Atlas of Anatomy. Head and Neuroanatomy. Michael Schuenke

4. Cranial Nerves

4.1 Overview of the Cranial Nerves

A Functional components of the cranial nerves

The twelve pairs of cranial nerves are designated by Roman numerals according to the order of their emergence from the brainstem (see topographical organization in C).

Note: The first two cranial nerves, the olfactory nerve (CN I) and optic nerve (CN II), are not peripheral nerves in the true sense but rather extensions of the brain, i.e., they are CNS pathways that are covered by meninges and contain cell types occurring exclusively in the CNS (oligodendrocytes and microglial cells).

Like the spinal nerves, the cranial nerves may contain both afferent and efferent axons. These axons belong either to the somatic nervous system, which enables the organism to interact with its environment (somatic fibers), or to the autonomic nervous system, which regulates the activity of the internal organs (visceral fibers). The combinations of these different general fiber types in spinal nerves result in four possible compositions that are found chiefly in spinal nerves but also occur in cranial nerves (see functional organization in C):

General somatic afferents (somatic sensation):

→ E.g., fibers convey impulses from the skin and striated muscle spindles

General visceral afferents (visceral sensation):

→ E.g., fibers convey impulses from the viscera and blood vessels

General visceral efferents (visceromotor function):

→ Fibers innervate the smooth muscle of the viscera, intraocular muscles, heart, salivary glands, etc.

General somatic efferents (somatomotor function):

→ Fibers innervate striated muscles

Additionally, cranial nerves may contain special fiber types that are associated with particular structures in the head:

Special somatic afferents:

→ E.g., fibers conduct impulses from the retina and from the auditory and vestibular apparatus

Special visceral afferents:

→ E.g., fibers conduct impulses from the taste buds of the tongue and from the olfactory mucosa

Special visceral efferents:

—» E.g., fibers innervate striated muscles derived from the branchial arches (branchiogenic efferents and branchiogenic muscles)

В Color coding used in subsequent units to indicate different fiber types

C Topographical and functional organization of the cranial nerves

Topographical origin

Name

Functional fiber type

Telencephalon

• Olfactory nerve (CN1)

• Special visceral afferent

Diencephalon

• Optic nerve (CN II)

• Special somatic afferent

Mesencephalon

• Oculomotor nerve

(CN III)*

• Somatic efferent

• Visceral efferent

(parasympathetic)

• Trochlear nerve (CN IV)*

• Somatic efferent

Pons

• Trigeminal nerve

(CNV)

• Special visceral efferent (first branchial arch)

• Somatic afferent

• Abducent nerve (CN VI)*

• Somatic efferent

• Facial nerve (CN VII)

• Special visceral efferent (second branchial arch)

• Special visceral afferent

• Visceral efferent

(parasympathetic)

• Somatic afferent

Medulla oblongata

• Vestibulocochlear nerve (CN VIII)

• Special somatic afferent

• Glossopharyngeal nerve (CN IX)

• Special visceral efferent (third branchial arch)

• Special visceral afferent

• Visceral afferent

(parasympathetic)

• Somatic afferent

• Vagus nerve (CN X)

• Special visceral efferent (fourth branchial arch)

• Special visceral afferent

• Visceral efferent

(parasympathetic)

• Visceral afferent

• Somatic afferent

• Accessory nerve (CN XI)*

• Special visceral efferent (fifth branchial arch)

• Somatic efferent

• Hypoglossal nerve

(CNXII)*

• Somatic efferent

* Note: Cranial nerves with somatic efferent fibers innervating the striated muscles also have somatic afferent fibers that conduct proprioceptive impulses from the muscle spindles and other structures (for clarity, not listed above).

A characteristic feature of the cranial nerves is that their sensory and motor fibers enter and exit the brainstem at the same sites. This differs from the spinal nerves, in which the sensory fibers enter the spinal cord through the dorsal roots while the motor fibers leave the spinal cord through the ventral roots.

4.2 Cranial Nerves:

Brainstem Nuclei and Peripheral Ganglia

A Overview of the nuclei of cranial nerves III -XII

Just as different fiber types can be distinguished in the cranial nerves (C.p.66), the nuclei of origin and nuclei of termination of the cranial nerves can also be classified according to different sensory and motor types and modalities. According to this scheme, the nuclei that belong to the parasympathetic nervous system are classified as general visceral efferent nuclei, while the nuclei of the branchial arch nerves are classified as special visceral efferent nuclei. The visceral afferent nuclei are considered either general (lower part of the solitary nuclei) or special (upper part, gustatory fibers). The somatic afferent nuclei can be differentiated in a similar way: the principal sensory nucleus of the trigeminal nerve is classified as general somatic afferent, while the nucleus of the vestibulocochlear nerve is special somatic afferent.

Motor nuclei: (give rise to efferent [motor] fibers, left in C)

Somatic efferent (somatic motor) nuclei (red):

• Nucleus of oculomotor nerve (CN III: eye muscles)

 Nucleus of trochlear nerve (CN IV: eye muscles)

 Nucleus of abducent nerve (CN VI: eye muscles)

 Nucleus of accessory nerve (CN XI, spinal root: shoulder muscles)

• Nucleus of hypoglossal nerve (CN XII: lingual muscles)

Visceral efferent (visceral motor) nuclei (blue):

Nuclei associated with the parasympathetic nervous system (light blue):

 Visceral oculomotor (Edinger-Westphal) nucleus (CN III: papillary sphincter and ciliary muscle)

 Superior salivatory nucleus (CN VII, facial nerve: submandibular and sublingual glands)

 Inferior salivatory nucleus (CN IX, glossopharyngeal nerve: parotid gland)

 Dorsal vagal nucleus (CN X: viscera)

Nuclei of the branchial arch nerves (dark blue):

 Trigeminal motor nucleus (CN V: muscles of mastication)

 Facial nucleus (CN VII: facial muscles)

 Nucleus ambiguus (CN IX, glossopharyngeal nerve; CN X, vagus nerve;

CN XI, accessory nerve [cranial root]: pharyngeal and laryngeal muscles)

Sensory nuclei: (where afferent [sensory] fibers terminate, right in C) * * * * •

Somatic afferent (somatic sensory) and vestibulocochlear nuclei (yellow):

Sensory nuclei associated with the trigeminal nerve (CN V):

• Mesencephalic nucleus (proprioceptive afferents from muscles of mastication)

• Principal (pontine) sensory nucleus (touch, vibration, joint position)

• Spinal nucleus (pain and temperature sensation in the head)

Nuclei of the vestibulocochlear nerve (CN VIII):

• Vestibular part (sense of balance):

— Medial vestibular nucleus

— Lateral vestibular nucleus

— Superior vestibular nucleus

— Inferior vestibular nucleus

• Cochlear part (hearing):

— Anterior cochlear nucleus

— Posterior cochlear nucleus

Visceral afferent (visceral sensory) nuclei (green):

« Nucleus of the solitary tract (nuclear complex):

— Superior part (special visceral afferents [taste] from CN VII [facial],

CN IX [glossopharyngeal], and CN X [vagus] nerves)

— Inferior part (general visceral afferents from CN IX [glossopharyngeal] and CN X [vagus] nerves)

В Arrangement of brainstem nuclear columns during embryonic development (after Herrick)

Cross-sections through the spinal cord and brainstem, superior view. The functional organization of the brainstem is determined by the location of the cranial nerve nuclei, which can be explained in terms of the embryonic migration of neuron populations.

a Initial form as seen in the spinal cord: The motor (efferent) neurons are ventral, and the sensory (afferent) neurons are dorsal (=dorso-ventral arrangement).

b early embryonic stage of brainstem development: the neurons of the alar plate (sensory nuclei) migrate laterally while the neurons of the basal plate (motor nuclei) migrate medially. This gives rise to a general mediolateral arrangement of the nuclear columns. The arrows indicate the directions of cell migration, c adult brainstem: features a medial-to-lateral arrangement of four longitudinal nuclear columns (one somatic efferent, one visceral efferent, one visceral afferent, and one somatic afferent). In each of these columns, nuclei that have the same function are arranged one above the other in a craniocaudal direction (see C). The nuclei in the somatic afferent and visceral afferent columns are differentiated into general and special afferent nuclei. Similarly, the visceral efferent nuclear column is differentiated into general (parasympathetic) and special (branchiogenic) efferent nuclei. This general/special subdivision is not present in the somatic efferent nuclear column.

