LAMINAR MICROELECTRODE STUDIES OF SPECIFIC SOMATOSENSORY CORTICAL POTENTIALS

Abstract

Microelectrode recording from successive depths of the posterior sigmoid gyrus in the cat suggests that the surface-positive potential evoked by stimulation of the sensory relay nucleus of the thalamus represents the summated depolarization of the afferent terminals and of primary sensory neurones in the cortex. The surface-negative potential probably represents electrotonic depolarization of the apical dendrites. The primary sensory neurones can be distinguished from those which do not have direct synaptic contacts with the afferent fiber terminals by their capability of rapid repetitive discharges in response to high frequency stimulation. The responsiveness of a single primary sensory neurone, as determined by paired shocks to the thalamic sensory relay nucleus, was most marked at intervals of 5-10 msec, after which it was depressed for 40-50 msec and was finally raised once again to a lesser stage of hyperexcitability. The latency measurements of a unitary spike response suggest that the synaptic delay may be of the order of 0.6 msec. Experimental results also imply that depolarization at the dendritic terminals may be capable of initiating impulses in the cortical neurones.