This research will investigate the effects of postembryonic growth on the ability of neurons to integrate and respond to synaptic inputs. During postembryonic development, most neurons elaborate new dendritic and axonal branches and increase the size of the soma, the lengths and diameters of the dendrites, and the length and diameter of the axon. Increases in neuronal size should decrease the input resistance and increase the input capacitance throughout the cell, and also increase the electrotonic separation of its different parts. These changes will alter the integrative and response characteristics of a neuron unless compensatory changes occur in the number and strength of inputs, in their spatial and temporal distribution, and in the nature and distribution of the cell's active and passive membrane. This project will examine this issue as it affects the lateral giant (LG) interneuron of crayfish during postembryonic growth of the animal from a 1 cm juvenile to a 10 cm adult. LG is a "command neuron" for the tailflip escape behavior, and is part of what is arguably the best-understood of all neural circuits. We propose to trace the growth of LG and its mechanosensory inputs to examine the suggestion that neuronal growth itself is responsible for the decline in LG's responsiveness to phasic mechanosensory stimuli, despite great increases in the number of afferent inputs the cell receives. We will determine how afferent synaptic terminals change during growth, and whether these changes succeed or fail to compensate for the growth of the postsynaptic cell. Finally, we will examine the hypothesis that the growth of the LG's electrotonic structure "tunes" the cell to respond to tactile stimuli of a size that matches the growing tailfan.