C Location of cranial nerves III -XII in the brainstem

a Posterior view (with cerebellum removed), b Midsagittal section, left lateral view.

Except for cranial nerves I and II, which are extensions of the brain rather than true nerves, all pairs of cranial nerves are associated with corresponding nuclei in the brainstem. The diagrams show the nerve pathways leading to and from these nuclei. The arrangement of the cranial nerve nuclei is easier to understand when we classify them into functional nuclear columns (see B). The efferent (motor) nuclei where the efferent fibers arise are shown on the left side in a. The afferent (sensory) nuclei where the afferent fibers end are shown on the right side.

D Ganglia associated with cranial nerves

Ganglia fall into two main categories: sensory and autonomic (parasympathetic). The sensory ganglia are analogous to the spinal ganglia in the dorsal roots of the spinal cord. They contain the perikarya of the pseudounipolar nerve cells (= primary afferent neuron). Their peripheral process comes from a receptor, and their central process terminates in the CNS. Synaptic relays do not occur in the sensory ganglia. The autonomic ganglia in the head are entirely parasympathetic. They contain the perikarya of the multipolar nerve cells (= second efferent, or postsynaptic, neuron). Unlike the sensory ganglia, these ganglia synapse with parasympathetic fibers from the brainstem (= first efferent, or preganglionic, neuron). Specifically they synapse with the perikarya of the second efferent (or postsynaptic) neuron, whose fibers are distributed to the target organ.

Cranial nerves

Sensory ganglia

Autonomic ganglia

 

Oculomotor nerve (CN III)

 

• Ciliary ganglion

 
 

Trigeminal nerve (CN V)

• Trigeminal ganglion

 
 

Facial nerve (CNVII)

• Geniculate ganglion

• Pterygopalatine ganglion

• Submandibular ganglion

 
 

Vestibulocochlear nerve (CN VIII)

• Spiral ganglion

   
 

• Vestibular ganglion

 

Glossopharyngeal nerve (CN IX)

• Superior ganglion

• Otic ganglion

 
 

• Inferior (petrosal) ganglion

   

Vagus nerve (CNX)

• Superior (jugular) ganglion

• Prevertebral and intramural ganglia

 
 
 

• Inferior

(nodose) ganglion

 

4.3 Cranial Nerves: Olfactory (CN I) and Optic (CN II)

A Olfactory bulb and olfactory tract on the basal surface of the frontal lobes of the brain

The unmyelinated axons of the primary bipolar sensory neurons in the olfactory mucosa are collected into approximately 20 fiber bundles (see B), which are referred to collectively as the olfactory nerve. These axon bundles pass from the nasal cavity through the cribriform plate of the ethmoid bone into the anterior cranial fossa (see B), and synapse in the olfactory bulb. The olfactory bulb and associated olfactory tract are not parts of a peripheral nerve but instead constitute an extension of the telencephalon that contains CNS-specific cell types (oligodendrocytes and microglia). The olfactory bulb and tract share with the telencephalon a meningeal covering that is removed here. Axons from second-order afferent neurons in the olfactory bulb pass through the olfactory tract and medial or lateral olfactory stria, ending in the cerebral cortex of the prepiriform area, in the amygdala, or in neighboring areas. By this short route, olfactory information is thus transmitted into the CNS and can be relayed directly to the cerebral cortex.

The primary sensory neurons of the olfactory mucosa have several unusual properties that should be noted. These neurons have a limited lifespan of up to several months, but are continuously replenished from a pool of precursor cells in the olfactory mucosa that undergo periodic mitosis. New olfactory receptors are thus generated throughout adult life, and their axons enter the olfactory bulb to form new synapses with existing CNS neurons. The regenerative capacity of the olfactory mucosa gradually diminishes with advancing age, however, resulting in a net loss of receptors and a slow decline in overall sensory function. Note: Injuries to the cribriform plate may damage the meningeal covering of the olfactory fibers, resulting in olfactory disturbances and cerebrospinal fluid leakage from the nose (“runny nose” after head trauma). There is an associated risk of ascending bacterial infection causing meningitis.

* The shaded structures are deep to the basal surface of the brain.

В Extent of the olfactory mucosa (olfactory region)

Portion of the left nasal septum and lateral wall of the right nasal cavity, viewed from the left side. The olfactory fibers on the septum and superior concha define the extent of the olfactory region (2-4cm2). The thin, unmyelinated olfactory fibers enter the skull through the cribriform plate of the ethmoid bone (see p.27) and pass to the olfactory bulb (see also pp. 116, 204, and 372).

C Eye, optic nerve, optic chiasm, and optic tract

a View of the base of the brain, b posterolateral view of the leftside of the brainstem. The termination of the optic tract in the lateral geniculate body is shown.

The optic nerve is not a true nerve but an extension of the brain, in this case of the diencephalon. Analogously to the olfactory bulb and tract (seeA), the optic nerve is sheathed by meninges (removed here) and contains CNS-specific cells (seeA). The optic nerve contains the axons of retinal ganglion cells. These axons terminate mainly in the lateral geniculate body of the diencephalon and in the mesencephalon (superior colliculus, pp. 234-235).

Note: Because the optic nerve is an extension of the brain, the clinician can directly inspect a portion of the brain with an ophthalmoscope. This examination is important in the diagnosis of many neurological diseases (ophthalmoscopy is described on p. 133).

The optic nerve passes from the eyeball through the optic canal into the middle cranial fossa (see D). Many, but not all, retinal cell ganglion axons cross the midline to the contralateral side of the brain in the optic chiasm (a). The optic tract extends from the optic chiasm to the lateral geniculate body (see also b).

D Course of the optic nerve in the right orbit

Lateral view. The optic nerve extends through the optic canal from the orbit into the middle cranial fossa. It exits the posterior side of the eyeball within the retro-orbital fat (removed here). The other cranial nerves enter the orbit through the superior orbital fissure (only CN is shown here).

4.4 Cranial Nerves of the Extraocular Muscles:

Oculomotor (CN III), Trochlear (CN IV), and Abducent (CN VI)

A Emergence of the nerves from the brainstem

Anterior view. All three nerves that supply the extraocular muscles emerge from the brainstem. The nuclei of the oculomotor nerve and trochlear nerve are located in the midbrain (mesencephalon), while the nucleus of the abducent nerve is located in the pons.

Note: Of these three nerves, the oculomotor (CN III) is the only one that contains somatic efferent and visceral efferent fibers and supplies several extraocular muscles (see C).

В Overview of the oculomotor nerve (CN III)

The oculomotor nerve contains somatic efferent and visceral efferent fibers.

