of the cell, carrying positive charge with it. The outflow of K causes
the membrane voltage to become less positive (Fig. 5.7). Enough K
flows out to return the cell to its resting potential of -65 mV, at which
point the potassium channels close and conditions are nearly back to
the way things were before the action potential started. Actually, the
membrane potential reaches about —-70 mV before the voltage-gated
K channels fully close, which is followed by a rapid adjustment to the
-65 mV resting potential.
An analogy can be made between an action potential and the flush-
ing of a toilet: once the lever is pushed down, water first flows into the
waste basin and then flows away, emptying it; then freshwater flows
into a storage basin to refill it, resetting the system—the flush runs to
completion and returns the system to where it started. Similarly, with
an action potential positive charge first flows into the nerve cell (Na)
and then flows back out (K*) flows back out, returning the neuronal
membrane potential to its resting value. Na/K pumps then function to
reestablish the inside/outside concentration differences.
- Voltage-gated Na* Voltage-gated Na
channels open ar channels close
+0
£ 7 of Voltage-gated K
a 0 ai channels open
c =
= = Voltage-gated K
& ®^5 = channels close
c © 4
(^2 5) -/0— od {
= 4
(^) TITTY TTT TT TTT TY TT TTY
1 2 3 4
Time (milliseconds)