Development of neuronal circuits and behaviors in the medicinal leech

We are studying the neuronal mechanisms responsible for establishing circui try underlying the local bending response in the medicinal leech. Local ben ding replaces an embryonic behavior, circumferential indentation, during th e time of initial chemical synaptogenesis in leech embryos. We found that t he electrical connections among the motor neurons are established first, ab out 5% of embryonic time (almost 2 full days) before chemical connections f orm. The inhibitory connections from muscle inhibitors to muscle exciters a re, we hypothesize, responsible for the emergence of local bending. We have also found that the central processes of the excitors- but not the inhibit ors- have much longer central processes when their peripheral processes are kept from contacting their target muscles. This system should allow us to test ideas about how individual neurons find their appropriate targets to f orm functional neuronal circuits. (C) 2001 Elsevier Science Inc.