This project is devoted to an interrelated neurophysiological and morphological analysis of the synaptic mechanisms that underlie fundamental forms of behavioral plasticity, with particular emphasis on habituation and sensitization. The basic rationale involves a biological "model-systems" approach, where neuronal processes of plasticity can be analyzed in simplified systems and the resulting mechanisms generalized to more complex systems. In this phase of the project we are focusing on habituation and sensitization in the monosynaptic lateral column-motoneuron system of the isolated frog spinal cord. Quantal analysis of transmitter release will permit localization of the mechanisms to pre- or postsynaptic processes and concomitant electron microscopic studies will characterize the morphological substrates of these processes.