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THE CRANIAL NERVES OUTSIDE THE SKULL

Seminar presented by:

Vicente Aige Gil DVM, PhD

Professor of Veterinary Anatomy

Universidad Autónoma de Barcelona. Spain

For the

Veterinary Neuroscience & Advanced Clinical Neurology/Neurosurgery Course

28th July - 8th August 2014 ECVN /ESVN

Bolognia (Italia)

There are twelve cranial nerves: olfactory nerves (I), optic nerve (II), oculomotor

nerve (III), trochlear nerve (IV), trigeminal nerve (V), abducent nerve (VI), facial

nerve (VII), vestibulocochlear nerve (VIII), glossopharyngeal nerve (IX), vagus

nerve (X), accessory nerve (XI) and hypoglossal nerve (XII).

Components of the cranial nerves

Functionally, the cranial nerves are classified as motor or sensory. Being each one

somatic and visceral.

The motor component:

The motor fibers that innervate muscles not associated with branchial arches as

the extrinsic muscles of the eye (III, IV and VI cranial nerves); the muscles derived

from the somites as the trapezius, omotransversarius, sternocephalicus and

cleidocephalicus muscles (XI cranial nerve); and the muscles of the tongue, and the

thyrohyoideus and geniohyoideus muscles (XII cranial nerve) form the general

somatic efferent component (GSE).

The motor fibers that innervate muscles associated with branchial arches as

the masticatory, facial, pharyngeal, laryngeal and some hyoideal muscles (V, VII, IX,

X, and XI cranial nerves) form the special visceral efferent component (SVE).

The autonomic nervous system fibers that innervate intrinsic muscles of the

eye, heart, vessels, viscera and glands (III, VII, IX, X and XI cranial nerves) form the

general visceral efferent component (GVE).

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The sensory component:

The sensory fibers coming from the skin and musculoskeletal receptors (V,

VII and X cranial nerves) form the general somatic afferent component (GSA).

The fibers for the sense of sight coming from the retina (II cranial nerve)

and for the senses of equilibrium and hearing coming from the inner ear (VIII

cranial nerve) form the special somatic afferent component (SSA).

The sensory fibers from viscera and heart (VII, IX and X cranial nerves)

make up the general visceral afferent component (GVA). The fibers for the sense of

taste (VII, IX and X cranial nerves) form the special visceral afferent component

(SVA). The sense of smell (I cranial nerve) is also considered an SVA.

It is important to point out that the components of some different cranial

nerves intermingle in order to reach their destinations.

The olfactory nerves (I cranial nerve)

Special visceral afferent (SVA)

The olfactory nerves are formed by unmyelinated axons (surrounded by olfactory

ensheating cells), whose bodies are located in the epithelium that covers half of the

ethmoid labyrinth and the dorsal nasal septum. The axons enter the cranial cavity

through the cribriform plate of the ethmoid bone and reach the olfactory bulb

(paleocortex) of the cerebrum. The olfactory nerves transmit odor sensations.

The vomeronasal organ is closely related to the sense of smell. It has

pheromone receptors to detect odors associated with reproductive functions. The

terminal and vomeronasal nerves also reach the olfactory bulb.

The optic nerve (II cranial nerve)

Special somatic afferent (SSA)

The optic nerve is the nerve of sight. It consists of the ganglion cell axons of the

retina. The optic nerve runs through the optic canal together with the internal

ophthalmic artery. A percentage of the optic fibers cross behind the jugum

sphenoidale, giving rise to the optic chiasm. After the chiasm, the optic fibers

(decussated and non decussated) form the optic tract that reaches the lateral

geniculate body of the thalamus.

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Oculomotor nerve (III cranial nerve)

General somatic efferent (GSE) and general visceral efferent (GVE)

The ocumolotor nerve leaves the brain stem between the cerebral peduncles and

runs rostrally in relation to the cavernous sinus and the trochlear, abducent and

ophthalmic nerves. It exits the cranial cavity through the orbital fissure, forming a

dorsal and a ventral branch.

The dorsal branch innervates the dorsal rectus and levator palpebrae

superioris muscles. The ventral branch innervates the medial rectus, ventral

rectus, and ventral oblique muscles. The GVE component is formed by fibers that

reach the ciliary ganglion. The parasympathetic postganglionic fibers that leave

this ganglion form the short ciliary nerves that run with the optic nerve to

innervate the ciliary and the pupillary sphincter muscles.

