12 Cranial Nerves/The cranial nerves provide afferent and efferent (sensory, motor, and autonomic) innervation to the structures of the head and neck. Unlike spinal nerves whose roots are neural fibers from the spinal grey matter, cranial nerves are composed of the neural processes associated with distinct brainstem nuclei and cortical structures. While the spinal grey matter is organized into a posterior sensory horn, an autonomic and interneuron intermediate grey, and an anterior motor horn, these cranial nerve nuclei are functionally organized into distinct nuclei within the brainstem. Typically, the more posterior and lateral nuclei tend to be sensory, and the more anterior of tend to be the motor. Cranial nerves I (olfactory), II (optic), and VIII (vestibulocochlear) are considered purely afferent. Cranial nerves III (oculomotor), IV (trochlear), VI (abducens), XI (spinal accessory), and XII (hypoglossal) are purely efferent. The remaining cranial nerves, V (trigeminal), VII (facial), IX (glossopharyngeal), and X (vagus), are functionally mixed (sensory and motor) (Figure 1A). Damage to the cranial nerves, their tracts, or nuclei results in stereotypical dysfunctions. While this is the classical way of organizing and indexing cranial nerves, the scientific reality is more complex and are still debated in the academic realm, including the classification and identification of the routes of distinct cranial nerves fibers and the presence or absence of other less recognized structures, such as the terminal nerve, also called nerve nulla or cranial nerve zero. The latter could be potentially classified as a pure afferent nerve following the traditional cranial nerves’ organization.
Nerves
Afferent Cranial Nerves and Clinical Implications
Cranial nerves I, II, and VIII are considered purely afferent nerves since they conduct sensory information from the olfactory region, the retina of the eye, and the inner ear structures, respectively. Cranial nerve I, the olfactory nerve, is composed of special visceral afferents (SVA). Chemo-sensory receptors in the olfactory mucosal lining bind to odorant molecules and conduct a signal through the nerves traveling through the cribriform plate of the ethmoid bone to synapse on the neurons of the olfactory bulb within the cranial vault. The central processes of these olfactory bulb neurons project through the olfactory trigone medially to the septal area and the contralateral bulb via the anterior commissure, while other fibers travel laterally to the amygdala and piriform cortex, also known as the primary olfactory cortex where conscious odorant sense is processed. Traumatic injury, especially “whiplash” from automobile collisions, can sever the olfactory projections through the cribriform plate, resulting in anosmia, which has been associated with the development of depression.[rx]rx][rx] The sense of olfaction also appears to have a non-conscious role in activating the limbic system, which may account for such an effect.[rx]
Cranial nerve II, the optic nerve, conveys special somatic afferent (SSA) visual sensory information from the rods and cones retinal sensory receptors to the thalamus, especially the lateral geniculate nucleus (LGN), and the superior colliculus (SC). Ganglion cells, whose cell bodies are located deep in the retina, have central projections that form the optic nerve fibers, which traverse the optic canal to enter the cranium. From there, fibers representing the medial visual fields travel posteriorly without crossing at the optic chiasm, while fibers of the lateral visual fields do cross within the chiasm. Therefore, regions of the visual field are retinotopically organized within the optic nerve and at their synapses in the LGN. Collaterals are also given off centrally to innervate the SC, responsible for the pupillary light reflex; and connections within the pulvinar of the thalamus,[rx] providing unconscious optic input responsible for the phenomenon of blindsight.[rx] In cortically blind individuals, these pulvinar collaterals from the optic nerve allow for unconscious eye movements in response to the detection of light as well as a weak directional sense of the location of the light within the visual field.[rx][rx][rx]
Cranial nerve VIII, the vestibulocochlear nerve, is responsible for the auditory sense and the vestibular sense of orientation of the head. This nerve conveys special sensory afferents (SSA) from the inner ear to the cochlear nuclei and the vestibular nuclei in the caudal medulla oblongata. Hair cells within the cochlear duct, semicircular canals, utricle, and saccule are polarized sensory receptor cells with apical ciliary extensions that transduce an electrochemical signal upon mechanical deformation. Ganglionic neurons within the cochlea and the vestibular nerve receive this signal peripherally and transmit it centrally through the internal auditory meatus before entering the medulla. Damage to the vestibular component of this nerve causes dizziness, while damage to the cochlear part causes peripheral, or sensorineural, hearing loss. The internal auditory meatus is a narrow canal of the temporal bone through which these nerves course and schwannoma of the vestibular or cochlear nerves in this meatus easily compresses and impinges these nerves. Early signs and symptoms are a progressive worsening hearing loss with tinnitus, imbalance, leading to a sense of pressure in the ear and facial weakness or paralysis.[rx] Vestibular schwannomas have an incidence rate of six to nine new cases per year per million people and are readily treatable with surgery or radiation.[rx] However, if the condition is left untreated, it can result in potentially large and life-threatening tumors.
