IMPULSE TRANSMISSION OF THALAMIC SOMATOSENSORY RELAY NUCLEI AS MODIFIED BY ELECTRICAL STIMULATION OF THE CEREBRAL CORTEX

Abstract

Effects of a single shock electrical stimulation of the cerebral cortex were examined upon the evoked potential of the thalamic somatosensory relay nucleus (ventrobasal complex) elicited by stimulation of the posterior funiculus at the level of the first cervical segment of the spinal cord. The animals, cats and dogs, were immobilized with tubocurarine or anesthetized lightly with pentobarbital sodium. When recorded monopolarly, the thalamic somatosensory evoked potential was a mono-phasic positive wave consisting of an initial, high amplitude, sharp deflection followed by a late discharge of low amplitude. When a single shock stimulation of the somatosensory cortex, ipsilateral to the thalamus explored, was applied preceding posterior funiculus stimulation by time intervals more than about 100 msec, the thalamic evoked potential was suppressed in its initial component and enhanced in its late discharge. Such modification of the thalamic evoked potential lasted with decreasing strength for several hundred msec, after cortical stimulation. The cortical points of the somatosensory area which were capable of exerting the most manifest effect upon the thalamic evoked potential coincided with those most strongly activated by the thalamocortical projection system. Evoked potentials recorded from the medial lemniscus upon posterior funiculus stimulation showed no detectable influences during electrostimulation of the somatosensory cortex. By recording spike discharges from the thalamic somatosensory relay nucleus with the use of fine electrodes, effects of electrostimulation of the somatosensory cortex were examined. When a peripheral stimulus was preceded by a cortical stimulus at about the same time intervals as in the observations with gross electrodes, there occurred a conspicuous increase in the number of spike discharges in response to peripheral stimulation. After an entire extent of the somatosensory cortex was aspirated acutely, the thalamic evoked potential was augmented in the intial component with the late one suppressed. This effect was strictly confined within the thalamus ipsilateral to the extirpated cortex.