Characterization of electrophysiological properties of intracellularly recorded neurons in the neocortex of awake cats: a comparison of the response to injected current in spike overshoot and undershoot neurons.

Intracellular recordings were obtained from 212 neurons of the coronal pericruciate cortex of 7 awake, untrained cats. Glass microelectrodes, filled with K+ citrate alone or K+ citrate with either cyclic GMP or 5'-GMP were used for recording and for injecting steady depolarizing and hyperpolarizing currents intracellularly. The effects of rectangular linearly rising (ramp) current pulses were also studied. Results were compared in spike overshoot* versus undershoot recordings. Spike overshoot recordings had action potentials (APs) larger than associated baseline shifts on penetration; undershoot recordings had APs smaller than associated baseline shifts on penetration. Undershoot recordings are more commonly encountered in mammalian neocortex than are overshoot recordings. (1) Except for sizes and slopes of APs and other effects consistent with the penetration of passive dendritic cables remote from regions of active spike initiation or propagation, no differences in response to current injection or in other electrophysiological properties were found between overshoot and undershoot recordings. (2) Injection of depolarizing currents produced de-reases in the amplitudes of APs, decreased rates of rise and fall of APs and increased frequencies of AP discharge. Injection of hyperpolarizing current produced slowing or cessation of AP discharge with little or only slight increases in AP amplitude when the resting potential was greater than 47 mV. (3) An effectively linear relationship was found between changes in AP size and the magnitude of weak injection depolarizing currents. This relationship provides a basis for measuring changes in cortical neuronal input resistance by the differential spike height method. (4) Most neurons showed little or no accommodative response to the injection of linearly rising, depolarizing currents. Simple or ceiling threshold-latency curves rather than minimal gradient curves were obtained in 83% of the cells in which ramp currents were injected. (5) Modal values of resting potentials between 47 and 53 mV, without increased rates of spontaneous discharge, indicate that most cells have a critical firing threshold near that reported for somatodendritic (SD) rather than initial segment (IS) generated spikes. The evidence suggests that undershoot recordings primarily reflect penetrations of passive dendritic regions rather than functional modification of neurocellular properties as a consequence of impalement.