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Lab 9 (ƒ 10) - Cranial Nerve Nuclei and Brain Stem Circulation

Cranial Nerve VIII - Vestibulocochlear Nerve (continued)

This section was cut from the caudal half of the pons. The lateral and superior vestibular nuclei (vestibular afferents in Cranial Nerve VIII) have replaced the inferior and medial vestibular nuclei and occupy the region lateral to the sulcus limitans.

The lateral vestibulospinal tract originates in the lateral vestibular nucleus and remains along its anterior surface.

The spinal trigeminal nucleus and tract (somatosensory afferents of Cranial Nerve V, VII, IX, X) continue to be located anterior to the vestibular nuclei.

The abducens nucleus (axons exit brain in Cranial Nerve VI) appears to form a bulge, the facial colliculus, in the floor of the fourth ventricle. The facial nucleus (axons exit brain in Cranial Nerve VII) is located lateral to the central tegmental tract and posterior to the superior olivary complex. Recall that on the gross brainstem specimen the facial nerve root exited the brain stem near the pontomedullary junction, just rostral to the vestibulocochlear nerve root entry. Within the pons, the corticofugal fibers are located in the anterior portion of the pons (basis pontis), and are surrounded by the pontine nuclei. Corticofugal fibers exert direct and indirect influence bilaterally on cell groups innervating upper facial muscles and predominantly contralaterally on cell groups innervating the lower face muscles.

The superior salivatory nucleus (axons exit the brain in Cranial Nerve VII), like its inferior counter-part, is too small to see in these sections. It is located medial and superior to the facial motor nucleus and sends its axons into the facial nerve.

Blood Supply

At this level of the brain stem, i.e., the pons, the vertebral arteries have joined to form a single basilar artery. The medial area of the basis pontis, which contains the pontine nuclei, corticofugal fibers, abducens nerve root, and the decussating pontine nuclei axons that travel laterally to form the middle cerebellar peduncle, is supplied by the paramedian branches of the basilar artery.

The short circumferential branches of the basilar artery supply more lateral areas of the basis pontis, i.e., the decussated axons of pontine nuclei that form the middle cerebellar peduncle. Branches of the anterior inferior cerebellar artery (AICA) also supply the middle cerebellar peduncle. The long circumferential branches of the basilar artery penetrate the pons tegmentum to supply the abducens nucleus, medial longitudinal fasciculus, tectospinal tract, facial nerve root, lateral and superior vestibular nuclei, spinal trigeminal tract and nucleus, pontine reticular formation, central tegmental tract, trapezoid body and superior olivary complex, facial nucleus, medial lemniscus, ventral trigeminal lemniscus, spinothalamic tracts, rubrospinal tract.