Figure 5.

Action potentials and electrical coupling. A, Left, Fifty action potentials collected during firing pattern evaluation and aligned on their rising phase (top). Notice that the spikes in the presynaptic cell (pre) evoke postsynaptic responses that are difficult to detect in the postsynaptic neuron (post) in single sweeps (middle). However, the average from 315 traces (average) clearly shows the presence of a spikelet in the postsynaptic neuron. Notice the slower kinetics of the spikelet compared with the presynaptic spike waveforms. Right, Distribution of the spike coupling coefficients calculated as the ratio of the amplitude of the averaged postsynaptic spikelet to the averaged presynaptic spike. B, Comparison of the cumulative distributions of DC versus spike-coupling (indicated as DC and s, respectively). Notice the much larger efficiency in the propagation of DC signals compared with spikes. C, Although DC coupling is consistently much larger than electrical spike transmission, the two variables are significantly correlated (p < 0.05). The regression and identity lines are shown for comparison (dotted lines). D, Dynamic modulation of spike coupling during an action potential train. Left, Notice that the train of spikes produced by a current step (645 ms duration, 80 pA above threshold amplitude level) results in strong modulation of the action potential waveform in the presynaptic neuron (pre), with much weaker impact on the amplitude of the spikelets in the postsynaptic cell (post). Middle, Presynaptic action potentials and postsynaptic spikelets aligned and superimposed. Right, Scatter plot showing the increasing spike-coupling coefficient during the action potential train.