Anatomy of Spinal Cord – Types, Structure, Functions

Anatomy of Spinal Cord – Types, Structure, Functions

Anatomy of Spinal Cord/The spinal cord is a long, thin, tubular structure made up of nervous tissue, which extends from the medulla oblongata in the brainstem to the lumbar region of the vertebral column. It encloses the central canal of the spinal cord, which contains cerebrospinal fluid. The brain and spinal cord together make up the central nervous system (CNS). In humans, the spinal cord begins at the occipital bone, passing through the foramen magnum and entering the spinal canal at the beginning of the cervical vertebrae. The spinal cord extends down to between the first and second lumbar vertebrae, where it ends. The enclosing bony vertebral column protects the relatively shorter spinal cord. It is around 45 cm (18 in) in men and around 43 cm (17 in) long in women. The diameter of the spinal cord ranges from 13 mm (12 in) in the cervical and lumbar regions to 6.4 mm (14 in) in the thoracic area.

Types of The Spinal Cord

Several classic patterns of injury are well described.

Complete Transection of the Spinal Cord 

  • These injuries typically demonstrate complete bilateral loss of motor function, pain sensation, temperature sensation, proprioception, vibratory sensation, and tactile sensation below the level of injury.
  • Lumbosacral injuries will present with paralysis and loss of sensation in the lower extremities. These injuries may also result in loss of bowel control, loss of bladder control, and sexual dysfunction.
  • Thoracic injuries lead to the same deficits as lumbosacral injuries and, in addition, may result in loss of function of the muscles of the torso, leading to difficulty maintaining posture.
  • Cervical injuries lead to the same deficits as thoracic injuries and, also, may result in loss of function of the upper extremities leading to tetraplegia. Injuries above C5 may also cause respiratory compromise due to loss of innervation of the diaphragm.

Central Cord Syndrome

  • This is the most common incomplete SCI.
  • Injury is caused by hyperextension of the neck leading to compression of the cervical spinal cord, causing damage primarily to the center of the cord.
  • This pattern of injury leads to weakness affecting the upper extremities more so than the lower extremities. This pattern occurs as the corticospinal tracts are arranged with those axons supplying the upper extremities located closer to the center of the spinal cord, while those supplying the lower extremities are closer to the periphery.
  • There may also be an associated loss of pain and temperature sensation below the level of injury.

Anterior Cord Syndrome

  • Classically due to compromise of blood flow from the anterior spinal artery.
  • Bilateral injury to the spinothalamic tracts leads to bilateral loss of pain and temperature sensation below the level of injury.
  • Bilateral injury to corticospinal tracts leads to weakness or paralysis below the level of injury.
  • As dorsal columns are unaffected, tactile sensation, proprioception, and vibratory sensation remain intact.

Posterior Cord Syndrome

  • This injury pattern rarely occurs due to trauma. More often, injury is due to infectious, toxic, or metabolic causes.
  • Damage to dorsal columns causes loss of tactile sensation, proprioception, and vibratory sensation.
  • As spinothalamic and corticospinal tracts are unaffected, there is the preservation of pain sensation, temperature sensation, and motor function.

Brown-Séquard Syndrome 

  • Injury results from right or left-sided hemisection of the spinal cord.
  • Transection of the corticospinal and dorsal column nerve tracts leads to ipsilateral loss of motor function, tactile sensation, proprioception, and vibratory sensation below the level of injury.
  • Transection of the spinothalamic tract leads to contralateral loss of pain and temperature sensation below the level of injury.

Conus medullaris Syndrome

  • It is caused by injury to the terminal aspect of the spinal cord, just proximal to the cauda equina.
  • It characteristically presents with loss of sacral nerve root functions. Loss of Achilles tendon reflexes, bowel and bladder dysfunction, and sexual dysfunction may be observable.

Neurogenic Shock 

  • It results from high cervical injuries affecting the cervical ganglia, which leads to a loss of sympathetic tone.
  • Loss of sympathetic tone results in a shock state characterized by hypotension and bradycardia.

Overview of the Spinal Cord

Cellular

The spinal cord is composed of gray matter and white matter that appears in cross-section as roughly H-shaped gray matter surrounded by white matter. The gray matter consists of the cell bodies of motor and sensory neurons, interneurons, and neuropils (neuroglia cells and mostly unmyelinated axons), while the white matter is composed of interconnecting fiber tracts, which are primarily myelinated sensory and motor axons. The supports of the gray matter’s “H” make up the right dorsal, right ventral, left dorsal, and left ventral horns. Running longitudinally through the center of the spinal cord is the central canal, which is continuous with the brain’s ventricles and filled with cerebrospinal fluid.

