Slide 4

Figure 4. Action potentials initiate in the Purkinje cell axon initial segment.

• (A) Voltage-sensitive dye fluorescence image of the soma-axon region in recording position shown on the left. Traces on the right: single trial recordings of AP signals, initiated by somatic current injection, from nine locations indicated on the image. Each trace is a spatial average of 4-5 same color pixels. Signals are scaled to the same peak height. Symbols represent actual data points; solid lines are reconstructed signals using cubic spline interpolation. The vertical line denotes the time at half-maximum amplitude at the edge of the soma (location 1). Note that the AIS segment traces (green and red) initiate the action potential first.
• (B) AP recordings from the AIS and the 1stnode of Ranvier scaled to the same height and compared on an expanded time scale.
• (C) The time to half-maximum amplitude subtracted from the time at half-max for the edge of the soma (”onset latency”) as a function of distance from the soma.
• (D) Group data (n=15) illustrating the level of consistency in the spatial localization of spike onset and the spike propagation velocity across cells. E. Group data statistics. The onset latency is lowest in the axon initial segment, and in all cells preceded the spike in the 1st node of Ranvier (p=0.000006, n=15). The diagonal line represents equity. Diamonds represent spikes initiated with a current pulse. Data obtained from spontaneous spikes are indicated by X’s. Data in red was obtained at 35º C, while data in blue was obtained at 23º C.