Accommodative properties of fast and slow pyramidal tract cells and their modification by different levels of their membrane potential

Abstract

1. Threshold-latency curves were determined by intracellular application of linearly rising currents to cat pyramidal tract (PT) cells under various conditions of impalement. The nature of the curves was found to vary considerably, depending mainly upon the resting potential level. Three different types of curves were distinguished: (1) the ceiling type at relatively high resting potentials, (2) the simple type at moderate levels and (3) the minimal gradient type under depolarized conditions. In the first type, the ceiling increased with increasing membrane potential level. The second type attained an extremely low ceiling. 2. After eliminating the effects of the resting potential, a significant difference was still found between the ceilings of fast and slow PT cells, the ceilings being higher in the former than in the latter. 3. Parameters determining the threshold-latency curve were analyzed by applying current steps intracellularly, as described in previous paper. The variations of the ceiling due to the resting potential level were found to be dominated by the first exponential component of the membrane, while those between fast and slow cells were ascribable to the second exponential component.