A sensory system initiating swimming activity in the medicinal leech.

1. Water-wave stimulation, which was previously shown to elicit swimming in intact leeches, can initiate swimming in a semi-intact leech preparation via activation of the sensillar movement receptors (SMRs), provided that 50 micron-serotonin is added to the physiological saline. 2. The neuronal responses resulting from near-field stimulation of the leech body wall with a vibrating probe were recorded in peripheral nerves and in nerve-cord connectives. The response in the dorsal posterior nerve to a single vibratory pulse consists of a graded compound action potential.The units contributing to this action potential have a much lower threshold for near-field stimulation than do touch cells. They appear to be the same sensory units, the SMRs, that mediate leech sensitivity to water waves. 3. The frequency domain of the SMR sensitivity extends as low as 1 Hz. Thus, leeches could receive self-stimulation from the water vibrations created by their own swimming movements. 4. Leech physiological saline containing 20-40 m-Mg² does not eliminate the SMR response to near-field stimulation recorded in the DP nerve;however, elevated Mg² concentrations do eliminate the neuronal responses in the nerve cord connectives. Thus, while no chemical synapse occurs between the peripherally situated SMRs and nerve cord ganglia, a synapse may be interposed between the SMRs and the intersegmental neurones activated by near-field stimulation.5. The swim-facilitating action of serotonin occurs at unidentified sites within the ventral nerve cord, since serotonin does not alter the sensitivity of the SMRs.