As the nervous system develops, synaptic connections between nerve cells are initially established in a highly specific manner. These initial contacts can then be modified to a limited extent during early postnatal life, increasing their specificity still further. A knowledge of the functional and morphological correlates of synaptic development in the central nervous system is basic to an understanding of how nerve cells originally make and then refine and maintain their proper connections. Synaptic connections in the spinal cord between muscle sensory afferent fibers and moto-neurons, connections which form the neuronal basis of the stretch reflex, provide an excellent experimental system for studying this problem at the level of single, functionally identified cells. The basic synaptic pattern is already established by birth, but the peripheral targets of sensory and motor cells may influence the final pattern of central synaptic connections. Studies of how peripheral targets may modulate central synapses can provide some clue concerning how appropriate connections might be re-established following spinal cord injury. My objectives during the next three years are to 1) develop the anatomical techniques necessary to determine the pattern of synaptic connections between identified muscle afferents and motoneurons, 2) physiologically determine the pattern of synaptic input a motoneuron receives from sensory axons in its own muscle, in synergistic muscles, and in antagonistic muscles, 3) study the formation and possible rearrangements of these synaptic contacts during normal development using both anatomical and physiological techniques, and 4) begin to assess the importance of appropriate peripheral targets in determining the final pattern of central synaptic connections.