Course: The nerve runs anteriorly from the mesencephalon (midbrain = highest level of the brainstem; see pp. 226, 228) and enters the orbit through the superior orbital fissure

Nuclei and distribution, ganglia:

 Somatic efferents: Efferents from a nuclear complex (oculomotor nucleus) in the midbrain (see C) supply the following muscles:

- Levator palpebrae superioris (acts on the upper eyelid)

- Superior, medial, and inferior rectus and inferior oblique (= extraocular muscles, all act on the eyeball).

 Visceral efferents: Parasympathetic preganglionic efferents from the visceral oculomotor (Edinger-Westphal) nucleus synapse with neurons in the ciliary ganglion that innervate the following intraocular muscles:

- Pupillary sphincter

- Ciliary muscle

Effects of oculomotor nerve injury:

Oculomotor palsy, severity depending on the extent of the injury.

 Effects of complete oculomotor palsy (paralysis of the extraocular and intraocular muscles and levator palpebrae):

- Ptosis (drooping of the lid)

- Downward and lateral gaze deviation in the affected eye

- Diplopia (in the absence of complete ptosis)

- Mydriasis (pupil dilated due to sphincter pupillae paralysis)

- Accommodation difficulties (ciliary paralysis - lens cannot focus).

C Topography of the oculomotor nucleus

Cross-section through the brainstem at the level of the oculomotor nucleus, superior view.

Note: the visceral efferent, parasympathetic nuclear complex (visceral oculomotor [Edinger-Westphal] nucleus) can be distinguished from the somatic efferent nuclear complex (nucleus of the oculomotor nerve).

D Overview of the trochlear nerve (CN IV)

The trochlear nerve contains only somatic efferent fibers.

Course: The trochlear nerve emerges from the posterior surface of the brainstem near the mid line, courses anteriorly around the cerebral peduncle, and enters the orbit through the superior orbital fissure.

Special features:

 The trochlear nerve is the only cranial nerve in which all the fibers cross to the opposite side (see A). Consequently, lesions of the nucleus or of nerve fibers very close to the nucleus, before they cross the mid line, result in tochlear nerve palsy on the side opposite to the lesion (contralateral palsy). A lesion past the site where the nerve crosses the midline leads to tochlear nerve palsy on the same side as the lesion (ipsilateral palsy).

 The trochlear nerve is the only cranial nerve that emerges from the dorsal side of the brainstem.

 It has the longest intradural course of the three extraocular motor nerves.

Nucleus and distribution: The nucleus of the trochlear nerve is located in the midbrain (mesencephalon). Its efferents supply motor innervation to one muscle, the superior oblique.

Effects of trochlear nerve injury:

 The affected eye is higher and is also deviated medially because the inferior oblique (responsible for elevation and abduction) becomes dominant due to loss of the superior oblique.

 Diplopia.

E Overview of the abducent nerve (CN VI)

The abducent nerve contains only somatic efferent fibers.

Course: The nerve follows a long extradural path before entering the orbit through the superior orbital fissure.

Nucleus and distribution:

 The nucleus of the abducent nerve is located in the pons (= midlevel brainstem), its fibers emerging at the inferior border of the pons.

 Its efferent fibers supply somatomotor innervation to a single muscle, the lateral rectus.

Effects of abducent nerve injury:

 The affected eye is deviated medially.

 Diplopia.

F Course of the nerves supplying the ocular muscles

a Lateral view. Right orbit, a Lateral view, b superior view (opened), c anterior view. All three cranial nerves leave the brainstem and enter the orbit through the superior orbital fissure, passing through the common tendinous ring of the extraocular muscles. The abducent nerve has the longest extradural course. Because of this, abducent nerve palsy may develop in association with meningitis and subarachnoid hemorrhage. Transient palsy may even occur in cases where lumbar puncture has caused an excessive fall of CSF pressure, with descent of the brainstem exerting traction on the nerve. The oculomotor nerve supplies para sympathetic innervation to intraocular muscles (its parasympathetic fibers synapse in the ciliary ganglion) as well as somatic motor innervation to most of the extraocular muscles and the levator palpebrae superioris. Oculomotor nerve palsy may affect the parasympathetic fibers exclusively, the somatic motor fibers exclusively, or both at the same time (seeB). Because the preganglionic parasympathetic fibers for the pupil lie directly beneath the epineurium after emerging from the brainstem, they are often the first structures to be affected by pressure due to trauma, tumors, or aneurysms.

4.5 Cranial Nerves:

Trigeminal (CN V), Nuclei and Distribution

A Nuclei and emergence from the pons.

a Anterior view. The larger sensory nuclei of the trigeminal nerve are distributed along the brainstem and extend downward into the spinal cord. The sensory root (major part) of the trigeminal nerve thus forms the bulk of the fibers, while the motor root (minor part) is formed by fibers arising from the small motor nucleus in the pons. They supply motor innervation to the muscles of mastication (see B). The following somatic afferent nuclei are distinguished: 

• Mesencephalic nucleus of the trigeminal nerve: proprioceptive fibers from the muscles of mastication. Special feature: the neurons of this nucleus are pseudounipolar ganglion cells that have migrated into the brain.

 Principal (pontine) sensory nucleus of the trigeminal nerve: chiefly mediates touch.

 Spinal nucleus of the trigeminal nerve: pain and temperature sensation, also touch. A small, circumscribed lesion of the trigeminal spinal sensory nucleus leads to characteristic sensory disturbances in the face (see D).

b Cross-section through the pons at the level of emergence of the trigeminal nerve, superior view.

В Overview of the trigeminal nerve (CN V)

The trigeminal nerve, the sensory nerve of the head, contains mostly somatic afferent fibers with a smaller proportion of special visceral efferent fibers. Its three major somatic divisions have the following sites of emergence from the middle cranial fossa:

 Ophthalmic division (CN Vj): enters the orbit through the superior orbital fissure.

 Maxillary division (CN VJ: enters the pterygopalatine fossa through the foramen rotundum.

 Mandibular division (CN VJ: passes through the foramen ovale to the inferior surfa ce of the base of the skull; only division containing motor fibers.

Nuclei and distribution:

 Special visceral efferent: Efferent fibers from the motor nucleus of the trigeminal nerve pass in the mandibular division (CN V3) to:

- Muscles of mastication (temporalis, masseter, medial and lateral pterygoid)

- Oral floor muscles: mylohyoid and anterior belly of the digastric

- Middle ear muscle: tensor tympani

- Pharyngeal muscle: tensor veli palatini

 Somatic afferent: The trigeminal ganglion contains pseudounipolar ganglion cells whose central fibers pass to the sensory nuclei of the trigeminal nerve (see A a). Their peripheral fibers innervate the facial skin, large portions of the nasopharyngeal mucosa, and the anterior two-thirds of the tongue (somatic sensation, see C).

 “Visceral efferent pathway”: The visceral efferent fibers of some cranial nerves adhere to branches or sub-branches of the trigeminal nerve, by which they travel to their destination:

- The lacrimal nerve (branch of CN V,) conveys parasympathetic fibers from the facial nerve along the zygomatic nerve (branch of CN V2) to the lacrimal gland.

- The auriculotemporal nerve (branch of CN V3) conveys parasympathetic fibers from the glossopharyngeal nerve to the parotid gland.

- The lingual nerve (branch of CN V3) conveys parasympathetic fibers from the facial nerve along the chorda tympani

to the submandibular and sublingual glands.

 “Visceral afferent pathway”: Gustatory fibers from the facial nerve (chorda tympani) travel by the lingual nerve (branch ofCN V3) to supply the anterior two-thirds of the tongue.

Developmentally, the trigeminal nerve is the nerve of the first branchial arch.