The white matter organizes into tracts. Ascending tracts carry information from the sensory receptors to higher levels of the central nervous system, while descending tracts carry information from the central nervous system to the periphery. The major tracts and their most defining features are as follows:

Ascending Tracts

  • Dorsal column: contains the gracile fasciculus and cuneate fasciculus, which together are the dorsal funiculus. The dorsal column is responsible for pressure and vibration sensation and two-point discrimination, movement sense, and conscious proprioception. The dorsal column decussates at the superior portion of the medulla oblongata and forms the medial lemniscus.
  • Lateral spinothalamic: carries pain and temperature information. The lateral spinothalamic tract decussates at the anterior commissure, two segments above the entry to the spinal cord.
  • Anterior spinothalamic: carries crude touch and pressure information. It decussates similar to the lateral spinothalamic tract.
  • Dorsal and ventral spinocerebellar: transmit unconscious proprioception sensory information to the cerebellum. The ventral spinocerebellar tract does not decussate, while the dorsal spinocerebellar tract decussates twice, making them both ipsilateral.

 Descending Tracts

  • Lateral and anterior corticospinal: involved in conscious control of the skeletal muscle. The majority of lateral corticospinal tract fibers decussate at the inferior portion of the medulla oblongata while anterior corticospinal descends ipsilaterally in the spinal cord and decussates at the segmental level. The lateral corticospinal tract, also called the pyramidal tract, innervates primarily contralateral muscles of the limbs, while the anterior innervates proximal muscles of the trunk.
  • Vestibulospinal: carries information from the inner ear to control head positioning and is involved in modifying muscle tone to maintain posture and balance. The vestibulospinal tract does not decussate.
  • Rubrospinal: is involved in the movement of the flexor and extensor muscles. The rubrospinal tract originates from the red nuclei in the midbrain and decussates at the start of its pathway.
  • Reticulospinal: originates from the reticular formation housed in the brainstem, and it facilitates, influences, and supplements the corticospinal tract. The reticulospinal tract does not decussate.

There is a laminar distribution of neurons in the gray matter, characterized by density and topography:

  • Lamina I is located at the tip of the dorsal horn and is composed of loosely packed neuropil along with neurons of low neuronal density. The most abundant neuron in lamina I is the Waldeyer cell: large, fusiform, and with a disk-shaped dendritic domain.
  • Lamina II is composed mainly of islet cells with rostrocaudal axes, which contain GABA and are thought to be inhibitory, and stalked cells with dorsoventral dendritic trees.
  • Lamina III has intermediate size cells, including antenna-like and radial neurons, many of which contain GABA or glycine and are also considered inhibitory.
  • Lamina IV contains antenna-like cells and transverse cells, with dendrites that mostly go to Laminas II and III and whose axons are mainly thought to enter the spinothalamic tract. Lateral from lamina IV is the lateral spinal nucleus, which sends signals to lamina IV from the midbrain and brainstem.
  • Lamina V and VI are composed of medium-sized multipolar neurons that can be fusiform or triangular. These neurons communicate with the reticular formation of the brainstem.
  • Lamina VII is composed of homogenous medium-sized multipolar neurons and contains, in individual segments, well-defined nuclei, including the intermediolateral nucleus (T1-L1), which has autonomic functions, and the dorsal nucleus of Clarke (T1-L2), which make up the dorsal spinocerebellar tract.
  • Lamina VIII consists of neurons with dorsoventrally polarized dendritic trees.
  • Lamina IX has the cell bodies of motor neurons, with dendrites extending dorsally into laminas as far as VI. Lamina IX also has Renshaw cells, inhibitory interneurons, placed at the medial border of motor nuclei.
  • Lamina X is the substantia grisea centralis, or the gray matter that surrounds the central canal. In the distal portion, lamina X consists of bipolar cells with fan-shaped dendritic trees, and in the ventral portion, lamina X consists of bipolar cells with poorly ramified longitudinal dendrites.

The spinal cord runs along the inside of the vertebral column and serves as the signaling conduit between the brain and the periphery.

Key Points

The spinal cord extends from the occipital bone of the skull until it terminates near the second lumbar vertebra.

The spinal cord is protected by three layers of meninges: the dura mater, the arachnoid mater, and the pia mater.

The central nervous system (CNS) is made up of the brain and spinal cord. The area between the arachnoid space and the pia mater contains cerebral spinal fluid (CSF).

The spinal cord is divided into 31 segments that send nerve rootlets out into the body through intervertebral foramen.

Each segment of the spinal cord is associated with a pair of ganglia called dorsal root ganglia, which are situated just outside of the spinal cord and contain cell bodies of sensory neurons. These neurons travel into the spinal cord via the dorsal roots.

Ventral roots consist of axons from motor neurons, which bring information to the periphery from cell bodies within the CNS. Dorsal roots and ventral roots come together and exit the intervertebral foramina as they become spinal nerves.

Key Terms

peripheral nervous system: The part of the nervous system that consists of the nerves and ganglia on the outside of the brain and spinal cord.

efferent: The conduction of impulses outward from the brain or spinal cord.

afferent: The conduction of impulses inwards to the brain or spinal cord.

cauda equina: A bundle of nerve roots at the base of the spinal column.

spinal cord: A thick, whitish cord of nerve tissue that is a major part of the vertebrate central nervous system. It extends from the brain stem down through the spine, with nerves branching off to various parts of the body.

