Resting potential and action potential

1The squid that explained the nerve impulse

Squid escape from predators by squirting water, a response driven by unusually wide axons, up to about 1 mm across. In the 1940s and 1950s Alan Hodgkin and Andrew Huxley pushed fine electrodes inside these giant axons and measured the voltage across the membrane while it carried an impulse. Their explanation of the impulse in terms of sodium and potassium ions earned them a share of the 1963 Nobel Prize in Physiology or Medicine, and it is still the model taught today. A nerve impulse is not a current of electrons along a wire: it is a brief, travelling reversal of the voltage across the axon membrane, caused by ions moving through protein channels.

2The resting potential

Three things together keep the inside of a resting axon at about $-70\mathrm{m}\mathrm{V}-70\backslash ,\backslash mathrm\{mV\}$:

1. The sodium–potassium pump. This carrier protein uses one ATP to move 3 Na⁺ out and 2 K⁺ in. It builds up steep concentration gradients: Na⁺ is about ten times more concentrated outside, K⁺ about thirty times more concentrated inside. Because it moves more positive charge out than in, it also makes the inside slightly negative directly.
2. Different permeability. At rest many potassium leak channels are open but almost all sodium channels are closed, so the membrane is far more permeable to K⁺ than to Na⁺. K⁺ diffuses out down its concentration gradient, carrying positive charge out of the cell. This is the main cause of the negative inside.
3. Large negative ions. Proteins and other large anions inside the axon cannot cross the membrane. As K⁺ leaves, the growing negative charge inside attracts K⁺ back, and an equilibrium is reached at about −70 mV, where the outward diffusion of K⁺ is balanced by the electrical pull inward.

3The action potential

Only a tiny fraction of the ions cross the membrane in one action potential, so the concentration gradients hardly change; the pump keeps them topped up over time.

4All-or-nothing and the refractory period

• All-or-nothing: any stimulus that reaches threshold produces a full action potential of the same size; a stimulus below threshold produces none. A stronger stimulus produces more frequent action potentials, not larger ones.
• Absolute refractory period (about 1 ms): the Na⁺ channels are inactivated and cannot reopen, so no stimulus, however strong, can start another action potential.
• Relative refractory period: the Na⁺ channels have recovered but the membrane is hyperpolarised, so only a stronger-than-usual stimulus can reach threshold.

The refractory period has three important results: action potentials are separate (they cannot merge); they travel in one direction only, because the membrane just behind the impulse cannot be re-excited; and there is a maximum frequency of impulses.