Clinical disorders of the trigeminal nerve:

Sensory disturbances and deficits may arise in various conditions:

 Sensory loss due to traumatic nerve lesions.

 Herpes zoster ophthalmicus (involvement of the territory of the first division of the trigeminal nerve, including the skin and/or the eye, by the varicella-zoster virus); herpes zoster of the face.

The afferent fibers of the trigeminal nerve (like the facial nerve, see p. 78) are involved in the corneal reflex (reflex closure of the eyelid; seeC, p. 361).

C Course and distribution of the trigeminal nerve

a Left lateral view. The three divisions of the trigeminal nerve and clinically important terminal branches are shown.

All three divisions of the trigeminal nerve supply the skin of the face (b) and the mucosa of the nasopharynx (c). The anterior two-thirds of the tongue (d) receives sensory innervation (touch, pain and thermal sensation, but not taste) via the lingual nerve, which is a branch of the mandibular division (CN V3). The muscles of mastication are supplied by the motor root of the trigeminal nerve, whose axons enter the mandibular division (e).

Note: The efferent fibers course exclusively in the mandibular division.

A peripheral trigeminal nerve lesion involving one of its divisions—ophthalmic (CN V-,), maxillary (CN V2), or mandibular (CN V3)—may cause loss of somatic sensation (touch, pain, and temperature) in the area innervated by the afferent nerve (seeb). This contrasts with the more concentric pattern, and more restricted modality, of sensory deficit produced by a central (CNS) lesion involving trigeminal nuclei and pathways (see D).

D Central trigeminal lesion

a Somatotopic organization of the spinal nucleus of the trigeminal nerve, b Facial zones in which sensory deficits (pain and temperature) arise when certain regions of the trigeminal spinal nucleus are destroyed. These zones follow the concentric Solder lines in the face. Their pattern indicates the corresponding portion of the trigeminal nucleus in which the lesion is located (matching color shades).

4.6 Cranial Nerves:

Trigeminal (CN V), Divisions

A Branches of the ophthalmic division (= first division of the trigeminal nerve, CN Vt) in the orbital region

Lateral view of the partially-opened right orbit. The first small branch arising from the ophthalmic division is the recurrent meningeal branch, which supplies sensory innervation to the dura mater. The bulk of the ophthalmic division fibers enter the orbit from the middle cranial fossa by passing through the superior orbital fissure. The ophthalmic division divides into three branches whose names indicate their distribution: the lacrimal nerve, frontal nerve, and nasociliary nerve.

Note: The lacrimal nerve receives postsynaptic, parasympathetic secretomotor fibers from the zygomatic nerve (maxillary division) via a communicating branch. These fibers travel to the lacrimal gland by the lacrimal nerve. Sympathetic fibers accompany the long ciliary nerves that arise from the nasociliary nerve, traveling in these nerves to the pupil. The ciliary nerves also contain afferent fibers that mediate the corneal reflex. Sensory fibers from the eyeball course in the nasociliary root, passing through the ciliary ganglion to the nasociliary nerve.

В Branches of the maxillary division (= second division of the trigeminal nerve, CN V2) in the maxillary region

Lateral view of the partially opened right maxillary sinus with the zygomatic arch removed. After giving off a meningeal branch, the maxillary division leaves the middle cranial fossa through the foramen rotundum and enters the pterygopalatine fossa, where it divides into the following branches:

 Zygomatic nerve

 Ganglionic branches to the pterygopalatine ganglion (sensory root of the pterygopalatine ganglion)

 Infraorbital nerve

The zygomatic nerve enters the orbit through the inferior orbital fissure. Its two terminal branches, the zygomaticofacial branch and zygomatic cotemporal branch (not shown here), supply sensory innervation to the skin over the zygomatic arch and temple. Parasympathetic, postsynaptic fibers from the pterygopalatine ganglion are carried to the lacrimal nerve by the communicating branch (see p. 81). The preganglionic fibers originally arise from the facial nerve. The infraorbital nerve also passes through the inferior orbital fissure into the orbit, from which it enters the infraorbital canal. Its fine terminal branches supply the skin between the lower eyelid and upper lip. Its other terminal branches form the superior dental plexus, which supplies sensory innervation to the maxillary teeth:

 Anterior superior alveolar branches to the incisors

 Middle superior alveolar branch to the premolars

 Posterior superior alveolar branches to the molars

C Branches of the mandibular division (= third division of the trigeminal nerve, CN V3) in the mandibular region

Right lateral view of the partially opened mandible with the zygomatic arch removed. The mixed afferent-efferent mandibular division leaves the middle cranial fossa through the foramen ovale and enters the infratemporal fossa on the external aspect of the base of the skull. Its meningeal branch reenters the middle cranial fossa to supply sensory innervation to the dura. Its sensory branches are as follows:

 Auriculotemporal nerve

 Lingual nerve

 Inferior alveolar nerve (also carries motor fibers, see below)

 Buccal nerve

The branches of the auriculotemporal nerve supply the temporal skin, the external auditory canal, and the tympanic membrane. The lingual nerve supplies sensory fibers to the anterior two-thirds of the tongue, and gustatory fibers from the chorda tympani (facial nerve branch) travel with it. The afferent fibers of the inferior alveolar nerve pass through the mandibular foramen into the mandibular canal, where they give off inferior dental branches to the mandibular teeth. The mental nerve is a terminal branch that supplies the skin of the chin, lower lip, and the body of the mandible. The efferent fibers that branch from the inferior alveolar nerve supply the mylohyoid muscle and the anterior belly of the digastric (not shown). The buccal nerve pierces the buccinator muscle and supplies sensory innervation to the mucous membrane of the cheek. The pure motor branches leave the main nerve trunk just distal to the origin of the meningeal branch. They are:

 Masseteric nerve (masseter muscle)

 Deep temporal nerves (temporalis muscle)

 Pterygoid nerves (pterygoid muscles)

 Nerve of the tensor tympani muscle

 Nerve of the tensor veli palatini muscle (not shown)

D Clinical assessment of trigeminal nerve function

Each of the three main divisions of the trigeminal nerve is tested separately during the physical examination. This is done by pressing on the nerve exit points with one finger to test the sensation there (local tenderness to pressure). The characteristic nerve exit points are as follows:

* For CN V1: the supraorbital foramen or supraorbital notch

* For CN V2: the infraorbital foramen

* For CN V3: the mental foramen

4.7 Cranial Nerves:

Facial (CN VII), Nuclei and Distribution

A Nuclei and principal branches of the facial nerve

a Anterior view of the brainstem, showing the site of emergence of the facial nerve from the lower pons, b Cross-section through the pons at the level of the internal genu of the facial nerve.

Note: each of the different fiber types (different sensory modalities) is associated with a particular nucleus.

From the facial nucleus, the special visceral efferent axons that innervate the muscles of facial expression first loop backward around the abducent nucleus, where they form the internal genu of the facial nerve. Then they run forward and emerge at the lower border of the pons. The superior salivatory nucleus contains visceromotor, presynaptic parasympathetic neurons. Together with viscerosensory (= gustatory) fibers from the nucleus of the solitary tract (superior part), they emerge from the pons as the nervus intermedius and then are bundled with the visceromotor axons from the facial motor nucleus to together form the facial nerve.

В Overview of the facial nerve (CN VII)

The facial nerve mainly conveys special visceral efferent (branchiogenic) fibers from the facial nerve nucleus which innervate the striated muscles of facial expression. The other visceral efferent (parasympathetic) fibers from the superior salivatory nucleus are grouped with the visceral afferent (gustatory) fibers from the nucleus of the solitary tract to form the nervus intermedius and aggregate with the visceral efferent fibers from the facial nerve nucleus.

