Lab 7 - Visual System

Visual Motor Pathways - Introduction

The perception of a well-focused visual image requires the interaction of various visual motor responses. For example, the eyes move conjugately (together and in the same direction) to focus the images of the visual target on the foveae of the two eyes. The size of the pupil is also adjusted to decrease or increase the amount of light entering the eye (i.e., the pupillary light reflex). In adapting for near vision, the pupil size is altered to adjust the depth of focus (pupillary accommodation reflex), the curvature of the lens is altered (lens accommodation), and the eyes converge (move nasally) to execute a change in fixation point.

The control of eye movements can be divided into infranuclear, nuclear, and supranuclear portions. The infranuclear portion of the oculomotor system includes the cranial nerves (III, IV, and VI) responsible for eye movements and the muscles controlled by them. The nuclear portion is the oculomotor cranial nerve nuclei themselves. Finally, the supranuclear portion coordinates vestibular, saccadic, pursuit, optokinetic, vergence, and fixation reflexive movements.

The andldquo;infraandrdquo; and andldquo;supraandrdquo; refer to the location of the pathways relative to the cranial nerve nuclei responsible for eye movement. The supranuclear portion refers to the higher order sensory and motor systems that control eye movements, including neural networks in the cerebral cortex, basal ganglia, cerebellum, and brainstem. The infranuclear portion refers to the nerves originating from these nuclei and the muscles they innervate.
In the next few sections we will consider each portion.