Fig. 8. A neuronal pathway for swim initiation in the medicinal leech. Body wall stimulation can initiate swimming through a well-defined neuronal pathway (see introduction). The present experiments demonstrate a role for glutamate in P cell activation of cell Tr1, as well as cell Tr1 activation of both the serotonergic Retzius cell and the swim-gating interneuron, cell 204. In contrast, glutamate is not involved in N cell activation of cell Tr1. Cell SE1 is included here as this cell provides strong input to cell 204; the monosynaptic connection from cell SE1 to cell 204 does not appear to be glutamatergic (P.D. Brodfuehrer and M.S.E. Thorogood, personal observations).