Abstract

Continuous, maximal (300/second) stimulation of the cervical spinal cord in decapitate animals (cat, rabbit, rat, hamster and mouse) results in a sequence of hindlimb movements identical with those seen in the maximal electroshock seizure in intact animals and in the grand mal epileptic convulsion in man. Thus the initial latent phase, the brief tonic flexion and the abrupt change from flexion to extension cannot be ascribed to specific supraspinal events, but appear to represent the natural response of the spinal cord to intense, general stimulation. The neural determinants of motor seizure patterns are discussed.

At low stimulus frequencies, either tonic flexion or phasic movements, resembling the manifestations of submaximal generalized seizure activity, can be elicited. Both clonic and tonic motor activity, once initiated, outlast the period of stimulation. Thus the spinal cord is capable of intense after-discharge which does not appear to be qualitatively different from that of supraspinal structures. This after-discharge is markedly reduced by low doses of anticonvulsant drugs.

Postictal depression, declining exponentially with time, is seen following cord stimulation. While such depression is less intense than that seen after a maximal electroshock seizure, it is probable that cord depression contributes to over-all postictal depression in the intact animal. Spinal postictal depression is markedly prolonged by trimethadione.

The effects of a number of anticonvulsant drugs as well as those of pentobarbital and mephenesin on responses to cord stimulation have been investigated. The anticonvulsants, in contrast to the other depressants, have little effect on the maximal cord convulsion in doses which are anticonvulsant for the intact animal. Mephenesin is distinct from the other depressants studied in that it selectively depresses alternating stepping movements produced by low-frequency stimulation.

Spinal cord stimulation provides a number of clear endpoints with which to study differential effects of drugs. It is suggested that this system, intermediate in complexity between simple reflex arcs and the entire central nervous system, may be usefully employed in studying loci and mechanisms of action of centrally-acting drugs.