Page 163 - The Miracle in the Cell Membrane

(b) MOVEMENT POTENTIAL
Stage 1: Disruption of polarity
Na+ ions move in at high speed.

Na+ ion gates
open in reaction to
a stimulus. Na+
rapidly enters the
cell by disrupting
polarity. The inner
part of the axon
rises from -70 mV
to +30 mV.

(C) MOVEMENT POTENTIAL:
Stage 2: Repolarization
K+ ions rapidly move out.
K+ ion gates open
shortly after, and
K+ ions begin
moving out of the
cell at high speed.
The cell is thus
repolarized, falling
to under -70 mV.

As the sodium-
potassium pump
moves Na+ ions
outside the cell, it
takes K+ ions in. It thus again
achieves resting potential. This
process requires ATP.

- At the beginning of movement potential, most sodium and potassium channels are closed.
- Electrical current passing by the axon or carrying a stimulus causes the potential of the
axon's cell membrane to fall.
- This opens some of the sodium channels, and sodium ions flow into the cell.
- Though positive charges enter in this way, the cell membrane potential is still negatively
charged. Therefore, sodium channels open, and more sodium ions enter.
- At this point, the exact opposite happens to the axon: Sodium channels begin closing one
after the other, and the inward flow of sodium is halted.
- In contrast, the axon's potassium channels begin opening and potassium ions leave.
- Finally, as the positive charge inside the cell declines, the cell membrane potential returns
to its former level.

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