EXAMPLES

  • A lumbar puncture (spinal tap) is an example of a medical procedure that directly targets the spinal cord.
  • The birth defect spina bifida is a failure of the vertebral arch to close, exposing the spinal cord.

The spinal cord is a long, thin, tubular bundle of nervous tissue and support cells that extends from the medulla oblongata of the brain to the level of the lumbar region. The brain and spinal cord together make up the central nervous system (CNS). The spinal cord, protected by the vertebral column, begins at the occipital bone and extends down to the space between the first and second lumbar vertebrae. The spinal cord has a varying width, ranging from 0.5 inch thick in the cervical and lumbar regions to 0.25 inch thick in the thoracic area. The length of the spinal cord is approximately 45 cm (18 in) in men and about 43 cm (17 in) long in women.

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Spinal Cord and Vertebrae.png: Relationship between the spinal cord and vertebral column, delineating the cervical, thoracic, and lumbar sections.

Layers and Regions of the Spinal Cord

The spinal cord is protected by three layers of tissue called meninges and divided into three regions.

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Spinal Cord Tissue Layers

The dura mater is the outermost layer of spinal cord tissue, forming a tough protective coating. The space between the dura mater and the surrounding bone of the vertebrae is called the epidural space. The epidural space is filled with adipose tissue and contains a network of blood vessels. The middle layer is called the arachnoid mater. The pia mater is the innermost protective layer and is tightly associated with the surface of the spinal cord. The space between the arachnoid and pia maters is called the subarachnoid space and is where the CSF is located. It is from this location at the level of the lumbar region that CSF fluid is obtained in a spinal tap.

Spinal Cord Regions

In cross-section, the peripheral region of the cord displays neuronal white matter tracts containing sensory and motor neurons. Internal to this peripheral region is the gray, butterfly-shaped central region made up of nerve cell bodies. This central region surrounds the central canal, which is an anatomic extension of the spaces in the brain known as the ventricles and like the ventricles, contains cerebrospinal fluid.

This cross-section of the spinal cord indicates the subarachnoid cavity, subdural cavity, posterior root, spinal ganglion, anterior root, spinal nerve, dura mater, arachnoid mater, and pia mater.

Spinal Cord Regions: Cross-sectional perspective of the spinal cord regions

The spinal cord is divided into cervical, thoracic, and lumbar regions. The cervical region is divided into eight levels that are related to different motor and sensory functions in the neck and the arms. The spinal nerves of the thoracic region supply the thorax and abdomen. The nerves of the lumbosacral spinal cord supply the pelvic region, legs, and feet.

Spinal Cord Nerve Branches

Thirty-one pairs of spinal nerves (sensory and motor) branch from the human spinal cord. Each spinal nerve is formed from the combination of nerve fibers from its posterior and anterior roots. The posterior root is the sensory (afferent) root that carries sensory information to the brain from other areas of the body. The anterior root is the motor (efferent) root that carries motor information to the body from the brain.

The spinal nerve emerges from the spinal column through the opening (intervertebral foramen) between adjacent vertebrae. An exception is the first spinal nerve pair (C1), which emerges between the occipital bone and the atlas (the first vertebra). The swelling found in the posterior root is the posterior (dorsal) root ganglion, which contains the cell bodies of sensory neurons. The anterior (ventral) root contains axons of motor neurons that conduct nerve impulses from the CNS to other parts of the body such as the muscles.

The cauda equina (“horse’s tail”) is the name for the collection of nerves in the vertebral column that extends beyond the cord. The nerves that compose the cauda equina supply the pelvic organs and lower limbs, including motor innervation for the hips, knees, ankles, feet, and internal and external anal sphincters. In addition, the cauda equina extends to sensory innervation of the perineum.

Primary Spinal Cord Function

The spinal cord functions primarily in the transmission of neural signals between the brain and the rest of the body, but it also contains neural circuits that can independently control numerous reflexes and central pattern generators. The three major functions of the spinal cord are the conduction of motor information traveling down the spinal cord, the conduction of sensory information in the reverse direction, and acting as the center for conducting certain reflexes. The spinal cord is the main pathway for information connecting the brain and peripheral nervous system.

The Spine and Spinal Cord

The spine encases the spinal cord for protection and support.

Key Points

The human spine consists of 24 articulating vertebrae grouped into cervical, thoracic, and lumbar regions. Nine more vertebrae make up the sacrum and coccyx.

Typical vertebrae consist of the anterior vertebral body and the posterior section, which contains the vertebral foramen through which the spinal cord passes.

There are four main curves of the spine: cervical, thoracic, lumbar, and pelvic.

Facets of the vertebrae restrict range of movement to prevent shearing of the spinal cord.

Blood vessels and nerves exit the spinal column at intervertebral foramina.

