Cells in the Mesencephalic Nucleus of the fifth nerve (MES V) are the somata of afferent fibers innervating spindles in jaw closing muscles or tooth-supporting tissues. Studies of the firing patterns of these afferents, in alert, behaving animals as well as in more conventional neurophysiological preparations, strongly suggest that these afferents """"""""encode"""""""" mandibular position, the timing and direction of mandibular movement, or the force on teeth and/or the maxilla produced by jaw muscle activity. We know, at present, almost nothing about where and how MES V afferents project within the central nervous system; the goal of our proposal is to provide this information. It will increase our understanding of how proprioception is involved in oral motor control. Besides being of interest to basic neuroscience, this subject is of interest to health-related disciplines such as communicative disorders and dentistry. Projections of MES V neurons will be studied anatomically in cats by injection of peroxidase-labeled lectin in the vicinity of MES V neurons after a previous kainic acid injection in this region has killed all but the MES V neurons (MES V neurons are reported to be insensitive to kainic acid). In another series of experiments, neurophysiological techniques with cars will be used to confirm the projection of MES V neurons to other structures, and to assess whether there is a differential projection based upon the functional properties of the afferents. Controlled natural stimuli will also be used to study the response properties of second-order and higher-order neurons in the projection pathway. In a concurrent study, we will study a projection of MES V neurons that we are sure exists, but one about which we have no detailed knowledge: to the face area of the precentral cortex of the monkey. We will study the responses of precentral neurons in unanesthetized monkeys trained to perform two different mandibular tasks. In one task, the animal will have to grade biting force, and in the other, will have to control mandibular position against a constant load. All cells studied will be tested in terms of how they respond to electrical stimulation of MES V and natural oral stimuli designed to selectively excite the three basic types of MES V afferents.
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