Sites of emergence: The facial nerve emerges in the cerebellopontine angle between the pons and olive. It passes through the internal acoustic meatus into the petrous part of the temporal bone, where it divides into its branches:

 The visceral efferent fibers pass through the stylomastoid foramen to the base of the skull to form the intraparotid plexus (see C).

 The parasympathetic, visceral efferent, and visceral afferent fibers pass through the petrotympanic fissure to the base of the skull (see A, p. 80). While still in the petrous bone, the facial nerve gives off the greater petrosal nerve, stapedial nerve, and chorda tympani.

Nuclei and distribution, ganglia:

 Special visceral efferent: Efferents from the facial nucleus supply the following muscles:

- Muscles of facial expression

- Stylohyoid

- Posterior belly of the digastric

- Stapedius (stapedial nerve)

 Visceral efferent (parasympathetic): Parasympathetic presynaptic fibers arising from the superior salivatory nucleus synapse with neurons in the pterygopalatine ganglion or submandibular ganglion. They innervate the following structures:

- Lacrimal gland

- Small glands of the nasal mucosa and of the hard and soft palate

- Submandibular gland

- Sublingual gland

- Small salivary glands on the dorsum of the tongue

 Special visceral afferent: Central fibers of pseudouni polar ganglion cells from the geniculate ganglion (corresponds to a spinal ganglion) synapse in the nucleus of the solitary tract. The peripheral processes of these neurons form the chorda tympani (gustatory fibers from the anterior two-thirds of the tongue).

 Somatic afferent neurons: Some sensory fibers that supply the auricle, the skin of the auditory canal, and the outer surface of the tympanic membrane travel by the facial nerve and geniculate ganglion to the trigeminal sensory nuclei. Their precise course is unknown.

Developmentally, the facial nerve is the nerve of the second branchial arch.

Effects of facial nerve injury: A peripheral facial nerve injury is characterized by paralysis of the muscles of expression on the affected side of the face (see D). Because the facial nerve conveys various fiber components that leave the main trunk of the nerve at different sites, the clinical presentation of facial paralysis is subject to subtle variations marked by associated disturbances of taste, lacrimation, salivation, etc. (see B, p.80).

C Facial nerve branches for the muscles of expression

Note the different fiber types. This unit focuses almost exclusively on the visceral efferent (branchiogenic) fibers for the muscles of facial expression. (The other fiber types are described on p. 80.)

The stapedial nerve (to the stapedius muscle) branches from the facial nerve while still in the petrous part of the temporal bone and is mentioned here only because it also contains visceral efferent fibers (its course is shown on p. 80). The first branch that arises from the facial nerve after its emergence from the stylomastoid foramen is the posterior auricular nerve; it supplies visceral efferent fibers to the posterior auricular muscles and the posterior belly of the occipitofrontalis. It also conveys somatosensory fibers from the external ear, whose pseudounipolar nerve cells are located in the geniculate ganglion (see p. 80). After leaving the petrous bone, the bulk of the remaining visceral efferent fibers of the facial nerve form the intraparotid plexus in the parotid gland, from which successive branches (temporal, zygomatic, buccal, and marginal mandibular) are distributed to the muscles of facia I expression. These facial nerve branches must be protected during the removal of a benign parotid tumor in order to preserve muscle function. Additionally, there are even smaller branches such as the digastric branch to the posterior belly of the digastric muscle and the stylohyoid branch to the stylohyoid muscle (not shown). The lowest branch arising from the intraparotid plexus is the cervical branch. It joins with the transverse cervical nerve, an anterior branch of the C3 spinal nerve.

D Central and peripheral facial paralysis

a The facial motor nucleus contains the cell bodies of lower motor neurons which innervate ipsilateral muscles of facial expression. The axons (special visceral efferent) of these neurons reach their muscle

targets through the facial nerve. These motor neurons are innervated in turn by upper motor neurons in the primary somatomotor cortex (precentral gyrus), whose axons enter corticonuclear fiber bundles to reach the facial motor nucleus in the brainstem.

Note: the facial nucleus has a “bipartite” structure, its upper part supplying the muscles of the forehead and eyes (temporal branches) while its lower part supplies the muscles in the lower half of the face. The upper part of the facial nerve nucleus receives bilateral innervation, the lower part contralateral innervation from cortical (upper) motor neurons, b Central (supranuclear) paralysis (loss of the upper motor neurons, in this case on the left side) presents clinically with paralysis of the contralateral muscles of facial expression in the lower half of the face, while the contralateral forehead and extra-ocular muscles remain functional. Thus, the corner of the mouth sags on the right (contralateral) side, but the patient can still wrinkle the forehead and close the eyes on both sides. Speech articulation is impaired.

c Peripheral (Intranuclear) paralysis (loss of lower motor neurons, in this case on the right side) is characterized by complete paralysis of the ipsilateral muscles. The patient cannot wrinkle the forehead, the corner of the mouth sags, articulation is impaired, and the eyelid cannot be fully closed. A Bell phenomenon is present (the eyeball turns upward and outward, exposing the sclera, when the patient attempts to close the eyelid), and the eyelid closure reflex is abolished. Depending on the site of the lesion, additional deficits may be present such as decreased lacrimation and salivation or loss of taste sensation in the anterior two- thirds of the tongue.

4.8 Cranial Nerves:

Facial (CN VII), Branches

A Facial nerve branches in the temporal bone

Lateral view of the right temporal bone, petrous portion (petrous bone). The facial nerve, accompanied by the vestibulocochlear nerve (CN VIII, not shown), passes through the internal acoustic meatus (not shown) to enter the petrous bone. Shortly thereafter it forms the external genu of the facial nerve, which marks the location of the geniculate ganglion. The bulk of the visceral efferent fibers for the muscles of expression pass through the petrous bone and leave it at the stylomastoid foramen (seep.79). The facial nerve gives off three branches between the geniculate ganglion and stylomastoid foramen:

 The parasympathetic greater petrosal nerve arises directly at the geniculate ganglion. This nerve leaves the anterior surface of the petrous pyramid at the hiatus of the canal for the greater petrosal nerve. It continues through the foramen lacerum (not shown), enters the pterygoid canal (see C), and passes to the pterygopalatine ganglion.

 The stapedial nerve passes to the muscle of the same name.

 The chorda tympani branches from the facial nerve above the stylomastoid foramen. It contains gustatory fibers as well as presynaptic parasympathetic fibers. It runs through the tympanic cavity and petrotympanic fissure and unites with the lingual nerve.

В Branching pattern of the facial nerve: diagnostic significance in temporal bone fractures

The principal signs and symptoms are different depending upon the exact site of the lesion in the course of the facial nerve through the bone.

Note: only the principal signs and symptoms associated with a particular lesion site are described. The more peripheral the site of the nerve injury, the less diverse the signs and symptoms become.

1 A lesion at this level affects the facial nerve in addition to the vestibulochochlear nerve. As a result, peripheral motor facial paralysis is accompanied by hearing loss (deafness) and vestibular dysfunction (dizziness).

2 Peripheral motor facial paralysis is accompanied by disturbances of taste sensation (chorda tympani), lacrimation, and salivation.

3 Motor paralysis is accompanied by disturbances of salivation and taste. Hyperacusis due to paralysis of the stapedius muscle has little clinical importance.