In addition, the terminal nerve or cranial nerve zero[rx] has been identified as a separate cranial nerve in the human brain dating back to 1914 but is overlooked by most current anatomical textbooks. Also called nervus terminals for its proximity to the lamina terminalis and nerve nulla (i.e., nothing, zero), cranial nerve zero is comprised of an independent central plexus of small unmyelinated (possibly special visceral afferent [SVA]) fibers located medially and in very close proximity to the olfactory tract by the olfactory trigone (Figure 1B). Its discrete positioning may explain its poor identification during standard dissection techniques. The cranial nerve zero fibers travel centrally to subcortical structures, it sends projections to the medial pre-commissural septum and the medial septal nucleus, among others. It appears to have a rich bundle of well-vascularized fibers ascending from the nasal submucosa and projecting to important limbic structures (e.g., amygdala, hypothalamic nuclei). Functionally it has been regarded as pheromones processing unconsciously by regulating autonomic responses through hypothalamic gonadotropin-releasing hormone (GnRH) possibly via the kisspeptin neuronal network. Clinically, a disruption of the normal embryological migratory pathway of GnRH neural crest cells of the olfactory placode and basal forebrain[rx] can result in Kallman syndrome, a genetic condition characterized by hypogonadotropic hypogonadism with partial or total anosmia, also resulting in abnormal sexual development in both sexes.
Efferent Cranial Nerves and Clinical Implications
Cranial nerves III, IV, VI, XI, and XII are considered purely efferent due to their motor output to the orbit, the neck, and the tongue. Cranial nerve III, IV, and VI (oculomotor, trochlear, and abducens nerves, respectively) are general somatic efferent (GSE) nerves responsible for innervating the extraocular muscles within the orbit. These nerves travel unilaterally from the brainstem to the calvarium through the superior orbital fissure from synonymous brainstem nuclei. The oculomotor nerve (CN III) travels through the common tendinous ring, the common attachment in the posterior orbit for the four extra-ocular recti muscles, along with abducens nerve (cranial nerve VI). Trochlear nerve (CN IV) travels into the orbit outside of the common tendinous ring to innervate the superior oblique muscle of the eye. The abducens nerve innervates the lateral rectus muscles only; thereby this nerve can be tested by evaluating the abduction of the eye gaze. Cranial nerve III innervates most of the eye muscles, by splitting into a superior and an inferior branch to innervate the remaining three recti muscles, the inferior oblique, and the skeletal muscle component of levator palpebrae superioris. However, cranial nerve III also has a general visceral efferent (GVE) component originating in the Edinger-Westphal nucleus (also called accessory oculomotor nucleus or visceral oculomotor nucleus). These fibers travel with cranial nerve III to synapse in the ciliary ganglion within the orbit. The post-ganglionic sympathetic fibers of the ciliary ganglion pierce the sclera of the eye to innervate the pupillary sphincter and ciliary smooth muscles responsible for pupillary constriction and lens accommodation. Pupillary constriction can be tested for by way of the pupillary light reflex[rx] through the efferents from the superior colliculus to the accessory oculomotor nucleus. The eye movements test (abduction, adduction, infraduction, supraduction) is an effective method to assess the viability of the GSE components of cranial nerves III, IV, and VI.
