Patterns of activity and the effects of activation of the fast conducting system on the behaviour of unrestrained leeches.

1. The neural activity of the nerve cord of Hirudo medicinalis has been recorded in unrestrained animals by means of chronically implanted electrodes. 2. The Fast Conducting System (FCS) is inactive both in motionless animals and during various kinds of active behaviour (creeping, swimming, ventilation). 3. Photic and tactile stimuli applied to a motionless animal elicit a FCS discharge, which may be followed by generalized shortening. 4. Photic and tactile stimuli applied during ventilation are followed by a reversible blockade of the ongoing activity only if they are able to elicit a FCS discharge. No such effect is observed on swimming. 5. An explanation of these findings in terms of the known connexions of leech neurones is offered and a role in the control of reafferent inputs is attributed to the rectifying synapses made by FCS and T cells on the L motor neurones.