RECEPTIVE-FIELDS, GEOMETRY AND CONDUCTION BLOCK OF SENSORY NEURONS IN CENTRAL NERVOUS-SYSTEM OF LEECH

In segmental ganglia of the leech [Hirudo medicinalis], the cutaneous mechanosensory neurons responding to touch innervated the skin of their own segment and of part of the anterior and posterior adjacent segments. Each touch receptive field could be divided into 3 non-overlapping areas: a central part innervated by the branches of the cell which ran in the nerve roots of the ganglion containing the cell body, and anterior and posterior parts innervated by its branches which ran in the nerve roots of the anterior and posterior adjacent ganglia. Impulses originating from the anterior and posterior parts of the receptive fields were susceptible to conduction block within the CNS when the touch cells fired repetitively at frequencies that could readily be elicited with weak mechanical stimulation. Impulses originating from the central part of the receptive fields were less susceptible to block. The morphology of touch cells revealed by intracellular injection of horseradish peroxidase suggested that conduction block occurred at specific bifurcation points where small cell processes joined the main process. Different physiological experiments supported this conclusion. In some touch cells bifurcation points with particularly low safety margins of conduction operated as low pass filters, limiting the frequency of impulses capable of invading certain branches. Mechanical stimuli likely to be encountered by the animal can probably lead to conduction block within its CNS and as a result modify its integrative activities.