The overall goal of the proposed research is to identify the premotor neurons and pathways that are necessary for controlling the firing rate of abducens motoneurons during the vestibulo-ocular reflex (VOR). The proposed experiments will be carried out in alert primates (squirrel monkeys) that have been trained to cancel their VOR and to make smooth pursuit eye movements.
The specific aims are to identify the signals carried by various classes of neurons projecting to the abducens nucleus and to determine their contributions to the VOR and its cancellation. Neurons in the medical vestibular nucleus, the ventrolateral vestibular nucleus, the propositus and the medullary reticular formation that project to the abducens nucleus will be identified by spike triggered averaging of lateral rectus muscle electromyographic (EMG) activity and by antidromic activation following microstimulation of the abducens nucleus. Confirmatory anatomical evidence will be obtained by injecting lectins (Phaseolus vulgaris leucoagglutinin or wheat germ agglutinin/HRP conjugate) in regions of the brainstem where premotor neurons are concentrated. The firing behavior of premotor neurons will be studied by recording their activity during vestibular and optokinetic nystagmus, cancellation of nystagmus, smooth pursuit, and saccadic eye movements. The labyrinth will be stimulated electrically in order to determine which types of premotor neurons receive direct inputs from the vestibular nerve. Those neurons will be identified by transynaptic activation following electrical stimulation of the vestibular nerve and spike triggered averaging of lateral rectus EMG activity. The results of these studies will increase our understanding of the anatomy and physiology of the central premotor pathways involved in controlling the vestibulo-ocular reflex.
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