Short note: Resting and action potentials

Resting potential

The electric potential difference across the membrane of a nerve or muscle cell when it is not excited. The inside is negative with respect to the outside: about −70 mV in nerve and −90 mV in skeletal muscle.

Cause: unequal concentrations of ions. K⁺ is high inside and Na⁺ is high outside; the membrane at rest is more permeable to K⁺ than Na⁺ (about 50 to 100 times). K⁺ leaks out, leaving negative charge inside. The Na⁺–K⁺ pump (3 Na⁺ out, 2 K⁺ in) maintains the gradients. For a single ion the potential is given by the Nernst equation, E = (RT/zF) ln(Co/Ci), e.g. EK ≈ −90 mV; for several ions, by the Goldman equation. The cell is said to be polarised.

Action potential

A brief, rapid change of membrane potential when a stimulus above the threshold (about −55 mV) is applied. It is an all-or-none response.

 mV
 +40 |        /\
   0 |       /  \
 -55 |------/----\      threshold
 -70 |_____/      \     ___ resting
 -80 |              \__/  after-hyperpolarisation
     +-------------------------> t (ms)
        depolar. repolar.

Phases:

1. Depolarisation: voltage-gated Na⁺ channels open, Na⁺ rushes in, potential rises to about +30 to +40 mV.
2. Repolarisation: Na⁺ channels close, K⁺ channels open, K⁺ leaves, and the potential returns to rest.
3. Hyperpolarisation: K⁺ channels close slowly, so the potential dips below rest, and then returns to normal.

The duration is about 1 to 2 ms in nerve (about 200 to 300 ms in cardiac muscle). In the absolute refractory period no new action potential can be produced; in the relative refractory period only a stronger stimulus can. The action potential travels along the fibre at 1 to 100 m/s without decay, and is the source of ECG, EEG and EMG signals.