Spikelets in Pyramidal Neurons: Action Potentials Initiated in the Axon Initial Segment That Do Not Activate the Soma
Fig 1
Somatic spikelets in a detailed biophysical model of a cortical pyramidal neuron in response to noisy input.
A: Morphology of the model neuron. Inset: excitatory (ge, red) and inhibitory (gi, blue) conductances are placed at the soma. Recording electrodes are placed at the soma (Vsoma, black) and the AIS (Vaxon,green). Basal dendrites were removed for clarity. B: Example three seconds of membrane voltage recorded at the soma (upper trace, black) and AIS (lower trace, green) during noisy stimulation. Somatic spikelets are marked with gray asterisks (*). Spikelets co-occur with APs at the AIS. C: Phase plot of ten somatic APs (black) and ten somatic spikelets (gray). D: Examples of a somatic AP (left, black) and a somatic spikelet (middle, gray) overlaid with the corresponding APs at the AIS (green traces). Right: overlay of the somatic AP (black) and the spikelet (gray). E: All somatic events generated during a 100 s simulation. Left: APs (N = 579, dark gray), aligned in time to crossing of the somatic voltage threshold (-10 mV, dashed line). The mean is shown in black. Right: spikelets (N = 63, light gray), aligned to the voltage threshold (-10 mV) crossing at the AIS. The mean is shown in dark gray. F: The all-or-none nature of APs (black) and spikelets (gray) is revealed in a plot of event amplitude against the maximum slope. G: Left: an example voltage trace recorded in a CA1 pyramidal neuron in a freely moving rat. Spikelets are marked with red asterisks (*). Right: Event amplitude plotted against the maximum slope of APs (dark green) and spikelets (red). Adapted from [2]. H: AP- and spikelet-triggered averages (solid and dashed lines, respectively), aligned to the time of crossing the voltage threshold in the AIS (vertical dashed line). H1: mean somatic AP (solid line) and mean somatic spikelet (dashed line) waveform. The horizontal dashed line accentuates the depolarization prior to AP and spikelet occurrence. H2: mean excitatory (red) and mean inhibitory (blue) AP-triggered (solid line) and spikelet-triggered (dashed) conductances. H3: the mean effective synaptic reversal potential combines mean excitatory and inhibitory conductances (see also Methods). During APs (solid line), the synaptic drive was stronger than during spikelets (dashed line).