurotransmitter and have excitatory effects on their targets. In the basal ganglia, however, the great majority of neurons uses gamma-aminobutyric acid (GABA) as the neurotransmitter and have inhibitory effects on their targets.

The inputs from the cortex and thalamus to the striatum and subthalamic nucleus are glutamatergic, but the outputs from the striatum, pallidum, and substantia nigra pars reticulata all use GABA. Thus, following the initial excitation of the striatum, the internal dynamics of the basal ganglia are dominated by inhibition and disinhibition.

Other neurotransmitters have important modulatory effects. Dopamine is used by the projection from the substantia nigra pars compacta to the dorsal striatum and also in the analogous projection from the ventral tegmental area to the ventral striatum (nucleus accumbens).

Acetylcholine also plays an important role, as it is used both by several external inputs to the striatum and by a group of striatal interneurons. Although cholinergic cells make up only a small fraction of the total population, the striatum has one of the highest acetylcholine concentrations of any brain structure.

Modulation of Movement by the Cerebellum

The cerebellum is important for motor control—specifically coordination, precision, and timing—as well as some forms of motor learning.

The cerebellum is a region of the brain that plays an important role in motor control. It may also be involved in some cognitive functions such as attention and language, and in regulating fear and pleasure responses, but its movement-related functions are the most solidly established. The cerebellum does not initiate movement, but it contributes to coordination, precision, and accurate timing.

It receives input from sensory systems of the spinal cord and from other parts of the brain, including the cerebral cortex, and integrates these inputs to fine-tune motor activity. Because of this fine-tuning function, damage to the cerebellum does not cause paralysis, but instead produces disorders in fine movement, equilibrium, posture, and motor learning.

The cerebellum differs from most other parts of the brain, especially the cerebral cortex, in regards to the ability of signals to move unidirectionally from input to output. This feedforward mode of operation means that the cerebellum cannot generate self-sustaining patterns of neural activity, in contrast to the cerebral cortex. However, the cerebellum can receive information from the cerebral cortex and processes this information to send motor impulses to the skeletal muscle.

Anatomy of the Cerebellum

In terms of anatomy, the cerebellum has the appearance of a separate structure attached to the bottom of the brain, tucked underneath the cerebral hemispheres. The surface of the cerebellum is covered with finely spaced parallel grooves, in striking contrast to the broad irregular convolutions of the cerebral cortex. These parallel grooves conceal the fact that the cerebellum is actually a continuous thin layer of tissue (the cerebellar cortex), tightly folded in the style of an accordion.

Within this thin layer are several types of neurons with a highly regular arrangement, the most important being Purkinje cells and granule cells. This complex neural network gives rise to a massive signal-processing capability, but almost all of its output is directed to a set of small, deep cerebellar nuclei lying in the interior of the cerebellum.

Function

Marr-Albus Theory

In addition to its direct role in motor control, the cerebellum is also necessary for several types of motor learning, the most notable one being learning to adjust to changes in sensorimotor relationships.

Several theoretical models have been developed to explain sensorimotor calibration in terms of synaptic plasticity within the cerebellum. Most of them derive from early models formulated by David Marr and James Albus, which were motivated by the observation that each cerebellar Purkinje cell receives two dramatically different types of input.

It receives input from thousands of parallel fibers, each individually very weak. However, each cerebellar Purkinje cell also gets input from one single climbing fiber, which is so strong that a single climbing fiber action potential will reliably cause a target Purkinje cell to fire a burst of action potentials.

The basic concept of the Marr-Albus theory is that the climbing fiber serves as a teaching signal, which induces a long-lasting change in the strength of synchronously activated parallel fiber inputs. Observations of long-term depression in parallel fiber inputs have provided support for theories of this type, but their validity remains controversial.

Insights from Cerebellar Dysfunction

The strongest clues to the function of the cerebellum have come from examining the consequences of damage to it. Animals and humans with cerebellar dysfunction show, above all, problems with motor control. They continue to be able to generate motor activity, but it loses precision, producing erratic, uncoordinated, or incorrectly timed movements.

A standard test of cerebellar function is to reach with the tip of the finger for a target at arm’s length. A healthy person will move the fingertip in a rapid straight trajectory, whereas a person with cerebellar damage will reach slowly and erratically, with many mid-course corrections.

Deficits in non-motor functions are more difficult to detect. Thus, the general conclusion reached decades ago is that the basic function of the cerebellum is not to initiate movements, or to decide which movements to execute, but rather to calibrate the detailed form of a movement.

The comparative simplicity and regularity of the cerebellar anatomy led to an early hope that it might imply a similar simplicity of computational function. Although a full understanding of cerebellar function remains elusive, at least four principles are identified as important: 1) feedforward processing, 2) divergence and convergence, 3) modularity, and 4) plasticity.

• Feedforward processing – Refers to the unidirectional movement of signals through the system from input to output, with the very little recurrent internal transmission. This means that the cerebellum, in contrast to the cerebral cortex, cannot generate self-sustaining patterns of neural activity. Signals enter the circuit, are processed by each stage in sequential order, and then leave.
• Divergence and convergence: The 1000 or so Purkinje cells belonging to a microzone may receive input from as many as 100 million parallel fibers, and focus their own output down to a group of less than 50 deep nuclear cells. Thus, the cerebellar network receives a modest number of inputs, processes them very extensively through its rigorously structured internal network, and sends out the results via a very limited number of output cells.
• Modularity: The cerebellar system is functionally divided into independent modules. All modules have a similar internal structure, but with different inputs and outputs. The output of one module does not appear to significantly influence the activity of other modules
• Plasticity: The synapses between parallel fibers and Purkinje cells, and the synapses between mossy fibers and deep nuclear cells, are both susceptible to modification of their strength. The influence of each parallel fiber on nuclear cells is adjustable. This arrangement gives tremendous flexibility for fine-tuning the relationship between the cerebellar inputs and outputs.

Functions of the Cerebellum in Integrating Movements

The cerebellum uses feedforward processing and modularity to process information.

Cerebellar Function

Feedforward Processing

The cerebellum differs from most other parts of the brain in that the signal processing is almost entirely feedforward—that is, signals move unidirectionally through the system from input to output, with very little recurrent internal transmission.

The small amount of recurrence that does exist consists of mutual inhibition; there are no mutually excitatory circuits. This feedforward mode of operation means that the cerebellum, in contrast to the cerebral cortex, cannot generate self-sustaining patterns of neural activity.

Signals enter the circuit, are processed by each stage in sequential order, and then leave. As Eccles, Ito, and Szentágothai wrote. This elimination in the design of all possibility of reverberatory chains of neuronal excitation is undoubtedly a great advantage in the performance of the cerebellum as a computer because what the rest of the nervous system requires from the cerebellum is presumably not some output expressing the operation of complex reverberatory circuits in the cerebellum, but the rather quick and clear response to the input of any particular set of information.”

Divergence and Convergence

In the human cerebellum, information from 200 million mossy fiber inputs is expanded to 40 billion granule cells, whose parallel fiber outputs then converge onto 15 million Purkinje cells. Because of the way that they are lined up longitudinally, the 1,000 or so Purkinje cells belonging to a microzone may receive input from as many as 100 million parallel fibers and focus their own output down to a group of less than 50 deep nuclear cells.

Thus, the cerebellar network receives a modest number of inputs, processes them very extensively through its rigorously structured internal network, and sends out the results via a very limited number of output cells.

Modularity

The cerebellar system is functionally divided into more or less independent modules, that probably number in the hundreds to thousands. All modules have a similar internal structure, but different inputs and outputs.

A module (a multizonal microcompartment in the terminology of Apps and Garwicz) consists of a small cluster of neurons in the inferior olivary nucleus, a set of long narrow strips of Purkinje cells in the cerebellar cortex (microzones), and a small cluster of neurons in one of the deep cerebellar nuclei.

Different modules share input from mossy fibers and parallel fibers, but in other respects they appear to function independently. The output of one module does not appear to significantly influence the activity of other modules.

Plasticity

The synapses between parallel fibers and Purkinje cells, and the synapses between mossy fibers and deep nuclear cells, are both susceptible to modification of their strength. In a single cerebellar module, input from as many as a billion parallel fibers converge onto a group of less than 50 deep nuclear cells, and the influence of each parallel fiber on those nuclear cells is adjustable. This arrangement gives tremendous flexibility for fine-tuning the relationships between the cerebellar inputs and outputs.

Peripheral Motor Endings

A neuromuscular junction exists between the axon terminal and the motor endplate of a muscle fiber where neurotransmitters are released.

A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, as shown in Figures 1 and 2. The highly excitable region of muscle fiber plasma membrane is responsible for initiation of action potentials across the muscle’s surface, ultimately causing the muscle to contract.

Invertebrates, the signal passes through the neuromuscular junction via the neurotransmitter acetylcholine.

Upon the arrival of an action potential at the presynaptic neuron terminal, voltage-dependent calcium channels open and Ca²⁺ ions flow from the extracellular fluid into the presynaptic neuron’s cytosol. This influx of Ca²⁺ causes neurotransmitter-containing vesicles to dock and fuse to the presynaptic neuron’s cell membrane, which results in the emptying of the vesicle’s contents (acetylcholine) into the synaptic cleft; this process is known as exocytosis.

Acetylcholine diffuses into the synaptic cleft and binds to the nicotinic acetylcholine receptors located on the motor endplate.

These receptors open to allow sodium ions to flow in and potassium ions to flow out of the muscle’s cytosol, producing a local depolarization of the motor endplate, known as an end-plate potential (EPP). This depolarization spreads across the surface of the muscle fiber and continues the excitation-contraction coupling to contract the muscle.

The action potential spreads through the muscle fiber’s network of T-tubules, depolarizing the inner portion of the muscle fiber. The depolarization activates L-type, voltage-dependent calcium channels (dihydropyridine receptors) in the T-tubule membrane, which are in close proximity to calcium-release channels (ryanodine receptors) in the adjacent sarcoplasmic reticulum.

As intracellular calcium levels rise, the motor proteins responsible for the contractile response are able to interact, as shown in Figure 3, to form cross-bridges and undergo shortening.

Overview of Motor Integration

A motor unit is comprised of a single alpha-motor neuron and all the muscle fibers it innervates.

Motor Unit Categories

Motor units are generally categorized based upon the similarities between several factors such as:

Physiological

Contraction speed in isometric contractions:

• Rate of rise of force.
• Time to peak of a twitch contraction (response to a single nerve impulse).

Biochemical

Histochemical (the oldest form of biochemical fiber typing):

• Glycolytic enzyme activity.
• Oxidative enzyme activity.
• Sensitivity of myosin ATPase to acid and alkali.

Immunohistochemical (a more recent form of fiber typing):

• Myosin heavy chain (MHC).
• Myosin light chain—alkali (MLC1).
• Myosin light chain—regulatory (MLC2).

Peripheral Motor Endings

A neuromuscular junction exists between the axon terminal and the motor endplate of a muscle fiber where neurotransmitters are released.

A neuromuscular junction is the synapse or junction of the axon terminal of a motor neuron with the motor end plate, as shown in Figures 1 and 2. The highly excitable region of muscle fiber plasma membrane is responsible for initiation of action potentials across the muscle’s surface, ultimately causing the muscle to contract.

Invertebrates, the signal passes through the neuromuscular junction via the neurotransmitter acetylcholine.

 

Figure 1. Detailed view of a neuromuscular junction: Detailed view of a neuromuscular junction: 1) Presynaptic terminal; 2) Sarcolemma; 3) Synaptic vesicle; 4) Nicotinic acetylcholine receptor; 5) Mitochondrion.

 

Figure 2. Neuromuscular junction: Electron micrograph showing a cross section through the neuromuscular junction. T is the axon terminal and M is the muscle fiber. The arrow shows junctional folds with basal lamina. Postsynaptic densities are visible on the tips between the folds. Scale is 0.3 µm.

Upon the arrival of an action potential at the presynaptic neuron terminal, voltage-dependent calcium channels open and Ca²⁺ ions flow from the extracellular fluid into the presynaptic neuron’s cytosol. This influx of Ca²⁺ causes neurotransmitter-containing vesicles to dock and fuse to the presynaptic neuron’s cell membrane, which results in the emptying of the vesicle’s contents (acetylcholine) into the synaptic cleft; this process is known as exocytosis.

Acetylcholine diffuses into the synaptic cleft and binds to the nicotinic acetylcholine receptors located on the motor endplate.

These receptors open to allow sodium ions to flow in and potassium ions to flow out of the muscle’s cytosol, producing a local depolarization of the motor endplate, known as an end-plate potential (EPP). This depolarization spreads across the surface of the muscle fiber and continues the excitation-contraction coupling to contract the muscle.

The action potential spreads through the muscle fiber’s network of T-tubules, depolarizing the inner portion of the muscle fiber. The depolarization activates L-type, voltage-dependent calcium channels (dihydropyridine receptors) in the T-tubule membrane, which are in close proximity to calcium-release channels (ryanodine receptors) in the adjacent sarcoplasmic reticulum.

As intracellular calcium levels rise, the motor proteins responsible for the contractile response are able to interact, as shown in Figure 3, to form cross-bridges and undergo shortening.

CLINICAL EXAMPLE of Motor Activity

Myasthenia gravis is an autoimmune disorder in which circulating antibodies block the nicotinic acetylcholine receptors on the motor endplate of the neuromuscular junction. This blockage of acetylcholine receptors causes muscle weakness, often first exhibiting drooping eyelids and expanding to include overall muscle weakness and fatigue.

The effects of myasthenia gravis illustrate the importance of effective and functioning neuromuscular junctions for communication between neurons and muscles to allow contraction and relaxation of muscle fibers.

 

Figure 3. Muscle contraction and actin-myosin interactions: Skeletal muscle contracts following activation by an action potential. The binding of acetylcholine at the motor endplate leads to intracellular calcium release and interactions between myofibrils to elicit contraction.

Overview of Motor Integration

A motor unit is comprised of a single alpha-motor neuron and all the muscle fibers it innervates.

 

Rectus femoris: The rectus femoris muscle is one of the four quadriceps muscles of the human body. These muscles may have as many as a thousand fibers in each motor unit.

 

The orbicularis oris (eye) muscle: These small motor units may contain only 10 fibers per motor unit. The more precise the action of the muscle, the fewer fibers innervated.

Motor Unit Categories

Motor units are generally categorized based upon the similarities between several factors such as:

Physiological

Contraction speed in isometric contractions:

• Rate of rise of force.
• Time to peak of a twitch contraction (response to a single nerve impulse).

Biochemical

Histochemical (the oldest form of biochemical fiber typing):

• Glycolytic enzyme activity.
• Oxidative enzyme activity.
• Sensitivity of myosin ATPase to acid and alkali.

