Overview of Cranial Nerves: Functions and Pathways

13 The Peripheral Nervous System and Reflex Activity: Part B

Cranial Nerves • Twelve pairs of nerves associated with the brain • Most are mixed in function; two pairs are purely sensory • Each nerve is identified by a number (I through XII) and a name “On occasion, our trusty truck acts funny—very good vehicle anyhow”

Filaments of olfactory nerve (I) Frontal lobe Olfactory bulb Olfactory tract Optic nerve (II) Temporal lobe Optic chiasma Infundibulum Optic tract Facial nerve (VII) Oculomotor nerve (III) Trochlear nerve (IV) Vestibulo- cochlear nerve (VIII) Trigeminal nerve (V) Glossopharyngeal nerve (IX) Abducens nerve (VI) Vagus nerve (X) Cerebellum Accessory nerve (XI) Medulla oblongata Hypoglossal nerve (XII) (a) Figure 13.5 (a)

Cranial nerves I – VI Sensory function Motor function PS* fibers I Olfactory Yes (smell) No No II Optic Yes (vision) No No III Oculomotor No Yes Yes IV Trochlear No Yes No V Trigeminal Yes (general sensation) Yes No VI Abducens No Yes No Cranial nerves VII – XII Sensory function Motor function PS* fibers VII Facial Yes (taste) Yes Yes VIII Vestibulocochlear Yes (hearing and balance) Some No IX Glossopharyngeal Yes (taste) Yes Yes X Vagus Yes (taste) Yes Yes XI Accessory No Yes No XII Hypoglossal No Yes No *PS = parasympathetic (b) Figure 13.5 (b)

I: The Olfactory Nerves • Arise from the olfactory receptor cells of nasal cavity • Pass through the cribriform plate of the ethmoid bone • Fibers synapse in the olfactory bulbs • Pathway terminates in the primary olfactory cortex • Purely sensory (olfactory) function

Table 13.2

II: The Optic Nerves • Arise from the retinas • Pass through the optic canals, converge and partially cross over at the optic chiasma • Optic tracts continue to the thalamus, where they synapse • Optic radiation fibers run to the occipital (visual) cortex • Purely sensory (visual) function

Table 13.2

III: The Oculomotor Nerves • Fibers extend from the ventral midbrain through the superior orbital fissures to the extrinsic eye muscles • Functions in raising the eyelid, directing the eyeball, constricting the iris (parasympathetic), and controlling lens shape

Table 13.2

IV: The Trochlear Nerves • Fibers from the dorsal midbrain enter the orbits via the superior orbital fissures to innervate the superior oblique muscle • Primarily a motor nerve that directs the eyeball

Table 13.2

V: The Trigeminal Nerves • Largest cranial nerves; fibers extend from pons to face • Three divisions • Ophthalmic (V1) passes through the superior orbital fissure • Maxillary (V2) passes through the foramen rotundum • Mandibular (V3) passes through the foramen ovale • Convey sensory impulses from various areas of the face (V1) and (V2), and supplies motor fibers (V3) for mastication

Table 13.2

VI: The Abducens Nerves • Fibers from the inferior pons enter the orbits via the superior orbital fissures • Primarily a motor, innervating the lateral rectus muscle

Table 13.2

VII: The Facial Nerves • Fibers from the pons travel through the internal acoustic meatuses, and emerge through the stylomastoid foramina to the lateral aspect of the face • Chief motor nerves of the face with 5 major branches • Motor functions include facial expression, parasympathetic impulses to lacrimal and salivary glands • Sensory function (taste) from the anterior two-thirds of the tongue

Table 13.2

VIII: The Vestibulocochlear Nerves • Afferent fibers from the hearing receptors (cochlear division) and equilibrium receptors (vestibular division) pass from the inner ear through the internal acoustic meatuses, and enter the brain stem at the pons-medulla border • Mostly sensory function; small motor component for adjustment of sensitivity of receptors

Table 13.2

IX: The Glossopharyngeal Nerves • Fibers from the medulla leave the skull via the jugular foramen and run to the throat • Motor functions: innervate part of the tongue and pharynx for swallowing, and provide parasympathetic fibers to the parotid salivary glands • Sensory functions: fibers conduct taste and general sensory impulses from the pharynx and posterior tongue, and impulses from carotid chemoreceptors and baroreceptors

Table 13.2

X: The Vagus Nerves • The only cranial nerves that extend beyond the head and neck region • Fibers from the medulla exit the skull via the jugular foramen • Most motor fibers are parasympathetic fibers that help regulate the activities of the heart, lungs, and abdominal viscera • Sensory fibers carry impulses from thoracic and abdominal viscera, baroreceptors, chemoreceptors, and taste buds of posterior tongue and pharynx

Table 13.2

XI: The Accessory Nerves • Formed from ventral rootlets from the C1–C5 region of the spinal cord (not the brain) • Rootlets pass into the cranium via each foramen magnum • Accessory nerves exit the skull via the jugular foramina to innervate the trapezius and sternocleidomastoid muscles

Table 13.2

XII: The Hypoglossal Nerves • Fibers from the medulla exit the skull via the hypoglossal canal • Innervate extrinsic and intrinsic muscles of the tongue that contribute to swallowing and speech

Table 13.2

Overview of Cranial Nerves: Functions and Pathways

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