ELECTROPHYSIOLOGY OF HIPPOCAMPAL NEURONS: II. AFTER-POTENTIALS AND REPETITIVE FIRING

Abstract

The pattern of spontaneous activity recorded intra-cellularly from hippocampal neurons of anesthetized cats consists of an admixture of solitary spikes and bursts. The solitary spikes were followed by a depolarizing after-potential (DAP) of approximately 10 mV. amplitude and 23 msec. duration. The DAPs are additive with repetitive firing but their time constant of decay is independent of the number of spikes in the burst. Despite a progressive increase in the local depolarization, frequency of firing, rate of rise of the action potential, and safety factor for AB invasion, all decrease progressively in later spikes of the burst. The burst is self-limited; the membrane potential repo-larizes in the presence of a long depolarizing pulse. These data are interpreted as indicative of inactivation of the inward current of both the A and the B spike generators. Repetitive firing can be produced as a rebound to a hyperpolarizing current pulse and may continue after the "off" of a depolarizing pulse. These findings are consistent with the concept of an intraneuronal maintenance of an excitatory state. They suggest that, once appropriately triggered, the pyramidal cell is capable of sustaining a brief period of repetitive firing by the summation of the DAPs.