Figure 7.
At subthreshold voltages, persistent Na+ current is generated predominately in the AIS. (a) A 1-s, 70-pA current step to just subthreshold voltage elicited a large [Na+]i increase in the AIS (red trace). A brief (5 ms) pulse that generated a single action potential generated a Na+ transient that rose sharply. (b) Subthreshold pulses of 0.3, 1.0 and 3.0 s each generated a [Na+]i increase that lasted the duration of the pulse. The rapid recovery at the end of the pulses indicates that the current was active throughout. A sharply rising Na+ transient elicited by a train of five action potentials at 50 Hz is shown for comparison. (c) A just subthreshold, 1-s stimulus elicited a large [Na+]i increase in the AIS (red trace), whereas the increase in the soma (black trace) was much smaller. The train of ten action potentials (40 Hz) caused sizable Na+ transients in both locations. (d) A 2-s voltage ramp from −70 to −40 mV elicited Na+ transients only in the axon. Interpolating along the ramp indicates that INaP and axonal [Na+]i both began to change at the same voltage (−57 ± 6 mV, n = 5). (e) A 2-s-long voltage ramp from −70 to 0 mV elicited Na+ signals that were clearly detectable in soma, basal and proximal apical dendrites. With this larger ramp, the membrane current and AIS optical signals still began to change at −57 ± 5 mV (n = 21), but the signals in the soma and basal dendrites began to change at −41 ± 5 mV (n = 21).