Atlas of Anatomy

31 Cranial Nerves

Cranial Nerves: Overview

Fig. 31.1   Cranial nerves
Inferior (basal) view. The 12 pairs of cranial nerves (CN) are numbered according to the order of their emergence from the brainstem. Note: The sensory and motor fibers of the cranial nerves enter and exit the brainstem at the same sites (in contrast to spinal nerves, whose sensory and motor fibers enter and leave through posterior and anterior roots, respectively).

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image The cranial nerves contain both afferent (sensory) and efferent (motor) axons that belong to either the somatic or the autonomic (visceral) nervous system (see pp. 622623). The somatic fibers allow interaction with the environment, whereas the visceral fibers regulate the autonomic activity of internal organs. In addition to the general fiber types, the cranial nerves may contain special fiber types associated with particular structures (e.g., auditory apparatus and taste buds). The cranial nerve fibers originate or terminate at specific nuclei, which are similarly classified as either general or special, somatic or visceral, and afferent or efferent.

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Fig. 31.2   Cranial nerve nuclei
The sensory and motor fibers of cranial nerves III to XII originate and terminate in the brainstem at specific nuclei.

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CN I & II: Olfactory & Optic Nerves

image The olfactory and optic nerves are not true peripheral nerves, but extensions (tracts) of the telencephalon and diencephalon, respectively. They are therefore not associated with cranial nerve nuclei in the brainstem.

Fig. 31.3   Olfactory nerve (CN I)
Fiber bundles in the olfactory mucosa pass from the nasal cavity through the cribriform plate of the ethmoid bone into the anterior cranial fossa, where they synapse in the olfactory bulb. Axons from second-order afferent neurons in the olfactory bulb pass through the olfactory tract and medial or lateral olfactory stria, terminating in the cerebral cortex of the prepiriform area, in the amygdala, or in neighboring areas. See p. 617 for the mechanisms of smell.

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Fig. 31.4   Optic nerve (CN II)
The optic nerve passes from the eyeball through the optic canal into the middle cranial fossa. The two optic nerves join below the base of the diencephalon to form the optic chiasm, before dividing into the two optic tracts. Each of these tracts divides into a lateral and medial root. Many retinal cell ganglion axons cross the midline to the contralateral side of the brain in the optic chiasm. See p. 619 for the mechanisms of sight.

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CN III, IV & VI: Oculomotor, Trochlear & Abducent Nerves

image Cranial nerves III, IV, and VI innervate the extraocular muscles (see p. 509). Of the three, only the oculomotor nerve (CN III) contains both somatic and visceral efferent fibers; it is also the only cranial nerve of the extraocular muscles to innervate multiple extra- and intraocular muscles.

Fig. 31.5   Nuclei of the oculomotor, trochlear, and abducent nerves
The trochlear nerve (CN IV) is the only cranial nerve in which all the fibers cross to the opposite side. It is also the only cranial nerve to emerge from the dorsal side of the brainstem and, consequently, has the longest intradural (intracranial) course of any cranial nerve.

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image Note: The oculomotor nerve supplies parasympathetic innervation to the intraocular muscles and somatic motor innervation to most of the extraocular muscles (also the levator palpebrae superioris). Its parasympathetic fibers synapse in the ciliary ganglion. Oculomotor nerve palsy may affect exclusively the parasympathetic or somatic fibers, or both concurrently.

Fig. 31.6   Course of the nerves innervating the extraocular muscles
Right orbit.

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CN V: Trigeminal Nerve

image The trigeminal nerve, the sensory nerve of the head, has three somatic afferent nuclei: the mesencephalic nucleus, which receives proprioceptive fibers from the muscles of mastication; the principal (pontine) sensory nucleus, which chiefly mediates touch; and the spinal nucleus, which mediates pain and temperature sensation. The motor nucleus supplies motor innervation to the muscles of mastication.

Fig. 31.7   Trigeminal nerve nuclei

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Fig. 31.8   Divisions of the trigeminal nerve (CN V)
Right lateral view.

