CORTICALLY EVOKED DEPOLARIZATION OF PRIMARY AFFERENT FIBERS IN THE SPINAL CORD

Abstract

Repetitive stimulation of soma-tosensory areas I and II of the cerebral cortex causes a depolarization of primary afferent fibers in the spinal cord which resembles the depolarization produced by afferent nerve volleys that is responsible for presynaptic inhibition. With both there are associated electrotonically conducted depolarizations of dorsal root fibers (the dorsal root potentials), positive waves of the cord dorsum (P-waves), and dorsal root reflexes. The SI arm and leg areas act specifically on the arm and leg afferent fibers respectively. Repetitive stimulation caused a large recruitment of the responses, particularly when at high frequency. Excitability testing of the central terminals of primary afferent fibers showed that the cortically induced depolarizing action occurred in large cutaneous fibers, and in Ib afferent fibers from muscle, but not in the Ia fibers from muscle. Similar results were given by intrafiber recording from primary afferent fibers. Cortically induced reduction of the synaptic action of primary afferent skin fibers was observed to have a time course typical of presynaptic inhibition.