Immunohistochemical (a more recent form of fiber typing):

• Myosin heavy chain (MHC).
• Myosin light chain—alkali (MLC1).
• Myosin light chain—regulatory (MLC2).

Cervical Plexus

The cervical plexus is the plexus of the ventral rami of the first four cervical spinal nerves.

Structure and Distribution

The cervical plexus is a plexus of the ventral rami of the first four cervical spinal nerves located from the C1 to C4 cervical segment in the neck. They are located laterally to the transverse processes between prevertebral muscles from the medial side and anterolateral to them. scalenus and m. levator scapulae.

There is anastomosis with the accessory nerve, hypoglossal nerve, and sympathetic trunk. It is located deep in the neck, near the sternocleidomastoid muscle.

Nerves formed from the cervical plexus innervate the back of the head, as well as some neck muscles. The branches of the cervical plexus emerge from the posterior triangle at the nerve point, a point that lies midway on the posterior border of the sternocleidomastoid.

Branches and Their Functions

The cervical plexus has two types of branches: cutaneous and muscular.

Cutaneous branches include

The lesser occipital nerve, or small occipital nerve, is a cutaneous spinal nerve that arises between the second and third cervical vertebrae, along with the greater occipital nerve. It innervates the scalp in the lateral area of the head posterior to the ear.

The great auricular nerve originates from the cervical plexus and is composed of branches from spinal nerves C2 and C3. It provides sensory innervation for the skin over the parotid gland and mastoid process, and both surfaces of the outer ear.

The transverse cervical nerve (superficial cervical or cutaneous cervical) arises from the second and third cervical nerves, turns around the posterior border of the sternocleidomastoideus about its middle, then passes obliquely forward beneath the external jugular vein to the anterior border of the muscle, where it perforates the deep cervical fascia and divides beneath the platysma into ascending and descending branches that are distributed to the anterolateral parts of the neck.

The supraclavicular nerves (descending branches) arise from the third and fourth cervical nerves. They emerge beneath the posterior border of the sternocleidomastoideus and descend in the posterior triangle of the neck beneath the platysma and deep cervical fascia.

Muscular branches include

• Ansa cervicalis (loop formed from C1–C3), geniohyoid (C1 only), thyrohyoid (C1 only), sternothyroid, sternohyoid, omohyoid: The ansa cervicalis is a loop of nerves that are part of the cervical plexus.
• The phrenic nerve (C3–C5, but primarily C4) is a nerve that originates in the neck and passes down between the lung and heart to reach the diaphragm.
• Segmental branches (C1–C4) innervate the anterior and middle scalenes.

There are two additional branches that are formed by the posterior roots of the spinal nerves

• Preauricular nerve (from the posterior roots of C2–C3).
• Postauricular nerve (from the posterior roots of C3–C4).

Brachial Plexus

The brachial plexus is formed by the four lower cervical spinal nerves and the first thoracic spinal nerve.

Formation and Distribution

Right brachial plexus: The right brachial plexus with its short branches, viewed from the front.

The brachial plexus is a network of nerve fibers that run from the spine that is formed by the ventral rami of the lower four cervical and first thoracic nerve roots (C5–C8, T1).

The brachial plexus proceeds through the neck, the axilla (armpit region), and into the arm. It is a collection of nerves passing through the cervical-axillary canal to reach the axilla and innervate the brachium, the antebrachium, and the hand.

The brachial plexus is responsible for cutaneous and muscular innervation of the entire upper limb, with two exceptions: the trapezius muscle is innervated by the spinal accessory nerve (CN XI) and an area of skin near the axilla is innervated by the intercostobrachial nerve. Lesions on the brachial plexus can lead to severe functional impairment.

Branches and Divisions

The brachial plexus is divided into roots, trunks, divisions, cords, and branches. There are five terminal branches and numerous other preterminal or collateral branches that leave the plexus at various points along its length. Its structure includes:

• Five roots: The five anterior rami of the spinal nerves, after they have given off their segmental supply to the muscles of the neck.
• These roots merge to form three trunks: The superior or upper (C5–C6), the middle (C7), and the inferior or lower (C8, T1).
• Each trunk then splits in two, to form six divisions: The anterior divisions of the upper, middle, and lower trunks and the posterior divisions of the upper, middle, and lower trunks.
• These six divisions will regroup to become the three cords. The cords are named by their position with respect to the axillary artery:
  • The posterior cord is formed from the three posterior divisions of the trunks (C5–C8,T1).
  • The lateral cord is the anterior division from the upper and middle trunks (C5–C7).
  • The medial cord is simply a continuation of the anterior division of the lower trunk (C8, T1).
  • Most branches branch from the cords, but a few branch directly from earlier structures. The five on the left are considered terminal branches.

Lumbar Plexus

The lumbar plexus is formed by the subcostal nerve and divisions of the first four lumbar nerves that arise from the middle to the lower back.

Structure and Distribution

The lumbar plexus is a nerve plexus in the lumbar region of the body that forms part of the lumbosacral plexus. It is formed by the ventral divisions of the first four lumbar nerves (L1–L4) and from contributions of the subcostal nerve (T12), which is the last thoracic nerve.

This plexus lies within the psoas major muscle. Nerves of the lumbar plexus serve the skin and the muscles of the lower abdominal wall, the thigh, and external genitals. The largest nerve of the plexus is the femoral nerve and it supplies the anterior muscles of the thigh and a part of skin distal to the inguinal ligament.

Lumbar plexus: An image of the lumbar plexus with its nerves highlighted in yellow.

Branches of the Lumbar Plexus

Iliohypogastric Nerve

Lumbar plexus: Schematic of the lumbar plexus.

Sacral and Coccygeal Plexuses

The sacral plexus is the plexus of the three sacral spinal nerves (S2–S4) that arise from the lower back just above the sacrum.

The sacral plexus is a nerve plexus that provides motor and sensory nerves for the posterior thigh, most of the lower leg, the entire foot, and part of the pelvis. It is part of the lumbosacral plexus and emerges from the sacral vertebrae (S2–S4).

The sacral plexus: The nerves of the sacral plexus are shown.

The largest and longest nerve of the human body, the sciatic nerve, is the main branch and gives rami to the motor innervation of the muscles of the foot, the leg, and the thigh.

The sacral plexus is formed by:

• The lumbosacral trunk.
• The anterior division of the first sacral nerve.
• Portions of the anterior divisions of the second and third sacral nerves.

The nerves forming the sacral plexus converge toward the lower part of the greater sciatic foramen and unite to form a flattened band from the anterior and posterior surfaces, from which several branches arise.

The band itself is continued as the sciatic nerve, which splits on the back of the thigh into the tibial nerve and the common fibular nerve. These two nerves sometimes arise separately from the plexus and, in all cases, their independence can be shown by dissection.

Often, the sacral plexus and the lumbar plexus are considered to be one large nerve plexus, the lumbosacral plexus. The lumbosacral trunk connects the two plexuses.

The coccygeal plexus originates from the S4, S5, and Co1 spinal nerves. It is interconnected with the lower part of the sacral plexus. The only nerve in this plexus is the anococcygeal nerve, which serves sensory innervation of the skin in the coccygeal region.

Sensory and Motor Tracts

The spinothalamic tract is a somatosensory tract and the corticospinal tract is a motor tract.

The Somatosensory Tract

The spinothalamic tract is a sensory pathway originating in the spinal cord. It transmits information to the thalamus about pain, temperature, itch, and crude touch. The pathway decussates at the level of the spinal cord.

The somatosensory organization is divided into the dorsal column–medial lemniscus tract (the touch, proprioception, vibration sensory pathway) and the anterolateral system, or ALS (the pain, temperature sensory pathway). Both sensory pathways use three different neurons to get information from sensory receptors at the periphery to the cerebral cortex.

These neurons are designated primary, secondary, and tertiary sensory neurons. In both pathways, primary sensory neuron cell bodies are found in the dorsal root ganglia, and their central axons project into the spinal cord.

Function

The types of sensory information transmitted via the spinothalamic tract are described as effective sensations. This means that the sensation is accompanied by a compulsion to act. For instance, an itch is accompanied by a need to scratch, and a painful stimulus makes us want to withdraw from the pain.

There are two subsystems:

• Direct (for direct, conscious appreciation of pain).
• Indirect (for affective and arousal impact of pain).

Indirect projections are further divided into

• Spino-reticulo-thalamocortical (part of the ascending reticular arousal system, also known as ARAS).
• Spino-mesencephalic-limbic (for the affective impact of pain).

The Corticospinal Tract

The corticospinal tract conducts impulses from the brain to the spinal cord. It contains mostly motor axons. The corticospinal tract is made up of two separate tracts in the spinal cord: the lateral corticospinal tract and the anterior corticospinal tract.

The corticospinal tract also contains the Betz cell (the largest pyramidal cells) that are not found in any other region of the body. An understanding of these tracts leads to an understanding of why one side of the body is controlled by the opposite side of the brain.

The corticospinal tract is concerned specifically with discrete, voluntary, skilled movements, such as the precise movement of fingers and toes. The brain sends impulses to the spinal cord that relay the message.

This is imperative in understanding that the left hemisphere of the brain controls the RIGHT side of the body, while the right hemisphere of the brain controls the LEFT side of the body. The signals cross in the medulla oblongata, and this process is also known as decussation.

Function

The primary purpose of the corticospinal tract is to maintain voluntary motor control of the body and limbs. However, connections to the somatosensory cortex suggest that the pyramidal tracts are also responsible for modulating sensory information from the body.

Some of these connections across the midline; therefore, each side of the brain is responsible for controlling muscles for the limbs on opposite sides of the body. However, control of trunk muscles is on the same side of the body.

After a patient’s pyramidal tracts are injured, the patient is paralyzed on the corresponding side of the body. Fortunately, they can re-learn some crude, basic motions, but not fine movements. This implies that the connections to these tracts are crucial for fine movement, and only partial recovery is possible if they are damaged.

References

Overview of the Spinal Nerves

Each spinal nerve is a mixed nerve, formed from the combination of nerve fibers from its dorsal and ventral roots. The dorsal root is the afferent sensory root and carries sensory information to the brain. The ventral root is the efferent motor root and carries motor information from the brain. The spinal nerve emerges from the spinal column through an opening (intervertebral foramen) between adjacent vertebrae. This is true for all spinal nerves except for the first spinal nerve pair (C1), which emerges between the occipital bone and the atlas (the first vertebra). Thus the cervical nerves are numbered by the vertebra below, except spinal nerve C8, which exists below vertebra C7 and above vertebra T1. The thoracic, lumbar, and sacral nerves are then numbered by the vertebra above. In the case of a summarized S1 vertebra (aka L6) or a sacralized L5 vertebra, the nerves are typically still counted to L5 and the next nerve is S1.

Outside the vertebral column, the nerve divides into branches. The dorsal ramus contains nerves that serve the posterior portions of the trunk carrying visceral motor, somatic motor, and somatic sensory information to and from the skin and muscles of the back (epaxial muscles). The ventral ramus contains nerves that serve the remaining anterior parts of the trunk and the upper and lower limbs (hypaxial muscles) carrying visceral motor, somatic motor, and sensory information to and from the ventrolateral body surface, structures in the body wall, and the limbs. The meningeal branches (recurrent meningeal or sinuvertebral nerves) branch from the spinal nerve and re-enter the intervertebral foramen to serve the ligaments, dura, blood vessels, intervertebral discs, facet joints, and periosteum of the vertebrae. The rami communicantes contain autonomic nerves that serve visceral functions carrying visceral motor and sensory information to and from the visceral organs.

Some anterior rami merge with adjacent anterior rami to form a nerve plexus, a network of interconnecting nerves. Nerves emerging from a plexus contain fibers from various spinal nerves, which are now carried together to some target location. Major plexuses include the cervical, brachial, lumbar, and sacral plexuses.

Spinal nerves, a part of the peripheral nervous system (PNS), are mixed nerves that send motor, sensory, and autonomic signals between the CNS and the body.

Spinal Nerve Anatomy

The term spinal nerve generally refers to a mixed spinal nerve that carries motor, sensory, and autonomic signals between the spinal cord and the body.

Humans have 31 left-right pairs of spinal nerves, each roughly corresponding to a segment of the vertebral column: eight cervical spinal nerve pairs (C1–C8), 12 thoracic pairs (T1–T12), five lumbar pairs (L1–L5), five sacral pairs (S1–S5), and one coccygeal pair. The spinal nerves are part of the peripheral nervous system (PNS).

Location

Intervertebral foramina: Intervertebral foramina are indicated by arrows.

Each spinal nerve is formed by the combination of nerve fibers from the dorsal and ventral roots of the spinal cord. The dorsal roots carry afferent sensory axons, while the ventral roots carry efferent motor axons.

The spinal nerve emerges from the spinal column through an opening (intervertebral foramen) between adjacent vertebrae.

This is true for all spinal nerves except for the first spinal nerve pair, which emerges between the occipital bone and the atlas (the first vertebra). Thus the cervical nerves are numbered by the vertebra below, except C8, which exists below C7 and above T1.

The thoracic, lumbar, and sacral nerves are then numbered by the vertebra above. In the case of a summarized S1 vertebra (i.e., L6) or a sacralized L5 vertebra, the nerves are typically still counted to L5 and the next nerve is S1.

Spinal Nerve Innervation

Outside the vertebral column, the nerve divides into branches. The dorsal ramus contains nerves that serve the dorsal portions of the trunk; it carries visceral motor, somatic motor, and somatic sensory information to and from the skin and muscles of the back (epaxial muscles).

The ventral ramus contains nerves that serve the remaining ventral parts of the trunk and the upper and lower limbs (hypaxial muscles); they carry the visceral motor, somatic motor, and sensory information to and from the ventrolateral body surface, structures in the body wall, and the limbs.

The meningeal branches (recurrent meningeal or sinuvertebral nerves) branch from the spinal nerve and re-enter the intervertebral foramen to serve the ligaments, dura, blood vessels, intervertebral discs, facet joints, and periosteum of the vertebrae.

The rami communicantes contain autonomic nerves that serve visceral functions, such as carrying visceral motor and sensory information to and from the visceral organs.

Cervical Nerves

The posterior distribution of the cervical nerves includes the suboccipital nerve (C1), the greater occipital nerve (C2), and the third occipital nerve (C3). The anterior distribution includes the cervical plexus (C1–C4) and brachial plexus (C5–T1).

The muscles innervated by the cervical nerves are the sternohyoid, sternothyroid, and omohyoid muscles.

A loop of nerves called ansa cervicalis is also part of the cervical plexus.