Proprioceptive fibers from the extra-ocular muscles travel in the III, IV and

VI cranial nerves and communicate with the ophthalmic nerve at the level of the

cavernous sinus. The soma of these neurons is located in the trigeminal ganglion.

Trochlear nerve (IV cranial nerve)

General somatic efferent (GSE)

The trochlear nerve is the only cranial nerve that leaves the brain stem

dorsolaterally at the level of the rostral medullary velum, where the left and right

nerves decussate (trochlear decussation). It runs, included in the dura mater,

through the lateral portion of the tentorium cerebelli and, then, in the lateral wall

of the cavernous sinus to exit the cranial cavity through the orbital fissure. Outside

of the skull, it is located between the oculomotor nerve and the ophthalmic branch

of the trigeminal nerve. It innervates the dorsal oblique muscle.

Trigeminal nerve (V cranial nerve)

Special visceral efferent (SVE) and general somatic afferent (GSA)

The trigeminal nerve leaves the brain stem at the level of the pons, rostral to the

trapezoid body. It passes through the trigeminal canal (located on the medial

aspect of the petrosal bone) and divides into three nerves: ophthalmic, maxillary

and mandibular. These nerves leave through foramina located in the wing of the

sphenoid bone. The ophthalmic nerve leaves the cranial cavity through the orbital

fissure together with the oculomotor, trochlear and abducent nerves. The

maxillary nerve enters the round foramen, that opens into the alar canal, and exits

through the rostral alar foramen. The mandibular nerve leaves through the oval

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foramen. The three nerves have a sensory component (GSA) but only the

mandibular has a motor component (SVE).

The sensory neurons have their bodies located in the trigeminal ganglion

(also called Gasser's ganglion or semilunar ganglion). This is located in a trigeminal

cavity of the dura mater in the rostral opening of the trigeminal canal in the

rostromedial surface of the petrous temporal bone. There is an exception in the

case of the proprioceptive neurons, whose fibers form the mesencephalic tract of

the trigeminal nerve inside the brain stem. The soma of these neurons is located in

the nucleus of the mesencephalic tract of the trigeminal nerve, situated lateral to

the periaqueductal gray matter of the mesencephalon.

. The ophthalmic nerve

Before going through the orbital fissure, the ophthalmic nerve gives off a sensory

meningeal branch. Just after leaving the cranial cavity, it enters the periorbita and

divides into the following nerves: frontal, lacrimal and nasociliary.

. The frontal nerve is sensory (GSA) to the upper eyelid.

. The lacrimal nerve is sensory to the upper eyelid, conjuntiva and lacrimal

gland. It carries postganglionic parasympathetic fibers from the pterogopalatine

ganglion to the lacrimal gland. These are GVE fibers from the facial nerve.

. The nasociliary nerve gives off the ethmoidal and infratrochlear nerves.

The ethmoidal nerve passes through the ethmoid foramen into the cranial cavity to

reach the cribriform plate and, then, enters the nasal cavity to innervate the nasal

mucosa and the skin of the nasal vestibule. The infratrochlear nerve is located

medial to the orbit. It is sensory to the skin of the medial canthus of the eye (GSA).

The nasociliary nerve sends a branch to the ciliary ganglion that (without

synapsing) forms part of the short ciliary nerves (GVE). The continuation of the

nasociliary nerve forms the long ciliary nerves that are sensory (GSA) to the

eyeball and carry postganglionic sympathetic fibers (from the cranial cervical

ganglion) to the dilator muscle of the pupil and to the smooth muscle fibers of the

periorbita.

. The maxillary nerve

After sending out a meningeal branch, the maxillary nerve leaves the cranial cavity

through the round foramen into the alar canal and, then, through the rostral alar

foramen. It divides into the zygomatic, pterygopalatine and infraorbital nerves.

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. The zygomatic nerve leaves the maxillary nerve at the level of the round

foramen. After exiting through the rostral alar foramen, it enters the periorbita and

divides into the zygomaticotemporal and zygomaticofacial nerves. The

zygomaticotemporal nerve runs over the orbital ligament. It is sensory to the skin

dorsal to the zygomatic arch and the lateral canthus of the eye. It carries

postganglionic parasympathetic fibers from the pterygopalatine ganglion to the

lacrimal gland (these are GVE fibers from the facial nerve). The zygomaticofacial

nerve runs under the orbital ligament. It is sensory to the lower eyelid and lateral

canthus of the eye.

