Sensory modification of leech swimming: interactions between ventral stretch receptors and swim-related neurons

The neuronal circuits that generate the leech swimming rhythm comprise osci llatory interneurons that provide appropriately phased output to drive swim -related motoneurons. Within ganglia, these interneurons express three phas es; between ganglia there exists a phase delay between homologs. Our earlie r experiments revealed that stretch receptors embedded in the body wall par ticipate in intersegmental coordination and setting intersegmental phases. To identify the basis for these sensory effects, we mapped interactions bet ween a ventral stretch receptor and swim-related neurons. Connections betwe en this receptor and motoneurons are weak and variable in quiescent prepara tions, but during fictive swimming stretch receptor activation modulates mo toneuron oscillations; hence, these effects are polysynaptic, mediated by i nterneurons. We identified a strong, nonrectifying, and apparently direct e lectrical connection between the stretch receptor and oscillator neuron 33. The ventral stretch receptor also interacts with most of the other oscilla tory interneurons, including inhibitory inputs to cells 28 and 208, excitat ory input to the contralateral cell 115, and mixed input to the ipsilateral cell 115. These direct and indirect interactions can account for previousl y described effects of body-wall stretch on motoneuron activity. They also could mediate the previously described modification of intersegmental phase relationships by appropriately phased stretch receptor activation.