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

Cranial Nerve III - Review (continued)

This section was taken at the level of the rostral midbrain. The mesencephalic trigeminal nucleus and tract (1° proprioceptive axons in Cranial Nerve V) continue to be located along the anterolateral margin of the periaqueductal gray. The oculomotor nucleus (axons exit brain in Cranial Nerve III) appears at the level of the superior colliculus. The oculomotor nucleus forms a prominent heart-shaped structure anterior to the cerebral aqueduct. The parasympathetic component of this nucleus (the Edinger-Westphal nucleus) is located along the midline in the rostral third of the oculomotor complex. The axons of the oculomotor nucleus course anteriorly and sweep through the red nucleus and leave the midbrain at the interpeduncular fossa. The cortical input to the oculomotor complex is indirect and involves midbrain (vertical gaze center for voluntary saccade and supraoculomotor area for accommodation) and pontine (abducens interneurons for voluntary horizontal saccades and vestibular nuclei for smooth pursuit) structures. The corticofugal fibers are located in the cerebral peduncles and the corticotectal fibers leave the cerebral peduncles to terminate in midbrain structures.

Blood Supply

At this level of the midbrain, branches of the basilar, posterior cerebral artery and posterior communicating artery supply the midbrain. The paramedian branches penetrate and supply the medial half of the anterior midbrain, which includes the cerebral peduncles, substantia nigra, the red nucleus, oculomotor nucleus and oculomotor nerve root fibers, medial longitudinal fasciculus, tectospinal tract, and fibers forming the rubrospinal tract. Branches of the superior cerebellar artery (which is a branch of the basilar artery) supply more lateral areas of the midbrain which contain the lateral half of the cerebral peduncles and substantia nigra, and part of the medial lemniscus. Branches of the posterior cerebral artery supply the more posterior aspect of the midbrain (i.e., the tectum), which at this level is formed by the superior colliculus and part of the periaqueductal gray. Branches of the posterior cerebral artery also supply part of the midbrain tegmentum that contains the pontine reticular formation, medial lemniscus, ventral trigeminal lemniscus, spinothalamic tracts, brachium of the inferior colliculus, medial geniculate body.