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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
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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
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
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
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
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
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
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
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
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
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
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
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