Lab 7 - Visual System

Supranuclear Motor Pathways - Smooth Pursuit

The major stimulus for a pursuit eye movement is a fixated target that moves; this evokes a following or tracking eye movement. Smooth pursuit eye movements are conjugate and under a control system capable of continuous modification of motor output in response to visual input (in contrast to discrete saccadic control). The movement is used to follow slow-moving, predictable targets. Damage to one-hemisphere results in a unidirectional pursuit abnormality toward the damaged hemisphere.

Smooth pursuit movements appear to be initiated by neurons in the ipsilateral parietal and temporal cortex. These cortical neurons are responsible for determining the speed and direction of pursuit movements. The frontal eye field is also involved in smooth pursuit; its neurons help determine the degree and rate of eye movement. The axons of all these cortical neurons descend as corticopontine fibers and terminate on dorsolateral pontine nucleus neurons. These pontine neurons send their axons to the cerebellum, which, in turn, control vestibular nuclear complex neurons. Recall that the vestibular nucleus neurons control the position and velocity of the eye via their input to the cranial motor neurons innervating the extraocular muscles. Notice that smooth pursuit does not involve neurons in the gaze centers.

The parieto-temporal eye fields are considered critical for smooth pursuit as they initiate and guide these types of eye movements. When there is damage to this area, eye movements tracking moving objects are not smooth and resemble saccades. In other words they are andquot;jerky.andquot;