Cranial nerve XI, the spinal accessory nerve, is responsible for the general somatic efferent (GSE) motor innervation of the trapezius and sternocleidomastoid muscles by way of the spinal nucleus of the accessory nerve. The spinal nucleus of the accessory nerve is located within the cervical spinal cord from the levels of C1 through approximately C5/6. The fibers emerge as independent roots, separate from the anterior or dorsal spinal roots of the central spinal grey matter, and ascend through the foramen magnum to enter the cranial cavity. These fibers then exit via the jugular foramen along with cranial nerve IX and X. Central root or nuclear damage to spinal accessory nerve results in ipsilateral flaccid paralysis of the sternocleidomastoid (with difficulty in turning the head against force) and partial ipsilateral trapezius paralysis leading to shoulder drop. The trapezius is innervated by anterior horn grey matter from cervical spinal regions C3 through C4/5 in addition to the spinal accessory nerve. Thus a complete paralysis of trapezius muscle will not occur following a simple focal lesion.[rx]
Cranial nerve XII, the hypoglossal nerve, is responsible for the general somatic efferent (GSE) innervation of the intrinsic and extrinsic muscles of the tongue, except palatoglossus muscle, from the nerve’s synonymous nucleus. This includes the genioglossus, geniohyoid, hyoglossus, and styloglossus muscles. Fibers from the hypoglossal nucleus exit the medulla from the sulcus between the pyramids and the olives as a collection of fibers that coalesce before enter the hypoglossal canal to exit the cranium. Damage to the nucleus or nerve fibers results in tongue deviation toward the side of the lesion, as the ipsilateral genioglossus muscle becomes weak or flaccid reducing its ability to protrude the tongue.
Mixed Cranial Nerves and Clinical Implications
Cranial nerves V, VII, IX, and X are considered mixed cranial nerves due to the presence of afferent and efferent fibers with both sensory and motor components. Cranial Nerve V is the trigeminal nerve responsible for the general somatic sensory innervation (GSA) of the face through its three main branches, V1, V2, and V3 (ophthalmic, maxillary, and mandibular, respectively). This cranial nerve (via V3) is also responsible for motor innervation (SVE) of the muscles of mastication, the anterior belly of the digastric, mylohyoid, and two tiny tensor muscles: the tensor veli palatini and tensor tympani. While no autonomic fibers travel with the fifth cranial nerve as it exits the pons, parasympathetic fibers from the other mixed cranial nerves will join with peripheral branches of cranial nerve V to innervate their respective target structures, such as the lacrimal, parotid, submandibular and sublingual glands. In this sense, central nuclear or supranuclear lesions may result in ipsilateral sensory or motor deficits, but parasympathetic functions will only be impaired by peripheral nerve damage to the respective branches.[rx]
Cranial nerve VII (facial nerve), has both motor and autonomic fibers with minor somatosensory components. Special visceral efferent (SVE) motor innervation is to the muscles of facial expression and exit the skull through the stylomastoid foramen deep to the parotid gland. Damage to these fibers results in ipsilateral facial paralysis (facial palsy). General visceral efferents (GVE) and special visceral afferents (SVA) fibers initially exit the brainstem as nervus intermedius, a separate nerve bundle that joins with the other components of the facial nerve within the facial canal. The GVE components from the superior salivary nucleus are responsible for parasympathetic innervation of the glands and mucosae of the face, with the exception of the parotid gland and the smaller buccal and labial glands. Taste fibers from the anterior two-thirds of the tongue travel centrally as the chorda tympani nerve to their cell body of origin in the geniculate ganglion before synapsing centrally in the solitary nucleus. Depending on the location of the lesion, these visceral components may also be impacted in lesions of the facial nerve. Somatic afferents (GSA) provide sensory innervation from the auricle and a small external portion of the auditory canal.
Cranial nerve IX (glossopharyngeal nerve), is responsible for motor (SVE) innervation of the stylopharyngeus and the pharyngeal constrictor muscles by the nucleus ambiguus. Inferior salivary nucleus fibers travel with cranial nerve IX to provide general visceral efferent (GVE) innervation to parotid, buccal and labial glands while visceral afferents (GVA and SVA) receive sensory information from the carotid body and carotid sinus, and taste from the posterior third of the tongue to synapse on the solitary nucleus. Whereas, the sensory afferents (GSA) receive information from the skin over the tongue, oropharynx, middle ear cavity, and auditory canal.[rx]
Cranial nerve X is the vagus nerve. The parasympathetic efferents (GVE) fibers from the dorsal vagal nucleus to the thoracic and abdominal viscera to the splenic flexure of the colon represent its major neural component. These fibers form a comprehensive plexus that travels along the esophageal serosa to the viscerae. It also has a considerable motor output (SVE) from the nucleus ambiguous to the pharyngeal and soft palate muscles, as well as the intrinsic laryngeal muscles via the superior and recurrent laryngeal nerves. Somatic afferents (GSA) supply the posterior cranial dura and a portion of the ear and external auditory canal epithelium. Visceral afferents (GVA) from the pharynx, larynx, aorta, thoracic and abdominal viscerae and taste buds from the root of the tongue and epiglottis (SVA) synapse on the solitary nucleus as well. Damage to the recurrent laryngeal branch of the vagus nerve can result in vocal hoarseness or acute dyspnea with bilateral avulsion.