. The pterygopalatine nerve passes over the dorsal surface of the medial

pterygoid muscle. It sends out the caudal nasal nerve and the major, accessory and

minor palatine nerves. These nerves incorporate postganglionic parasympathetic

fibers from the pterygopalatine ganglion (these are GVE and GVA fibers from the

facial nerve) to innervate the nasal and palatine glands. The caudal nasal nerve

enters the nasal cavity through the sphenopalatine foramen. The major palatine

nerve enters the palatine canal through the caudal palatine foramen and

innervates the hard palate. The minor palatine nerve innervates the soft palate,

and the accessory palatine nerve innervates the caudal portion of the hard palate.

. The infraorbital nerve is a continuation of the maxillary nerve. At the level

of the pterygopalatine fossa, the infraorbital nerve sends out the caudal superior

alveolar branches that enter the alveolar foramina of the maxilla (to supply the

caudal upper teeth). Then the infraorbital nerve enters the infraorbital canal via

the maxillary foramen. Within the canal, it sends out the middle superior alveolar

branches which enter the alveolar canals (to supply the middle upper teeth). Just

before leaving the canal, through the infraorbital foramen, the maxillary nerve

sends out the rostral superior alveolar branches through the incisivo-maxillary

canal (to the upper canine and incisor teeth). Once out of the infraorbital canal, the

infraorbital nerve divides into nasal and superior labial branches.

The mandibular nerve

After sending out a meningeal branch, the mandibular nerve exits the cranial

cavity through the oval foramen and divides into the following nerves: masticator,

buccal, lateral pterygoid, medial pterygoid, inferior alveolar, lingual, mylohyoid,

tensor tympani, tensor veli palatine and auriculotemporal.

. The masticator divides into the masseteric and deep temporal nerves to

the masseter and temporal muscles respectively.

. The buccal nerve is sensory for the mucousa of the cheeks. It is also

sensory to the skin located ventrally to the zygomatic arch and dorsocaudally to

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the commissure of the mouth. It incorporates postganglionic parasympathetic

fibers from the otic ganglion to innervate the salivary zygomatic gland (these are

GVE nerve fibers from the glossopharyngeal nerve).

. The lateral and medial pterygoid nerves innervate the lateral and medial

pterygoid muscles respectively.

. The inferior alveolar nerve enters the mandibular canal through the

mandibular foramen to innervate the lower teeth. The rostral, middle and caudal

mental nerves are rostral extensions of the inferior alveolar nerve that exit

through the mental foramina. These nerves are sensory to the lower lip.

. The lingual nerve is sensory for the rostral two thirds of the tongue. The

GSA component is formed by trigeminal fibers. The GVE, GVA and SVA (taste)

components are facial fibers of the chorda tympani that join the lingual nerve in

the dorsomedial surface of the medial pterygoid muscle.

. The mylohyoid nerve is motor to the mylohyoid and rostral belly of the

digastricus muscles. It is also sensory to the skin of the intermandibular region

and, through a communicating branch with the ventral buccal branch of the facial

nerve, to the caudal portion of the lower lip and ventrolateral area of the cheek

(caudal to the region innervated by the mental branches).

. The tensor tympani nerve innervates the tensor tympani muscle. When

this muscle contracts, the manubrium of the malleus moves medially and tightens

the tympanic membrane.

. The tensor veli palatini nerve innervates the tensor veli palatini muscle.

When this muscle acts in combination with the levator veli palatini (innervated by

the glossopharyngeal and vagus nerves), it opens the orifice of the auditory tube

into the pharynx.

. The auriculotemporal nerve leaves the mandibular nerve at the level of the

oval foramen and runs caudally to the retroarticular process of the temporal bone.

It sends out the external acoustic meatus nerve and a branch to the tympanic

membrane. They are sensory to the external acoustic meatus and to the tympanic

membrane respectively. The auriculotemporal nerve receives postganglionic

parasympathetic fibers from the otic ganglion to innervate the salivary parotid

gland (these are GVE fibers from the glossopharyngeal nerve). The rostral

auricular nerves of the auriculotemporal nerve are sensory to the skin of the ear

and the skin that covers the surface of the temporal muscle and the zygomatic

arch. The transverse facial branch is formed by auriculotemporal nerve fibers that

are sensory to the skin located laterally and ventrally to the zygomatic arch. A

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communicating ramus between the auriculotemporal nerve and the dorsal buccal

branch runs dorsal to the masseter muscle. It is sensory to the skin of the cheek,

ventral to the zygomatic arch.

