Thin‐fibre receptors responding to mechanical, chemical, and thermal stimulation in the skeletal muscle of the dog

Abstract

Unitary activities of muscular thin fiber afferents, which were not sensitive to muscle stretching, were recorded from the nerve of the medial gastrocnemius muscle of the dog. Polymodal receptors which responded to all types of stimulation existed in the thin fiber afferents of the muscle. The receptive area of these units tested by mechanical stimulation was spot-like and appeared to be located not only on the surface but in the midst of the muscle. The mechanical response varied among these units with respect to the threshold and the pattern of discharges. In these units, NaCl, KCl and bradykinin consistently evoked responses, with differences in the latencies and discharge patterns, while solutions of histamine, acetylcholine and sodium citrate caused responses less consistently and less effectively. In the stretch receptors, chemical stimulation applied in the same way as tested in the thin fiber afferents produced quite different features in their responses. Heating the receptive area of the muscle surface caused responses in 25 out of 36 units, which were sensitive both to mechanical and to chemical stimulations. The threshold varied from 38.0 to 48.3.degree. C, with a mean of 43.1.degree. C for C fiber units, which were sensitive both to mechanical and to chemical stimulations. The threshold varied from 38.0 to 48.3.degree. C, with a mean of 43.1.degree. C for C fiber units and 41.degree. C for A-.delta. fiber units. The responses to heating were consistently obtained in the units responding to the surface application of chemical solutions. This response was never obtained in the units which did not respond to surface chemical stimulation but responded to intra-arterial injection. These results suggest a large population of polymodal receptors in the muscular thin fiber afferents.