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Fig. 31.9   Course of the trigeminal nerve divisions
Right lateral view.

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CN VII: Facial Nerve

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

Fig. 31.10   Facial nerve nuclei

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Fig. 31.11   Branches of the facial nerve
Right lateral view.

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Fig. 31.12   Course of the facial nerve
Right lateral view. Visceral efferent (parasympathetic) and special visceral afferent (taste) fibers shown in black.

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CN VIII: Vestibulocochlear Nerve

image The vestibulochochlear nerve is a special somatic afferent nerve that consists of two roots. The vestibular root transmits impulses from the vestibular apparatus (balance, see p. 618); thecochlear root transmits impulses from the auditory apparatus (hearing, see p. 616).

Fig. 31.13   Vestibulocochlear nerve: Vestibular part

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Fig. 31.14   Vestibulocochlear nerve: Cochlear part

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Fig. 31.15   Vestibular and cochear (spiral) ganglia
Note: The vestibular and cochlear roots are still separate structures in the petrous part of the temporal bone.

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Fig. 31.16   Vestibulocochlear nerve in the temporal bone

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CN IX: Glossopharyngeal Nerve

Fig. 31.17   Glossopharyngeal nerve nuclei

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Fig. 31.18   Course of the glossopharyngeal nerve
Left lateral view. Note: Fibers from the vagus nerve (CN X) cornbine with fibers from CN IX to form the pharyngeal plexus and supply the carotid sinus.

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Table 31.7 Glossopharyngeal nerve branches

1

Tympanic n.

2

Branch to carotid sinus

3

Branch to stylopharyngeus muscle

4

Tonsillar branches

5

Lingual branches

6

Pharyngeal branches

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Fig. 31.19   Glossopharyngeal nerve in the tympanic cavity
Left anterolateral view. The tympanic nerve contaqins visceral efferent (presynaptic parasympathetic) fibers for the otic ganglion, as well as somatic afferent fibers for the tympanic cavity and pharyngotympanic tube. It joint with sympathetic fibers from the internal carotid plexus (via the carticotympanic nerve) to from the tympanic plexus.

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Fig. 31.20   Visceral efferent (parasympathetic) fibers of CNIX

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CN X: Vagus Nerve

Fig. 31.21   Vagus nerve nuclei

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Fig. 31.22   Course of the vagus nerve
The vagus nerve gives off four major branches in the neck. The inferior laryngeal nerves are the terminal branches of the recurrent laryngeal nerves. Note: The left recurrent laryngeal nerve winds around the aortic arch, while the right nerve winds around the subclavian artery.

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Table 31.10 Vagus nerve branches in the neck

1

Pharyngeal branches

2

Superior laryngeal n.

3R

Right recurrent laryngeal n.

3L

Left recurrent laryngeal n.

4

Cervical cardiac branches

CN XI & XII: Accessory & Hypoglossal Nerves

image The traditional "cranial root" of the accessory nerve (CN XI) is now considered a part of the vagus nerve (CN X) that travels with the spinal root for a short distance before splitting. The cranial fibers are distributed via the vagus nerve while the spinal root fibers continue on as the (spinal) accessory nerve (CN XI).

Fig. 31.23   Accessory nerve
Posteriorview of the brainstem with the cerebellum removed. Note: For didactic reasons, the muscles are displayed from the right side.

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Fig. 31.24   Accessory nerve lesions
Lesion of the right accessory nerve.

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Fig. 31.25   Hypoglossal nerve
Posterior view of the brainstem with the cerebellum removed. Note: C1, which innervates the thyrohyoid and geniohyoid, runs briefly with the hypoglossal nerve.

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Fig. 31.26   Hypoglossal nerve nuclei
Note: The nucleus of the hypoglossal nerve is innervated by cortical neurons from the contralateral side.

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Fig. 31.27   Hypoglossal nerve lesions
Superior view.

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