Fig. 1.

The neuronal basis of swimming in the leech (Friesen, 1989a; Brodfuehrer et al., 1995a). A, Schematic diagram of the swim circuit. Identified neurons from each functional level are given in the boxes. The synaptic flow is mostly top down: the swim-activating neurons excite the gating neurons, which excite the oscillator neurons, which in turn drive the motor neurons. This is denoted schematically by thearrows. There is some feedback from the oscillator to the gating level, indicated by the smaller arrow. Interneurons are assigned to a given level on the basis of their functional properties as well as their synaptic inputs and outputs. The top-level neurons, trigger cell 1 (Tr1) and swim excitor 1 (SE1), can initiate prolonged swim episodes when briefly depolarized. The gating neurons, at the next level down, require sustained current injection to initiate and maintain a swim episode. At the lowest interneuronal level are the oscillator neurons, which do not initiate swimming when depolarized; these cells make up the central pattern generator for the behavior. Although cell SE1 has the “trigger” property of being able to elicit swimming when briefly activated, it has been hypothesized that this cell normally functions as a “gain-control” for swim-initiating inputs rather than as a trigger (Brodfuehrer et al., 1995b). Trigger cell 2 (Tr2) is not included in the swim-activating category because a recent study has shown that this cell is more effective at terminating swimming than initiating it (O’Gara and Friesen, 1995). B, A swim-initiating stimulus excited cells Tr1 and 204 before the onset of swimming. The stimulus (indicated by the horizontal bar, with individual pulses indicated by the vertical tics; see Materials and Methods) was delivered to the posterior dorsal skin of a semi-intact preparation, between segments 17 and 18 (seg. 17–18). The activation of cell Tr1 is brief, but cell 204 remains active for the duration of the swim episode, receiving some cyclic feedback from the oscillator level. The stimulus caused cell Tr1 to fire eight spikes. The motor pattern for swimming is characterized by alternating bursts in dorsal excitor (DE) and ventral excitor (VE) motor neurons, which can be seen in the extracellular recordings (see Materials and Methods). This preparation had an intact posterior portion, with the nerve cord exposed from the head brain to segment 15. It was bathed in saline containing 50 μm serotonin to lower the threshold for swimming (Willard, 1981). In this and subsequent figures, the numbers in parentheses in the trace labels indicate from which segmental ganglion the recording was made.