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

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Abstract
1. Unitary activities of muscular thin fibre afferents, which were not sensitive to muscle stretching, were recorded from the nerve of the medial gastrocnemius muscle of the dog. Responses to mechanical stimulation, intra‐arterial injection and local application of chemical solutions, and thermal stimulation of the surface of the muscle were studied. It was observed that polymodal receptors which responded to all types of stimulation existed in the thin fibre afferents of the muscle.2. 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.3. The mechanical response varied among these units with respect to the threshold and the pattern of discharges.4. 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 fibre afferents produced quite different features in their responses.5. Heating the receptive area of the muscle surface caused responses in twenty‐five out of thirty‐six units, which were sensitive both to mechanical and to chemical stimulations. The threshold varied from 38·0 to 48·3 °C, with a mean of 43·1 °C for C fibre units and 41 °C for A‐δ fibre units.6. The responses to heating were consistently obtained in the units responding to the surface application of chemical solutions. However, the above 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 fibre afferents.