Thoracic Nerves

Thoracic nerve branches exit the spine and go directly to the paravertebral ganglia of the autonomic nervous system, where they are involved in the functions of organs and glands in the head, neck, thorax, and abdomen.

Anterior Divisions

The intercostal nerves come from thoracic nerves T1–T11 and run between the ribs. The subcostal nerve comes from nerve T12 and runs below the twelfth rib.

Posterior Divisions

The medial branches (ramus medialis) of the posterior branches of the upper six thoracic nerves run between the semispinalis dorsi and multifidus, which they supply.

They then pierce the rhomboid and trapezius muscles and reach the skin by the sides of the spinous processes. This branch is called the medial cutaneous ramus.

The medial branches of the lower six thoracic nerves are distributed chiefly to the multifidus and longissimus dorsi, occasionally they give off filaments to the skin near the middle line. This sensitive branch is called the posterior cutaneous ramus.

Lumbar Nerves

The lumbar nerves are divided into posterior and anterior divisions.

Posterior Divisions

The medial branches of the posterior divisions of the lumbar nerves run close to the articular processes of the vertebrae and end in the multifidus muscle. The lateral branches supply the erector spinae muscles.

Anterior Divisions

The anterior divisions of the lumbar nerves (rami anteriores) consist of long, slender branches that accompany the lumbar arteries around the sides of the vertebral bodies, beneath the psoas major.

The first and second, and sometimes the third and fourth, lumbar nerves are each connected with the lumbar part of the sympathetic trunk by a white ramus communicans.

The nerves pass obliquely outward behind the psoas major, or between its fasciculi, distributing filaments to it and the quadratus lumborum.

The first three and the greater part of the fourth are connected by anastomotic loops and form the lumbar plexus.

The smaller part of the fourth joins with the fifth to form the lumbosacral trunk, which assists in the formation of the sacral plexus. The fourth nerve is named the furcal nerve, from the fact that it is subdivided between the two plexuses.

Sacral Nerves

There are five paired sacral nerves, half of them arising through the sacrum on the left side and the other half on the right side. Each nerve emerges in two divisions: one division through the anterior sacral foramina and the other division through the posterior sacral foramina.

The sacral nerves have both afferent and efferent fibers, thus they are responsible for part of the sensory perception and the movements of the lower extremities of the human body.

The pudendal nerve and parasympathetic fibers arise from S2, S3, and S4. They supply the descending colon and rectum, urinary bladder, and genital organs. These pathways have both afferent and efferent fibers.

Coccygeal Nerve

The coccygeal nerve is the 31st pair of spinal nerves and arises from the conus medullaris. Its anterior root helps form the coccygeal plexus.

Function

Spinal nerve motor functions are summarized in the table below.

Branches of Spinal Nerves

The spinal nerves branch into the dorsal ramus, ventral ramus, the meningeal branches, and the rami communicantes.

Course and branches of thoracic spinal nerve: This diagram depicts the course and branches of a typical thoracic spinal nerve. The posterior division (dorsal ramus) is labeled at the top right.

Outside the vertebral column, the spinal nerves divide into branches.

• The dorsal ramus: Contains nerves that serve the dorsal portions of the trunk carrying visceral motor, somatic motor, and sensory information to and from the skin and muscles of the back.
• The ventral ramus: Contains nerves that serve the remaining ventral parts of the trunk and the upper and lower limbs carrying visceral motor, somatic motor, and sensory information to and from the ventrolateral body surface, structures in the body wall, and the limbs. Some ventral rami merge with adjacent ventral rami to form a nerve plexus, a network of interconnecting nerves. Nerves emerging from a plexus contain fibers from various spinal nerves, which are now carried together to some target location. Major plexuses include the cervical, brachial, lumbar, and sacral plexuses.
• The meningeal branches (recurrent meningeal or sinuvertebral nerves): These branch from the spinal nerve and re-enter the intervertebral foramen to serve the ligaments, dura, blood vessels, intervertebral discs, facet joints, and periosteum of the vertebrae.
• The rami communicantes: Contain autonomic nerves that carry visceral motor and sensory information to and from the visceral organs.

Types of Spinal Nerve fibers

• Somatic efferent fibers originate in the anterior/ventral column of central grey matter in the spinal cord. They pass through the anterior root of the spinal nerve. They are responsible for the motor innervation of the skeletal muscles.
• Somatic afferent fibers carry sensory information from the skin, joints, and muscle to the posterior/dorsal column of grey matter in the spinal cord. These fibers pass through the dorsal root ganglion.
• Visceral efferent fibers are autonomic fibers that supply the organs. They are divided into sympathetic and parasympathetic fibers. Sympathetic fibers originate from the thoracic spinal nerves as well as L1 and L2. Parasympathetic nerves come from the S2, S3, and S4 spinal nerves only to supply the pelvic and lower abdominal viscera. The remainder of the parasympathetic nerves come from extensions of the cranial nerves into the thoracic and abdominal cavities.
• Visceral afferent fibers carry sensory information through the dorsal root ganglion and to the dorsal column of grey matter in the spinal cord.

Plexuses

A nerve plexus is a network of intersecting nerves that serve the same part of the body.

A nerve plexus is a network of intersecting nerves; multiple nerve plexuses exist in the body. Nerve plexuses are composed of afferent and efferent fibers that arise from the merging of the anterior rami of spinal nerves and blood vessels.

There are five spinal nerve plexuses—except in the thoracic region—as well as other forms of autonomic plexuses, many of which are a part of the enteric nervous system.

Spinal Plexuses

Cervical Plexus—Serves the Head, Neck, and Shoulders

• The cervical plexus is formed by the ventral rami of the upper four cervical nerves and the upper part of the fifth cervical ventral ramus. The network of rami is located deep within the neck.

Brachial Plexus—Serves the Chest, Shoulders, Arms, and Hands

• The brachial plexus is formed by the ventral rami of C5–C8 and the T1 spinal nerves, and lower and upper halves of the C4 and T2 spinal nerves. The plexus extends toward the armpit (axilla).

Lumbar Plexus—Serves the Back, Abdomen, Groin, Thighs, Knees, and Calves

• The lumbar plexus is formed by the ventral rami of L1–L5 spinal nerves with a contribution of T12 form the lumbar plexus. This plexus lies within the psoas major muscle.

Sacral Plexus—Serves the Pelvis, Buttocks, Genitals, Thighs, Calves, and Feet

• The sacral plexus is formed by the ventral rami of L4-S3, with parts of the L4 and S4 spinal nerves. It is located on the posterior wall of the pelvic cavity.

Coccygeal Plexus—Serves a Small Region over the Coccyx

• The coccygeal plexus serves a small region over the coccyx and originates from S4, S5, and Co1 spinal nerves. It is interconnected with the lower part of the sacral plexus.

In addition, the celiac plexus serves the internal organs, and Auerbach’s plexus serves the gastrointestinal tract.

Autonomic Plexuses

• Celiac plexus (solar plexus)—Serves internal organs.
• Auerbach’s plexus—Serves the gastrointestinal tract.
• Meissner’s plexus (submucosal plexus)—Serves the gastrointestinal tract.

Brachial plexus: Cervical (C5–C8) and thoracic (T1) nerves comprise the brachial plexus, which is a nerve plexus that provides sensory and motor function to the shoulders and upper limbs.

Lumbar plexus: The lumbar plexus is comprised of the ventral rami of the lumbar spinal nerves (L1–L5) and a contribution from the thoracic nerve (T12). The posterior (green) and anterior (yellow) divisions of the lumbar plexus are shown in the diagram.

Intercostal Nerves

The anterior divisions of the thoracic spinal nerves (T1–T11) are called the intercostal nerves.

The intercostal nerves are part of the somatic nervous system and arise from anterior divisions (rami anterior, ventral divisions) of the thoracic spinal nerves T1 to T11. The intercostal nerves are distributed chiefly to the thoracic pleura and abdominal peritoneum.

Intercostal nerves: An image of the intercostal brachial nerves.

They differ from the anterior divisions of the other spinal nerves in that each pursues an independent course without plexus formation.

First Thoracic Nerve

The anterior division of the first thoracic nerve divides into two branches:

• The larger branch leaves the thorax in front of the neck of the first rib and enters the brachial plexus.
• The other smaller branch, the first intercostal nerve, runs along with the first intercostal space and ends on the front of the chest as the first anterior cutaneous branch of the thorax.

The Upper Thoracic Nerves (2nd–6th)

These are limited in their distribution to the parietes (wall) of the thorax. The anterior divisions of the second, third, fourth, fifth, and sixth thoracic nerves, and the small branch from the first thoracic are confined to the walls of the thorax and are named thoracic intercostal nerves.

Near the sternum, they cross in front of the internal mammary artery and transversus thoracic muscle, pierce the intercostales interni, the anterior intercostal membranes, and pectoralis major, and supply the integument of the front of the thorax and over the mamma, forming the anterior cutaneous branches of the thorax.

The branch from the second nerve unites with the anterior supraclavicular nerves of the cervical plexus.

The Lower Thoracic Nerves (7th–12th)

The seventh intercostal nerve terminates at the xiphoid process, at the lower end of the sternum.

The anterior divisions of the seventh, eighth, ninth, tenth, and eleventh thoracic intercostal nerves are continued anteriorly from the intercostal spaces into the abdominal wall; hence they are named thoracoabdominal nerves or thoracoabdominal intercostal nerves.

The tenth intercostal nerve terminates at the umbilicus.

The twelfth (subcostal) thoracic nerve is distributed to the abdominal wall and groin.

Unlike the nerves from the autonomic nervous system that innervate the visceral pleura of the thoracic cavity, the intercostal nerves arise from the somatic nervous system. This enables them to control the contraction of muscles, as well as provide specific sensory information regarding the skin and parietal pleura.

This explains why damage to the internal wall of the thoracic cavity can be felt as a sharp pain localized in the injured region. Damage to the visceral pleura is experienced as an unlocalized ache.

Dermatomes

A dermatome is an area of skin that is supplied by a single spinal nerve, and a myotome is a group of muscles that a single spinal nerve root innervates.

A dermatome is an area of skin that is supplied by a single spinal nerve. There are eight cervical nerves, twelve thoracic nerves, five lumbar nerves and five sacral nerves. Each of these nerves relays sensation, including pain, from a particular region of the skin to the brain.

Dermatomes: Dermatomes are areas of skin supplied by sensory neurons that arise from a spinal nerve ganglion. Dermatomes and the associated major cutaneous nerves are shown here in a ventral view.

Along the thorax and abdomen, the dermatomes are like a stack of discs, with each section supplied by a different spinal nerve. Along the arms and the legs, the pattern is different. The dermatomes run longitudinally along the limbs, so that each half of the limb has a different dermatome.

Although the general pattern is similar in all people, the precise areas of innervation are as unique to an individual as fingerprints.

Dermatomes have clinical significance, especially in the diagnosis of certain diseases. Symptoms that follow a dermatome, such as pain or a rash, may indicate a pathology that involves the related nerve root. Examples include dysfunction of the spine or a viral infection.

Viruses that remain dormant in nerve ganglia, such as the varicella zoster virus that causes both chickenpox and shingles, often cause either pain, rash, or both in a pattern defined by a dermatome.

Shingles rash: The shingles rash appears across a dermatome. In this patient, one of the dermatomes in the arm is affected, restricting the rash to the length of the back of the arm.

Shingles is one of the only diseases that cause a rash in a dermatomal pattern, and as such, this is its defining symptom. The rash of shingles is almost always restricted to a specific dermatome, such as on the chest, leg, or arm caused by the residual varicella-zoster virus infection of the nerve that supplies that area of skin. Shingles typically appear years or decades after recovery from chickenpox.

Myotome

A myotome is the group of muscles that a single spinal nerve root innervates. The myotome is the motor equivalent of a dermatome.

The myotome distributions of the upper and lower extremities are listed below:

• C1/C2: Neck flexion / extension
• C3: Neck lateral flexion
• C4: Shoulder elevation
• C5: Shoulder abduction
• C6: Elbow flexion/wrist extension
• C7: Elbow extension/wrist flexion
• C8: Finger flexion
• T1: Finger abduction
• L2: Hip flexion
• L3: Knee extension
• L4: Ankle dorsiflexion
• L5: Great toe extension
• S1: Ankle plantar flexion/ankle eversion/hip extension
• S2: Knee flexion
• S3–S4: Anal reflex

The testing of myotomes provides the clinician with information about the level in the spine where a lesion may be present. During testing, the clinician looks for muscle weakness of a particular group of muscles. Results may indicate lesions to the spinal cord nerve root, or intervertebral disc herniation that presses on the spinal nerve roots.

Function and Physiology of the Spinal Nerves

Spinal nerves connect the brain and spinal cord to the limbs and organs of the body.

Review of Peripheral Nervous System Structure

The peripheral nervous system (PNS) consists of the nerves and ganglia outside of the brain and spinal cord. The main function of the PNS is to connect the central nervous system (CNS) to the limbs and organs.

Unlike the CNS, the PNS is not protected by the bones of the spine and skull, or by the blood –brain barrier, leaving it exposed to toxins and mechanical injuries. The peripheral nervous system is divided into the somatic nervous system and the autonomic nervous system.

The peripheral nervous system includes 12 cranial nerves and 31 pairs of spinal nerves that provide communication from the CNS to the rest of the body by nerve impulses to regulate the functions of the human body. The term spinal nerve generally refers to a mixed spinal nerve, which carries motor, sensory, and autonomic signals between the spinal cord and the body.

Spinal Nerve Correspondences

Each pair of spinal nerves roughly correspond to a segment of the vertebral column: 8 cervical spinal nerve pairs (C1–C8), 12 thoracic pairs (T1–T12), 5 lumbar pairs (L1–L5), 5 sacral pairs (S1–S5), and 1 coccygeal pair.

• The first 4 cervical spinal nerves, C1 through C4, split and recombine to produce a variety of nerves that subserve the neck and back of the head.
• The spinal nerve C1 (suboccipital nerve) provides motor innervation to muscles at the base of the skull.
• C2 and C3 form many of the nerves of the neck, and provides both sensory and motor control. These include the greater occipital nerve that provides sensation to the back of the head, the lesser occipital nerve that provides sensation to the area behind the ears, the greater auricular nerve, and the lesser auricular nerve.
• The phrenic nerve arises from nerve roots C3, C4, and C5. It innervates the diaphragm to enable breathing. If the spinal cord is transected above C3, then spontaneous breathing is not possible.
• The last four cervical spinal nerves, C5 through C8, and the first thoracic spinal nerve, T1, combine to form the brachial plexus, or plexus brachialis, a tangled array of nerves, splitting, combining and recombining to form the nerves that subserve the upper limb region and upper back. Although the brachial plexus may appear tangled, it is highly organized and predictable with little variation among people.