Classification of Cranial Nerves
Every cranial nerve (CN) is assigned a Roman numeral as a name. The numbering is based on the order in which the CN emerges from the brain, from ventral to dorsal. The name indicates the function or the course.
List of CNs
- I Olfactory
- II Optic
- III Oculomotor
- IV Trochlear
- V Trigeminal
- VI Abducens
- VII Facial
- VIII Vestibulocochlear
- IX Glossopharyngeal
- X Vagus
- XI Accessory
- XII Hypoglossal
Following is a mnemonic to remember the names of the CNs: ooh, ooh, ooh to touch and feel very good velvet. Always heavenly!
How do the CNs differ in terms of fibers?
The 12 pairs of CNs originate from the nose (CN I), the eyes (CN II), the inner ear (CN VIII), the brainstem (CN III-XII), and the spinal cord (part of XI).
CNs have sensory (afferent) and motor (efferent) functions. CN I and CN II convey sensory information. The olfactory and optic nerves arise from the cerebrum and diencephalon, respectively.
All the other CNs originate from the brainstem nuclei (the hypoglossal nerve is located at the border of the spinal cord) and include sensory axons as well as motor axons. The oculomotor nerve (III) and the trochlear nerve (IV) emerge from the mesencephalon. The CNs V to XII originate from the pons (bridge) and the medulla oblongata (extended spinal cord).
The CNs III, IV, VI, XI, and XII are mainly motor nerves and provide innervation of the skeletal muscles. However, the CNs I and II mentioned before and the vestibulocochlear nerve are purely sensory, but there are also CNs (CN V, VII, IX, X) that are both sensory and motor nerves.
For this reason, you should memorize the following sentence: (s = sensory, m = motor, b = both):
Some students make money, but my brother says Boris Becker makes more.
The oculomotor nerve, the facial nerve, the glossopharyngeal nerve, and the vagus nerve also include somatic and autonomic axons. The somatic part ensures the innervation of the skeletal muscles and the autonomic part belonging to the parasympathetic nervous system innervates the glands, the smooth muscles, and the cardiac muscle.
CN I: Olfactory Nerve
Quality and course
The olfactory nerve is part of the olfactory pathway and is a purely sensory nerve. The olfactory mucosa, with its olfactory cells, is located in the superior nasal meatus (meatus nasi superius).The olfactory cells are nerve cells in which the unmyelinated axons are bundled and emerge through the openings of the cribriform plate (lamina cribrosa, part of the ethmoid bone) and the dura mater located on top into the anterior cranial fossa. The approximately 40 bundles make up the right and the left olfactory nerve.
The olfactory nerve is the shortest nerve that does not travel via the brainstem. The olfactory nerves end in the brain in two paired masses of grey matter and the olfactory bulb, where they are switched to the second neuron. The axon terminals (synaptic knobs) of the olfactory receptors (olfactory cells) compose the next neurons on the olfactory pathway in the respective bulbus synapses with dendrites and soma.
These axons that emerge from the neurons of the olfactory bulb are located in the olfactory tract, which leads to the olfactory cortex (rhinencephalon/smell brain), where the olfactory sensation is perceived and then is linked to emotions.
Function of the olfactory nerve
- It functions to transmit smells and more advanced aspects of taste.
CN II: Optic Nerve
Quality and course
The optic nerve is the visual nerve (optikos = vision) and it is purely sensory in function. The optic nerve is formed by the convergence of axons from the retinal ganglion cells. These cells, in turn, receive impulses from the photoreceptors of the eye. For the most part, the optic nerve is encased in the three meninges. From the retina of the eye, the visual impulses are transmitted to the diencephalon via the optic nerve. Fibers pass from the visual center of the thalamus to the occipital visual cortex, where the image of the event is created.
