Fig. 10.

Spike initiation sites change for short and long current pulses. Dashed vertical lines indicate electrode position, end of AIS and the beginning of myelination. Membrane voltages profiles are shown for 50 μs (green) and 5 ms (blue) pulse stimulation at selected times. (A) Intracellular stimulation of Neuron 2 at the main dendrite. At the end of a 50 μs pulse with threshold intensity (20 nA) a dendritic spike (green curve) is generated. Although during a 5 ms pulse just above the 0.79 nA threshold the maximum membrane potential occurs close to the electrode position (thick blue curves), the spike arises at a quite distant position (∼300 μm) at the thin segment of the axon. (B–D) Extracellular anodic stimulation of Neuron 2 for electrode positions as defined in Fig. 1A. (B) Electrode positions above the soma (black line) and the third node of Ranvier (blue line) cause combined strength–duration curves as spike initiation site depends on pulse duration in these cases. (C) Stimulus electrode above soma. Thick lines show the situation at the end of the pulses. For better recognition the 10 μs response of the short pulse stimulation is shown in black. (D) Stimulus electrode above node of Ranvier.