Lumbosacral Plexus

The anterior divisions of the lumbar, sacral, and coccygeal nerves form the lumbosacral plexus, the first lumbar nerve being frequently joined by a branch from the twelfth thoracic. For descriptive purposes, this plexus is usually divided into three parts: lumbar plexus, sacral plexus, and pudendal plexus.

Autonomic Nervous System Function (ANS)

The sympathetic division typically functions in actions that need quick responses. The parasympathetic division functions with actions that do not require immediate reaction.

The sympathetic system is often considered the fight or flight system, while the parasympathetic system is often considered the rest and digest or feed and breed system.

Some typical actions of the sympathetic and parasympathetic systems are listed below.

Sympathetic Nervous System

• Diverts blood flow away from the gastrointestinal (GI) tract and skin via vasoconstriction.
• Enhances blood flow to skeletal muscles and the lungs.
• Dilates bronchioles of the lung by circulating epinephrine to allow for greater alveolar oxygen exchange.
• Increases the heart rate and contractility of cardiac muscle for enhanced blood flow to skeletal muscles.
• Dilates pupils and relaxes the ciliary muscle to the lens for far vision.
• Provides vasodilation for the coronary vessels of the heart.
• Constricts all the intestinal sphincters and the urinary sphincter.
• Inhibits peristalsis.
• Stimulates orgasm.

Parasympathetic Nervous System

• Dilates blood vessels that lead to the GI tract to increase blood flow; this is important following food consumption due to the greater metabolic demands placed on the body by the gut.
• Constricts the bronchiolar diameter when the need for oxygen has diminished.
• Manages heart control via dedicated cardiac branches of the vagus and thoracic spinal accessory nerves.
• Constricts the pupil and contracts the ciliary muscles to facilitate accommodation for closer vision.
• Stimulates salivary gland secretion and accelerates peristalsis to mediate the digestion of food.
• PNS nerves are involved in the erection of genital tissues via the pelvic splanchnic nerves 2–4. They are also responsible for stimulating sexual arousal.

Neurotransmitters

• Acetylcholine is the preganglionic neurotransmitter for both divisions of the ANS, as well as the postganglionic neurotransmitter of parasympathetic neurons.
• Nerves that release acetylcholine are said to be cholinergic. In the parasympathetic system, ganglionic neurons use acetylcholine as a neurotransmitter to stimulate muscarinic receptors.
• At the adrenal medulla, there is no postsynaptic neuron. Instead, the presynaptic neuron releases acetylcholine to act on nicotinic receptors.
• Stimulation of the adrenal medulla releases adrenaline (epinephrine) into the bloodstream, which acts on adrenoceptors, producing a widespread increase in sympathetic activity.

Somatic Nervous System Function (SoNS)

The somatic nervous system consists of afferent and efferent nerves and is associated with the voluntary control of skeletal muscle movements. The afferent nerves are responsible for relaying sensations from the body to the central nervous system (CNS), while the efferent nerves are responsible for sending out commands from the CNS to the body to stimulate muscle contraction.

Upper motor neurons release acetylcholine. Acetylcholine is released from the axon terminal knobs of alpha motor neurons and received by postsynaptic receptors (nicotinic acetylcholine receptors) of muscles, thereby relaying the stimulus to contract muscle fibers.

Blood Supply and Lymphatics

The spinal cord receives both longitudinal and segmental blood supply.  The vasculature is highly complex and anastomotic which ensures adequate delivery to the entire structure.  Longitudinally, the anterior spinal artery (ASA) and a pair of posterior spinal arteries (PSAs) are the predominant supply of the spinal cord.  The three arteries branch from the distal vertebral arteries at the base of the skull.  The three vessels connect circumferentially around the cord as the vaso-corona to supply the entire periphery of the cord.[rx] Numerous radicular arteries are also arranged segmentally which provide additional blood flow to the entire vertebral column.  Many of these are branches of segmental intercostal and lumbar arteries.[rx][rx]

or

Three lumbar vertebral arteries surround each lumbar vertebral body. Lumbar vertebral arteries are direct branches off of the aorta. Spinal arterial branches differentiate into radicular and segmental arteries. Segmental branches supply vertebral bodies, and posterior arterial branches supply vertebral arches. Spinal branches enter the vertebral canal through intervertebral foramina to supply the bones, periosteum, ligaments, and meninges. Radicular and segmental arteries supply spinal nerve roots, spinal nerves, and the spinal cord. Venous drainage of the vertebral column parallels the arterial system. Additionally, there is an internal anterior lumbar venous plexus; and a posterior external venous plexus.[rx]

References

Brief Overview of Cranial Nerves

The characteristics of this nerve are presented in the table.

The peripheral nervous system has 12 pairs of cranial nerves that control much of the motor and sensory functions of the head and neck.

Cranial nerves are the nerves that emerge directly from the brain (including the brainstem). In contrast, spinal nerves emerge from segments of the spinal cord. Cranial nerves relay information between the brain and parts of the body, primarily to and from regions of the head and neck.

Cranial Nerve Anatomy and Terminology

Spinal nerves emerge sequentially from the spinal cord with the spinal nerve closest to the head (C1) emerging in the space above the first cervical vertebra. The cranial nerves emerge from the central nervous system above this level.

Each cranial nerve is paired and is present on both sides. The numbering of the cranial nerves is based on the order in which they emerge from the brain, front to back (brainstem).

The terminal nerves, olfactory nerves (I) and optic nerves (II) emerge from the cerebrum or forebrain, and the remaining ten pairs arise from the brainstem, which is the lower part of the brain. The cranial nerves are considered components of the peripheral nervous system.

However, on a structural level, the olfactory, optic, and terminal nerves are more accurately considered part of the central nervous system.

The twelve cranial nerves are shown in the figure below followed by brief descriptions.

• The olfactory nerve (I): This is instrumental for the sense of smell, it is one of the few nerves that are capable of regeneration.
• The optic nerve (II): This nerve carries visual information from the retina of the eye to the brain.
• The oculomotor nerve (III): This controls most of the eye’s movements, the constriction of the pupil, and maintains an open eyelid.
• The trochlear nerve (IV): A motor nerve that innervates the superior oblique muscle of the eye, which controls rotational movement.
• The trigeminal nerve (V): This is responsible for sensation and motor function in the face and mouth.
• The abducens nerve (VI): A motor nerve that innervates the lateral rectus muscle of the eye, which controls lateral movement.
• The facial nerve (VII): This controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity.
• The vestibulocochlear nerve (VIII): This is responsible for transmitting sound and equilibrium (balance) information from the inner ear to the brain.
• The glossopharyngeal nerve (IX): This nerve receives sensory information from the tonsils, the pharynx, the middle ear, and the rest of the tongue.
• The vagus nerve (X): This is responsible for many tasks, including heart rate, gastrointestinal peristalsis, sweating, and muscle movements in the mouth, including speech and keeping the larynx open for breathing.
• The spinal accessory (XI): This nerve controls specific muscles of the shoulder and neck.
• The hypoglossal nerve (XII): This nerve controls the tongue movements of speech, food manipulation, and swallowing.

There are many mnemonic devices to remember the cranial nerves. One that may be helpful is: Old Opie Occasionally Tries Trigonometry And Feels Very Gloomy, Vague And Hypoactive.

Olfactory (I) Nerve

The olfactory nerve, or cranial nerve I, is the first of 12 cranial nerves and is responsible for the sense of smell.

Olfactory bulb: Sagittal section of human head showing the olfactory bulb.

The olfactory nerve, or cranial nerve I, is the first of the 12 cranial nerves. It is instrumental in the sense of smell. The olfactory nerve is the shortest of the 12 cranial nerves and only one of two cranial nerves (the other being the optic nerve) that do not join with the brainstem.

The specialized olfactory receptor neurons of the olfactory nerve are located in the olfactory mucosa of the upper parts of the nasal cavity. The olfactory nerves consist of a collection of many sensory nerve fibers that extend from the olfactory epithelium to the olfactory bulb, passing through the many openings of the cribriform plate of the ethmoid bone.

Olfactory receptor neurons continue to emerge throughout life and extend new axons to the olfactory bulb. Olfactory-ensheathing glia wrap bundles of these axons and are thought to facilitate their passage into the central nervous system.

The sense of smell (olfaction) arises from the stimulation of olfactory (or odorant) receptors by small molecules of different spatial, chemical, and electrical properties that pass over the nasal epithelium in the nasal cavity during inhalation. These interactions are transduced into electrical activity in the olfactory bulb, which then transmits the electrical activity to other parts of the olfactory system and the rest of the central nervous system via the olfactory tract.

Optic (II) Nerve

The optic nerve (cranial nerve II) receives visual information from photoreceptors in the retina and transmits it to the brain.

The optic nerve is also known as cranial nerve II. It transmits visual information from the retina to the brain.

Each human optic nerve contains between 770,000 and 1.7 million nerve fibers. The eye’s blind spot is a result of the absence of photoreceptors in the area of the retina where the optic nerve leaves the eye.

The optic nerve is the second of twelve paired cranial nerves. It is considered by physiologists to be part of the central nervous system, as it is derived from an outpouching of the diencephalon during embryonic development.

As a consequence, the fibers are covered with myelin produced by oligodendrocytes, rather than Schwann cells that are found in the peripheral nervous system. The optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium, perineurium, and endoneurium found in the peripheral nerves.

The fiber tracks of the mammalian central nervous system are incapable of regeneration. As a consequence, optic nerve damage produces irreversible blindness.

The optic nerve leaves the orbit, which is also known as an eye socket, via the optic canal, running posteromedially toward the optic chiasm, where there is a partial decussation (crossing) of fibers from the nasal visual fields of both eyes.

Most of the axons of the optic nerve terminate in the lateral geniculate nucleus (where information is relayed to the visual cortex), while other axons terminate in the pretectal nucleus and are involved in reflexive eye movements.

The optic nerve transmits all visual information including brightness perception, color perception, and contrast. It also conducts the visual impulses that are responsible for two important neurological reflexes: the light reflex and the accommodation reflex.

The light reflex refers to the constriction of both pupils that occurs when light is shone into either eye; the accommodation reflex refers to the swelling of the lens of the eye that occurs when one looks at a near object, as in reading.

Oculomotor (III) Nerve

The oculomoter nerve (cranial nerve III) controls eye movement, such as constriction of the pupil and open eyelids.

The oculomotor nerve is the third paired cranial nerve. It enters the orbit via the superior orbital fissure and controls most of the eye’s movements, including constriction of the pupil and maintaining an open eyelid by innervating the levator palpebrae superiors muscle.

The oculomotor nerve is derived from the basal plate of the embryonic midbrain. Cranial nerves IV and VI also participate in the control of eye movement.

There are two nuclei for the oculomotor nerve:

1. The oculomotor nucleus originates at the level of the superior colliculus. The muscles it controls are the striated muscle in the levator palpebrae superioris and all extraocular muscles, except for the superior oblique muscle and the lateral rectus muscle.
2. The Edinger-Westphal nucleus supplies parasympathetic fibers to the eye via the ciliary ganglion and controls the pupillae muscle (affecting pupil constriction) and the ciliary muscle (affecting accommodation).

Sympathetic postganglionic fibers also join the nerve from the plexus on the internal carotid artery in the wall of the cavernous sinus and are distributed through the nerve, for example, to the smooth muscle of the levator palpebrae superioris.

Emergence from Brain

On emerging from the brain, the oculomotor nerve is invested with a sheath of pia mater and enclosed in a prolongation from the arachnoid mater. It passes between the superior cerebellar and posterior cerebral arteries, and then pierces the dura mater anterior and lateral to the posterior clinoid process (to give attachment to the tectorium cerebella), passing between the free and attached borders of the tentorium cerebelli.

It then runs along the lateral wall of the cavernous sinus, above the other orbital nerves, receiving in its course one or two filaments from the cavernous plexus of the sympathetic nervous system, and a communicating branch from the ophthalmic division of the trigeminal nerve.

It then divides into two branches that enter the orbit through the superior orbital fissure, between the two heads of the lateral rectus (a muscle on the lateral side of the eyeball in the orbit). Here the nerve is placed below the trochlear nerve and the frontal and lacrimal branches of the ophthalmic nerve, while the nasociliary nerve is placed between its two rami (the superior and inferior branch of oculomotor nerve).

Trochlear (IV) Nerve

The trochlear nerve (cranial nerve IV) is a motor nerve that innervates a single muscle: the superior oblique muscle of the eye.

The trochlear nerve (cranial nerve IV) is a motor nerve that innervates a single muscle: the superior oblique muscle of the eye.

The trochlear nerve: The trocheal nerve and where it innervates.

The trochlear nerve is unique among the cranial nerves in several respects.

• It is the smallest nerve in terms of the number of axons it contains and it has the greatest intracranial length.
• Other than the optic nerve (cranial nerve II), it is the only cranial nerve that decussates (crosses to the other side) before innervating its target.
• It is the only cranial nerve that exits from the dorsal aspect of the brainstem.

The nucleus of the trochlear nerve is located in the caudal mesencephalon beneath the cerebral aqueduct. It is immediately below the nucleus of the oculomotor nerve (III) in the rostral mesencephalon.

The trochlear nucleus is unique in that its axons run dorsally and cross the midline before emerging from the brainstem—so a lesion of the trochlear nucleus affects the contralateral eye. Lesions of all other cranial nuclei affect the ipsilateral side (except of course the optic nerve, cranial nerve II, which innervates both eyes).

Homologous trochlear nerves are found in all jawed vertebrates. The unique features of the trochlear nerve, including its dorsal exit from the brainstem and its contralateral innervation, are seen in the primitive brains of sharks.

The human trochlear nerve is derived from the basal plate of the embryonic midbrain.

Clinical Syndromes

There are two major clinical syndromes that can manifest through damage to the trochlear nerve:

• Vertical diplopia: Injury to the trochlear nerve causes weakness of downward eye movement with consequent vertical diplopia (double vision).
• Torsional diplopia: Weakness of intorsion results in torsional diplopia, in which two different visual fields, tilted with respect to each other, are seen at the same time. To compensate for this, patients with trochlear nerve palsies tilt their heads to the opposite side, in order to fuse the two images into a single visual field.

The clinical syndromes can originate from both peripheral and central lesions. A peripheral lesion is damage to the bundle of nerves, in contrast to a central lesion, which is damage to the trochlear nucleus.

Trigeminal (V) Nerve

The trigeminal nerve is the fifth cranial nerve and it is responsible for sensation and motor function in the face and mouth.

The trigeminal nerve (cranial nerve V), and it contains both sensory and motor fibers. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing.

Trigeminal nerve: Schematic illustration of the trigeminal nerve (labeled Sensory root above) and the structures it innervates in the face and mouth.

