Resting Membrane Potential and Repolarization

On average, an intracellular electrode records a value of $-70$ millivolts.
The resting membrane potential depends on two factors:
- Presence of sodium and potassium gradients across the plasma membrane.
  - More sodium ions ( $Na^+$ ) outside the neuron than inside.
  - More potassium ions ( $K^+$ ) inside the neuron than outside.
- Differential permeability of the plasma membrane to sodium and potassium ions.

Leak channels in the plasma membrane allow sodium and potassium ions to diffuse down their concentration gradients.
The membrane contains many more potassium leak channels than sodium leak channels.
The membrane is much more permeable to potassium ions.
As positively charged potassium ions leak out of the neuron, the inside surface of the membrane becomes negatively charged compared to the outside surface.
If potassium was the only ion moving, the potential will stabilize at $-90$ millivolts.
Positively charged sodium ions leak into the neuron, which slightly offsets the negative charge and raises the voltmeter reading to $-70$ millivolts.
Sodium-potassium pumps actively transport sodium ions out of the neuron and potassium ions back in, compensating for the sodium and potassium leaks.

Two key events occur:
- Inactivation gates of voltage-gated sodium channels close.
- Voltage-gated potassium channels open.
These two events mark the beginning of phase two of the action potential known as repolarization.
Membrane potential briefly hyperpolarizes.
Voltage-gated potassium channels close, and the membrane returns to the resting state of $-70$ millivolts.