Postural reflexes are evoked and controlled by a variety of sensory inputs. The vestibulo-spinal (VS) pathways carry sensory vestibular signals to widespread regions of the spinal cord where they interact with motor nuclei to produce reflexive movement. The vestibulo-collic reflexes (VCR) are an important subset of postural reflexes that avert potential neck injury by reorienting the head during perturbations of the body. The neck motor nuclei involved in producing the VCR receive input from two distinct populations of VS neurons that are anatomically distinguished by the absence or presence of collateral projections to the oculomotor nucleus. We hypothesize that these two distinct VS populations are important in controlling both reflexive head movements and reflexive gaze shifts (combined eye and head). To test this hypothesis we will record the firing behavior of VS neurons located in the vestibular nuclei during reflexive, voluntary, and passive head movements in the awake squirrel monkey. The monkey will be seated on a vestibular turntable and will be permitted to produce different combinations of voluntary and reflexive head movements only in the plane of the horizontal semicircular canals. Responses during voluntary and reflexive head movements will be quantified with respect to the response during rotations when the head is not permitted to move with respect to the body. Secondary VS neurons will be orthodromically identified from the vestibular nerve and antidromically identified as projecting to the spinal cord. VS neurons projecting to the oculomotor nucleus will be identified by the presence of eye movement related signals in their firing behavior and by antidromic activation following stimulation of the oculomotor nucleus. The goal of these studies is to characterize the signal processing for each of the VS neural populations with respect to the different functional roles of the VCR. The signal processing will be quantified and related to behavioral goals of the reflex to determine functional roles of each pathway. The results of these studies are not only significant to our understanding of the sensory-motor control of head movements but are also globally applicable to the control of all vestibular postural reflexes because their mechanisms likely utilize similar neural processing strategies.
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