However, both of the optic nerves do not run separately to their ipsilateral part of the thalamus. In the part of the sphenoid bone, the sella turcica (Turkish chair), they join into the optic decussation (chiasma opticum). There, the fibers of both of the optic nerves mix until new fiber tracts form and continue to the diencephalon. As for the chiasma, the nerve bundles are not called nerves any longer, but optic tracts.
The transmission of the field of vision leads to a mixture of fibers. In the optic tract, the part of the optic nerve transmitting the lateral (temporary) field of vision proceeds on the same side. In the optic decussation, the medial parts of the field of vision are switched to the contralateral side and cross to the opposite side.
Function of the optic nerve
- It functions to transmit visual information from the retina to the vision centers of the brain through electrical impulses.
CN III: Oculomotor Nerve
Quality and course
The oculomotor nerve (oculus= eye) is a mixed CN that is mainly motor, while the motor nucleus is located in the ventral mesencephalon. It innervates most of the outer (extraocular) eye muscles and passes from the midbrain (mesencephalon) to the bony eye socket (orbit).
It extends ventrally and divides into a superior branch and an inferior branch. The rectus superior muscle (one of the outer eye muscles) and the levator palpebrae superioris muscle (muscle of the upper eyelid) are innervated by the axons of the superior branch of the oculomotor nerve. The rectus medialis muscle, the rectus inferior muscle, and the obliquus inferior muscle, which are all outer eye muscles, are supplied by the axons of the inferior branch.
Function of the motor axons
- They function to transmit movement of the upper eyelid and the eye.
The oculomotor nerve serves the voluntary motor function, but it also contains parasympathetic fibers.
The inferior branch serves the parasympathetic innervation of the inner eye muscles and contains the ciliary muscle and the iris sphincter muscle. Parasympathetic impulses, reaching the ciliary ganglion, originate from the oculomotor nucleus in the mesencephalon, which is the ‘circuit center’ of the autonomous nervous system.
Function of the parasympathetic axons
Parasympathetic axons extend from the ciliary ganglion to the ciliary muscle, which is responsible for the following:
- Accommodation (adaption to the distance of an object)
Further parasympathetic fibers serve the following:
- The stimulation of the iris sphincter muscle (constriction of the pupil in bright light)
- The stimulation of the iris dilator muscle (dilatation of the pupil)
Function of the sensory axons
- Their function is proprioception (part of self-perception).
The sensory part consists of afferent neurons of the proprioceptors of the outer eye muscles toward the mesencephalon. These axons bring together information about the non-visual perception of body movement and position in space (proprioception), as well as the location and position of single body parts to each other. It controls the muscles that allow for visual tracking and fixation by the eye. Visual tracking is the ability to follow an object as it moves across the field of vision.
CN IV: Trochlear Nerve
Quality and course
The trochlear nerve is the smallest of the 12 CNs and is the only nerve exiting the dorsal aspect of the brainstem with mostly motor axons. The trochlear nerve arises from the trochlear nucleus of the brain, emerging from the posterior aspect of the midbrain. It is the only CN to exit from the posterior midbrain.
Beneath the tectum of the mesencephalon (quadrigeminal plate), the nerve exits the midbrain (mesencephalon) and, as the olfactory nerve, it passes through the wall of the cavernous sinus.
Function of the trochlear nerve
Somatomotor functions include the following:
- Eye movement and innervation of the obliquus superior muscle
Sensory functions include the following:
- Proprioception
CN V: Trigeminal Nerve
Quality and course
The trigeminal nerve is a mixed CN with mainly sensory parts. It is regarded as the largest CN. It runs from the pons to the petrous portion (pars petrosa) of the temporal bone (Os temporale), where it converges the trigeminal ganglion. As the name suggests, the trigeminal nerve has three major branches consisting of nerve cell clusters:
- Ophthalmic nerve
- Maxillary nerve
- Mandibular nerve
The ophthalmic nerve passes through the superior orbital fissure. It is the smallest branch. The maxillary nerve is located between the ophthalmic nerve and the mandibular nerve. It is a medium-sized nerve that passes through the foramen rotundum. The mandibular nerve passes through the oval foramen and is the largest of the three trigeminal branches.
