The basic goal of the research is to identify and study the anatomy and physiology of the pathways by which neurons of the Mesencephalic Nucleus of the Fifth Nerve (MES V) convey sensory information to higher neural structures and reflexly affect jaw muscle motoneurons. MES V cells are the somata of first-order afferents from jaw muscle spindles or from tooth mechanoreceptors. Research associated with this project has demonstrated the existence of cells in the Nucleus Supratrigeminalis that appear to receive selective input from jaw muscle spindle differents. The proposed research is designed to learn more about the specific projection of functionally-identified MES V cells to brainstem neurons by intracellularly injecting MES V cells with horseradish peroxidase. Another aim is to identify the projection areas of the jaw muscle spindle-driven cells in Nucleus Supratrigeminalis by attempting to antidromically excite them from electrodes placed in the cerebellum, thalamus, and trigeminal motor nucleus. Another goal is to study the morphology of trigeminal motoneurons by intracellular injection of horseradish peroxidase. A last goal is to study the synaptic distribution of MES V cells and interneurons in Nucleus Supratrigeminalis to trigeminal motoneurons. Intracellular recordings from identified trigeminal motoneurons will be averaged with respect to activity of these jaw sensory neurons. It seems possible from past research that the "monosynaptic" jaw stretch reflex includes a component that is largely dependent upon a disynaptic pathway. The proposed research should test this possibility. If evidence of a substantial disynaptic excitatory pathway is found, it would radically affect or general understanding of the synaptic control of jaw motoneurons.