Trochlear Nucleus

The large motoneurons of the trochlear nucleus are located within the central gray adjacent to the aqueduct of Sylvius and dorsal to the medial longitudinal fasciculus at the level of the inferior colliculus (Fig. 1). The axons of the motoneurons course caudally along the aqueduct and then ascend to the dorsal lateral aspect of the central gray. The nerves decussate completely within the anterior medullary velum (the roof of the aqueduct) to exit from the dorsal aspect of the brain stem just caudal to the inferior colliculus. The fourth cranial nerve then wraps around the pons lying between the superior cerebellar and posterior cerebral arteries. The nerve then ascends in the prepontine cistern along the free edge of the tentorium cerebelli before piercing the dural attachment of the tentorium to reach the cavernous sinus. The trochlear nerve enters the orbit through the superior orbital fissure to innervate a single muscle, the superior oblique. This muscle is a primary intorter of the eye in primary and abducted positions and a primary depressor of the eye in adducted (toward the nose) positions. Within the brain stem the trochlear nuclei receive direct projections from the rostral interstitial nucleus of the medial longitudinal fasciculus, which is the premotor nucleus for the coordination of vertical eye movements.

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