Electrical coupling between pyramidal cells in adult cortical regions.

Recently, intense interest has focussed on electrical coupling between interneurones in cortical regions and their contributions towards oscillatory network activity. Despite mounting circumstantial evidence that pyramidal cells are also coupled, the paucity of direct evidence has made this controversial. Dual intracellular recordings from pairs of cortical and hippocampal pyramids demonstrated strong, but sparse coupling. Approximately 70% of CA1 pyramids close to the stratum radiatum border were coupled to another pyramid, but only to one or two of their very closest neighbours. On average 25% of the steady state and 10% of the peak action potential voltage change in one cell transferred to the other, supporting synchrony and promoting burst firing. The very high incidence of convergent inputs from coupled pyramids onto single targets provided additional evidence that 'spikelets' reflected full action potentials in a coupled cell, since the EPSPs activated by APs and by 'spikelets' had significantly different amplitude distributions.