There are four main curves of the spine: cervical, thoracic, lumbar and pelvic.

Key Terms

vertebrae: The bones that make up the spinal column.

laminae: Plates of bone that form the posterior walls of each vertebra.

pedicle: A segment of bone connecting the lamina to the vertebral body.

vertebral foramen: Formed by the vertebral body and vertebral arch and containing the spinal cord.

vertebral column: The series of vertebrae that protect the spinal cord; the spinal column.

EXAMPLES

  • Kyphosis is an exaggerated concave (kyphotic) curvature of the thoracic vertebral column; it is commonly known as “humpback.”
  • Lordosis is an exaggerated convex (lordotic) curvature of the lumbar region; it is commonly known as “swayback.”
  • Scoliosis is an abnormal lateral curvature of the vertebral column.

Number of Vertebrae

This diagram of the vertebral column delineates the sacral curve, cervical curve, thoracic curve, lumbar curve, coccygeal vertebrae, sacrum, lumbar vertebrae, thoracic vertebrae, cervical vertebrae.

Vertebral Column: The sections of the vertebral column.

In human anatomy, the vertebral column (backbone or spine) usually consists of 24 articulating vertebrae and nine fused vertebrae in the sacrum and the coccyx. Situated in the dorsal aspect of the torso and separated by intervertebral discs, it houses and protects the spinal cord in its spinal canal. There are normally 33 vertebrae in humans, including the five that are fused to form the sacrum, the four coccygeal bones that form the tailbone, and the others separated by intervertebral discs. The upper three regions comprise the remaining 24, and are grouped as cervical (seven vertebrae), thoracic (12 vertebrae) and lumbar (five vertebrae).

Vertebral Shape

A typical vertebra consists of the vertebral body and vertebral arch. These parts together enclose the vertebral foramen that contains the spinal cord. The vertebral arch is formed by a pair of pedicles and a pair of laminae. Two transverse processes and one spinous process are posterior to (behind) the vertebral body. The spinous process projects toward the posterior direction, while one transverse process projects to the left and the other to the right. The spinous processes of the cervical and lumbar regions can be felt through the skin. Facet joints are located above and below each vertebra. These restrict the range of movement. Between each pair of vertebrae are two small openings called intervertebral foramina through which the spinal nerves exit.

This oblique view of a cervical vertebra delineates the spinal cord, spinal nerve, nucleus pulpus, disc annulus, superior articular process, spinous process, posterior tubercle of transverse process, foramen transversium, anterior tubercle of transverse process, vertebral body.

Vertebrae: Oblique view of cervical vertebrae.

Vertebral Curvature

When viewed laterally, the vertebral column presents several curves corresponding to the different regions of the column: cervical, thoracic, lumbar, and pelvic.

Cervical and Thoracic Curves

The cervical curve convexes forward and begins at the apex of the odontoid (tooth-like) process. It ends at the middle of the second thoracic vertebra. The thoracic curve convexes dorsally begins at the middle of the second thoracic vertebra and ends at the middle of the 12th thoracic vertebra.

Lumbar and Pelvic Curves

The lumbar curve, which is more pronounced in women than in men, begins at the middle of the last thoracic vertebra and ends at the sacrovertebral angle. It is convex anteriorly with the lower three vertebrae much more convex than the upper two. This curve is described as a lordotic curve. The pelvic curve begins at the sacrovertebral articulation and ends at the point of the coccyx; its concavity is directed downward and forward.

Primary and Secondary Curves

The thoracic and sacral curvatures are termed primary curves because they are present in the fetus and remain the same in the adult. As the child grows, lifts the head, and begins to assume an upright position, the secondary curves (cervical and lumbar) develop. The cervical curve forms when the infant is able to hold up his or her head (at three or four months) and sit upright (at nine months). The lumbar curve forms between twelve to eighteen months when the child begins to walk.

Spinal Cord Grey Matter and Spinal Roots

The grey matter of the spinal cord contains neuronal cell bodies, dendrites, axons, and nerve synapses.

Key Points

Each segment of the spinal cord is associated with a pair of ganglia called dorsal root ganglia, situated just outside of the spinal cord.

The dorsal root ganglia contain the cell bodies of sensory neurons. Axons of these sensory neurons travel into the spinal cord via the dorsal roots.

The grey matter in the center of the cord contains interneurons and the cell bodies of motor neurons, axons, and dendrites.

Projections of the grey matter (the “wings”) are called horns. Together, the grey horns and the grey commissure form the H-shaped grey matter.

The dorsal root ganglia develops in the embryo from neural crest cells. The spinal ganglia can thus be regarded as grey matter of the spinal cord that was translocated to the periphery.

Key Terms

neural crest: A strip of ectodermal material in the early vertebrate embryo inserted between the prospective neural plate and the epidermis.

grey matter: A major component of the central nervous system consisting of neuronal cell bodies, neuropil (dendrites and unmyelinated axons), glial cells (astroglia and oligodendrocytes), and capillaries.

neural tube: The embryonic precursor to the central nervous system (CNS).