Structure

The trigeminal nerve is the largest of the cranial nerves. Its name, trigeminal, means three twins. It is derived from the fact that each nerve, one on each side of the pons, has three major branches: the ophthalmic nerve (V1 in the illustration below), the maxillary nerve (V2), and the mandibular nerve (V3).

The ophthalmic and maxillary nerves are purely sensory. The mandibular nerve has both sensory and motor functions.

The three branches converge on the trigeminal ganglion that is located within the trigeminal cave in the brain; it contains the cell bodies of incoming sensory nerve fibers. The trigeminal ganglion is analogous to the dorsal root ganglia of the spinal cord, which contain the cell bodies of incoming sensory fibers from the rest of the body.

Areas of the face innervated by the trigeminal nerve: The ophthalmic nerve branch (V1) innervates the bright red area, the maxillary nerve branch (V2) innervates the light red area, and the mandibular nerve branch (V3) innervates the yellow area.

From the trigeminal ganglion, a single large sensory root enters the brainstem at the level of the pons. Immediately adjacent to the sensory root, a smaller motor root emerges from the pons at the same level.

Motor fibers pass through the trigeminal ganglion on their way to peripheral muscles, but their cell bodies are located in the nucleus of the trigeminal nerve, deep within the pons.

Function

The sensory function of the trigeminal nerve is to provide tactile, proprioceptive, and nociceptive afferents to the face and mouth. The motor component of the mandibular division (V3) of the trigeminal nerve controls the movement of eight muscles, including the four muscles of mastication: the masseter, the temporal, and the medial and lateral pterygoids.

The other four muscles are the tensor veli palatini, the mylohyoid, the anterior belly of the digastric, and the tensor tympani. With the exception of the tensor tympani, all of these muscles are involved in biting, chewing and swallowing, and all have bilateral cortical representation.

The function of the trigeminal nerve

The sensory functions of the trigeminal nerve are the transmission of impulses for touch, pain, temperature, proprioception, and the somatomotor function with mandibular movement.

The ophthalmic nerve (V1) has mainly sensory fibers and has the following functions:

• Sensory innervation of the skin of the forehead, the eyes, the nose, and the nasal mucosa
• Innervation of the iris, the cornea, the conjunctiva, and the lacrimal gland
• Supply of the skin of the forehead and transmission of pressure pain at the exit point of the supraorbital foramen

The maxillary nerve (V2) innervates the facial skin with sensory fibers and a parasympathetic part and has the following tasks:

• Supply of the lower eyelid, its conjunctiva, and the upper lip
• Supply of the teeth (upper molar, incisor, and canine), nasal cavity gums of the upper jaw and the palate mucosa (including the uvula), and tonsils
• Supply of the lacrimal gland and nasal glands by the parasympathetic part

In addition to the sensory fibers, the mandibular nerve also contains motor fibers of the trigeminal nerve. It has the following tasks:

• Innervation of all the muscles of mastication (e.g., masseter muscle) and the suprahyoid muscles by the motor fibers
• Supply of the teeth (lower molar, incisor, and canine teeth), the gums of the lower jaw, the buccal mucosa, the dorsum of the tongue, and the external acoustic meatus, including the eardrum, by the sensory fibers

Abducens (VI) Nerve

The abducens nerve (cranial nerve VI) controls the lateral movement of the eye through innervation of the lateral rectus muscle.

The abducens nerve (cranial nerve VI) is a somatic efferent nerve that, in humans, controls the movement of a single muscle: the lateral rectus muscle of the eye that moves the eye horizontally. In most other mammals it also innervates the musculus retractor bulbi, which can retract the eye for protection. Homologous abducens nerves are found in all vertebrates except lampreys and hagfishes.

Abducens nerve: Schematic of cranial nerves showing cranial nerve VI, the abducens nerve.

The abducens nerve leaves the brainstem at the junction of the pons and the medulla, medial to the facial nerve. In order to reach the eye, it runs upward (superiorly) and then bends forward (anteriorly).

The nerve enters the subarachnoid space when it emerges from the brainstem. It runs upward between the pons and the clivus, and then pierces the dura mater to run between the dura and the skull.

At the tip of the petrous temporal bone, it makes a sharp turn forward to enter the cavernous sinus. In the cavernous sinus it runs alongside the internal carotid artery. It then enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle of the eye.

The long course of the abducens nerve between the brainstem and the eye makes it vulnerable to injury at many levels. For example, fractures of the petrous temporal bone can selectively damage the nerve, as can aneurysms of the intracavernous carotid artery.

Mass lesions that push the brainstem downward can damage the nerve by stretching it between the point where it emerges from the pons and the point where it hooks over the petrous temporal bone.

Facial (VII) Nerve

The facial nerve (cranial nerve VII) determines facial expressions and the taste sensations of the tongue.

The facial nerve: Illustration of the facial nerve and its branches.

The facial nerve is the seventh (cranial nerve VII) of the 12, paired cranial nerves. It emerges from the brainstem between the pons and the medulla and controls the muscles of facial expression.

It also functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity, and it supplies preganglionic parasympathetic fibers to several head and neck ganglia.

Location

The motor part of the facial nerve arises from the facial nerve nucleus in the pons, while the sensory part of the facial nerve arises from the nervus intermedius. The motor and sensory parts of the facial nerve enter the petrous temporal bone into the internal auditory meatus (intimately close to the inner ear), then runs a tortuous course (including two tight turns) through the facial canal, emerges from the stylomastoid foramen, and passes through the parotid gland, where it divides into five major branches.

Although it passes through the parotid gland, it does not innervate the gland (this is the responsibility of cranial nerve IX, the glossopharyngeal nerve). The facial nerve forms the geniculate ganglion prior to entering the facial canal.

The path of the facial nerve can be divided into six segments.

• The intracranial (cisternal) segment.
• The meatal segment (brainstem to the internal auditory canal).
• The labyrinthine segment (internal auditory canal to geniculate ganglion),
• The tympanic segment (from geniculate ganglion to pyramidal eminence).
• The mastoid segment (from pyramidal eminence to stylomastoid foramen).
• The extratemporal segment (from stylomastoid foramen to post parotid branches).

Function

Bell’s Palsy: A person attempting to show his teeth and raise his eyebrows with Bell’s palsy on his right side (left side of the image).

Motor functions include the following:

• Innervation of the entire mimic facial musculature (platysma and muscles of the auricle)
• Eyelid movement and closing of the eye by the orbicularis oculi muscle
• Movement and closing of the mouth by the orbicularis oris muscle
• Precise adjustment of the auditory ossicles by the stapedius muscle
• Movement of the mandible by the mentalis muscle

Sensory and parasympathetic functions include the following:

• Sense of taste in the anterior two-thirds of the tongue
• Innervation of the three large salivary glands, the lacrimal glands, and the nasal glands
• External acoustic meatus

Voluntary facial movements, such as wrinkling the brow, showing teeth, frowning, closing the eyes tightly (inability to do so is called lagophthalmos), pursing the lips, and puffing out the cheeks, all test the facial nerve. There should be no noticeable asymmetry.

In an upper motor neuron lesion, called central seven (central facial palsy ), only the lower part of the face on the contralateral side will be affected due to the bilateral control to the upper facial muscles (frontalis and orbicularis oculi).

Lower motor neuron lesions can result in cranial nerve VII palsy (Bell’s palsy is the idiopathic form of facial nerve palsy), manifested as both upper and lower facial weakness on the same side of the lesion.

Taste can be tested on the anterior 2/3 of the tongue. This can be tested with a swab dipped in a flavored solution, or with electronic stimulation (similar to putting your tongue on a battery).

In regards to the corneal reflex, the afferent arc is mediated by the general sensory afferents of the trigeminal nerve. The efferent arc occurs via the facial nerve.

The reflex involves the consensual blinking of both eyes in response to stimulation of one eye. This is due to the facial nerve’s innervation of the muscles of facial expression, namely the orbicularis oculi, responsible for blinking. Thus, the corneal reflex effectively tests the proper functioning of both cranial nerves V and VII.

Vestibulocochlear (VIII) Nerve

The vestibulocochlear nerve (cranial nerve VIII) carries information about hearing and balance.

The vestibulocochlear nerve (also known as the auditory vestibular nerve and cranial nerve VIII) has axons that carry the modalities of hearing and equilibrium.

It consists of the cochlear nerve that carries information about hearing, and the vestibular nerve that carries information about balance.

This is the nerve along which the sensory cells (the hair cells) of the inner ear transmit information to the brain. It emerges from the pons and exits the inner skull via the internal acoustic meatus (or internal auditory meatus) in the temporal bone.

Vestibular system’s semicircular canal: An illustration of the inner ear showing its semicircular canal, hair cells, ampulla, cupula, vestibular nerve, and fluid.

The vestibulocochlear nerve consists mostly of bipolar neurons and splits into two large divisions: the cochlear nerve and the vestibular nerve. The cochlear nerve travels away from the cochlea of the inner ear where it starts as the spiral ganglia.

Processes from the organ of Corti (the receptor organ for hearing) conduct afferent transmission to the spiral ganglia. It is the inner hair cells of the organ of Corti that are responsible for activating the afferent receptors in response to pressure waves reaching the basilar membrane through the transduction of sound.

The vestibular nerve travels from the vestibular system of the inner ear. The vestibular ganglion houses the cell bodies of the bipolar neurons and extends processes to five sensory organs.

Three of these are the cristae, located in the ampullae of the semicircular canals. Hair cells of the cristae activate afferent receptors in response to rotational acceleration.

The other two sensory organs supplied by the vestibular neurons are the maculae of the saccule and utricle. Hair cells of the maculae activate afferent receptors in response to linear acceleration.

The vestibulocochlear nerve has axons that carry the modalities of hearing and equilibrium. Damage to the vestibulocochlear nerve may cause hearing loss, vertigo, a false sense of motion, loss of equilibrium in dark places, nystagmus, motion sickness, and gaze-evoked tinnitus.

A benign primary intracranial tumor of the vestibulocochlear nerve is called a vestibular schwannoma (also called acoustic neuroma).

Functions of the vestibular nerve include the following:

• Regulation of the hair cells of the organ of Corti for the adjustment of sensitivity (spatial position)
• Transmission of impulses for the sense of balance, maintaining equilibrium

Functions of the cochlear nerve include the following:

• Regulation of the hair cells of the organ of Corti for the adjustment of sensitivity (regarding sound waves)
• Transmission of impulses for hearing

Glossopharyngeal (IX) Nerve

The glossopharyngeal nerve (cranial nerve IX) serves many distinct functions, including providing sensory innervation to various head and neck structures.

Structure

The glossopharyngeal nerve is the ninth of 12 pairs of cranial nerves. It exits the brainstem out from the sides of the upper medulla, just rostral (closer to the nose) to the vagus nerve.

Glossopharyngeal nerve: Image of head structures including the glossopharyngeal nerve.

Function

There are a number of functions of the glossopharyngeal nerve. It controls muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva.

Along with taste, the glossopharyngeal nerve relays general sensations from the pharyngeal walls. The various functions of the glossopharyngeal nerve are that:

• It receives general sensory fibers (ventral trigeminothalamic tract) from the tonsils, the pharynx, the middle ear, and the posterior 1/3 of the tongue.
• It receives special sensory fibers (taste) from the posterior 1/3 of the tongue.
• It receives visceral sensory fibers from the carotid bodies, carotid sinus.
• It supplies parasympathetic fibers to the parotid gland via the otic ganglion.
• It supplies motor fibers to the stylopharyngeus muscle.
• It contributes to the pharyngeal plexus.

Motor functions include the following:

• Innervation of the palate muscles and the muscles of the pharynx by the vagus nerve
• Dilatation of the pharynx during swallowing and speaking

Sensory functions include the following:

• Innervation of the mucosa of the middle ear, the mastoid, and the eardrum
• Supply of the velum, including the palatine tonsil and the posterior third of the tongue
• Taste and somatic perception (touch, pain, and temperature) of the posterior third of the tongue
• Proprioception in the swallowing musculature
• Blood pressure regulation
• Regulation of the oxygen and carbon dioxide content of the blood for the control of ventilation

Parasympathetic functions include the following:

• Stimulation of salivation
• Carotid glomus—contains chemoreceptors responsible for the oxygen content, as well as pressure receptors that are important for the regulation of blood pressure

Five Functional Components

The glossopharyngeal nerve consists of five components with distinct functions:

• Branchial motor (special visceral efferent): Supplies the stylopharyngeus muscle.
• Visceral motor (general visceral efferent): Provides parasympathetic innervation of the parotid gland.
• Visceral sensory (general visceral afferent): Carries visceral sensory information from the carotid sinus and body.
• General sensory (general somatic afferent): Provides general sensory information from the skin of the external ear, the internal surface of the tympanic membrane, upper pharynx, and the posterior 1/3 of the tongue.
• Special sensory (special afferent): Provides taste sensation from the posterior 1/3 of the tongue.

Vagus (X) Nerve

The vagus nerve (cranial nerve X) is responsible for parasympathetic output to the heart and visceral organs.

Vagus Nerve Anatomy

The vagus nerve, also known as the pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves. Upon leaving the medulla between the medullary pyramid and the inferior cerebellar peduncle, it extends through the jugular foramen, then passes into the carotid sheath between the internal carotid artery and the internal jugular vein below the head, to the neck, chest and abdomen, where it contributes to the innervation of the viscera.

Vagus nerve: Diagram demonstrating the course of the vagus nerve.

Besides output to the various organs in the body, the vagus nerve conveys sensory information about the state of the body’s organs to the central nervous system.  Eighty to 90% of the nerve fibers in the vagus nerve are afferent (sensory) nerves that communicate the state of the viscera to the brain.

The vagus nerve includes axons that emerge from or converge onto four nuclei of the medulla.

• The dorsal nucleus of vagus nerve: Sends parasympathetic output to the viscera, especially the intestines.
• The nucleus ambiguus: Sends parasympathetic output to the heart (slowing it down).
• The solitary nucleus: Receives afferent taste information and primary afferents from visceral organs.
• The spinal trigeminal nucleus: Receives information about deep/crude touch, pain, and temperature of the outer ear, the dura of the posterior cranial fossa, and the mucosa of the larynx.

Function

The vagus nerve supplies motor parasympathetic fibers to all the organs, except the suprarenal (adrenal) glands, from the neck down to the second segment of the transverse colon. The vagus also controls a few skeletal muscles, most notably:

• Cricothyroid muscle.
• Levator veli palatini muscle.
• Salpingopharyngeus muscle.
• Palatoglossus muscle.
• Palatopharyngeus muscle.
• Superior, middle, and inferior pharyngeal constrictors.
• Muscles of the larynx (speech).

