Lab 7 - Visual System

Supranuclear Control of Eye Movement - Introduction

The three cranial nuclei supplying the muscles of each eye constitute the direct motor innervation of the extraocular musculature. The responses of the motor neurons supplying the extraocular muscles are controlled by various supranuclear structures located in the brain stem, cerebellum, and cerebral cortex. Supranuclear control of the precise coordinated actions of the extraocular muscles permits conjugate and vergence movements of both eyes.

The medial longitudinal fasciculus (MLF) contains ascending and descending fibers of the abducens nucleus, trochlear nucleus and oculomotor nucleus that are involved in coordinating the activity of antagonist muscles (e.g., the medial and lateral rectus muscles) for conjugate lateral and vertical gaze. The MLF also contains vestibular nuclear fibers that control eye movements that compensate for changes in head position (vestibulo-ocular reflexes).

The supranuclear pathways controlling extraocular movements are not fully understood. The following is an operational outline of the pathways involved in the control of guided and reflex saccades and smooth pursuit, based on animal experimentation and clinicopathologic correlations in patients with eye movement disorders. In the following discussion, keep in mind that the entire visual pathway, the retina, lateral geniculate body and visual cortex, is involved in determining whether the image is in focus on the fovea.