Presynaptic depolarization of cutaneous mechanoreceptor afferents after mechanical skin stimulation.

Abstract

1. An investigation was made into the presynaptic depolarization of the spinal cord terminals of mechanoreceptor units of the hind foot of the cat after short mechanical displacement of the skin. The depolarization was measured by testing the excitability of the primary afferent fibres. 2. The following types of mechanoreceptor units were investigated: touch receptors of the central foot pad, hair follicle receptors, and touch corpuscles of the hairy skin. They were all depolarized by mechanical stimuli to the central pad or to the hairy skin. No difference has been found between receptor units which had or did not have a collateral in the dorsal columns. 3. The amount of depolarization depended on the amplitude of the mechanical stimulus. With weak mechanical pulses there was a close relation between the amplitude of the pulse and the size of the depolarization, but there was little additional increase of the depolarization with skin indentations exceeding 10 to 15 μ. No spatial facilitation could be demonstrated when two mechanical pulses were applied simultaneously. 4. The presynaptic depolarization of cutaneous mechanoreceptor afferents has a ‘surround’ pattern of organization: with mechanical pulses of constant amplitude the depolarization was largest when the pulse was applied at or close to the receptive field of the unit and decreased with increasing distance between the middle of the receptive field and the stimulus point. It seemed unimportant whether or not the unit under study was excited by the conditioning stimulus. 5. There is a discussion of the functional significance of these findings.