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
- 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
CN VI: Abducens Nerve
Quality and course
The abducens nerve arises from the abducens nucleus of the pons (bridge) and is one of the mixed CNs with mainly motor fibers. From the nucleus, the axons emerge to the rectus lateralis muscle (outer eye muscle), pass through the superior orbital fissure, and end in the orbit.
Function of the abducens nerve
Somatomotor functions include the following:
- Eye movement and abduction of the eyes
Sensory functions include the following:
- Proprioception
A short overview of the CNs VII-XIIi in[
CN VII: Facial Nerve
Quality and course
The facial nerve is a mixed CN with motor, sensory, and parasympathetic parts. The sensory axons extend from the taste buds of the anterior two-thirds of the tongue through the geniculate ganglion, to the pons (bridge). Somatomotor neurons, on the other hand, originate from a nucleus of the pons and pass through the temporal bone.
The axons of the parasympathetic neurons of the facial nerve end in the two parasympathetic ganglions (pterygopalatine ganglion and submandibular ganglion).
Function of the facial nerve
The facial nerve has mainly motor functions. However, a part of its fibers, which is called an intermediate nerve, has sensory functions.
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
CN VIII: Vestibulocochlear Nerve
Quality and course
The vestibulocochlear nerve is a mainly sensory nerve that has its nucleus in the pons and passes together with the facial nerve to the internal acoustic meatus. There, it splits into two divisions:
- Vestibular nerve
- Cochlear nerve
In the vestibular nerve, the sensory fibers of the semi-circular canal, the saccule and the utricle of the inner ear, pass to the vestibular ganglion ending in the vestibular nuclei of the pons.
Axons originating from the motor neurons project from the pons to the hair cells of the semi-circular canal, the saccule, and the utricle.
The sensory fibers, located in the cochlear ganglion and passing to the nuclei of the medulla oblongata, originate from the cochlear nerve in the organ of Corti (spiral organ) located in the cochlea of the inner ear.
Axons of motor neurons extend from the pons to the hair cells of the spiral organ of Corti.
Function of the vestibulocochlear nerve
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
CN IX: Glossopharyngeal Nerve
Quality and course
The glossopharyngeal nerve is a mixed CN with motor, sensory, and parasympathetic fibers. It exits the medulla oblongata behind the olivary bodies, together with the vagus nerve and the accessory nerve.
In the area of the posterior cranial fossa, the sensory fibers exit the cranial cavity and pass between the internal carotid artery and the internal jugular vein until they laterally reach the root of the tongue. The motor fibers pass through the jugular foramen from the nuclei of the medulla.
Function of the glossopharyngeal nerve
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
CN X: Vagus Nerve
Quality and course
The vagus nerve is the parasympathetic main nerve that is responsible for large parts of the body and innervates almost all of the organs of the thorax and the abdomen. The vagus nerve has the longest course of all CNs.
After exiting the medulla oblongata, the vagus nerve passes through the skull base. Near the carotid artery, it passes to the intrapleural space in the neck, where it branches off to the base of the heart (to the atria) and the hilum of the lung.
Sensory parts of the vagus nerve originate from the skin of the outer ear. They can cause a cough and nausea if they receive a certain stimulus (cotton bud). A few sensory parts originate in the epiglottis and the pharynx; proprioceptors originate in the muscles of the neck and pharynx and in chemoreceptors in the carotid glomus near the aortic arch. Further branches pass to the recurrent laryngeal nerve, which passes back to the laryngeal cartilage to supply the vocal folds. In addition, there are axons of viscerosensory receptors of thoracic and abdominal organs.
Additionally, parasympathetic fibers extend from the nuclei of the medulla to the lung and the heart. The pressure receptors at the carotid glomus contain fibers of the vagus nerve that are necessary for blood pressure regulation. The vagus gives rise to a network of branches around the esophagus to enter the abdomen at the esophageal hiatus.
The glands of the gastrointestinal tract, the smooth musculature of the respiratory tract, the stomach, the esophagus, the gallbladder, the small intestine, and most parts of the large intestine are supplied by the parasympathetic axons.
The skeletal muscles of the inner and outer neck are innervated by somatomotor neurons.
Function of the vagus nerve
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
CN XI: Accessory Nerve
Quality and course
The accessory nerve is the main motor, mixed CN, which emerges from the brainstem and the spinal cord.