Sensory functions include the following:

• Taste and sensation (touch, pain, temperature, etc.) in the epiglottis and the pharynx
• Blood pressure regulation
• Regulation of the oxygen and carbon dioxide content in the blood for the control of ventilation
• Sensation in visceral, thoracic, and abdominal organs

Somatomotor functions include the following:

• Swallowing
• Coughing
• Voice production

Parasympathetic functions include the following:

• Contraction and relaxation of the smooth musculature of the gastrointestinal tract
• Reduction of the heart rate
• Secretion of digestive juices

This means that the vagus nerve is responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, and quite a few muscle movements in the mouth, including speech (via the recurrent laryngeal nerve), swallowing, and keeping the larynx open for breathing (via action of the posterior cricoarytenoid muscle, the only abductor of the vocal folds).

It also has some afferent fibers that innervate the inner (canal) portion of the outer ear, via the auricular branch (also known as Alderman’s nerve) and part of the meninges. This explains why a person may cough when tickled on the ear (such as when trying to remove ear wax with a cotton swab).

Afferent vagus nerve fibers that innervate the pharynx and back of the throat are responsible for the gag reflex. In addition, 5-HT3 receptor-mediated afferent vagus stimulation in the gut due to gastroenteritis and other insults is a cause of vomiting.

Cardiovascular Influence

Parasympathetic innervation of the heart is partially controlled by the vagus nerve and is shared by the thoracic ganglia. Activation of the vagus nerve typically leads to a reduction in heart rate and/or blood pressure.

This occurs commonly in cases of viral gastroenteritis, acute cholecystitis, or in response to stimuli such as the Valsalva maneuver or pain. Excessive activation of the vagal nerve during emotional stress can also cause vasovagal syncope due to a sudden drop in cardiac output, causing cerebral hypoperfusion.

Accessory (XI) Nerve

The accessory nerve (cranial nerve XI) controls the muscles of the shoulder and neck.

Anatomic Description

The accessory nerve (cranial nerve XI) controls the sternocleidomastoid and trapezius muscles of the shoulder and neck. It begins in the central nervous system (CNS) and exits the cranium through a foramen.

Unlike the other 11 cranial nerves, the accessory nerve begins outside the skull. In fact, most of the fibers of the nerve originate in neurons situated in the upper spinal cord.

The accessory nerve: Upon exiting the skull via the jugular foramen, the spinal accessory nerve pierces the sternocleidomastoid muscle before terminating on the trapezius muscle.

Hypoglossal (XII) Nerve

The hypoglossal nerve (cranial nerve XII) controls the muscles of the tongue.

Structure and Location

The hypoglossal nerve is the twelfth cranial nerve (XII) and innervates all extrinsic and intrinsic muscles of the tongue, except for the palatoglossus. The hypoglossal nerve emerges from the medulla oblongata in the preolivary sulcus where it separates the olive (olivary body) and the pyramid (medullary pyramid).

It goes on to traverse the hypoglossal canal and, upon emerging, it branches and merges with a branch from the anterior ramus of C1. It passes behind the vagus nerve and between the internal carotid artery and internal jugular vein which lies on the carotid sheath. After passing deep to the posterior belly of the digastric muscle it proceeds to the submandibular region to enter the tongue.

Sensory functions include the following:

• Proprioception

Somatomotor functions include the following:

• Innervation of the sternocleidomastoid muscle and the trapezius muscle to control the movement of the head and the shoulders
• Swallowing movement

Function

The hypoglossal nerve controls tongue movements of speech, food manipulation, and swallowing. It supplies motor fibers to all of the muscles of the tongue, with the exception of the palatoglossus muscle, which is innervated by the vagus nerve (cranial nerve X) or, according to some classifications, by fibers from the glossopharyngeal nerve (cranial nerve IX) that hitchhike within the vagus.

While the hypoglossal nerve controls the tongue’s involuntary activities of swallowing to clear the mouth of saliva, most of the functions it controls are voluntary, meaning that the execution of these activities requires conscious thought.

Proper function of the hypoglossal nerve is important for executing the tongue movements associated with speech. Many languages require specific and sometimes unusual uses of the nerve to create unique speech sounds, which may contribute to the difficulties some adults encounter when learning a new language. Several corticonuclear-originating fibers supply innervation and aid in the unconscious movements required upon engaging in speech and articulation.

Progressive bulbar palsy is neuromuscular atrophy associated with the combined lesions of the hypoglossal nucleus and the nucleus ambiguous, upon the atrophy of the motor nerves of the pons and medulla. This condition causes dysfunctional tongue movements that lead to speech and chewing impairments and swallowing difficulties. Tongue muscle atrophy may also occur.

Somatomotor functions include the following:

• Innervation of tongue muscles
• Transmission of impulses for speaking and swallowing

Sensory functions include the following:

• Proprioception

Associated Conditions

The possible conditions associated with the cranial nerves are as follows:

• Abducens Nerve Diseases – Also known as sixth cranial disorders, this palsy kind of disease results in diabetics, mostly. This condition is a part of Mononeuritis Multiplex, which may result in ischemia, hypertension (sometimes), or compression of the nerve by lesions in the cavernous sinus (eg, nasopharyngeal tumors), orbit, or base of the skull.
• Accessory Nerve Diseases Of Cranial Nerves – Weakness in these muscles may result to a general disease process such as amyotrophic lateral sclerosis, Guillain-Barre syndrome or poliomyelitis. Injuries in these nerves can result in neck dissection and lymph node excision.
• Glossopharyngeal Nerve Diseases – Glossopharyngeal neuralgia is a state wherein one can experience unbearable, frequent pain in the throat, ears, tongue, and tonsils. However, this pain can stay around a few minutes.
• Cranial Nerve Injuries – These are commonly traumatic injuries.
• Cranial Nerve Malignant Neoplasm – Malignant cranial nerve sheath tumors are uncommon and deals with weak prognosis as those of spinal nerves at other places.
• Facial Nerve Diseases – This conditions pull down your facial expressions and cause weakness on your face. Bell’s palsy, Ramsay Hunt syndrome, Lyme disease are the conditions associated with facial nerve disorders.
• Hypoglossal Nerve Diseases – This condition results in tight oral musculature, making it difficult for the person to chew or eat.
• Ocular Motility Disorders – This causes impairment of eye movements.
• Olfactory Nerve Diseases – This is a result of a physical blockage in the nose and this can cause loss of smell.
• Optic Nerve Diseases – These disorders are related to loss of vision.
• Trochlear Nerve Diseases – Injuries in the nucleus of the midbrain can be counted within this.
• Vagus Nerve Diseases – These are caused by the damage of stomach muscles.
• Vestibulocochlear Nerve Diseases – Disorders related to this affects the inner ear and weaken the functions related to it.

References

Brief Overview of Cranial Nerves

The characteristics of this nerve are presented in the table.

The peripheral nervous system has 12 pairs of cranial nerves that control much of the motor and sensory functions of the head and neck.

Cranial nerves are the nerves that emerge directly from the brain (including the brainstem). In contrast, spinal nerves emerge from segments of the spinal cord. Cranial nerves relay information between the brain and parts of the body, primarily to and from regions of the head and neck.

Cranial Nerve Anatomy and Terminology

Spinal nerves emerge sequentially from the spinal cord with the spinal nerve closest to the head (C1) emerging in the space above the first cervical vertebra. The cranial nerves emerge from the central nervous system above this level.

Each cranial nerve is paired and is present on both sides. The numbering of the cranial nerves is based on the order in which they emerge from the brain, front to back (brainstem).

The terminal nerves, olfactory nerves (I) and optic nerves (II) emerge from the cerebrum or forebrain, and the remaining ten pairs arise from the brainstem, which is the lower part of the brain. The cranial nerves are considered components of the peripheral nervous system.

However, on a structural level, the olfactory, optic, and terminal nerves are more accurately considered part of the central nervous system.

The twelve cranial nerves are shown in the figure below followed by brief descriptions.

• The olfactory nerve (I): This is instrumental for the sense of smell, it is one of the few nerves that are capable of regeneration.
• The optic nerve (II): This nerve carries visual information from the retina of the eye to the brain.
• The oculomotor nerve (III): This controls most of the eye’s movements, the constriction of the pupil, and maintains an open eyelid.
• The trochlear nerve (IV): A motor nerve that innervates the superior oblique muscle of the eye, which controls rotational movement.
• The trigeminal nerve (V): This is responsible for sensation and motor function in the face and mouth.
• The abducens nerve (VI): A motor nerve that innervates the lateral rectus muscle of the eye, which controls lateral movement.
• The facial nerve (VII): This controls the muscles of facial expression, and functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity.
• The vestibulocochlear nerve (VIII): This is responsible for transmitting sound and equilibrium (balance) information from the inner ear to the brain.
• The glossopharyngeal nerve (IX): This nerve receives sensory information from the tonsils, the pharynx, the middle ear, and the rest of the tongue.
• The vagus nerve (X): This is responsible for many tasks, including heart rate, gastrointestinal peristalsis, sweating, and muscle movements in the mouth, including speech and keeping the larynx open for breathing.
• The spinal accessory (XI): This nerve controls specific muscles of the shoulder and neck.
• The hypoglossal nerve (XII): This nerve controls the tongue movements of speech, food manipulation, and swallowing.

There are many mnemonic devices to remember the cranial nerves. One that may be helpful is: Old Opie Occasionally Tries Trigonometry And Feels Very Gloomy, Vague And Hypoactive.

Olfactory (I) Nerve

The olfactory nerve, or cranial nerve I, is the first of 12 cranial nerves and is responsible for the sense of smell.

Olfactory bulb: Sagittal section of human head showing the olfactory bulb.

The olfactory nerve, or cranial nerve I, is the first of the 12 cranial nerves. It is instrumental in the sense of smell. The olfactory nerve is the shortest of the 12 cranial nerves and only one of two cranial nerves (the other being the optic nerve) that do not join with the brainstem.

The specialized olfactory receptor neurons of the olfactory nerve are located in the olfactory mucosa of the upper parts of the nasal cavity. The olfactory nerves consist of a collection of many sensory nerve fibers that extend from the olfactory epithelium to the olfactory bulb, passing through the many openings of the cribriform plate of the ethmoid bone.

Olfactory receptor neurons continue to emerge throughout life and extend new axons to the olfactory bulb. Olfactory-ensheathing glia wrap bundles of these axons and are thought to facilitate their passage into the central nervous system.

The sense of smell (olfaction) arises from the stimulation of olfactory (or odorant) receptors by small molecules of different spatial, chemical, and electrical properties that pass over the nasal epithelium in the nasal cavity during inhalation. These interactions are transduced into electrical activity in the olfactory bulb, which then transmits the electrical activity to other parts of the olfactory system and the rest of the central nervous system via the olfactory tract.

Optic (II) Nerve

The optic nerve (cranial nerve II) receives visual information from photoreceptors in the retina and transmits it to the brain.

The optic nerve is also known as cranial nerve II. It transmits visual information from the retina to the brain.

Each human optic nerve contains between 770,000 and 1.7 million nerve fibers. The eye’s blind spot is a result of the absence of photoreceptors in the area of the retina where the optic nerve leaves the eye.

The optic nerve is the second of twelve paired cranial nerves. It is considered by physiologists to be part of the central nervous system, as it is derived from an outpouching of the diencephalon during embryonic development.

As a consequence, the fibers are covered with myelin produced by oligodendrocytes, rather than Schwann cells that are found in the peripheral nervous system. The optic nerve is ensheathed in all three meningeal layers (dura, arachnoid, and pia mater) rather than the epineurium, perineurium, and endoneurium found in the peripheral nerves.

The fiber tracks of the mammalian central nervous system are incapable of regeneration. As a consequence, optic nerve damage produces irreversible blindness.

The optic nerve leaves the orbit, which is also known as an eye socket, via the optic canal, running posteromedially toward the optic chiasm, where there is a partial decussation (crossing) of fibers from the nasal visual fields of both eyes.

Most of the axons of the optic nerve terminate in the lateral geniculate nucleus (where information is relayed to the visual cortex), while other axons terminate in the pretectal nucleus and are involved in reflexive eye movements.

The optic nerve transmits all visual information including brightness perception, color perception, and contrast. It also conducts the visual impulses that are responsible for two important neurological reflexes: the light reflex and the accommodation reflex.

The light reflex refers to the constriction of both pupils that occurs when light is shone into either eye; the accommodation reflex refers to the swelling of the lens of the eye that occurs when one looks at a near object, as in reading.

Oculomotor (III) Nerve

The oculomoter nerve (cranial nerve III) controls eye movement, such as constriction of the pupil and open eyelids.

The oculomotor nerve is the third paired cranial nerve. It enters the orbit via the superior orbital fissure and controls most of the eye’s movements, including constriction of the pupil and maintaining an open eyelid by innervating the levator palpebrae superiors muscle.

The oculomotor nerve is derived from the basal plate of the embryonic midbrain. Cranial nerves IV and VI also participate in the control of eye movement.

There are two nuclei for the oculomotor nerve:

1. The oculomotor nucleus originates at the level of the superior colliculus. The muscles it controls are the striated muscle in the levator palpebrae superioris and all extraocular muscles, except for the superior oblique muscle and the lateral rectus muscle.
2. The Edinger-Westphal nucleus supplies parasympathetic fibers to the eye via the ciliary ganglion and controls the pupillae muscle (affecting pupil constriction) and the ciliary muscle (affecting accommodation).

Sympathetic postganglionic fibers also join the nerve from the plexus on the internal carotid artery in the wall of the cavernous sinus and are distributed through the nerve, for example, to the smooth muscle of the levator palpebrae superioris.

Emergence from Brain

On emerging from the brain, the oculomotor nerve is invested with a sheath of pia mater and enclosed in a prolongation from the arachnoid mater. It passes between the superior cerebellar and posterior cerebral arteries, and then pierces the dura mater anterior and lateral to the posterior clinoid process (to give attachment to the tectorium cerebella), passing between the free and attached borders of the tentorium cerebelli.

It then runs along the lateral wall of the cavernous sinus, above the other orbital nerves, receiving in its course one or two filaments from the cavernous plexus of the sympathetic nervous system, and a communicating branch from the ophthalmic division of the trigeminal nerve.

It then divides into two branches that enter the orbit through the superior orbital fissure, between the two heads of the lateral rectus (a muscle on the lateral side of the eyeball in the orbit). Here the nerve is placed below the trochlear nerve and the frontal and lacrimal branches of the ophthalmic nerve, while the nasociliary nerve is placed between its two rami (the superior and inferior branch of oculomotor nerve).

