Central effects of centripetal impulses in axons of spinal ventral roots.

THE EXPERIMENTS of Muller (18) and others demonstrated that stimulation of the central end of a severed ventral spinal root does not result in the production of muscular movements; that is to say, an antidromic volley of impulses in a group of motor axons does not result in the discharge of other motoneurons. However, such antidromic volleys are not without effect on the spinal cord. It is well known that they condition reflex discharges of the attached motoneurons (9, 10, 13). Three additional phenomena have recently been demonstrated: (i) the retrograde axonal impulses spread over the somas of the attached motoneurons, producing action currents (16, 17, 21); (ii) centripetal impulses are then set up, after a brief latency, in a fraction of the stimulated motoneurons (20); and (iii) tested two-neuron arc reflex discharges into groups of motor axons other than those occupied by the antidromic volley are conditioned, i.e., facilitated or inhibited as the case may be (20). The discovery of the latter phenomenon has prompted a search for evidences of neural activity in the cord during the period subsequent to the arrival of a volley of impulses travelling centripetally in axons of ventral roots. Recordings made with micro-electrodes have now revealed relatively prolonged bursts of action potentials, which are believed to originate in interneurons of the ventral horn. Although this activity has not been exhaustively investigated, information concerning it is presented at this time because of its possible importance in the economy of the ventral gray matter and, more particularly, because the pattern of discharge of the individual neurons is sufficiently unusual to be of general interest.