Afferent connections to the fast conduction pathway in the central nervous system of the leech Hirudo medicinalis.

A burst of all-or-none action potentials, identical in size and shape, can be recorded from the ventral cord of H. medicinalis following both photic and mechanical stimulation of the skin. This response propagates both anteriorly and posteriorly from its point of origin at the same conduction velocity of about 1.3 m/sec. The action potential elicited by electrical stimulation of the cord collides by refractoriness with the action potentials elicited in response to photic and mechanical stimulation. The cord response to photic and mechanical stimulation is reversibly suppressed by perfusion with high Mg++ solutions, whereas the afferent discharges recorded from the segmental nerves remain unaffected. Lesion experiments show that the cord responses to mechanical and photic stimuli, travel along the median connective (Faivre's nerve). It is concluded that afferent impulses originating from mechanoreceptors and photoreceptors converge with chemical excitatory synapses onto a fast conducting pathway in the ventral cord. This fast conducting pathway is coextensive with the one which is excited by electrical stimulation of the ventral cord (1, 3).