Trochlear (IV) Nerve

The trochlear nerve (cranial nerve IV) is a motor nerve that innervates a single muscle: the superior oblique muscle of the eye.

The trochlear nerve (cranial nerve IV) is a motor nerve that innervates a single muscle: the superior oblique muscle of the eye.

The trochlear nerve: The trocheal nerve and where it innervates.

The trochlear nerve is unique among the cranial nerves in several respects.

• It is the smallest nerve in terms of the number of axons it contains and it has the greatest intracranial length.
• Other than the optic nerve (cranial nerve II), it is the only cranial nerve that decussates (crosses to the other side) before innervating its target.
• It is the only cranial nerve that exits from the dorsal aspect of the brainstem.

The nucleus of the trochlear nerve is located in the caudal mesencephalon beneath the cerebral aqueduct. It is immediately below the nucleus of the oculomotor nerve (III) in the rostral mesencephalon.

The trochlear nucleus is unique in that its axons run dorsally and cross the midline before emerging from the brainstem—so a lesion of the trochlear nucleus affects the contralateral eye. Lesions of all other cranial nuclei affect the ipsilateral side (except of course the optic nerve, cranial nerve II, which innervates both eyes).

Homologous trochlear nerves are found in all jawed vertebrates. The unique features of the trochlear nerve, including its dorsal exit from the brainstem and its contralateral innervation, are seen in the primitive brains of sharks.

The human trochlear nerve is derived from the basal plate of the embryonic midbrain.

Clinical Syndromes

There are two major clinical syndromes that can manifest through damage to the trochlear nerve:

• Vertical diplopia: Injury to the trochlear nerve causes weakness of downward eye movement with consequent vertical diplopia (double vision).
• Torsional diplopia: Weakness of intorsion results in torsional diplopia, in which two different visual fields, tilted with respect to each other, are seen at the same time. To compensate for this, patients with trochlear nerve palsies tilt their heads to the opposite side, in order to fuse the two images into a single visual field.

The clinical syndromes can originate from both peripheral and central lesions. A peripheral lesion is damage to the bundle of nerves, in contrast to a central lesion, which is damage to the trochlear nucleus.

Trigeminal (V) Nerve

The trigeminal nerve is the fifth cranial nerve and it is responsible for sensation and motor function in the face and mouth.

The trigeminal nerve (cranial nerve V), and it contains both sensory and motor fibers. It is responsible for sensation in the face and certain motor functions such as biting, chewing, and swallowing.

Trigeminal nerve: Schematic illustration of the trigeminal nerve (labeled Sensory root above) and the structures it innervates in the face and mouth.

Structure

The trigeminal nerve is the largest of the cranial nerves. Its name, trigeminal, means three twins. It is derived from the fact that each nerve, one on each side of the pons, has three major branches: the ophthalmic nerve (V1 in the illustration below), the maxillary nerve (V2), and the mandibular nerve (V3).

The ophthalmic and maxillary nerves are purely sensory. The mandibular nerve has both sensory and motor functions.

The three branches converge on the trigeminal ganglion that is located within the trigeminal cave in the brain; it contains the cell bodies of incoming sensory nerve fibers. The trigeminal ganglion is analogous to the dorsal root ganglia of the spinal cord, which contain the cell bodies of incoming sensory fibers from the rest of the body.

Areas of the face innervated by the trigeminal nerve: The ophthalmic nerve branch (V1) innervates the bright red area, the maxillary nerve branch (V2) innervates the light red area, and the mandibular nerve branch (V3) innervates the yellow area.

From the trigeminal ganglion, a single large sensory root enters the brainstem at the level of the pons. Immediately adjacent to the sensory root, a smaller motor root emerges from the pons at the same level.

Motor fibers pass through the trigeminal ganglion on their way to peripheral muscles, but their cell bodies are located in the nucleus of the trigeminal nerve, deep within the pons.

Function

The sensory function of the trigeminal nerve is to provide tactile, proprioceptive, and nociceptive afferents to the face and mouth. The motor component of the mandibular division (V3) of the trigeminal nerve controls the movement of eight muscles, including the four muscles of mastication: the masseter, the temporal, and the medial and lateral pterygoids.

The other four muscles are the tensor veli palatini, the mylohyoid, the anterior belly of the digastric, and the tensor tympani. With the exception of the tensor tympani, all of these muscles are involved in biting, chewing and swallowing, and all have bilateral cortical representation.

The function of the trigeminal nerve

The sensory functions of the trigeminal nerve are the transmission of impulses for touch, pain, temperature, proprioception, and the somatomotor function with mandibular movement.

The ophthalmic nerve (V1) has mainly sensory fibers and has the following functions:

• Sensory innervation of the skin of the forehead, the eyes, the nose, and the nasal mucosa
• Innervation of the iris, the cornea, the conjunctiva, and the lacrimal gland
• Supply of the skin of the forehead and transmission of pressure pain at the exit point of the supraorbital foramen

The maxillary nerve (V2) innervates the facial skin with sensory fibers and a parasympathetic part and has the following tasks:

• Supply of the lower eyelid, its conjunctiva, and the upper lip
• Supply of the teeth (upper molar, incisor, and canine), nasal cavity gums of the upper jaw and the palate mucosa (including the uvula), and tonsils
• Supply of the lacrimal gland and nasal glands by the parasympathetic part

In addition to the sensory fibers, the mandibular nerve also contains motor fibers of the trigeminal nerve. It has the following tasks:

• Innervation of all the muscles of mastication (e.g., masseter muscle) and the suprahyoid muscles by the motor fibers
• Supply of the teeth (lower molar, incisor, and canine teeth), the gums of the lower jaw, the buccal mucosa, the dorsum of the tongue, and the external acoustic meatus, including the eardrum, by the sensory fibers

Abducens (VI) Nerve

The abducens nerve (cranial nerve VI) controls the lateral movement of the eye through innervation of the lateral rectus muscle.

The abducens nerve (cranial nerve VI) is a somatic efferent nerve that, in humans, controls the movement of a single muscle: the lateral rectus muscle of the eye that moves the eye horizontally. In most other mammals it also innervates the musculus retractor bulbi, which can retract the eye for protection. Homologous abducens nerves are found in all vertebrates except lampreys and hagfishes.

Abducens nerve: Schematic of cranial nerves showing cranial nerve VI, the abducens nerve.

The abducens nerve leaves the brainstem at the junction of the pons and the medulla, medial to the facial nerve. In order to reach the eye, it runs upward (superiorly) and then bends forward (anteriorly).

The nerve enters the subarachnoid space when it emerges from the brainstem. It runs upward between the pons and the clivus, and then pierces the dura mater to run between the dura and the skull.

At the tip of the petrous temporal bone, it makes a sharp turn forward to enter the cavernous sinus. In the cavernous sinus it runs alongside the internal carotid artery. It then enters the orbit through the superior orbital fissure and innervates the lateral rectus muscle of the eye.

The long course of the abducens nerve between the brainstem and the eye makes it vulnerable to injury at many levels. For example, fractures of the petrous temporal bone can selectively damage the nerve, as can aneurysms of the intracavernous carotid artery.

Mass lesions that push the brainstem downward can damage the nerve by stretching it between the point where it emerges from the pons and the point where it hooks over the petrous temporal bone.

Facial (VII) Nerve

The facial nerve (cranial nerve VII) determines facial expressions and the taste sensations of the tongue.

The facial nerve: Illustration of the facial nerve and its branches.

The facial nerve is the seventh (cranial nerve VII) of the 12, paired cranial nerves. It emerges from the brainstem between the pons and the medulla and controls the muscles of facial expression.

It also functions in the conveyance of taste sensations from the anterior two-thirds of the tongue and oral cavity, and it supplies preganglionic parasympathetic fibers to several head and neck ganglia.

Location

The motor part of the facial nerve arises from the facial nerve nucleus in the pons, while the sensory part of the facial nerve arises from the nervus intermedius. The motor and sensory parts of the facial nerve enter the petrous temporal bone into the internal auditory meatus (intimately close to the inner ear), then runs a tortuous course (including two tight turns) through the facial canal, emerges from the stylomastoid foramen, and passes through the parotid gland, where it divides into five major branches.

Although it passes through the parotid gland, it does not innervate the gland (this is the responsibility of cranial nerve IX, the glossopharyngeal nerve). The facial nerve forms the geniculate ganglion prior to entering the facial canal.

The path of the facial nerve can be divided into six segments.

• The intracranial (cisternal) segment.
• The meatal segment (brainstem to the internal auditory canal).
• The labyrinthine segment (internal auditory canal to geniculate ganglion),
• The tympanic segment (from geniculate ganglion to pyramidal eminence).
• The mastoid segment (from pyramidal eminence to stylomastoid foramen).
• The extratemporal segment (from stylomastoid foramen to post parotid branches).

Function

Bell’s Palsy: A person attempting to show his teeth and raise his eyebrows with Bell’s palsy on his right side (left side of the image).

Motor functions include the following:

• Innervation of the entire mimic facial musculature (platysma and muscles of the auricle)
• Eyelid movement and closing of the eye by the orbicularis oculi muscle
• Movement and closing of the mouth by the orbicularis oris muscle
• Precise adjustment of the auditory ossicles by the stapedius muscle
• Movement of the mandible by the mentalis muscle

Sensory and parasympathetic functions include the following:

• Sense of taste in the anterior two-thirds of the tongue
• Innervation of the three large salivary glands, the lacrimal glands, and the nasal glands
• External acoustic meatus

Voluntary facial movements, such as wrinkling the brow, showing teeth, frowning, closing the eyes tightly (inability to do so is called lagophthalmos), pursing the lips, and puffing out the cheeks, all test the facial nerve. There should be no noticeable asymmetry.

In an upper motor neuron lesion, called central seven (central facial palsy ), only the lower part of the face on the contralateral side will be affected due to the bilateral control to the upper facial muscles (frontalis and orbicularis oculi).

Lower motor neuron lesions can result in cranial nerve VII palsy (Bell’s palsy is the idiopathic form of facial nerve palsy), manifested as both upper and lower facial weakness on the same side of the lesion.

Taste can be tested on the anterior 2/3 of the tongue. This can be tested with a swab dipped in a flavored solution, or with electronic stimulation (similar to putting your tongue on a battery).

In regards to the corneal reflex, the afferent arc is mediated by the general sensory afferents of the trigeminal nerve. The efferent arc occurs via the facial nerve.

The reflex involves the consensual blinking of both eyes in response to stimulation of one eye. This is due to the facial nerve’s innervation of the muscles of facial expression, namely the orbicularis oculi, responsible for blinking. Thus, the corneal reflex effectively tests the proper functioning of both cranial nerves V and VII.

Vestibulocochlear (VIII) Nerve

The vestibulocochlear nerve (cranial nerve VIII) carries information about hearing and balance.

The vestibulocochlear nerve (also known as the auditory vestibular nerve and cranial nerve VIII) has axons that carry the modalities of hearing and equilibrium.

It consists of the cochlear nerve that carries information about hearing, and the vestibular nerve that carries information about balance.

This is the nerve along which the sensory cells (the hair cells) of the inner ear transmit information to the brain. It emerges from the pons and exits the inner skull via the internal acoustic meatus (or internal auditory meatus) in the temporal bone.

Vestibular system’s semicircular canal: An illustration of the inner ear showing its semicircular canal, hair cells, ampulla, cupula, vestibular nerve, and fluid.

The vestibulocochlear nerve consists mostly of bipolar neurons and splits into two large divisions: the cochlear nerve and the vestibular nerve. The cochlear nerve travels away from the cochlea of the inner ear where it starts as the spiral ganglia.

Processes from the organ of Corti (the receptor organ for hearing) conduct afferent transmission to the spiral ganglia. It is the inner hair cells of the organ of Corti that are responsible for activating the afferent receptors in response to pressure waves reaching the basilar membrane through the transduction of sound.

The vestibular nerve travels from the vestibular system of the inner ear. The vestibular ganglion houses the cell bodies of the bipolar neurons and extends processes to five sensory organs.

Three of these are the cristae, located in the ampullae of the semicircular canals. Hair cells of the cristae activate afferent receptors in response to rotational acceleration.

The other two sensory organs supplied by the vestibular neurons are the maculae of the saccule and utricle. Hair cells of the maculae activate afferent receptors in response to linear acceleration.

The vestibulocochlear nerve has axons that carry the modalities of hearing and equilibrium. Damage to the vestibulocochlear nerve may cause hearing loss, vertigo, a false sense of motion, loss of equilibrium in dark places, nystagmus, motion sickness, and gaze-evoked tinnitus.

A benign primary intracranial tumor of the vestibulocochlear nerve is called a vestibular schwannoma (also called acoustic neuroma).

Functions of the vestibular nerve include the following:

• Regulation of the hair cells of the organ of Corti for the adjustment of sensitivity (spatial position)
• Transmission of impulses for the sense of balance, maintaining equilibrium

Functions of the cochlear nerve include the following:

• Regulation of the hair cells of the organ of Corti for the adjustment of sensitivity (regarding sound waves)
• Transmission of impulses for hearing

Glossopharyngeal (IX) Nerve

The glossopharyngeal nerve (cranial nerve IX) serves many distinct functions, including providing sensory innervation to various head and neck structures.

Structure

The glossopharyngeal nerve is the ninth of 12 pairs of cranial nerves. It exits the brainstem out from the sides of the upper medulla, just rostral (closer to the nose) to the vagus nerve.

Glossopharyngeal nerve: Image of head structures including the glossopharyngeal nerve.

Function

There are a number of functions of the glossopharyngeal nerve. It controls muscles in the oral cavity and upper throat, as well as part of the sense of taste and the production of saliva.

Along with taste, the glossopharyngeal nerve relays general sensations from the pharyngeal walls. The various functions of the glossopharyngeal nerve are that:

• It receives general sensory fibers (ventral trigeminothalamic tract) from the tonsils, the pharynx, the middle ear, and the posterior 1/3 of the tongue.
• It receives special sensory fibers (taste) from the posterior 1/3 of the tongue.
• It receives visceral sensory fibers from the carotid bodies, carotid sinus.
• It supplies parasympathetic fibers to the parotid gland via the otic ganglion.
• It supplies motor fibers to the stylopharyngeus muscle.
• It contributes to the pharyngeal plexus.

