1. 1.

    Behavioural responses to electrical stimulation of mechanosensory neurones were characterized in two species of leeches, Hirudo medicinalis and Macrobdella decora.

  2. 2.

    Depending upon the site and intensity of stimulation, the stimuli elicited one or a combination of five different responses: local bending, curling, shortening, whole-body bending or swimming.

  3. 3.

    The electrical threshold for activating identified mechanosensory neurones, T (touch) cells and P (pressure) cells, was the same in all regions of the body.

  4. 4.

    The voltage range over which the electrical stimuli produced progressively more mechanosensory impulses was the same as the range that produced different behavioural responses.

  5. 5.

    These results suggest that the T and P mechanosensory neurones provide the entire sensory input for all the behavioural responses. The production of different behavioural responses to stimuli of different intensities at the same location are attributable to different firing rates of the same sensory cells, and different responses to the same stimulus at different locations suggest different interneuronal targets for the T and P cells in different body regions.

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