Correlation between mass and unitary potentials evoked in the rat cerebral cortex by peripheral stimulation.

Rats were anaesthetized with urethane or trilene. Electrical stimuli were applied to the forepaw and responses recorded from the primary somatosensory cortex. The cortical response and its variability were studied firstly in an attempt to identify and control variability in subsequent experiments. Depth and type of anaesthesia were relatively unimportant, but recording position was critical when results from different experiments were compared. The main variability was that due to interaction of evoked responses with ongoing spontaneous activity. It was found useful to divide all responses into two groups according to the presence or absence of ECoG activity at the time of stimulation. The variability of cortical evoked potentials in these experiments apparently reflected a cortical stabilizing mechanism. The suppression by ongoing cortical activity of a widespread surface positive/depth negative component in the response was associated with the suppression of unitary responses in the deep layers of the cortex. A simple postsynaptic modulation of evoked IPSPs may be largely responsible for the suppression of these action potential responses. Postsynaptic modulation of PSPs is also considered in the context of another cortical gating mechanism - direction specificity in responses to moving stimuli. A striking correlation was found between unitary action potentials, local negative waves and surface positive waves in both the stable and variable components of the response to forepaw stimulation, thus supporting the hypothesis that evoked potentials could, at least in part, be due to summation of soma action potentials. The expected contribution of soma action currents in pyramidal cells, to cortical surface potentials was estimated and found to be significant.