Summary: Origin of the "kink" in Somatic Action Potentials
Figure 15. Comparison of the different currents during the generation of an action potential in the model containing a dendrite, soma, and axon reveal that at the beginning of the axonal spike, only the small amount of Na+ current flowing into the axon is available. This results in the axonal spike rising smoothly as the spike initiates. The somatic spike, however, is delayed. At the threshold of the spike in the soma there is a large regenerative Na+ current that is occurring in the axon. The rapid rate of increase of Na+ current in the axon at this point causes the initial part of the spike in the soma to have an unusually rapid rate of rise, and results in the first "bump" in the dV/dt and phase plot of dV/dt versus membrane potential. Thus, the large and regenerative ion flow into the axon is responsible for the rapid depolarization of the soma (via capacitative current).
Following this initial and rapid "bump", the soma then actively generates an action potential through the local opening of Na+ channels. In conclusion, the "Kink" at spike initiation in the soma is generated by propagation of the action potential from the axon into the soma.