4 Peripheral motor paralysis is accompanied by disturbances of taste and salivation.

5 Peripheral motor (facial) paralysis is the only manifestation of a lesion at this level.

C Parasympathetic visceral efferents and visceral afferents (gustatory fibers) of the facial nerve

The presynaptic, parasympathetic, visceral efferent neurons are located in the superior salivatory nucleus. Their axons enter and leave the pons with the visceral efferent axons as the nervus intermedius, then travel with the visceral efferent fibers arising from the facial motor nucleus. These preganglionic parasympathetic axons exit the brainstem in the facial nerve and branch from it in the greater petrosal nerve, then mingle with postganglionic sympathetic axons (from the superior cervical ganglion, via the deep petrosal nerve) in the nerve of the pterygoid canal. This nerve enters the pterygopalatine ganglion, where the preganglionic parasympathetic motor axons synapse; the sympathetic axons pass through uninterrupted to innervate local blood vessels. The pterygopalatine ganglion supplies the lacrimal gland, nasal glands, and nasal, palatine, and pharyngeal mucosa. Fibers from this ganglion enter the maxillary division and travel with it to innervate the lacrimal gland. Visceral afferent axons (gustatory fibers) for the anterior two-thirds of the tongue run in the chorda tympani. The gustatory fibers originate from pseudounipolar sensory neurons in the geniculate ganglion, which corresponds to a spinal sensory (dorsal root) ganglion. The chorda tympani also conveys the presynaptic parasympathetic visceral efferent fibers for the submandibular gland, sublingual gland, and small salivary glands in the anterior two-thirds of the tongue. These fibers travel with the lingual nerve (CN V3) and are relayed in the submandibular ganglion. Glandular branches are then distributed to the respective glands.

D Nerves of the petrous bone

Greater petrosal nerve

Presynaptic parasympathetic branch from CN VII to the pterygopalatine ganglion (lacrimal gland, nasal glands)

Deep petrosal nerve

Postsynaptic sympathetic branch from the internal carotid plexus; unites with the greater petrosal nerve to form the nerve of the pterygoid canal, then

Lesser petrosal nerve

Presynaptic parasympathetic branch from CN IX to the otic ganglion (parotid gland, buccal and labial glands, see p. 85)

 

continues to the pterygopalatine ganglion and supplies the same territory as the greater petrosal nerve (see C).

4.9 Cranial Nerves:

Vestibulocochlear (CN VIII)

A Nuclei of the vestibulocochlear nerve (CN VIII)

Cross-sections through the upper medulla oblongata.

a Vestibular nuclei. Four nuclear complexes are distinguished:

 Superior vestibular nucleus (of Bechterew)

 Lateral vestibular nucleus (of Deiters)

 Medial vestibular nucleus (of Schwalbe)

 Inferior vestibular nucleus (of Roller)

Note: The inferior vestibular nucleus does not appear in a cross-section at this level (see the location of the cranial nerve nuclei in the brainstem, p.228).

Most of the axons from the vestibular ganglion terminate in these four nuclei, but a smaller number pass directly through the inferior cerebellar peduncle into the cerebellum (seeEa). The vestibular nuclei appear as eminences on the floor of the rhomboid fossa (see Eb, p. 227). Their central connections are shown in Ea.

b Cochlear nuclei. Two nuclear complexes are distinguished:

 Anterior cochlear nucleus

 Posterior cochlear nucleus

Both nuclei are located lateral to the vestibular nuclei (see Aa, p. 228). Their central connections are shown in Eb.

В Overview of the vestibulocochlear nerve (CN VIII)

The vestibulocochlear nerve is a special somatic afferent (sensory) nerve that consists anatomically and functionally of two components:

 The vestibular root transmits impulses from the vestibular apparatus.

 The cochlear root transmits impulses from the auditory apparatus.

These roots are surrounded by a common connective-tissue sheath. They pass from the inner ear through the internal acoustic meatus to the cerebellopontine angle, where they enter the brain.

Nuclei and distribution, ganglia:

 Vestibular root: The vestibular ganglion contains bipolar ganglion cells whose central processes pass to the four vestibular nuclei on the floor of the rhomboid fossa of the medulla oblongata. Their peripheral processes begin at the sensory cells of the semicircular canals, saccule, and utricle.

 Cochlear root: The spiral ganglion contains bipolar ganglion cells whose central processes pass to the two cochlear nuclei, which are lateral to the vestibular nuclei in the rhomboid fossa. Their peripheral processes begin at the hair cells of the organ of Corti.

Every thorough physical examination should include a rapid assessment of both nerve components (hearing and balance tests). A lesion of the vestibular root leads to dizziness, while a lesion of the cochlear root leads to hearing loss (ranging to deafness).

C Acoustic neuroma in the cerebellopontine angle

Acoustic neuromas (more accurately, vestibular schwannomas) are benign tumors of the cerebellopontine angle arising from the Schwann cells of the vestibular root of CN VIII. As they grow, they compress and displace the adjacent structures and cause slowly progressive hearing loss and gait ataxia. Large tumors can impair the egress of CSFfrom the fourth ventricle, causing hydrocephalus and symptomatic intracranial hypertension (vomiting, impairment of consciousness).

E Nuclei of the vestibulocochlear nerve in the brainstem

Anterior view of the medulla oblongata and pons. The inner ear and its connections with the nuclei are shown schematically.

a Vestibular part: The vestibular ganglion contains bipolar sensory cells whose peripheral processes pass to the semicircular canals, saccule, and utricle. Their axons travel as the vestibular root to the four vestibular nuclei on the floor of the rhomboid fossa (further connections are shown on p.368). The vestibular organ processes information concerning orientation in space. An acute lesion of the vestibular organ is manifested clinically by dizziness (vertigo).

b Cochlear part: The spiral ganglia form a band of nerve cells that follows the course of the bony core of the cochlea. It contains bipolar sensory cells whose peripheral processes pass to the hair cells of the organ of Corti. Their central processes unite on the floor of the internal auditory canal to form the cochlear root and are distributed to the two nuclei that are posterior to the vestibular nuclei. Other connections of the nuclei are shown on0 366.

4.10 Cranial Nerves:

Glossopharyngeal (CN IX)

A Nuclei of the glossopharyngeal nerve

a Medulla oblongata, anterior view, b Cross-section through the medulla oblongata at the level of emergence of the glossopharyngeal nerve. For clarity, the nuclei of the trigeminal nerve are not shown (see В for further details on the nuclei).

В Overview of the glossopharyngeal nerve (CN IX)

The glossopharyngeal nerve contains general and special visceral efferent fibers in addition to visceral obèrent and somatic afferent fibers.

Sites of emergence: The glossopharyngeal nerve emerges from the medulla oblongata and leaves the cranial cavity through the jugular foramen.

Nuclei and distribution, ganglia:

 Special visceral efferent (branchiogenic:): The nucleus ambiguus sends its axons to the constrictor muscles of the pharynx (= pharyngeal branches, join with the vagus nerve to form the pharyngeal plexus) and to the stylopharyngeus (see C).

 General visceral efferent (parasympathetic): The inferior salivatory nucleus sends parasympathetic presynaptic fibers to the otic ganglion. Postsynaptic axons from the otic ganglion are distributed to the parotid gland and to the buccal and labial glands (see a and E).

 Somatic afferent: Central processes of pseudounipolar sensory ganglion cells located in the intracranial superior ganglion or extracranial inferior ganglion of the glossopharyngeal nerve terminate in the spinal nucleus of the trigeminal nerve. The peripheral processes of these cells arise from:

- the posterior third of the tongue, soft palate, pharyngeal mucosa, and tonsils (afferent fibers for the gag reflex), see b and c

- the mucosa of the tympanic cavity and eustachian tube (tympanic plexus), see d

- the skin of the external ear and auditory canal (blends with the territory supplied by the vagus nerve) and the internal surface of the tympanic membrane (part of the tympanic plexus).

