Electrophysiological properties of so-called inactivation response and their relationship to dendritic activity in hippocampal pyramidal cells of rabbits.

(1) The so-called inactivation response (IR), i.e. a prolonged (duration, 40-80 msec) and large (magnitude, more than 20 mV) depolarization of the cell membrane with superposed spike burst exhibiting a strong tendency of spike inactivation, was studied with an intracellular microelectrode in hippocampal pyramidal cells of rabbits, which were anesthetized with Nembutal and, in addition, curarized. (2) Evidence was presented that an all-or-nothing process was involved in the mechanism of production of the IR. (3) The threshold of the spontaneous IR was lower by more than 10 mV than that of the IR induced with depolarizing current injected into the cell body. From this, it was inferred that the IR originated at a site away from the cell body, possibly in the dendrites. (4) Two kinds of small fast spikes (magnitude, 10-20 mV) and spike-like potentials (magnitude, 8-22 mV; duration, 6-15 msec) were shown to exist. All of them were of high threshold and not related in magnitude to the membrane potential of the cell body. The dendrites were suggested as possible sites of their origin.