Oculomotor Complex

The oculomotor complex lies on the midline of the rostral midbrain beneath the aqueduct (Fig. 1) and contains the motoneurons whose fibers comprise the third cranial nerve innervating four of the six extrao-cular muscles. Medial rectus motoneurons are distributed throughout the rostral-caudal extent of the third nerve nucleus. There are three subgroups of medial rectus motoneurons. Large motoneurons of about 26 mm in diameter (cell group A) are located in the ventral portion of the rostral two-thirds of the oculomotor nucleus. Neurons of group B, again of large size averaging 30 mm in diameter, are situated in a dorsal lateral position and occupy the caudal two-thirds of the oculomotor complex. The motoneurons of groups A and B innervate the global "inner" muscle fibers situated close to the sclera of the eye. Smaller motoneurons 18 mm in diameter (cell group C) are a third population of motoneurons located in the dorsal portion of the rostral two-thirds of the oculomotor complex. Cell group C innervates the smaller "outer" fibers (i.e., the orbital muscle surface away from the globe of the eye) of the medial rectus muscle. The orbital portion of the medial rectus muscle inserts not on the globe, but on a specialized portion of the orbital tissue made up of collagen and connective tissue. This region serves as a pulley to permit shifts in the axis of rotation of the eye. Additional groups of large and medium-sized motoneurons in the oculomotor complex innervate the inferior rectus, superior rectus, and inferior oblique extraocular muscles. Interestingly, the inferior rectus motoneurons are intermingled with the medial rectus group C motoneurons in the dorsal portion of the rostral two-thirds of the oculomotor complex which participate in the generation of convergence and downward saccades. Evidence that small motoneurons of group C innervate primarily the orbital portion of the medial rectus muscle suggests that these fibers could participate in a caudal shift of the oculomotor pulley (located in the orbital tissues), thus shifting the muscle pulling directions during vergence or saccades from initial eye positions outside of primary (straight-ahead) position (i.e., secondary or tertiary positions).

A separate group of small interneurons resides within the oculomotor complex and has intracranial projections. The targets of these projections are varied but include the abducens motoneurons, reticularis tegmenti pontis, facial nucleus, cerebellum, spinal trigeminal nucleus, dorsal column nuclei, principal and dorsal accessory olivary nuclei, rostral medulla, parabrachial region, and cervical spinal cord. A role in divergence has been hypothesized for neurons projecting to the abducens nuclei, but the roles for the other projections have not been formulated.

The major inputs to the oculomotor complex include projections from the abducens nucleus and the nucleus prepositus hypoglossi (the location of the horizontal neural integrator). The terminals of the abducens internuclear neurons reach the midbrain by crossing through the contralateral abducens nucleus to the contralateral medial longitudinal fasciculus, which ascends to midbrain levels. They synapse directly on the motoneurons in the oculomotor complex, with the vast majority terminating on medial rectus motoneur-ons. Projections from the rostral interstitial nucleus of the medial longitudinal fasciculus (see later discussion) and the interstitial nucleus of Cajal have been demonstrated to reach the motoneurons of the primary vertical movers of the eye, including the inferior rectus, superior rectus, and inferior oblique extraocular muscles. The circuitry for the light reflex includes the crossed projections from the pretectal olivary nucleus via the posterior commissure to reach the Edinger-Westphal nucleus.

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