Sensory modification of leech swimming: Rhythmic activity of ventral stretch receptors can change intersegmental phase relationships

For segmented animals to generate optimal locomotory movements, appropriate phase relationships between segmental oscillators are crucial. Using swimm ing leeches, we have investigated the role of sensory input in establishing such relationships. We found that the stretch receptors associated with ve ntral longitudinal muscles encode the information of muscle contraction dur ing swimming via membrane potential oscillations, with amplitudes of up to 10 mV at our recording site. We subsequently modified the activity of ventr al stretch receptors (VSRs) by injecting rhythmic current at different phas es of the swim cycle and determined intersegmental phase lags by comparing the delay between the discharges of serially homologous motoneurons in thre e adjacent segments of isolated nerve cords. When no current was injected, the phase lag between neighboring segments was 8.6 +/- 0.8 degrees (mean +/ - SEM; n = 20), with large phase variations from cycle to cycle, between di fferent episodes, and between different preparations. When the phase of str etch receptor activity was set to 90-150 degrees by current injection, the phase of the motoneuron activity in the ganglion was consistently retarded by similar to 5 degrees. It was advanced by similar to 5 degrees when the V SR phase was set to 240-300 degrees. Therefore, the rhythmic activity of th e ventral stretch receptor generated during swimming can change intersegmen tal phase lags of leech ganglia in a phase-dependent manner. These stretch receptors may set the optimal intersegmental phases during swimming movemen t in intact leeches.