EXAMPLES

  • The spine acts as the conduit to relay information to and from the brain from the rest of the body.
  • Damage to the grey matter (eg, the ventral gray horn) may lead to tingling and muscle weakness.

The spinal cord is the main pathway for information connecting the brain and peripheral nervous system. The spinal cord is much shorter in length than the bony spinal column. The human spinal cord extends from the foramen magnum of the occipital bone of the skull and continues to the conus medullaris near the second lumbar vertebra, terminating in a fibrous extension known as the filum terminale.

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Spinal Cord Topography and Roots

This cross-section of the spinal cord delineates the grey commissure, posterior and anterior nerve roots, posterior median sulcus, posterior median septum, postero-lateral sulcus, posterior column, anterior column, formatio reticularus, and anterior median fissure.

Cross-Section of Spinal Cord, Mid-Thoracic Level: Note the commissure, rendered in grey.

The spinal cord is compressed dorsoventrally, giving it an elliptical shape. The cord has grooves in the dorsal and ventral sides. The posterior median sulcus is the groove in the dorsal side, and the anterior median fissure is the groove in the ventral side.

Each segment of the spinal cord is associated with a pair of ganglia called dorsal root ganglia, situated just outside of the spinal cord. These ganglia contain cell bodies of sensory neurons. Axons of these sensory neurons travel into the spinal cord via the dorsal roots.

The grey matter, in the center of the cord, is shaped like a butterfly and consists of cell bodies of interneurons and motor neurons, as well as neuroglia cells and unmyelinated axons. Projections of the grey matter (the “wings”) are called horns. Together, the grey horns and the grey commissure form the H-shaped grey matter.

Dorsal and Ventral Roots

The dorsal root ganglia lie along the vertebral column by the spine. The dorsal root ganglia develops in the embryo from neural crest cells, not the neural tube. Hence, the spinal ganglia can be regarded as grey matter of the spinal cord that became translocated to the periphery.

The axons of dorsal root ganglion neurons are known as afferents. In the peripheral nervous system, afferents refer to the axons that relay sensory information into the central nervous system. These neurons are of the pseudo-unipolar type, meaning that they have an axon with two branches that act as a single axon, often referred to as distal and proximal processes. Ventral roots consist of axons from motor neurons, which bring information to the periphery from cell bodies within the CNS. Dorsal roots and ventral roots come together and exit the intervertebral foramina as they become spinal nerves.

The nerve endings of dorsal root ganglion neurons have a variety of sensory receptors that are activated by mechanical, thermal, chemical, and noxious stimuli. In these sensory neurons, a group of ion channels thought to be responsible for somatosensory transduction has been identified. Compression of the dorsal root ganglion by a mechanical stimulus lowers the voltage threshold needed to evoke a response and causes action potentials to be fired. This firing may even persist after the removal of the stimulus.

Impulse Transmission

The dendrite receives information from another neuron’s axon at the synapse, and the axon sends information to the next neuron’s dendrites. Unlike the majority of neurons found in the CNS, an action potential in a dorsal root ganglion neuron may initiate in the distal process in the periphery, bypass the cell body, and continue to propagate along the proximal process until reaching the synaptic terminal in the dorsal horn of the spinal cord.

The distal section of the axon may either be a bare nerve ending or encapsulated by a structure that helps relay specific information to nerve. For example, a Meissner’s corpuscle or a Pacinian corpuscle may encapsulate the nerve ending, rendering the distal process sensitive to mechanical stimulation, such as stroking or vibration.

Ion Channels

Two distinct types of mechanosensitive ion channels have been found in the dorsal root ganglia, broadly classified as either high-threshold (HT) or low-threshold (LT). As their names suggest, they have different thresholds as well as different sensitivities to pressure. These are cationic channels whose activity appears to be regulated by the proper functioning of the cytoskeleton and cytoskeleton-associated proteins. The presence of these channels in the dorsal root ganglion gives reason to believe that other sensory neurons may contain them as well.

High-threshold channels have a possible role in nociception. These channels are found predominantly in smaller sensory neurons in the dorsal root ganglion cells and are activated by higher pressures, two attributes that are characteristic of nociceptors. Also, the threshold of HT channels was lowered in the presence of PGE2 (a compound that sensitizes neurons to mechanical stimuli and mechanical hyperalgesia), which further supports a role for HT channels in the transduction of mechanical stimuli into nociceptive neuronal signals.

Spinal Cord White Matter

The white matter of the spinal cord is composed of bundles of myelinated axons.

Key Points

White matter is one of the two components of the central nervous system and consists mostly of glial cells and myelinated axons.

The white matter is white because of the fatty substance ( myelin ) that surrounds the nerve fibers. Myelin acts as an electrical insulation. It allows the messages to pass quickly from place to place.

Cerebral and spinal white matter do not contain dendrites, which can only be found in grey matter along with neural cell bodies, and shorter axons.

