Sensory and Motor Neurones Responsible for the Local Bending Response In Leeches

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Abstract
Intracellular recordings were made from identified mechanosensory neurones (T and P cells) and longitudinal muscle motor neurones of leeches Hirudo medicinalis and Macrobdella decora while the skin was electrically stimulated to produce local bending responses. The stimulus intensity required to produce local bending was found to activate the mechanosensory neurones at physiological firing rates. For a given stimulation frequency, intracellular activation of the mechanosensory neurones produced the same local bending response as did skin stimulation. Hyperpolarization sufficient to block the propagation of the afferent impulses into the central nervous system eliminated the local bending response to skin stimulation. Stimulating identified longitudinal muscle motor neurones at frequencies observed during the local bending response produced body wall movements similar to those seen in local bending. Hyperpolarization of the motor neurones to block impulse initiation abolished local bending. Mechanosensory neurone to longitudinal muscle motor neurone connexions were demonstrated to be effective and reliable, but polysynaptic for all but the previously documented monosynaptic connexions from mechanosensory neurones onto the L motor neurone (Nicholls & Purves, 1970). It is concluded that the previously identified mechanosensory and motor neurones are exclusively responsible for the local bending response.