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Midbrain Ascending sensory systems are closely associated with each other and continue toward their thalamic termination. The medial lemniscus shifts laterally and dorsally with the spinothalamic tracts at its posterior tip; the lateral lemniscus is dorsal to the spinothalamic tract before terminating in the inferior colliculus. The trigeminothalamic tracts lie between the central gray, which surrounds the central canal, and the medial lemniscus. Corticospinal and corticobulbar tracts are somatotopically arranged in the medial 3/5ths of the crus cerebri with the head represented medially and the feet laterally; corticopontine fibers are on either side. At the level of the inferior colliculus, lying ventrally in the central gray, is the trochlear nucleus. Fibers of CN IV, serving the superior oblique muscle, pass dorsally and caudally to cross and exit through the superior medullary velum. Associated with the superior colliculus level is the oculomotor nerve, serving the a) inferior oblique, medial, superior, and inferior rectus muscles and b) the parasympathetic fibers to the iris and ciliary muscles. Oculomotor fibers exit in the interpeduncular fossa. The MLF occupies its usual position at the ventral edge of the central gray connecting the 3rd, 4th and 6th nuclei. The vertical gaze center, the rostral interstitial nucleus of the MLF, lies rostral to the oculomotor nucleus as the midbrain joins the diencephalon. The superior colliculus serves as a target nucleus for some incoming fibers from the optic nerves and for visual reflexes. Just rostral to the superior colliculus, at the junction of midbrain and diencephalon, is the pretectal area containing the center for the pupillary light reflex; the posterior commissure connects both sides. The inferior colliculus is a relay nucleus receiving auditory information from the lateral lemniscus and participates in auditory reflexes. The substantia nigra lies between the crus cerebri and the tegmentum and serves motor functions associated with the basal ganglia.
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