Our long term goal is to define the neural mechanisms that transform sensory inputs into appropriate motor outputs. Here we propose to approach this goal by defining the roles of vestibulospinal and reticulospinal neurons (VSNs, RSNs) in the vestibulocolic reflex (VCR) and in head orienting. Parallel experiments in alert and decerebrate animals will explore the neural mechanisms that underlie the dynamic and kinematic organization of head movements produced by the VCR or by stimulation of the superior colliculus.
Our specific aims are: 1. Define spatial and dynamic properties of response of VSNs and RSNs to vestibular stimulation. These properties are: a) the direction of rotation in 3-dimensional space that maximally excites the neuron, b) its frequency response and c) the latency of its response to a step change of angular velocity. Collectively these 3 properties define the transformation of vestibular signals that occurs between labyrinth and neuron and suggest which pathways may be involved in generating those transformations. 2. Relate response of RSNs elicited by stimulation of superior colliculus with neck motor activation produced by the same stimulus. This relation will indicate the spatial properties of the head orienting signal carried by that RSN. Timing of the response will suggest the pathways involved in generating it. 3. Determine connections of VSNs, RSNs with neck motor pools. Four approaches will be used to measure these connections: morphological examination of terminal arbors of VSNs and RSNs, spike-triggered averaging of changes in neck EMG activity following discharge of a VSN or RSN, recording muscle responses to microstimulation of VS or RS axons, and computation of correlation and other measures of the relatedness of neural and motoneuron discharge. Combined data from all 3 aims will completely characterize each neuron by determining how it collects input signals that originated in specific labyrinthine receptor or output of neurons in the well-defined superior colliculus motor map and how it distributes these signals to specific neck motor pools. This information will allow us to analyze the role of each vestibulospinal and reticulospinal neuron group in producing the overall sensorimotor transformation that occurs in the VCR or tecto-collic systems. We hope that our successes in this endeavor will establish a trend for performing similar comprehensive analyses of other somatomotor systems where analogous input/output transformations must occur.
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