White matter modulates the distribution of action potentials, acting as a relay and coordinating communication between different brain regions.

White matter in the spinal cord functions as the “wiring”; primarily to carry information.

Key Terms

myelin: A white, fatty, material composed of lipids and lipoproteins, that surrounds the axons of nerves.

white matter: A region of the central nervous system containing myelinated nerve fibers and no dendrites.

cerebral ventricles: Interconnected cavities in the brain where the cerebrospinal fluid is produced.

glial cell: A type of cell, in the nervous system, that provides support for the neurons.

White matter is one of the two components of the central nervous system. It consists mostly of glial cells and myelinated axons and forms the bulk of the deep parts of the brain and the superficial parts of the spinal cord. It is the tissue through which messages pass between different areas of grey matter within the nervous system.

Composition of White Matter

White matter is composed of bundles of myelinated nerve cell processes (or axons). The axons connect various grey matter areas (the locations of nerve cell bodies) of the brain to each other and carry nerve impulses between neurons. The axonal myelin acts as an insulator and increases the speed of transmission of all nerve signals. White matter does not contain dendrites, which are only found in grey matter along with neural cell bodies and shorter axons.

In a freshly cut brain, the tissue of white matter appears pinkish white to the naked eye because myelin is composed largely of lipid tissue that contains capillaries. In nonelderly adults, 1.7-3.6% of the white matter is blood. Myelin is found in almost all long nerve fibers and acts as electrical insulation. This is important because it allows the messages to pass quickly from place to place.

Spinal Cord Columns

The spinal cord white matter is subdivided into columns. The dorsal columns carry sensory information from mechanoreceptors (cells that respond to mechanical pressure or distortion). The axons of the lateral columns ( corticospinal tracts ) travel from the cerebral cortex to contact spinal motor neurons. The ventral columns carry sensory pain and temperature information and some motor information.

Function of White Matter

Long thought to be passive tissue, white matter actively affects how the brain learns and functions. While grey matter is primarily associated with processing and cognition, white matter modulates the distribution of action potentials, acting as a relay and coordinating communication between different brain regions. The brain in general (and especially a child’s brain) can adapt to white-matter damage by finding alternative routes that bypass the damaged white-matter areas; therefore, it can maintain good connections between the various areas of grey matter. Using a computer network as an analogy, the grey matter can be thought of as the actual computers themselves, whereas the white matter represents the network cables connecting the computers together.

Axon Tracts

Within white matter, there are three different kinds of tracts or bundles of axons that connect one part of the brain to another and to the spinal cord:

  • Projection tracts extend vertically between the higher and lower brain and spinal cord centers. They carry information between the cerebrum and the rest of the body. The corticospinal tracts, for example, carry motor signals from the cerebrum to the brainstem and spinal cord.
  • Commissural tracts cross from one cerebral hemisphere to the other through bridges called commissures. Commissural tracts enable the left and right sides of the cerebrum to communicate with each other.
  • Association tracts connect different regions within the same hemisphere of the brain. Among their roles, association tracts link perceptual and memory centers of the brain.

White Matter-Grey Matter Interactions

White matter forms the bulk of the deep parts of the brain and the superficial parts of the spinal cord. Aggregates of grey matter, such as the basal ganglia and brain stem nuclei, are spread within the cerebral white matter. The cerebellum is structured in a similar manner as the cerebrum, with a superficial mantle of the cerebellar cortex, deep cerebellar white matter (called the “arborvitae”), and aggregates of grey matter surrounded by deep cerebellar white matter (dentate nucleus, globose nucleus, emboliform nucleus, and fastigial nucleus). The fluid-filled cerebral ventricles (lateral ventricles, third ventricle, cerebral aqueduct, and fourth ventricle) are also located deep within the cerebral white matter.

Blood Supply and Lymphatics

Arterial Supply of the Spinal Cord

The spinal cord receives its blood supply from three longitudinal channels that extend along the length of the spinal cord, i.e., one anterior spinal artery and two posterior spinal arteries. The anterior spinal artery runs along the anterior median fissure, and the smaller two posterior spinal arteries pass through the posterolateral sulcus on either side. The vertebral arteries are the main source from where these spinal arteries branch. But it should be remembered that the blood coming from the vertebral arteries supply only the cord’s cervical segments. In the lower down region, the spinal arteries receive blood through radicular arteries that reach the cord and the roots of the spinal nerves. The radicular arteries are actually branches from many arteries like the vertebral, cervical, intercostal, lumbar, and even sacral arteries. The largest radicular feeder from the left posterior intercostal artery to the anterior spinal artery is between T9 and T12, called the artery of Adamkewicz. All the radicular and spinal arteries anastomose with each other to form an anastomotic pial plexus called vasocorona. But the better part of the spinal cord is supplied by the anterior spinal artery and its branches.

