Primate cutaneous sensory units with unmyelinated (C) afferent fibers

Abstract

Recordings of single unitary discharges were made from fine filaments prepared from monkey (Macaca mulatta and Saimiri sciureus) saphenous, sural and median nerves innervating hairy skin. Units were selected on the basis of a discharge conducting at C-velocity to supramaximal stimulation of the nerves to survey receptive characteristics. The majority of these monkey C-fiber cutaneous receptors (70/88) were of the polymodal nociceptive type; they responded to mechanical, heat and chemical stimulation of noxious intensity. Excitation of polymodal nociceptors by noxious heat usually was followed by an enhanced response at a given temperature, and/or lowered threshold on subsequent skin heating, and the initiation of background discharge. Low-threshold C-mechanoreceptors were less common in the monkey than in the cat, particularly in nerves distributed to the distal part of the limb; these mechanoreceptors had the same predilection for low-velocity skin deformation noted for the similar cat receptors. Several (3/88) typical warm receptors exhibited the close parallel between discharge frequency and skin temperature reported by others. Observations were reported on a few other receptors with either C or slowly conducting myelinated afferent fibers.