Abducent nerve (VI cranial nerve)

General somatic efferent (GSE)

The abducent nerve leaves the brain stem at the level of the trapezoid body, lateral

to the pyramids. It runs rostrally to leave the cranial cavity through the orbital

fissure. It innervates the retractor bulbi and lateral rectus muscles.

Facial nerve (VII cranial nerve)

Special visceral efferent (SVE), general visceral efferent (GVE), general visceral

afferent (GVA), special visceral afferent (SVA) and general somatic afferent (GSA)

The soma of the neurons of the SVE component of the facial nerve form the motor

nucleus of the facial nerve. This is located ventrolaterally in the medulla oblongata,

rostral to the nucleus ambiguus. The axons ascend dorsomedially to form a loop

(named internal genu of the facial nerve) around the abducent nucleus and they

descend, ventrally to the nucleus of the spinal tract of the trigeminal nerve, to leave

the brain stem forming the facial motor root. The facial fibers for the GSA, GVE,

GVA and SVA components form the intermediate nerve.

The motor and intermediate nerve fibers of the facial nerve leave the brain

stem ventrolaterally at the rostral portion of the trapezoid body. They are

surrounded by a common sheath of dura mater together with the

vestibulocochlear nerve. After a short distance, they enter the facial canal (located

in the petrosal part of the temporal bone) through the internal acoustic meatus.

Inside the facial canal, the nerve turns caudally and laterally, forming the external

genu of the facial nerve. At this point the geniculate ganglion is located. This

ganglion contains the bodies of the afferent neurons.

The major petrosal nerve (GVE and GVA) leaves the facial nerve at the level

of the geniculate ganglion. It is joined by sympathetic postganglionic fibers from

the cranial cervical ganglion via the internal carotid plexus. These sympathetic

fibers form the deep petrosal nerve. Both nerves (the major and the deep)

constitute the nerve of the pterygoid canal, which leaves the skull through a small

foramen on the rim of the rostral alar foramen. This nerve reaches the

pterogopalatine ganglion where the parasympathetic neurons synapse on

postganglionic neurons that innervate the lacrimal gland by means of the lacrimal

nerve (a branch of the ophthalmic nerve) and the zygomaticotemporal nerve (a

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branch of the maxillary nerve). Other postganglionic fibers are incorporated to the

pterygopalatine nerve (also a branch of the maxillary nerve) to innervate the

palate and nasal glands.

The facial nerve continues along the facial canal and gives off the stapedial

nerve (SVE) to innervate the stapedius muscle. This muscle modifies the position

of the stapes in the vestibular window.

The next branch to leave the facial nerve within the facial canal is the

chorda tympani (GVE, GVA and SVA). This nerve passes through the canaliculus of

the chorda tympani to reach the middle ear cavity. It runs medial to the tympanic

membrane and exits the skull through the petrotympanic fissure to join the lingual

nerve (a branch of the mandibular nerve). The efferent fibers (GVE) are

parasympathetic preganglionic neurons that reach the mandibular and sublingual

ganglia in order to innervate the mandibular and sublingual salivary glands

respectively. The afferent fibers transmit sensations from the rostral two thirds of

the tongue including taste (SVA). The soma of the GVA and SVA neurons is located

at the geniculate ganglion of the facial nerve (inside the facial canal).

The facial nerve is joined by the auricular branch of the vagus nerve. This

leaves the vagus nerve at the level of the jugular foramen and constitute the lateral

internal auricular nerve (a branch of the facial nerve) that innervates the external

ear canal.

Once the facial nerve has become external through the stylomastoid

foramen, it gets thicker because the epineurium increases. As the facial nerve runs

rostrally, it gives rise to the following nerves: the caudal auricular nerve (SVE for

the platysma and caudal auricular muscles), the caudal and middle internal

auricular nerve (GSA fibers for the auricular concha), the lateral internal auricular

nerve (formed by GSA vagal fibers to the external ear canal), the digastric branch

(SVE for the caudal belly of the digastricus muscle) and the stylohyoid branch (SVE

for the stylohyoid and jugulohyoid muscles).

When the facial nerve passes the caudal border of the mandible on the

surface of the masseter muscle, it sends out the following SVE branches: cervical,

buccal and auriculopalpebral. The cervical branches innervate the

parotidoauricularis and the sphincter colli muscles before joining the ventral

branch of the second cervical spinal nerve. The buccal nerve divides into dorsal

and ventral branches. These innervate the buccinator, the orbicularis oris and the

nasolabial muscles. The dorsal and ventral buccal branches receive GSA fibers

through communicating branches of the auriculotemporal nerve and mylohyoid

nerves respectively (these nerves are branches of the mandibular nerve). The

auriculopalpebral nerve divides into palpebral and rostral auricular nerves. The

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palpebral nerve forms a plexus that innervates the rostral auricular, nasolabial and

palpebral muscles. The rostral auricular branch innervates the scutuloauricularis

muscles.

