Lab 7 - Visual System

The Visual/Ocular Motor Pathways - Midbrain

In this section we will review the ocular/motor pathways in the midbrain. The next section will cover the pons.

Figure1, Layer A is at the level of the inferior colliculus. The inferior colliculi are the external gross manifestations of the nucleus of the inferior colliculus. The inferior colliculus makes up the midbrain tectum here. The trochlear nuclei appear as pale andquot;capsandquot; on the heads of the two kissing elephants formed by the medial longitudinal fasciculi. The axons of the trochlear nucleus pass posteriorly over the periaqueductal gray, decussate, and exit the brain stem posteriorly near the ponto-midbrain junction. The tectopontine fibers from the superior colliculus continue to travel with the tectospinal tract (which is just anterior to the medial longitudinal fasciculus) down to the pons.

Figure 1, Layer B is a section through the rostral midbrain. The superior colliculus at this level is lateral to the posterior commissure. Pretectal area neurons synapse with fibers in the brachium of the superior colliculus; i.e., optic tract fibers that bypassed the lateral geniculate body. In turn, the pretectal neurons send their axons bilaterally in the posterior commissure to the Edinger-Westphal nuclei and play a role in the pupillary light reflex. Notice the proximity of the pineal gland to the posterior commissure.

The superior colliculus forms the rostral part of the midbrain tectum (i.e., the roof of the cerebral aqueduct). The superior colliculus receives input from the eye (via optic tract fibers in the brachium of the superior colliculus), cortical areas and from the basal ganglia. It sends some of its axons to two nuclei in the periaqueductal gray that are called the andquot;vertical gaze centersandquot;. These two nuclei, the rostral interstitial nucleus of the medial longitudinal fasciculus andinterstitial nucleus of Cajal, send their axons to the trochlear and oculomotor nuclei to control vertical eye movements.

Figure 1, Layer C is more caudal. The oculomotor nuclear complex, which includes the Edinger-Westphal nucleus, is located between and posterior to the medial longitudinal fasciculus. The Edinger-Westphal nucleus provides preganglionic parasympathetic fibers to the ciliary ganglion. The ciliary ganglion, in turn, provides postganglionic parasympathetic fibers to the sphincter of the iris and ciliary muscle of the ciliary body. Note that the oculomotor nerve fibers innervating the superior rectus muscles decussate in the midbrain and exit contralateral to their cells of origin. Those nerve fibers innervating the superior levator palpebrae (for eyelid elevation) undergo partial decussation and, as a result, the superior levator palpebra of one eye is innervated by neurons in both (left and right) oculomotor nuclei. Consequently, section of the left oculomotor nerve will result in paralysis of the left superior levator palpebrae. The left eyelid will droop because the oculomotor nerve fibers normally innervating it are destroyed. However, if the left oculomotor nuclei were destroyed, eyelid opening will not be absent. However, it may be less brisk and weaker because half of the innervation of the left superior palpebrae is destroyed

The superior colliculus is important for controlling reflex saccades - e.g., eye movements to a sudden noise or light flash and voluntary vertical saccades. It provides controlling inputs to the midbrain andquot;vertical gaze centersandquot; and the pons andquot;horizontal gaze centersandquot;. The superior collicular axons descend as tectopontine fibers within the tectospinal tract. The tectopontine fibers leave the tectospinal tract in the pons to terminate in the pons horizontal gaze center.

The supraoculomotor area (SOA) is located immediately posterior to the oculomotor nuclear complex in the periaqueductal gray. The supraoculomotor area contains neurons that provide control signals for the accommodation or near response, (i.e., adjustments in eye position, convergence, pupil size and lens shape to view near objects). These SOA neurons send their axons bilaterally to motor neurons (oculomotor and abducens) controlling the medial and lateral rectus muscles and to Edinger-Westphal neurons controlling the lens shape and position and the pupil size.