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

Sensory Modification of Leech Swimming: Rhythmic Activity of Ventral Stretch Receptors Can Change Intersegmental Phase Relationships