CORTICAL INTRACELLULAR POTENTIALS DURING AUGMENTING AND RECRUITING RESPONSES: I. EFFECTS OF INJECTED HYPERPOLARIZING CURRENTS ON EVOKED MEMBRANE POTENTIAL CHANGES

Abstract

Intracellular potentials of cortical neurons associated with thalamocortical evoked responses in motor cortex have been studied during membrane hyperpolarization produced by intracellular injection of hyperpolarizing currents. Synaptic depolarizations observed during augmenting or recruiting responses were enhanced, whereas evoked hyperpolarizing potentials were inverted to depolarizing potentials during induced membrane hyperpolarization. These results are similar to those expected following changes in resting membrane potential relative to the equilibrium potentials for ionic movements generating EPSP''s [excitatory post-synaptic potentials] and IPSP''s [inhibitory PSP''s], respectively. Elimination of spikes and their sequelae has disclosed complex temporal sequences of EPSP''s and IPSP''s during different components of cortical surface augmenting and recruiting responses. The data establish that PSP''s are responsible for the major transmembrane potential changes involved in the production of evoked cortical potentials. Differences in the time course of PSP''s evoked during augmenting and recruiting responses and their modifications by intracellular current injection are discussed with respect to the mode of engagement of cortical neurons by specific and nonspecific thalamocortical projections.