Mechanisms of Sustained Increases of Firing Rate of Neurones in the Rat Cerebral Cortex after Polarization: Reverberating Circuits or Modification of Synaptic Conductance?

Abstract

IF a rat is anaesthetized with urethane (1.8 g/kg given intraperitoneally) and prepared for microelectrode recording from the cerebral cortex¹, cells discharge in bursts. The mean frequency, that is, total number of spikes/s, of these bursts of firing will stay constant for many hours provided the animal is not stimulated in any way and kept at a steady temperature. The passage of a polarizing current between a wick on the cortical surface and an indifferent electrode in the orbit, so that the cortical surface is positive with respect to the indifferent, will cause the rate of discharge of the cells to be increased. Furthermore, if the current maintains the increased rate of discharge for more than 5 min then, on turning the current off, the discharge rate does not return to the preexisting value but remains at an increased level¹. I shall refer to this prolonged increase in activity as an “aftereffect” for convenience. Bindman² and I and Lippold³ have shown that the after-effect is a result of the increase in neuronal discharge rate during stimulation and not of the specific nature of the stimulus itself.