 Special visceral afferent: Central processes of pseudounipolar ganglion cells from the inferior ganglion terminate in the superior part of the nucleus of the solitary tract. Their peripheral processes originate in the posterior third of the tongue (gustatory fibers, see e).

 Visceral efferent: Sensory fibers from the following receptors terminate in the inferior part of the nucleus of the solitary tract:

- Chemoreceptors in the carotid body

- Pressure receptors in the carotid sinus (see f)

Developmentally, the glossopharyngeal nerve is the nerve of the third branchial arch.

Isolated lesions of the glossopharyngeal nerve are rare. Lesions of this nerve are usually accompanied by lesions of CN Xand XI (vagus nerve and accessory nerve, cranial part) because all three nerves emerge jointly from the jugular foramen and are all susceptible to injury in basal skull fractures.

C Branches of the glossopharyngeal nerve beyond the skull base

Left lateral view.

Note the close relationship of the glossopharyngeal nerve to the vagus nerve (CN X). The carotid sinus is supplied by both nerves.

The most important branches of CN IX seen in the diagram are as follows:

 Pharyngeal branches: three or four branches for the pharyngeal plexus.

 Branch to the stylopharyngeus muscle.

 Branch to the carotid sinus: supplies the carotid sinus and carotid body.

 Tonsillar branches: for the mucosa of the pharyngeal tonsil and its surroundings.

 Lingual branches: somatosensory fibers and gustatory fibers for the posterior third of the tongue.

D Branches of the glossopharyngeal nerve in the tympanic cavity

Left anterolateral view. The tympanic nerve, which passes through the tympanic canaliculus into the tympanic cavity, is the first branch of the glossopharyngeal nerve. It contains visceral efferent (presynaptic parasympathetic) fibers for the otic ganglion and somatic afferent fibers for the tympanic cavity and pharyngotympanic (Eustachian) tube. It joins with sympathetic fibers from the carotid plexus (via the caroticotympanic nerve) to form the tympanic plexus. The parasympathetic fibers travel as the lesser petrosal nerve to the otic ganglion (see p. 99), which provides parasympathetic innervation to the parotid gland.

E Visceral efferent (parasympathetic) fibers of the glossopharyngeal nerve

The presynaptic parasympathetic fibers from the inferior salivatory nucleus leave the medulla oblongata with the glossopharyngeal nerve and branch off as the tympanic nerve immediately after emerging from the base of the skull. The tympanic nerve divides within the tympanic cavity to form the tympanic plexus (see A.p. 144), which is joined by postsynaptic sympathetic fibers from the plexus on the middle meningeal artery (not shown). The tympanic plexus gives rise to the lesser petrosal nerve, which leaves the petrous bone through the hiatus of the canal for the lesser petrosal nerve and enters the middle cranial fossa. Coursing beneath the dura, it passes through the foramen lacerum to the otic ganglion. Its fibers enter the auriculotemporal nerve, pass to the facial nerve, and its autonomic fibers are distributed to the parotid gland via facial nerve branches.

4.11 Cranial Nerves: Vagus (CN X)

A Nuclei of the vagus nerve.

a Medulla oblongata, anterior view showing the site of emergence of the vagus nerve.

b Cross-section through the medulla oblongata at the level of the superior olive. Note the various nuclei of the vagus nerve and their functions.

The nucleus ambiguus contains the somatic efferent (branchiogenic) fibers for the superior and inferior laryngeal nerves. It has a somatotopic organization ,i.e.P the neurons for the superior laryngeal nerve are above, and those for the inferior laryngeal nerve are below. The dorsal nucleus of the vagus nerve is located on the floor of the rhomboid fossa and contains presynaptic, parasympathetic visceral efferent neurons. The somatic afferent fibers whose pseudounipolar ganglion cells are located in the superior (jugular) ganglion of the vagus nerve terminate in the spinal nucleus of the trigeminal nerve. They use the vagus nerve only as a means of conveyance. The central processes of the pseudounipolar ganglion cells from the inferior (nodose) ganglion are gustatory fibers and visceral afferent fibers. They terminate in the nucleus of the solitary tract.

В Overview of the vagus nerve (CN X)

The vagus nerve contains general and special visceral efferent fibers as well as visceral afferent and somatic afferent fibers. It has the most extensive distribution of all the cranial nerves (vagus = “vagabond”) and consists of cranial, cervical, thoracic, and abdominal parts. This unit deals mainly with the vagus nerve in the head and neck (its thoracic and abdominal parts are described in the volume on the Neck and Internal Organs).

Site of emergence: The vagus nerve emerges from the medulla oblongata and leaves the cranial cavity through the jugular foramen.

Nuclei and distribution, ganglia:

 Special visceral efferent (branchiogenic:): Efferent fibers from the nucleus ambiguus supply the following muscles:

- Pharyngeal muscles (pharyngeal branch, joins with glossopharyngeal nerve to form the pharyngeal plexus) and muscles of the soft palate (levator veli palatini, muscle of uvula).

- All laryngeal muscles: The superior laryngeal nerve supplies the cricothyroid, while the inferior laryngeal nerve supplies the other laryngeal muscles (the origin of the fibers is described on p. 88).

 General visceral efferent (parasympathetic, see Dg): Parasympathetic presynaptic efferents from the dorsal vagal nucleus nerve synapse in prevertebral or intramural ganglia with postsynaptic fibers to supply smooth muscles and glands of:

- thoracic viscera and

- abdominal viscera as far as the left colic flexure (Cannon-Bohm point).

 Somat/c afferent: Central processes of pseudounipolar ganglion cells located in the superior (jugular) ganglion of the vagus nerve terminate in the spinal nucleus of the trigeminal nerve. The peripheral fibers originate from:

- the dura in the posterior cranial fossa (meningeal branch, see Df),

- a small area of the skin of the pinna (see Db) and external auditory canal (auricular branch, see Dc). The auricular branch is the only cutaneous branch of the vagus nerve.

 Special visceral afferent: Central processes of pseudounipolar ganglion cells from the inferior nodose ganglion terminate in the superior part of the nucleus of the solitary tract. Their peripheral processes supply the taste buds on the epiglottis (see Dd).

 General visceral afferent: The perikarya of these afferents are also located in the inferior ganglion. Their central processes terminate in the inferior part of the nucleus of the solitary tract. Their peripheral processes supply the following areas:

- Mucosa of the lower pharynx at its junction with the esophagus (see Da)

- Laryngeal mucosa above (superior laryngeal nerve) and below (inferior laryngeal nerve) the glottic aperture (see Da)

- Pressure receptors in the aortic arch (see De)

- Chemoreceptors in the para-aortic body (see De)

- Thoracic and abdominal viscera (see Dg)

Developmentally, the vagus nerve is the nerve of the fourth and sixth branchial arch.

A structure of major clinical importance is the recurrent laryngeal nerve, which supplies visceromotor innervation to the only muscle that abducts the vocal cords, the posterior cricoarytenoid. Unilateral destruction of this nerve leads to hoarseness, and bilateral destruction leads to respiratory distress (dyspnea).

D Visceral and sensory distribution of the vagus nerve (CN X)

4.12 Cranial Nerves:

Accessory (CN XI) and Hypoglossal (CN XII)

A Nucleus and course of the accessory nerve

Posterior view of the brainstem (with the cerebellum removed). For didactic reasons, the muscles are displayed from the right side (see C for further details).