Motor functions include the following:

• Innervation of the palate muscles and the muscles of the pharynx by the vagus nerve
• Dilatation of the pharynx during swallowing and speaking

Sensory functions include the following:

• Innervation of the mucosa of the middle ear, the mastoid, and the eardrum
• Supply of the velum, including the palatine tonsil and the posterior third of the tongue
• Taste and somatic perception (touch, pain, and temperature) of the posterior third of the tongue
• Proprioception in the swallowing musculature
• Blood pressure regulation
• Regulation of the oxygen and carbon dioxide content of the blood for the control of ventilation

Parasympathetic functions include the following:

• Stimulation of salivation
• Carotid glomus—contains chemoreceptors responsible for the oxygen content, as well as pressure receptors that are important for the regulation of blood pressure

Five Functional Components

The glossopharyngeal nerve consists of five components with distinct functions:

• Branchial motor (special visceral efferent): Supplies the stylopharyngeus muscle.
• Visceral motor (general visceral efferent): Provides parasympathetic innervation of the parotid gland.
• Visceral sensory (general visceral afferent): Carries visceral sensory information from the carotid sinus and body.
• General sensory (general somatic afferent): Provides general sensory information from the skin of the external ear, the internal surface of the tympanic membrane, upper pharynx, and the posterior 1/3 of the tongue.
• Special sensory (special afferent): Provides taste sensation from the posterior 1/3 of the tongue.

Vagus (X) Nerve

The vagus nerve (cranial nerve X) is responsible for parasympathetic output to the heart and visceral organs.

Vagus Nerve Anatomy

The vagus nerve, also known as the pneumogastric nerve or cranial nerve X, is the tenth of twelve paired cranial nerves. Upon leaving the medulla between the medullary pyramid and the inferior cerebellar peduncle, it extends through the jugular foramen, then passes into the carotid sheath between the internal carotid artery and the internal jugular vein below the head, to the neck, chest and abdomen, where it contributes to the innervation of the viscera.

Vagus nerve: Diagram demonstrating the course of the vagus nerve.

Besides output to the various organs in the body, the vagus nerve conveys sensory information about the state of the body’s organs to the central nervous system.  Eighty to 90% of the nerve fibers in the vagus nerve are afferent (sensory) nerves that communicate the state of the viscera to the brain.

The vagus nerve includes axons that emerge from or converge onto four nuclei of the medulla.

• The dorsal nucleus of vagus nerve: Sends parasympathetic output to the viscera, especially the intestines.
• The nucleus ambiguus: Sends parasympathetic output to the heart (slowing it down).
• The solitary nucleus: Receives afferent taste information and primary afferents from visceral organs.
• The spinal trigeminal nucleus: Receives information about deep/crude touch, pain, and temperature of the outer ear, the dura of the posterior cranial fossa, and the mucosa of the larynx.

Function

The vagus nerve supplies motor parasympathetic fibers to all the organs, except the suprarenal (adrenal) glands, from the neck down to the second segment of the transverse colon. The vagus also controls a few skeletal muscles, most notably:

• Cricothyroid muscle.
• Levator veli palatini muscle.
• Salpingopharyngeus muscle.
• Palatoglossus muscle.
• Palatopharyngeus muscle.
• Superior, middle, and inferior pharyngeal constrictors.
• Muscles of the larynx (speech).

Sensory functions include the following:

• Taste and sensation (touch, pain, temperature, etc.) in the epiglottis and the pharynx
• Blood pressure regulation
• Regulation of the oxygen and carbon dioxide content in the blood for the control of ventilation
• Sensation in visceral, thoracic, and abdominal organs

Somatomotor functions include the following:

• Swallowing
• Coughing
• Voice production

Parasympathetic functions include the following:

• Contraction and relaxation of the smooth musculature of the gastrointestinal tract
• Reduction of the heart rate
• Secretion of digestive juices

This means that the vagus nerve is responsible for such varied tasks as heart rate, gastrointestinal peristalsis, sweating, and quite a few muscle movements in the mouth, including speech (via the recurrent laryngeal nerve), swallowing, and keeping the larynx open for breathing (via action of the posterior cricoarytenoid muscle, the only abductor of the vocal folds).

It also has some afferent fibers that innervate the inner (canal) portion of the outer ear, via the auricular branch (also known as Alderman’s nerve) and part of the meninges. This explains why a person may cough when tickled on the ear (such as when trying to remove ear wax with a cotton swab).

Afferent vagus nerve fibers that innervate the pharynx and back of the throat are responsible for the gag reflex. In addition, 5-HT3 receptor-mediated afferent vagus stimulation in the gut due to gastroenteritis and other insults is a cause of vomiting.

Cardiovascular Influence

Parasympathetic innervation of the heart is partially controlled by the vagus nerve and is shared by the thoracic ganglia. Activation of the vagus nerve typically leads to a reduction in heart rate and/or blood pressure.

This occurs commonly in cases of viral gastroenteritis, acute cholecystitis, or in response to stimuli such as the Valsalva maneuver or pain. Excessive activation of the vagal nerve during emotional stress can also cause vasovagal syncope due to a sudden drop in cardiac output, causing cerebral hypoperfusion.

Accessory (XI) Nerve

The accessory nerve (cranial nerve XI) controls the muscles of the shoulder and neck.

Anatomic Description

The accessory nerve (cranial nerve XI) controls the sternocleidomastoid and trapezius muscles of the shoulder and neck. It begins in the central nervous system (CNS) and exits the cranium through a foramen.

Unlike the other 11 cranial nerves, the accessory nerve begins outside the skull. In fact, most of the fibers of the nerve originate in neurons situated in the upper spinal cord.

The accessory nerve: Upon exiting the skull via the jugular foramen, the spinal accessory nerve pierces the sternocleidomastoid muscle before terminating on the trapezius muscle.

Hypoglossal (XII) Nerve

The hypoglossal nerve (cranial nerve XII) controls the muscles of the tongue.

Structure and Location

The hypoglossal nerve is the twelfth cranial nerve (XII) and innervates all extrinsic and intrinsic muscles of the tongue, except for the palatoglossus. The hypoglossal nerve emerges from the medulla oblongata in the preolivary sulcus where it separates the olive (olivary body) and the pyramid (medullary pyramid).

It goes on to traverse the hypoglossal canal and, upon emerging, it branches and merges with a branch from the anterior ramus of C1. It passes behind the vagus nerve and between the internal carotid artery and internal jugular vein which lies on the carotid sheath. After passing deep to the posterior belly of the digastric muscle it proceeds to the submandibular region to enter the tongue.

Sensory functions include the following:

• Proprioception

Somatomotor functions include the following:

• Innervation of the sternocleidomastoid muscle and the trapezius muscle to control the movement of the head and the shoulders
• Swallowing movement

Function

The hypoglossal nerve controls tongue movements of speech, food manipulation, and swallowing. It supplies motor fibers to all of the muscles of the tongue, with the exception of the palatoglossus muscle, which is innervated by the vagus nerve (cranial nerve X) or, according to some classifications, by fibers from the glossopharyngeal nerve (cranial nerve IX) that hitchhike within the vagus.

While the hypoglossal nerve controls the tongue’s involuntary activities of swallowing to clear the mouth of saliva, most of the functions it controls are voluntary, meaning that the execution of these activities requires conscious thought.

Proper function of the hypoglossal nerve is important for executing the tongue movements associated with speech. Many languages require specific and sometimes unusual uses of the nerve to create unique speech sounds, which may contribute to the difficulties some adults encounter when learning a new language. Several corticonuclear-originating fibers supply innervation and aid in the unconscious movements required upon engaging in speech and articulation.

Progressive bulbar palsy is neuromuscular atrophy associated with the combined lesions of the hypoglossal nucleus and the nucleus ambiguous, upon the atrophy of the motor nerves of the pons and medulla. This condition causes dysfunctional tongue movements that lead to speech and chewing impairments and swallowing difficulties. Tongue muscle atrophy may also occur.

Somatomotor functions include the following:

• Innervation of tongue muscles
• Transmission of impulses for speaking and swallowing

Sensory functions include the following:

• Proprioception

Associated Conditions

The possible conditions associated with the cranial nerves are as follows:

• Abducens Nerve Diseases – Also known as sixth cranial disorders, this palsy kind of disease results in diabetics, mostly. This condition is a part of Mononeuritis Multiplex, which may result in ischemia, hypertension (sometimes), or compression of the nerve by lesions in the cavernous sinus (eg, nasopharyngeal tumors), orbit, or base of the skull.
• Accessory Nerve Diseases Of Cranial Nerves – Weakness in these muscles may result to a general disease process such as amyotrophic lateral sclerosis, Guillain-Barre syndrome or poliomyelitis. Injuries in these nerves can result in neck dissection and lymph node excision.
• Glossopharyngeal Nerve Diseases – Glossopharyngeal neuralgia is a state wherein one can experience unbearable, frequent pain in the throat, ears, tongue, and tonsils. However, this pain can stay around a few minutes.
• Cranial Nerve Injuries – These are commonly traumatic injuries.
• Cranial Nerve Malignant Neoplasm – Malignant cranial nerve sheath tumors are uncommon and deals with weak prognosis as those of spinal nerves at other places.
• Facial Nerve Diseases – This conditions pull down your facial expressions and cause weakness on your face. Bell’s palsy, Ramsay Hunt syndrome, Lyme disease are the conditions associated with facial nerve disorders.
• Hypoglossal Nerve Diseases – This condition results in tight oral musculature, making it difficult for the person to chew or eat.
• Ocular Motility Disorders – This causes impairment of eye movements.
• Olfactory Nerve Diseases – This is a result of a physical blockage in the nose and this can cause loss of smell.
• Optic Nerve Diseases – These disorders are related to loss of vision.
• Trochlear Nerve Diseases – Injuries in the nucleus of the midbrain can be counted within this.
• Vagus Nerve Diseases – These are caused by the damage of stomach muscles.
• Vestibulocochlear Nerve Diseases – Disorders related to this affects the inner ear and weaken the functions related to it.

References

Structure of a Nerve

A nerve is the primary structure of the peripheral nervous system and is composed of bundles of axons.

Nerves are the organs that make up the peripheral nervous system. It consists of a cord-like structure with multiple nerve fibers (also called axons) wrapped in layers of tissue and fat. This axon has layers of connective tissue around it. This connective tissue is called the endoneurium. This entire nerve is further enclosed in another layer of connective tissue called the epineurium.

The structure of a nerve is explained below:

• A group of neurons is organized into bundles inside the nerves. This bundle is known as fascicles.
• The perineurium surrounds and holds together each fascicle.
• The perineurium is concentrically laminated and composed of flattened cells collagen fibers and basement membranes.
• Neurons and blood vessels are held inside the fascicles by a loose connective tissue known as endoneurium. It covers and holds the outer surface of the nerves together.
• Arteries and veins are present between the fascicles. These blood vessels supply nutrients and gases to the neurons inside the fascicles.

Nerve Anatomy

Nerves: An illustration of the main nerves of the arm.

A nerve is an enclosed, cable-like bundle of axons (the projections of neurons) in the peripheral nervous system (PNS). A nerve provides a structured pathway that supports the electrochemical nerve impulses transmitted along each of the axons.

In the central nervous system, the analogous structures are known as tracts. Neurons are sometimes referred to as nerve cells, although this term is misleading since many neurons do not occupy nerves, and nerves also include non-neuronal support cells (glial cells) that contribute to the health of enclosed neurons.

Each nerve contains many axons that are sometimes referred to as fibers. Within a nerve, each axon is surrounded by a layer of connective tissue called the endoneurium. The axons are bundled together into groups called fascicles. Each fascicle is wrapped in a layer of connective tissue called the perineurium.

Finally, the entire nerve is wrapped in a layer of connective tissue called the epineurium. See the following illustrations of these structures.

The endoneurium consists of an inner sleeve of material called the glycocalyx and a mesh of collagen. Nerves are bundled along with blood vessels, which provide essential nutrients and energy to the enclosed and metabolically demanding, neurons.

Within the endoneurium, individual nerve fibers are surrounded by a liquid called the endoneurial fluid. The endoneurium has properties analogous to the blood-brain barrier. It prevents certain molecules from crossing from the blood into the endoneurial fluid.

Classification of Nerves

Nerves are primarily classified based on their direction of travel to or from the CNS, but they are also subclassified by other nerve characteristics.

Nerve Classifications

The direction of Signal Transmission

Nerves are categorized into three, primary groups based on the direction of signal transmission within the nervous system.

• Afferent nerves conduct signals from sensory neurons to the central nervous system, for example from mechanoreceptors in skin.
• Efferent nerves conduct signals away from the central nervous system to target muscles and glands.
• Mixed nerves contain both afferent and efferent axons, and thus conduct both incoming sensory information and outgoing muscle commands in the same nerve bundle.

Central Nervous System Connection

Nerves can be further categorized based on where they connect to the central nervous system. Spinal nerves innervate much of the body and connect through the spinal column to the spinal cord. Spinal nerves are assigned letter-number designations according to the vertebra where they connect to the spinal column. Cranial nerves innervate parts of the head and connect directly to the brain. Cranial nerves are typically assigned Roman numerals from 0 to 12.

Diameter, Conduction Velocity, Myelination State

Peripheral nerve fibers are grouped based on the diameter, signal conduction velocity, and myelination state of the axons. These classifications apply to both sensory and motor fibers. Fibers of the A group have a large diameter, high conduction velocity, and are myelinated.

The A group is further subdivided into four types (A-alpha, A-beta, A-delta, and A-gamma fibers) based on the information carried by the fibers and the tissues they innervate.

• A-alpha fibers are the primary receptors of the muscle spindle and golgi tendon organ.
• A-beta fibers act as secondary receptors of the muscle spindle and contribute to cutaneous mechanoreceptors.
• A-delta fibers are free nerve endings that conduct painful stimuli related to pressure and temperature.
• A-gamma fibers are typically motor neurons that control the intrinsic activation of the muscle spindle.

Fibers of the B group are myelinated with a small diameter and have a low conduction velocity. The primary role of B fibers is to transmit autonomic information. Fibers of the C group are unmyelinated, have a small diameter, and low conduction velocity. The lack of myelination in the C group is the primary cause of their slow conduction velocity.

Saltatory conduction: Demonstrates the faster propagation of an action potential in myelinated neurons than that of unmyelinated neurons.

C fiber axons are grouped together into what is known as Remak bundles. These occur when an unmyelinated Schwann cell bundles the axons close together by surrounding them. The Schwann cell keeps them from touching each other by squeezing its cytoplasm between the axons.

C fibers are considered polymodal because they can often respond to combinations of thermal, mechanical, and chemical stimuli.

A-delta and C fibers both contribute to the detection of diverse painful stimuli. Because of their higher conduction velocity, A-delta fibers are responsible for the sensation of sharp, initial pain and respond to a weaker intensity of the stimulus.

These nerve fibers are associated with acute pain and therefore constitute the afferent portion of the reflex arc that results in pulling away from noxious stimuli.  An example is a retraction or your hand from a hot stove. Slowly conducting, unmyelinated C fibers, by contrast, carry slow, longer-lasting pain sensations.

References