Figure 5. The TTX-I current may originate at a distance from the soma.

A, depolarizing steps were applied from a holding potential of −80 mV. Reponses to steps to −50, −30 −10, +10 and to +30 mV are shown. For just-threshold steps the current onset was delayed (–30 mV, arrow). B, average delay to peak was voltage dependent. C and D, voltage dependence of deactivation. The TTX-insensitive current was activated by a 100 ms step depolarization to −20 mV, followed by a 1400 ms duration step to various potentials. Holding potential between trials was −60 mV. Sample traces in C show a complex deactivation that occurred in multiple steps. D, voltage dependence of deactivation at two time points, 1 ms after offset of the test potential (blue circles, linear I–V relation) and after 1300 ms (red triangles). The current response to a hyperpolarizing voltage ramp from +70 to −110 mV recorded in the same cell is shown in black. Note the inflection during deactivation. E, current response to triangular-shaped upward then downward voltage command from −140 to +70 mV then back to −140 mV. The holding potential was −60 mV. F, hysteresis in responses to rising and falling ramp. Same current trace as in E, plotted as a function of voltage. The inward current on the downward ramp is smaller than on the ascending ramp due to partial inactivation. Voltage for activation (red dot, ascending ramp) was more depolarized than the voltage for channel closure upon hyperpolarization (red dot, descending ramp).