Cranial root
The cranial root arises from the nuclei of the medulla oblongata. It passes through the jugular foramen and supplies the musculature of the pharynx, the larynx, and the velum to enable the swallowing process.
Spinal root
The spinal root contains mixed, but mainly motor, axons. The motor axons pass through the foramen magnum and exit the jugular foramen together with the cranial fibers. Motor impulses are transmitted via the spinal root to the sternocleidomastoid muscle and the trapezius muscle, which control the movement of the head.
Sensory axons originate from the proprioceptors of the muscles. They supply the motor neurons and end in the medulla oblongata.
Function of the accessory nerve
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
CN XII: Hypoglossal Nerve
Quality and Course
The hypoglossal nerve is a mixed CN with mainly motor functions.
The somatomotor axons passing through the hypoglossal canal arise from a nucleus of the medulla oblongata. However, the sensory part consists of proprioceptor axons of the tongue muscles and ends in the medulla oblongata.
Function of the hypoglossal nerve
Somatomotor functions include the following:
- Innervation of tongue muscles
- Transmission of impulses for speaking and swallowing
Sensory functions include the following:
- Proprioception
Overview of the 12 CNs
| CN | Function | Nuclei | Quality |
| I—olfactory nerve (smell) | Transmits signals from the olfactory organ (nose) to the brain | – | Somatosensory and afferent |
| II—optic nerve (vision) | Transmits visual signals from the retina to the brain | – | Somatosensory and afferent |
| III—oculomotor nerve (eye movement) | Controls four out of six outer eye muscles and the lifting muscle of the upper eyelid; accommodation; contraction of the pupil | Oculomotor nucleus, Erdinger-Westphal nucleus | Somatomotor, vegetative, and efferent |
| IV—trochlear nerve (eye rotation/movement) | Controls the superior oblique muscle | Trochlear nucleus | Somatomotor and efferent |
| V—trigeminal nerve (facial sensation) | Subdivides into the eye nerve (ophthalmic nerve), upper jaw nerve (maxillary nerve), and lower jaw nerve (mandibular nerve); receives sensation from the facial area and transmits it to the brain; innervates the muscles of mastication | Spinal accessory nucleus, principal (sensory) nucleus of the trigeminal nerve, mesencephalic nucleus of the trigeminal nerve, spinal nucleus of the trigeminal nerve | Somatosensory, visceromotor, efferent, and afferent |
| VI—abducent nerve (eye movement) | Innervates the lateral eye muscle | Abducens nucleus | Somatomotor and efferent |
| VII—facial nerve (facial expression) | Provides motor innervation to the muscles of facial expression and the stapedius muscle; also transmits gustatory perception from the anterior two-thirds of the tongue; innervates all of the glands of the head, except the parotid gland | Nucleus of the facial nerve, superior salivary nucleus, and solitary nucleus | Visceromotor, vegetative, sensory, efferent, and afferent |
| VIII—vestibulocochlear nerve (hearing and balance) | Carries sensory information from the cochlea and the vestibular system | Cochlear nuclei and vestibular nuclei | Sensory and afferent |
| IX—glossopharyngeal nerve (oral sensation, taste, salivation) | Carries information from the posterior part of the tongue to the brain; innervates pharyngeal muscles; controls deglutition; innervates the parotid gland | Ambiguous nucleus, inferior salivary nucleus, nucleus of the solitary tract, and spinal nucleus of the trigeminal nerve | Sensory, visceromotor, vegetative, efferent, and afferent |
| X—vagus nerve (‘wandering nerve’) | Main nerve of the parasympathetic nervous system; regulates the activity of the internal organs | Ambiguous nucleus, dorsal nucleus of the vagus nerve, nucleus of the solitary tract, and spinal nucleus of the trigeminal nerve | Sensory, visceromotor, vegetative, efferent, and afferent |
| XI—accessory nerve (shoulder and head movement) | Controls the trapezius muscle and the sternocleidomastoid muscle | Ambiguous nucleus and spinal accessory nucleus | Somatomotor and efferent |
| XII—hypoglossal nerve (tongue movement; hypo = under, glossus = tongue) | Provides motor innervation of the muscles of the tongue | Hypoglossal nucleus | Somatomotor and efferent |
References