Figure 4.

Action potentials propagate in a saltatory manner. A, Spontaneously initiated action potential. Bottom frame, Voltage-sensitive dye fluorescence image of the soma–axon region with the AIS, first and second nodes as indicated. Colored frames, A time sequence of color-coded spatial map of relative membrane potential during initiation of AP shown in C. Signals from detectors centered over the main axon and collaterals are shown. The color scale is in relative units with resting membrane potential shown in blue/green, half-maximum amplitude in red, and peak of the AP in yellow. Individual frames are separated by 50 μs (obtained from cubic spline interpolation of original data points recorded at 100 μs intervals) (Fig. 2B). At ∼50 μs, the spike initiated in the AIS and propagated down the axon. At ∼100 μs, the first node was discharged to half-maximum amplitude before the immediately adjacent axonal regions. At ∼200 μs, the region of the second axon collateral (second node) is approaching half-maximal. B, A time sequence of changing spatial profiles of membrane potential during initiation and propagation of an action potential. The spatial plots constructed by dividing the axon into 20 regions of equal length. The average relative voltage for each region plotted as a function of distance from the soma. The time of each spatial plot in the sequence corresponds to time points indicated for color-coded frames in A. Note how the action potential propagates from the AIS to the first node, then to the second node, followed by propagation into the internodal regions. C, A temporal average of 26 spontaneous action potentials for the AIS, first node, and second node with time lines corresponding to the data in A and B. See also supplemental Movie 3 and supplemental Figure 4 (available at www.jneurosci.org as supplemental material).