Nervous outflow from skeletal muscle following chemical noxious stimulation

Abstract

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 K+) was studied in anesthetized cats using a single fiber recording technique. By administration of algesic agents in doses which produce pain in man and pain reactions in animals, about half of the group IV and 2/3 of the group III muscle afferents could be activated. Group II and group I afferent units were usually not excited by chemical noxious stimulation. If any effect occurred in the thick myelinated afferents, it was a depression of the fiber activity rather than an activation. 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. 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.