CENTRAL EFFECTS OF CENTRIPETAL IMPULSES IN AXONS OF SPINAL VENTRAL ROOTS

Abstract

Centripetal volleys of impulses which enter the spinal cord over [alpha] fibers of ventral (motor) roots in cats and rabbits evoke in the ipsilateral ventral horn action potentials (spikes) which persist, in progressively decreasing numbers, for 30-50 msec. The action potentials do not represent repetitive centripetal discharges from the periphery, and no comparable centrifugal impulses in motor axons have been detected. It is, therefore, inferred that they represent the activity of interneurons located in the ventral horn. The available evidence suggests that the discharges are not injury effects associated with the presence of the recording micro-electrode. Impulses in many motor axons regulate the discharges. In general, as the size of an antidromic volley is increased, individual neurons respond with an increasing number of spikes at increasing frequencies and decreasing latencies. The first action potential has a minimum latency, measured from the time of arrival of the centripetral volley at the somas of the motoneurons, of 0.7 msec. The first two or three action potentials are sometimes spaced at intervals as short as 0.6-0.7 msec, i.e. the frequency is about 1500/sec. The succeeding impulses, which may total as many as 15, are spaced in a regular pattern at progressively increasing inter-vals. A neuron''s discharge to a centripetal volley in one deafferented motor nerve can be conditioned (augmented or decreased) by simultaneous or preceding volleys in a 2d deafferented motor nerve. The neurons frequently are not discharged by dorsal root volleys sufficing to activate relatively few motoneurons; in other instances the same neuron can be thrown into activity by either an antidromic motor volley or a dorsal root volley. It is reasonable to extrapolate the present findings to instances in which motoneusons are syn-aptically rather than antidromically stimulated. Thus the internuncial system in the ventral horn may act as a significant correlating system. Attention is directed to the regular pattern of discharge at surprisingly high initial frequencies and it is suggested that some types of interneurons may normally exhibit this type of activity.