Where are spikes initiated in cortical pyramidal cells?
Figure 2. We recorded simultaneously from the apical dendrite and soma (A-C) and the axon and soma (E-F) of layer 5 pyramidal cells while they generated spikes in response to synaptic activity of the Up state in vitro. We found that all action potentials initiated in the axon initial segment and then back-propagated into the soma and dendrites. (G) Similarly, we found that all action potentials occurring in response to the intrasomatic injection of either current pulses or noise, or during epileptiform activity, were also initiated in the axon initial segment, about 50 microns from the cell body, and that these spikes then propagate antidromically into the soma and dendrites. Thus, the axon initial segment is the preferred site of spike initiation in cortical pyramidal cells. A full length paper on this can be obtained here.