Figure 4.
The shape of spike-evoked Na+ transients constrains the ratio of Na+ channel densities in different compartments. (a) Na+ transients elicited by 10 (left) and 100 (right) action potentials at the soma (black) in a basal dendrite (20 µm from the edge of the soma, blue) and in the AIS (20 µm from the hillock, red) of a representative neuron. Arrowheads indicate the points where the steady state and the end-of-train fluorescence values were measured and the numbers are the ratios of the steady-state to end-of-train ΔF/F values. (b) Na+ transients in models with different process-to-soma ratios of Na+ channel density. Somatic Na+ channel density was kept constant and Na+ channel density in the process was varied over the range of 0–30-fold of the somatic density (numbers along the left side of the figure). The numbers to the right of the traces are the calculated ratios of the steady-state to end-of-train signal. The model required the basal dendrite Na+ channel density to be 0.1–0.3-fold larger than the somatic density and the AIS density to be one- to threefold larger than the somatic density to match the experimentally determined signals.