Veinous Drainage of the Spinal Cord

The veins draining the spinal cord are arranged in the form of a network of six longitudinal channels, i.e., an anteromedian vein, a posteromedian vein, and a pair of anterolateral and posterolateral veins. As the name indicates, the anteromedian and posteromedian channels will be present in the midline anteriorly and posteriorly, respectively, relative to the spinal cord. The anterolateral and posterolateral channels are paired and lie as per their names on either side of the spinal cord. All these channels are interconnected to form a plexus of veins of venous vasocorona. The venous blood from these veins drains into radicular veins. These radicular veins finally drain into the segmental veins.

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Nerves

Spinal Nerves

The nerves are of two types based on their origin – the cranial and spinal nerves. Those nerves which begin in the cerebrum or the brain stem are called cranial nerves. There are twelve pairs of cranial nerves. The nerves coming from the spinal cord are called the spinal nerves. There are 31 pairs of spinal nerves coming from the spinal cord and spread on either side of the vertebral column. They are classified into cervical, thoracic, lumbar, sacral, and coccygeal nerves.  The cervical nerves are in eight pairs; the thoracic are twelve pairs, the lumbar and sacral nerves are in five pairs, and one pair of coccygeal nerves.

Spinal Tracts

When we study the transverse section of a spinal cord, we can appreciate many spinal tracts within the white matter. They are categorized into two types – ascending and descending tracts. The descending tracks are mainly motor, and the ascending tracks are sensory.

Some of the most essential descending (motor) tracts passing through the spinal cord are as follows:

  • Cortico-spinal tract (This is also known as the pyramidal tract, which is the most important of the motor tracts)
  • Tectospinal tract
  • Rubrospinal tract
  • Vestibulospinal tract
  • Reticulospinal tracts
  • Olivospinal tract

Some of the most important ascending (sensory) tracts are as follows:

  • Lateral spinothalamic tract (spinal lemniscus)
  • Anterior spinothalamic tract (medial lemniscus)
  • Trigeminal lemniscus
  • Lateral lemniscus
  • Spinotectal tract
  • Spino-olivary tract
  • Ventral spinocerebellar tract
  • Dorsal spinocerebellar tract
  • Dorsolateral tract (of Lissauer)
  • Spino-cervical-thalamic tract

Here we will explain one descending tract in detail to better understand the whole course.

The cortico-spinal tract arises mainly from the motor area of the cerebral cortex (area 4) with some contribution from the premotor area (area 6), somatosensory area (area 3, 2, 1), and parietal cortex (area 5). The fibers run through the internal capsule’s posterior limb and then occupy the middle part of the midbrain’s crus cerebri. The tract then occupies the ventral part of the pons and descends through the pyramids in the medulla. At the lower end of the medulla, almost 70 to 80% of the fibers cross to the opposite side of the spinal cord, called the pyramidal (motor) decussation. These crossed fibers enter the lateral funiculus of the spinal cord and continue as the lateral corticospinal tract. They end at various levels of the spinal cord’s grey matter by synapsing with internuncial neurons of dorsal and ventral columns. The remaining 20 to 30% of the corticospinal tract fibers, which have not crossed, will descend within the anterior funiculus as the anterior corticospinal tract. Finally, these anterior corticospinal tract fibers too cross to the opposite side at the appropriate levels. Thus all fibers of the corticospinal tract eventually cross to the opposite side and connect the cerebral cortex of one side with the ventral horn cells of the opposite part of the spinal cord. These fibers are called upper motor neurons. The fibers starting in the ventral horn cells and descending downwards will be called lower motor neurons. This knowledge is vital as the clinical presentation of upper and motor neuron lesions are quite different. Any minor injury of these fibers will lead to widespread paralysis of the muscles supplied.

Spinal Cord Lesions

Lesions of the spinal cord fall into the following categories:

  • Lesions of the afferent system:

    • Dorsal nerve roots
    • Spinothalamic tract
    • Posterior white funiculus
    • Syringomyelia
  • Lesions of the efferent system:

    • Upper motor neuron lesions
    • Lower motor neuron lesions
  • Lesions involving upper and lower motor neurons:
  • Lesions involving posterior and lateral funiculi
  • Thrombosis of the spinal artery
  • Hemisections (Brown-Sequard syndrome)
  • Transections
  • Hereditary diseases: Friedreich ataxia

Clinical Significance

Any spinal cord lesion can generally be localized quite easily by understanding the anatomy. There is usually impairment or sensory or motor functions in the lower extremities. It is always important to identify the motor or sensory level to determine the lesion’s exact site. In general, the best imaging modality for any suspected spinal cord lesion is magnetic resonance imaging(MRI).

