Sensory innervation of the raccoon forepaw: 1. Receptor types in glabrous and hairy skin and deep tissue.

Electrophysiological recordings were made from the median, ulnar, radial, and dorsal ulnar nerves to determine the types of mechanosensory receptors serving glabrous and hairy skin surfaces of the raccoon forepaw. In addition to the cutaneous mechanoreceptors, fibers innervating deep tissues were also recorded from each of these nerves. These included sensory fibers innervating muscles, joints, claws, and the subcutaneous pulp. The array of receptors serving raccoon glabrous skin was the same as found in monkeys and humans: Rapidly adapting (RA), slowly adapting (SA), and Pacinian (Pc) fibers were characterized. Pacinian fibers have been rarely described in previous physiological studies of the raccoon peripheral nerves, but in the present study they composed between 14% and 18% of the glabrous skin mechanoreceptors recorded. A distal-proximal gradient in the density of skin innervation was evident for all three types of receptors. Receptors characterized in the hairy skin of the dorsal paw were similar to those described in other mammals, and included both down and guard hair afferents, non-hair-associated RA fibers, and SA I and SA II fibers. The relative proportions of these fibers differed from those generally reported for the hairy skin of other mammals. SA hair-associated afferent fibers, which have been reported previously only in primate hairy skin, were also found in large numbers in the raccoon. Similarities and differences in the frequency and types of receptors innervating the raccoon forepaw, the forepaws of other mammals, and the hands of primates (including humans) are discussed.