Nervous outflow from skeletal muscle following chemical noxious stimulation.

1. In order to determine the nervous outflow from skeletal muscle during chemically induced muscle pain, the impulse activity of various types of muscle afferents in response to close intra-arterial injections of pain-producing substances (bradykinin, 5-hydroxytryptamine, histamine and potassium) was studied in anaesthetized cats using a single fibre recording technique.2. By administration of algesic agents in doses which produce pain in man and pain reactions in animals, about half of the group IV and two thirds of the group III muscle afferents could be activated. In contrast, group II and group I afferent units were usually not excited by chemical noxious stimulation. If effects at all occurred in the thick myelinated afferents, they consisted of a depression of the fibre activity rather than of an activation.3. The qualitative features of the discharges of group III muscle afferents induced by chemical stimulation resembled those of the group IV units very closely. The group III units differed from the group IV afferents in that their responses to a given dose of bradykinin were of greater magnitude.4. It is concluded that the chemically induced muscle pain is probably mediated by certain portions of the group IV and group III afferents, whereas the reactions of group II and group I units to algesic agents are such that a contribution to muscular chemo-nociception seems improbable.