Intracellular stimulation of sensory cells elicits swimming activity in the medicinal leech.

Intracellular stimulation of each of three different types of mechanoreceptors, the T, P and N cells, evokes swimming behavior in leech preparations. Stimulation of an individual N cell or P cell evoked swimming in 75% and 53% respectively, of the preparations tested. Stimulation of an individual T cell was ineffective in eliciting swimming; however, simultaneous stimulation of two T cells evoked swimming in 59% of our preparations. Stimulation of mechanosensory neurons elicited swimming activity for a limited number of trials; i.e. the response habituated. The number of swim episodes evoked before habituation to criterion did not differ significantly for the different types of mechanoreceptors. The duration of swim episodes declined significantly over the course of N cell stimulation. The tendency for swim length to decline with repeated stimulation was present as well for swim episodes elicited by P or T cell stimulation. Swim initiation recovered spontaneously following habituation resulting from T cell stimulation. Spontaneous recovery following N cell stimulation was not demonstrated. However, N cell stimulation evoked swimming again after DP nerve shock or to a limited extent, after cell 204 stimulation. Spontaneous recovery of swim initiation to P cell stimulation was not investigated. A previous study detailed habituation of swimming activity to mechanical stimulation of the body wall (Debski and Friesen 1985). Only the T cells are activated significantly by this stimulus. Stimulation of sensory receptors other than mechanoreceptors was not effective in eliciting swimming in our preparation. We conclude that T cells mediate swim initiation elicited by stroking of the body wall and that the cessation of swimming to this stimulus is not due to sensory adaptation.