How nerve signals travel

An action potential swings the inside of a neuron from -70 mV to +30 mV and back in about 2 milliseconds.

A tiny battery across the membrane

Every neuron keeps a small electrical difference across its membrane. It is measured on the inside compared with the outside, in millivolts (mV). At rest the inside sits at about -70 mV. Two things set this up. Sodium ions (Na+) are about ten times more concentrated outside the cell than inside, while potassium ions (K+) are more concentrated inside. The sodium-potassium pump keeps those differences in place by moving Na+ out and K+ in, and because it works against the gradients it uses energy in the form of ATP. The resting membrane is like a charged battery waiting for a switch.

Threshold and the all-or-none spike

A stimulus, such as a neurotransmitter from another cell or pressure on a touch receptor, lets some Na+ in, and the inside becomes less negative. This is depolarization. If it stays small it simply fades away. If it reaches about -55 mV, the threshold, voltage-gated sodium channels open in large numbers, Na+ rushes in and the inside climbs to about +30 mV. Then voltage-gated potassium channels open, K+ flows out and the membrane falls back. It overshoots below -70 mV for a moment, a short hyperpolarization, before settling at rest. The whole action potential takes about 2 milliseconds, and the swing from -70 to +30 is a change of 100 mV, or 0.1 volt.

Because of the threshold, an action potential is all or none. A weak stimulus that never reaches -55 mV produces no action potential at all. A strong stimulus does not make a bigger one, because every action potential peaks at the same voltage. What a stronger stimulus does is start action potentials more often. The nervous system therefore codes intensity by how frequently a neuron fires, not by how tall each spike is. During the absolute refractory period, the same membrane segment cannot initiate another action potential. This refractory period also keeps the signal moving one way, toward the axon terminals.

Creeping and jumping along the axon

On an axon without myelin, the depolarization spreads a little, opens the next sodium channels, spreads a little more, and so on. This continuous conduction is slow. On a myelinated axon there are voltage-gated sodium channels only at the nodes of Ranvier. The charge spreads quickly under the myelin and the action potential is renewed at each node, so it seems to leap from node to node. This is saltatory conduction, from the Latin saltare, to leap. Wider axons also conduct faster. Thin unmyelinated fibres carry signals at roughly 0.5 to 2 metres per second; the thickest myelinated fibres reach about 120 metres per second.

Crossing the gap: the synapse

At the end of the axon the electrical signal has to reach another cell. A few synapses are electrical: the two cells are joined so that ions pass straight across. Most are chemical. When an action potential reaches the synaptic end bulb, voltage-gated calcium channels open and Ca2+ flows in. The calcium helps vesicles, tiny sacs full of neurotransmitter, merge with the membrane and release their contents into the synaptic cleft, the narrow gap between the cells. The neurotransmitter diffuses across and binds to receptors on the receiving cell. Receptors fit one neurotransmitter each, like a lock and a key.

Binding changes the voltage of the receiving membrane. An excitatory postsynaptic potential pushes it toward threshold; an inhibitory one pushes it away. A single input is rarely enough. Signals from many synapses, or several in quick succession from one synapse, add up at the trigger zone, and only if the total reaches threshold does the next neuron fire. Finally the neurotransmitter is cleared so the signal can end: acetylcholine is broken down by the enzyme acetylcholinesterase, and amino acids such as glutamate are pumped back into neurons or glial cells and reused. According to NINDS, acetylcholine makes muscles contract, glutamate is a major excitatory neurotransmitter and GABA is inhibitory.

This is educational material about how the body is built and works. It is not medical advice and is not for diagnosing or treating anyone.