FIGURE 1. Properties of spike initiation in the soma and axon of cortical pyramidal cells.

a, Somatic spike exhibits a 'kink' at its onset. b, Phase plot (dV/dt versus V) and close-up of rapid initiation (inset) of the spike shown in a. c, Close-up of the phase plot of somatic spike initiation during noisy intrasomatic current injection⁹, showing a broad distribution of thresholds (green lines). d, Whole-cell axonal recording (50 m from the soma). e, Phase plot of the axonal spike. Note the smoothly rising dV/dt. f, Overlay of dV/dt versus V for the onset of axonal spikes, showing lower variability (compare with the soma) of spike threshold (green lines).

Methods. Simultaneous axonal and somatic whole-cell recordings were obtained with the multiclamp 700B amplifier from ferret prefrontal cortical layer-5 pyramidal cells in slices maintained in vitro at 36°C (ref. 6). Spikes shown in a, d, as well as in c, f, were recorded simultaneously. Spikes occurred either during spontaneous synaptic activity⁶ or in response to the intrasomatic injection of a noisy (10–15 mV) current injection⁹. Whole-cell axonal recordings obtained through patching the cut end of the axon (terminal bleb) do not result in abnormal smoothness of spikes because spikes recorded from distal (> 100 m) axonal sites also show an onset kink owing to spike propagation (see also http://www.mccormicklab.org).