В Lesion of the accessory nerve (on the right side)

a Posterior view. Paralysis of the trapezius muscle causes drooping of the shoulder on the affected side.

b Right anterolateral view. With paralysis of the sternocleidomastoid muscle, it is difficult for the patient to turn the head to the opposite side against a resistance.

C Overview of the accessory nerve (CN XI)

The accessory nerve is considered by some authors to be an independent part of the vagus nerve (CN X). It contains both visceral and somatic efferent fibers, and has one cranial and one spinal root.

Sites of emergence: The spinal root emerges from the spinal cord, passes superiorly, and enters the skull through the foramen magnum, where it joins with the cranial root from the medulla oblongata. Both roots then leave the skull together through the jugular foramen. While still within the jugular foramen, fibers from the cranial root pass to the vagus nerve (internal branch). The spinal portion descends to the nuchal region as the external branch of the accessory nerve.

Nuclei and distribution:

 Cranial root: The special visceral efferent fibers of the accessory nerve that arise from the caudal part of the nucleus ambiguus join the vagus nerve and are distributed with the recurrent laryngeal nerve. They innervate all of the laryngeal muscles except the cricothyroid.

 Spinal root: The spinal nucleus of the accessory nerve forms a narrow column of cells in the anterior horn of the spinal cord at the level of C2—C5/6. After emerging from the spinal cord, its somatic efferent fibers form the external branch of the accessory nerve, which supplies the trapezius and sternocleidomastoid muscles.

Effects of accessory nerve injury

A unilateral lesion results in the following deficits:

 Trapezius paralysis, characterized by drooping of the shoulder and difficulty raising the arm above the horizontal (the trapezius supports the serratus anterior in elevating the arm past 90°). The part of the accessory nerve that supplies the trapezius is vulnerable during operations in the neck (e.g., lymph node biopsies). Because the lower portions of the muscle are also innervated by segments C3 and C4/5, an injury of the accessory nerve will not result in complete trapezius paralysis.

 Sternocleidomastoid paralysis, characterized by torticollis (wry neck, i.e., difficulty turning the head to the opposite side). Because this muscle is supplied exclusively by the accessory nerve, an injury to that nerve causes flaccid paralysis. With bilateral lesions, it is difficult for the patient to hold the head in an upright position.

D Nuclei of the hypoglossal nerve

a Cross-section through the medulla oblongata at the level of the olive. This section passes through the nucleus of the hypoglossal nerve. It can be seen that the nucleus lies just beneath the rhomboid fossa and raises the floor of the fossa to form the hypoglossal trigone. Because each nucleus

is close to the midline, it is common for more extensive lesions to involve the nuclei on both sides, producing the clinical manifestations of a bilateral nuclear lesion, b Anterior view. The neurons contained in this nuclear column correspond to the alpha motor neurons of the spinal cord.

E Overview of the hypoglossal nerve (CN XII)

The hypoglossal nerve is a purely somatic efferent nerve that supplies the musculature of the tongue.

Nucleus and site of emergence: The nucleus of the hypoglossal nerve is located in the floor of the rhomboid fossa. Its somatic efferent fibers emerge from the medulla oblongata, leaving the cranial cavity through the hypoglossal canal and descending lateral to the vagus nerve. The hypoglossal nerve enters the root of the tongue above the hyoid bone and distributes its fibers there.

Distribution: The hypoglossal nerve supplies all intrinsic and extrinsic muscles of the tongue (except for the palatoglossus, CN X). It can be considered a “zeroth” ventral root rather than a true cranial nerve. The ventral fibers of Cl and C2 travel with the hypoglossal nerve but leave it again after a short distance to form the superior root of the (deep) ansa cervicalis.

Effects of hypoglossal nerve injury:

 Central hypoglossal paralysis (supranuclear): The tongue deviates away from the side of the lesion.

 Nuclear or peripheral paralysis: The tongue deviates toward the affected side due to a preponderance of muscular action on the healthy side.

F Distribution of the hypoglossal nerve

a Central and peripheral course, b Function of the genioglossus muscle, c Deviation of the tongue toward the paralyzed side.

The nucleus of the hypoglossal nerve is innervated (upper motor neurons) by cortical neurons from the contralateral side. With a unilateral nuclear or peripheral lesion of the hypoglossal nerve, the tongue deviates toward the side of the lesion when protruded because of the relative dominance of the healthy genioglossus muscle (c). When both nuclei are injured, the tongue cannot be protruded (flaccid paralysis).

4.13 Neurovascular Pathways through the Base of the Skull, Synopsis

A Sites where nerves and vessels pass through the skull base

Left half of drawing: internal view of the base of the skull. Right half of drawing: external view of the base of the skull. Because the opening into the cranium is not identical to the site of emergence on the external aspect of the base of the skull for some neurovascular structures, the site of entry into the cranuim is shown on the left side and the site of emergence is shown on the right side.

 

Opening

Transmitted structures

Internal view, base of the skull

 

Anterior cranial fossa

• Cribriform plate

• Olfactory fibers (collected to form CN 1)

• Anterior and posterior ethmoidal artery

Middle cranial fossa

• Optic canal

• Optic nerve (CN II)

• Ophthalmic artery

• Superior orbital fissure

• Oculomotor nerve (CN III)

• Trochlear nerve (CN IV)

• Ophthalmic nerve (CN Vj

• Abducent nerve (CN VI)

• Superior ophthalmic vein

• Foramen rotundum

• Maxillary nerve (CNV2)

• Foramen ovale*

• Mandibular nerve (CNV3)

• Foramen spinosum

• Middle meningeal artery

• Meningeal branch of CN V3

• Carotid canal

• Internal carotid artery

• Carotid sympathetic plexus

• Hiatus of canal for greater petrosal nerve

• Greater petrosal nerve

• Hiatusof canal for lesser petrosal nerve

• Lesser petrosal nerve

• Superior tympanic artery

Posterior cranial fossa

• Internal acoustic meatus

• Facial nerve (CN VII)

• Vestibulocochlear nerve (CN VIII)

• Labyrinthine artery

• Labyrinthine veins

• Jugularforamen

• Superior bulb of internal jugular vein

• Glossopharyngeal nerve (CN IX)

• Vagus nerve (CN X)

• Accessory nerve (CN XI)

• Posterior meningeal artery

• Hypoglossal canal

• Hypoglossal nerve (CN XII)

• Foramen magnum

• Meninges

• Medulla oblongata, spinal cord

• Vertebral arteries

• Anterior spinal artery

• Posterior spinal arteries

• Accessory nerve (CN XI): entering spinal roots

• Spinal vein

External aspect, base of the skull (where different from internal aspect)

• Incisive canal

• Greater palatine foramen

• Nasopalatine nerve

• Greater palatine nerve

• Greater palatine artery

• Lesser palatine foramen

• Lesser palatine nerves

• Lesser palatine arteries

• Foramen lacerum

• Deep petrosal nerve

• Greater petrosal nerve

• Petrotympanic fissure

• Chorda tympani

• Anterior tympanic artery

• Stylomastoid foramen

• Facial nerve

• Stylomastoid artery

• Condylar canal

• Condylar emissary vein

• Mastoid foramen

• Emissaryvein

B Principal sites where neurovascular structures pass through the skull base

Note: The external opening of the foramen rotundum is located in the pterygopalatine fossa, which is located deep on the lateral surface of the base of the skull and is not visible here.

* This foramen has an oval shape because it transmits the motor roots of the trigeminal nerve (CNV) for the muscles of mastication.