Synaptic basis of swim initiation in the leech

Summary

• 1. Cell 208 is an unpaired, intersegmental interneuron (Fig. 1) present in many, and probably all, segmental ganglia of the leech. By the criteria that it is cyclically active during swimming (Figs. 4, 5, 8, 13) and that current pulses injected into it can reset the swim motor pattern (Fig. 5), cell 208 is judged to be a member of the central pattern generator (CPG) for swimming.

• 2. The cell 208 population is strongly excited interganglionically by the swim-initiating interneurons, cells 204 and 205 (Figs. 2, 3). These are the first synaptic connections between swim-initiating and pattern-generating interneurons to be described in this system. Because of these inputs, the cell 208 population is active during the ‘preparatory’ phase of swimming which precedes the start of rhythmic undulations (Fig. 4); this early activation of cell 208 may facilitate induction of the oscillatory mode in the CPG circuit during swim initiation.

• 3. Cell 208 makes a variety of segmentally repeated, interganglionic mono- and polysynaptic connections with swim motor neurons which are appropriate for generating a sinusoidal body posture (Figs. 6, 7, 9, 10). Furthermore, in some segments cell 208 has almost complete control over the strength of the dorsal flexion phase of swimming (Fig. 8).

• 4. When released from injected hyperpolarizing current, cell 208 exhibits pronounced delayed hyperpolarizing waves in its membrane potential, termed ‘H-waves’ (Fig. 11), which are hypothesized to result from limited oscillation between cell 208 and some other member(s) of the CPG circuit (Fig. 15). H-waves are associated with specific motor effects (Fig. 12) which combine with those produced by cell 208 (Fig. 10) to generate short bouts of motor activity indistinguishable by many criteria from swimming (Fig. 13). Occasionally, self-sustaining swim episodes can be triggered by evoking H-waves in cell 208, and in these cases, H-waves clearly correspond to the hyperpolarized phase of cell 208's oscillatory swim cycle (Fig. 13).

• 5. Cell 208 is polysynaptically inhibited by pressure or painful stimuli applied to the body wall, or by intracellular stimulation of P (pressure) or N (nociceptive) mechanosensory neurons (Fig. 14), suggesting an involvement in the sensory modulation of the swim motor pattern.