Nature and Significance of the Reflex Connections Established by Large Afferent Fibers of Muscular Origin1

A CENTRIPETAL impulse volley traversing the myelinated afferent fibers in the nerve to a given muscle, or fraction thereof, is known to provoke a variety of reflex effects (4). By direct, or monosynaptic impingement upon motor nuclei it promotes the discharge of homonymous motoneurons, which is to say motoneurons supplying that given muscle or muscle fraction; it facilitates the response of motoneurons that supply the muscle remainder or synergists; it inhibits the response of motoneurons that supply antagonists. Such direct actions are mediated by the group of largest afferent fibers (group 1) peculiar to muscle nerves, and are exerted in the service of, and in association with, myotatic reflexes. By internuncially relayed impingement upon motor nuclei of action engendered in the medium (group 11) and smaller (group 111) afferent fibers, an afferent volley brings about reflex discharge predominantly to flexor muscles; facilitation of response in flexor nuclei; and inhibition of response in extensor nuclei. These polysynaptic reflex effects, widespread throughout the limb, are manifestations of flexor reflexes. A systematic re-examination of the conditioning potentialities of afferent volleys arising in muscle nerves has been made, and the results herewith presented, because considerable reason existed for supposing that the list of established reflex actions enumerated above was far from complete. In a recent study on the actions of muscle afferent fibers, however, Brock, Eccles and Rall (5) found no evidence for postulating actions not listed.