There is no evidence for proprioceptive afferents in the facial nerve that

innervates the superficial muscles of the face, most likely the trigeminal nerve

contains afferents from intramuscular and cutaneous receptors involved in mimic

movements.

Vestibulocochlear nerve (VII cranial nerve)

Special somatic afferent (SSA)

The vestibulocochlear nerve consists of fibers that transmit stimuli from receptors

located in the inner ear.

The vestibulocochlear nerve enters the cranial cavity through the internal

acoustic meatus (located in the medial surface of the petrosal part of the temporal

bone) and enters the brain stem at the level of the trapezoid body. The fibers of the

vestibular root (sense of balance) end in the vestibular nuclei and the fibers of the

cochlear root (sense of hearing) reach the cochlear nuclei. Same cochlear fibers are

efferent to the organ of Corti.

Glossopharyngeal nerve (IX cranial nerve)

Special visceral efferent (SVE), general visceral efferent (GVE), general visceral

afferent (GVA) and special visceral afferent (SVA)

The glossopharyngeal nerve leaves the brain stem at the level of the medulla

oblongata, caudally to the trapezoid body.

The parasympathetic preganglionic fibers of the glossopharyngeal nerve

form the tympanic nerve (GVE). This nerve enters the middle ear to form the

tympanic plexus. Fibers from this plexus form the minor petrosal nerve that leaves

the skull through a small foramen (located dorsocaudally to the oval foramen) to

reach the otic ganglion. The parasympathetic postganglionic fibers join the

auriculotemporal and the buccal nerves (both are branches of the mandibular

nerve) to innervate the parotid and zygomatic salivary glands respectively.

The remaining fibers of the glossopharyngeal nerve exit the cranial cavity

through the jugular foramen and, then, through the tympano-occipital fissure,

along with the vagus and accessory nerves. The glossopharyngeal nerve sends a

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sensory branch (GVA) to the carotid sinus and carotid body. Subsequently, it

divides into lingual and pharyngeal branches.

The lingual branch (GVE, GVA and SVA) innervates the tonsils and the root

of the tongue. The pharyngeal branch and the vagus nerve form the pharyngeal

plexus, which innervates the muscles and mucosa of the pharynx. This plexus is

composed of motor fibers (SVE and GVE) and sensory fibers (GVA), and is joined

by sympathetic postganglionic fibers from the cranial cervical ganglion.

In the dog, the soma of the sensory neurons is located in a ganglion at the

level of the jugular foramen.

Vagus nerve (X cranial nerve)

Special visceral efferent (SVE), general visceral efferent (GVE), general visceral

afferent (GVA), special visceral afferent (SVA) and general somatic afferent (GSA)

The vagus nerve leaves the brain stem at the level of the medulla oblongata, caudal

to the glossopharyngeal nerve.

Most of the vagal fibers are visceral sensory (GVA). The rest are made up of

visceral motor (GVE), special visceral motor (SVE), sensory to the taste buds of the

epiglottis (SVA) and somatic sensory (GSA) to the ear canal.

The vagus nerve leaves the cranial cavity through the jugular foramen and,

then, through the tympano-occipital fissure. At the level of the jugular foramen it

sends a sensory auricular branch (GSA) to join the facial nerve in order to reach

the skin of the ear canal through the lateral internal auricular nerve.

The proximal vagal ganglion (also named jugular ganglion) is located at the

level of the jugular foramen. It is formed by the soma of the somatic afferent

neurons (GSA). The soma of the GVA and SVA neurons is located at the distal vagal

ganglion (also named nodose ganglion). This is situated outside the tympano-

occipital fissure and topographically related with the glossopharyngeal, accessory

and hypoglossal nerves, and the cranial cervical ganglion.

The pharyngeal ramus (SVE, GVE and GVA) leaves the vagus nerve at the

level of the distal vagal ganglion. It is joined by fibers from the glossopharyngeal

ramus and sympathetic postganglionic fibers (from the cranial cervical ganglion)

to form the pharyngeal plexus. This innervates the pharyngeal muscles and the

cranial part of the esophagus.

