The Action Potential

The resting membrane potential

Before a neuron can fire, it must first sit in a poised, ready state. A resting neuron is not electrically neutral: there is a steady voltage across its membrane, with the inside slightly negative compared with the outside. This resting membrane potential is typically around minus 70 millivolts, and it is the baseline from which every action potential begins.

This voltage arises from an uneven distribution of charged particles, or ions, across the membrane. Sodium ions are far more concentrated outside the cell, while potassium ions are more concentrated inside. Two factors set up and hold this arrangement. First, the sodium-potassium pump works continuously, pushing sodium out and drawing potassium in, at the cost of energy. Second, the resting membrane is much more permeable to potassium than to sodium, so potassium tends to leak outward, leaving the inside of the cell relatively negative. The balance of these forces settles the membrane at its resting value.

It is worth pausing on this point: maintaining the resting potential costs energy, and a large share of the brain's fuel is spent simply keeping neurons ready to fire. The resting state is not passive but actively held, a loaded spring waiting to be released.

Threshold and the all-or-nothing principle

A neuron does not fire in response to just any nudge. Small inputs shift the membrane voltage a little, but the voltage soon drifts back to rest. Only when the inputs together push the membrane up to a critical level, the threshold, usually around minus 55 millivolts, does the neuron fire a full impulse. Threshold is the tipping point that separates a passing ripple from a committed signal.

Crucially, the action potential obeys an all-or-nothing rule. Once threshold is reached, the impulse fires completely, always to the same size and shape, no matter how far past threshold the stimulus pushed. A stronger stimulus does not produce a bigger spike. What it does instead is make the neuron fire more frequently. This has a deep consequence for how the nervous system encodes information: the strength of a signal, the brightness of a light or the force of a touch, is carried by the rate of firing, the number of spikes per second, rather than by the size of any one spike.

The phases of the action potential

Once threshold is crossed, the action potential unfolds as a fast, stereotyped sequence of ion movements. The whole event lasts only a millisecond or two. Following it phase by phase shows how the neuron's voltage swings from negative to positive and back again.

1. Resting state

   The membrane sits at about minus 70 millivolts. Voltage-gated sodium and potassium channels are closed, and the ion gradients built by the sodium-potassium pump stand ready. The neuron is poised but quiet.

2. Depolarisation, the upstroke

   When the voltage reaches threshold, voltage-gated sodium channels snap open. Sodium ions, driven by both concentration and charge, rush into the cell. Their positive charge drives the inside voltage upward fast, past zero and on toward roughly plus 40 millivolts. This rapid reversal is the sharp rising edge of the spike.

3. Repolarisation, the downstroke

   The rise is short-lived. Sodium channels quickly inactivate, cutting off the inflow, while slower voltage-gated potassium channels now open fully. Potassium ions flow out of the cell, carrying positive charge with them, and the voltage falls back down toward its resting level.

4. After-hyperpolarisation, the undershoot

   Potassium channels are slow to close, so a little too much potassium leaves. For a brief moment the inside becomes even more negative than the resting value, dipping below minus 70 millivolts. This undershoot is called after-hyperpolarisation.

5. Return to rest

   The potassium channels finally close and the membrane settles back to its resting potential. The sodium-potassium pump quietly restores the original balance of ions over the following moments, and the neuron is ready to fire again.

Two definitions are worth fixing here, as they name the two halves of the spike and recur throughout neuroscience.

The refractory period

Immediately after firing, a neuron enters a brief recovery window called the refractory period, during which it cannot readily fire again. This period has two parts. In the absolute refractory period, no new action potential is possible at all, because the sodium channels have inactivated and simply cannot reopen until the voltage has fallen. In the following relative refractory period, firing is possible but harder, needing a stronger than usual stimulus, because the membrane is still hyperpolarised and some channels have not yet reset.

The refractory period also sets an upper limit on how fast a neuron can fire. Because each spike must be followed by this recovery, there is a ceiling on the number of action potentials per second, which in turn bounds how much information a single neuron can carry through its firing rate. In practice this ceiling is high enough for most purposes, allowing hundreds of spikes each second in many cells, but it is a real limit, and it is set directly by how long the channels take to reset after each impulse.

Propagation along the axon

An action potential is not much use if it stays in one spot. Its purpose is to travel the length of the axon and reach the terminals, and the way it does so explains one of the most important features of nerve signalling: the impulse does not weaken with distance.

The key is that the action potential is regenerated at every step rather than simply passed along. When one patch of membrane depolarises, the inflow of sodium there spreads a little current to the neighbouring patch ahead, nudging it up to threshold. That neighbour then fires its own full-sized action potential, which in turn triggers the next patch, and so on down the axon. Each point rebuilds the signal to its full strength, so the spike that arrives at the far end of a metre-long axon is exactly as large as the one that set out.

This is the answer to why the signal does not fade. A passive electrical signal would die away over distance, as a voltage does along a wire. The action potential escapes this fate by actively remaking itself at every point, trading a little speed for perfect reliability over long distances. It is a striking design: the neuron gives up the simplicity of passing charge along passively, and in return gains a signal that arrives at the far terminal every bit as sharp and full as it began, no matter how long the journey.

Saltatory conduction and myelin

Regenerating the impulse at every single patch of membrane is reliable but slow. Many axons speed things up dramatically by wrapping themselves in myelin, a fatty insulating sheath laid down in segments along the fibre. Between these segments lie small bare gaps called the nodes of Ranvier, and it is only at these nodes that the voltage-gated channels are concentrated.

The insulation lets the electrical signal skip quickly from one node to the next, so the action potential is regenerated only at the nodes rather than continuously along the whole fibre. Because it appears to jump from node to node, this process is called saltatory conduction, from the Latin for leaping. Skipping the insulated stretches is far quicker than crawling along every point, so myelinated axons can conduct at well over 100 metres per second, while unmyelinated fibres of similar size manage only around one. Myelin is thus one of the nervous system's great speed tricks, letting signals cross the body in a fraction of the time.

The action potential by the numbers

These values are typical figures rather than fixed constants; the exact numbers vary between neurons and conditions, but they capture the scale of the event. The quantitative account of how sodium and potassium currents produce these voltages was worked out by Alan Hodgkin and Andrew Huxley in the early 1950s, in experiments on the giant axon of the squid. Their model of the action potential remains a foundation of neuroscience and earned them a share of the Nobel Prize.

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