REGULATION OF THE SEGMENTAL SWIM-GENERATING SYSTEM BY A PAIR OF IDENTIFIED INTERNEURONS IN THE LEECH HEAD GANGLION

1. The aim of this study was to identify neurons that modulate activity of segmental swim gating interneurons. We found a pair of bilaterally symmetrical interneurons cells SE1, whose activity level directly influences three groups of segmental neurons associated with generating swimming in the medicinal leech. 2. The somata of cells SEI are located on the dorsal surface of the subesophageal ganglion. Their axons extend most, if not the entire, length of the ventral nerve cord and appear to make identical connections with the same group of swim-generating neurons in all segmental ganglia. 3. Cells SE1 excite monosynaptically all segmental swim gating interneurons, cells 204, examined. The level of excitation in cell 204 is directly correlated with the firing frequency of cell SE1. In most quiescent preparations (when the preparation is not swimming) hyperpolarization of a single cell SE1 eliminates all excitatory synaptic input to cells 204. 4. Cells SE1 excite monosynaptically three swim oscillatory interneurons, cells 115, 28, and 208. The strength of the connection from cell SE1 to cell 115 is stronger than the connection from cell SE1 to either cells 28 or 208. The level of excitation in cell 115 is dirt directly correlated with the firing frequency of cell SE1. In most quiescent preparations. hyperpolarization of a single cell SE1 eliminates all excitatory synaptic input to cell 115 but has only a minor effect on the level of activity in cells, 208 and 28. 5. Due most likely to the strong and direct connections cells SE1 have with swim gating and oscillatory interneurons, brief stimulation of cell SE1 can elicit swimming. Swimming generally occurs within 1 s after stimulation of cell SE1. During swimming, the membrane potential of cell SE1 depolarizes by 2-5 mV, and its firing frequency increases. Brief depolarization of cell SE1 during swimming reliably shifts the phase of the swimming rhythm, whereas longer periods of depolarization increase both swim period and burst duration. 6. Excitatory motor neurons to the dorsal longitudinal muscles, cells 3, 5, and 7, are strongly excited by stimulation of cell SE1. The firing frequency of cell 3 is positively correlated with the firing frequency of cell SE1. 7. The results of this study indicate that cells SE1 can modulate the level of excitation in three groups of neurons associated with generating leech swimming. We hypothesize that cells SE1 function as swim exciters or gain control interneurons for the initiation of swimming and, as such, could play a critical role in determining whether a given sensory input initiates leech swimming.