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The cranial laryngeal nerve branches off from the vagus nerve also at the

level of the distal vagal ganglion. It divides into an external ramus and an internal

ramus. The external ramus (SVE) innervates the cricothyroideus muscle. The

internal ramus (GVE and GVA) innervates the mucosa of the larynx, rostral to the

vocal folds, and the taste buds of the epiglottis (SVA).

The vagus nerve descends into the thoracic cavity together with ascending

sympathetic fibers, forming the vagosympathetic trunk. At the level of the middle

cervical ganglion, the vagus nerve leaves the sympathetic fibers and sends out

cardiac branches formed by parasympathetic preganglionic fibers (GVE) and

sensory fibers (GVA). On the left side, the nerve forms the left recurrent laryngeal

nerve that passes around the aortic arch. On the right side, the right recurrent

laryngeal nerve turns around the right subclavian artery. Both recurrent laryngeal

nerves run cranially becoming the left and the right caudal laryngeal nerves. They

innervate the trachea (GVE and GVA), the esophagus (SVE, GVE and GVA), and the

laryngeal muscles (SVE), except for the cricothyroid muscle that is innervated by

the external ramus of the cranial laryngeal nerve. The laryngeal nerves also

innervate the mucosa of the larynx caudal to the vocal folds (GVE and GVA).

At the middle mediastinum, the vagus nerve sends branches to the bronchi,

esophagus and heart. Just caudal to the bronchi, each left and right vagus nerves

form a dorsal and a ventral branch. The dorsal branches (left and right) unite, as do

the ventral ones, to form the dorsal and ventral vagal trunks respectively. In the

abdominal cavity, the dorsal vagal trunk joins the celiacomesenteric plexus so the

fibers may follow the vessels to reach the viscera. The ventral vagal trunk

innervates the lesser curvature of the stomach and the liver. These fibers are made

of preganglionic parasympathetic (GVE) and visceral sensory (GVA) neurons.

Although the pharyngeal and laryngeal muscles were considered lacking of

spindles, spindle-like receptors to stretch have been found indicating the

possibility of GSA fibers for proprioception in the glossopharyngeal and vagus

nerves.

Accessory nerve (XI cranial nerve)

General somatic efferent (GSE) and special visceral efferent (SVE)

The accessory nerve consists of cranial and spinal roots. The neurons that form the

cranial roots have their bodies in the nucleus ambiguus in the medulla oblongata.

The neurons of the spinal roots have their bodies in the motor nucleus of the

accessory nerve, located in the dorsolateral portion of the ventral horn of the

cervical spinal cord segments (from C1 to C7).

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The spinal roots leave the spinal cord between the dorsal and ventral roots

of the cervical spinal nerves, and lie dorsal to the denticulate ligament (under the

dorsal spinal roots). They run rostrally to enter the cranial cavity, through the

foramen magnum, and join the cranial roots. These leave the medulla oblongata

vetrolaterally. Cranial and spinal accessory nerve roots unite to become the

external branch of the accessory nerve (GSE). Some fibers from the cranial roots

join the vagus nerve to form the internal branch of the accessory nerve. This

branch becomes part of the recurrent laryngeal nerve (SVE).

Upon exiting the cranial cavity through the jugular foramen and, then,

through the tympano-occipital fissure, the external branch of the accessory nerve

descends to innervate the trapezius, omotransversarius, sternocephalicus and

cleidocephalicus muscles. The latter two muscles are also innervated by ventral

branches of the cervical spinal nerves.

As there are communications between the accessory nerve and the cervical

spinal nerves, it has been proposed that the soma of sensory cells (GSA) for

proprioception may be located in the dorsal root ganglia.

Hypoglossal nerve (XII cranial nerve)

General Somatic efferent (GSE)

The hypoglossal nerve is motor to muscles of the tongue (intrinsic and extrinsic)

and genihyoideus and thyrohyoideus muscles. It leaves the medulla oblongata

laterally to the pyramids by means of several rootlets. The nerve exits the skull

through the hypoglossal canal and goes ventrorostrally, lateral to the external

carotid artery, to run parallel to the lingual artery. In its course, the hypoglossal

nerve communicates with the ventral branch of the first cervical spinal nerve

forming, the cervical loop (ansa cervicalis).

Although the hypoglossal nerve is considered a motor nerve, there are

afferents that respond to stretch. As this nerve has no sensory ganglion, it has been

proposed that the soma of the afferent neurons may be located at the dorsal root

ganglion of C1 (the connection will be through the ansa cervicalis).

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