  • Meningitis: It is the infection of meninges (coverings) of the brain. It can be bacterial or viral. Some of the most common being:

    • Bacterial meningitis: It can be because of bacterias’ infection such as Streptococcus pneumoniaeNeisseria meningitidesListeria monocytogenesE. coliPseudomonas aeruginosaKlebsiellaEnterobacterStaphylococcus aureus, and Staphylococcus albusStreptococcus pneumoniae and Neisseria meningitides are known to be the most common.
    • Tubercular meningitis: It is due to infection by Mycobacterium tuberculosis. It presents with typical features like a stiff neck, fever, increased intracranial pressure, and headache. The CSF shows increased proteins and decreased glucose levels. It is common in children with primary tuberculosis, patients with malnourishment, and immunodeficiencies like HIV and cancer. Even though rare compared to bacterial meningitis, this can lead to high morbidity and mortality if not detected early and treated.
  • Traumatic injuries of the spinal cord: It is the commonest, accounting for almost 90% of all spinal cord injuries. It frequently results from road traffic accidents, falls, and sports injuries. They can have devastating effects on the life of a person. Lesions in the lower thoracic region lead to paraplegia, and that in the cervical area leads to quadriplegia.

    • Compression: It can be due to intervertebral disc herniation or the vertebras’ dislocation leading to compression of the spinal cord. Symptoms due to compression can be paresis to paralysis.
    • Hemisection: Traumatic injuries can lead to an incomplete section of a part of the spinal cord. A classical presentation of a hemisection is the Brown-Sequard syndrome. There is ipsilateral motor loss below the section in this syndrome, contralateral loss of pain, and temperature sensations, with no loss of ipsilateral light touch sensations.
    • Complete section: It is a condition where there is the absence of sacral sparing with no sensation in the segment of S4-5 or the lack of voluntary contraction of the anal sphincter.
  • Vascular injuries of the spinal cord:

    • Anterior cord syndrome: When the anterior spinal artery is blocked, it results in ischemia of the anterior two-thirds of the spinal cord’s area supplied by this artery. This ischemia is the commonest cause of spinal cord infarction, often occurs due to aortic manipulation or dissection. The commonest site is the mid-thoracic level due to the best supplies from the origin from the vertebral artery and the artery of Ademkiewicz near its lower end. It presents itself as an incomplete motor paralysis below the site of the lesion. Also seen is the sensory loss relative to pain and temperature; this is called anterior cord syndrome. It may be more easily recognized if the whole cord is affected, sparing only the dorsal columns.
    • Posterior cord syndrome: It is a syndrome that develops due to ischemia of the posterior spinal artery, affecting the area of the spinal cord supplied by it. It presents itself with an absence of proprioception and vibration sensation, hypotonia, ataxic gait, positive Romberg sign, and the lack of deep tendon reflexes.
    • Central cord syndrome: This is a non-vascular injury of the spinal cord, especially seen in a hyperextended neck after a road traffic accident. It is said to be the most common among the incomplete spinal cord injuries. It presents itself with severe sensory and motor function loss in the case of the upper limbs compared with the lower limbs.
  • Development anomalies of the spinal cord:

    • Spina bifida: It is a developmental anomaly of the vertebrates where the laminae fail to fuse with the spinal process. It is said to be one of the most frequent developmental defects of the neural tube. It is usually seen in the lumbosacral region and is identifiable by a tuft of hairs in this region. No other external visible abnormality is visible.
    • Meningocele: In this condition, too, laminae fail to cover the spinal cord leading to protruding of arachnoid and pia mater as a cystic swelling in this region covered by skin.
    • Meningomyelocele: This is an extension of meningocele where the cystic swelling will contain a part of the spinal cord with relevant nerves.
    • Syringo-myelocele: Further to the above, here we see distension of the central canal.
    • Rachischisis: It is a condition where the neural tube fails to close or incompletely closed. This anomaly can develop both in the brain and spinal cord. When it affects the brain, it is called anencephalus, and when the spinal cord, then it is myelocele.
  • Herniation: Herniation of the spinal cord is less frequent and usually misdiagnosed for other conditions.
  • Malignancy: They can present themselves as ependymomas, astrocytomas, and hemangioblastomas. Others include lipomas, lymphomas, germ cell tumors, gangliogliomas, and germinomas. They can also rarely metastases from different parts of the body like breast, bone, etc.
  • Syringomyelia: It is a condition where there is a fluid-filled cavity within the spinal cord. This condition can be due to a disturbance of CSF flow, an intramedullary tumor, or spinal cord tethering.
  • Subacute combined degeneration: It is a rare condition arising due to the deficiency of vitamin B12. It leads to neurological complications with demyelination of the lateral and dorsal spinal cord. Similar pathological findings can also be found in patients with copper deficiency, zinc excess, or HIV infection with myelopathy.
  • Tabes dorsalis: this is selective pathology affecting only the dorsal columns as a late manifestation of neurosyphilis.
  • Transverse myelitis: this is a condition caused by inflammation of the spinal cord. It presents with variable involvement of all functional modalities at and below the site of inflammation. It can occur by itself as an immune-mediated post-infectious problem. It is also commonly involved by an acute relapse of multiple sclerosis. Other inflammatory diseases may also affect the spinal cord, such as Sjogren disease, Behcet’s disease, or neuromyelitis optica spectrum disorder.

 

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