How a Neuron Works: Nerve Impulses and Synapses

How a neuron works in one sentence: a neuron collects signals on its dendrites, and if they push the voltage across its membrane past a threshold, it fires an electrical impulse (an action potential) down its axon, which makes the axon terminal release chemicals that pass the message across a gap (the synapse) to the next cell. Your brain has about 86 billion of these cells, and every thought, movement and feeling is carried by them.

What are the parts of a neuron?

A neuron is a nerve cell specialised to carry electrical signals. A typical motor neuron, the kind that makes your muscles move, has five main parts:

Part	What it does
Dendrites	Short branches that receive signals from other neurons
Cell body	Contains the nucleus and keeps the cell alive
Axon	A long fibre that carries the impulse away from the cell body, up to 1 m long
Myelin sheath	Fatty insulation wrapped round the axon by Schwann cells, with gaps called nodes of Ranvier
Axon terminals	Swollen endings that release neurotransmitter at the synapse

There are three types you need to know. Sensory neurons carry signals from receptors (in the skin, eyes or ears) to the central nervous system. Relay neurons connect neurons inside the brain and spinal cord. Motor neurons carry signals from the central nervous system to muscles and glands.

What is the resting potential?

The resting potential is the voltage across a neuron's membrane when it is not firing: about −70 mV, with the inside negative compared with the outside. It is kept there by the sodium–potassium pump, which uses ATP to push three sodium ions (Na⁺) out for every two potassium ions (K⁺) it brings in, and by potassium leaking back out through open channels.

So at rest there is a lot of Na⁺ outside, waiting to rush in. That stored difference is what powers the impulse.

How does a nerve impulse travel?

A nerve impulse travels as a wave of voltage change that sweeps along the axon. At each point the same four steps happen:

1. Stimulus: something (a touch, a signal from another neuron) lets a little Na⁺ in, raising the voltage.
2. Threshold: if it reaches about −55 mV, voltage-gated sodium channels snap open.
3. Depolarisation: Na⁺ floods in and the inside shoots up to about +40 mV.
4. Repolarisation: sodium channels close, potassium channels open, K⁺ flows out and the voltage drops, briefly overshooting to about −80 mV (hyperpolarisation) before the pump restores −70 mV.

The Na⁺ rushing in at one spot spreads sideways and pushes the next piece of membrane past threshold, so the action potential moves along. It can't go backwards, because the membrane it has just left is in its refractory period: for about 1 to 2 ms its sodium channels can't open again.

What does "all or nothing" mean?

An action potential is all or nothing: if the stimulus doesn't reach threshold, nothing travels; if it does, the impulse is always the same size, about 110 mV from bottom to top. A stronger stimulus doesn't make a bigger impulse. It makes more impulses per second, and that frequency is how your brain tells a light touch from a hard press.

Why does myelin make impulses faster?

Myelin makes impulses faster because it insulates the axon, so the action potential can only happen at the nodes of Ranvier, about 1 mm apart. The impulse jumps from node to node (saltatory conduction) instead of creeping along every bit of membrane.

Axon	Typical speed
Thin, unmyelinated (slow pain fibres)	0.5–2 m/s
Myelinated motor neuron	70–120 m/s
Squid giant axon (very wide, no myelin)	about 25 m/s

Wider axons are faster too, which is why the squid evolved a 1 mm giant axon. Vertebrates use myelin instead, and get far more speed from a much thinner fibre. In multiple sclerosis the immune system damages myelin, and signals slow down or fail.

Worked example: how long does an impulse take?

An impulse travels 1.2 m from your spinal cord to your foot at 100 m/s.

time = distance ÷ speed = 1.2 ÷ 100 = 0.012 s = 12 ms

Without myelin, at 1 m/s, the same journey would take 1.2 seconds: far too slow to catch yourself when you trip.

What happens at a synapse?

A synapse is the junction between two neurons, with a gap of about 20 nm. The electrical impulse can't jump it, so the message is passed on by chemicals:

1. The impulse arrives at the axon terminal and calcium ions (Ca²⁺) flow in.
2. Vesicles full of neurotransmitter (such as acetylcholine) fuse with the membrane and release it into the gap.
3. The neurotransmitter diffuses across and binds to receptors on the next neuron.
4. This lets Na⁺ into the next neuron. If enough arrives to reach threshold, it fires its own action potential.
5. Enzymes break the neurotransmitter down (or it is taken back up), so the signal stops.

Synapses only work in one direction, because only the axon terminal has vesicles and only the next cell has receptors. They also let the nervous system add up signals from many neurons, which is why they are where many drugs act: caffeine, nicotine, painkillers and antidepressants all work at synapses.

How do neurons make a reflex?

A reflex is a fast, automatic response that doesn't wait for the brain. Touch a hot pan and the signal goes: receptor → sensory neuron → relay neuron in the spinal cord → motor neuron → muscle. Your hand pulls away in about 50 ms. The message also travels up to the brain, so you feel the pain a moment after you have already moved. You can follow the touch pathway, and see which brain areas it reaches, in the 3D brain and neuron lab.

Common mistakes

• Saying the impulse is "electricity flowing like in a wire". It is a wave of ions crossing the membrane, regenerated at every point, not electrons flowing along the axon.
• Thinking stronger stimuli make bigger impulses. They make more frequent impulses; each one is the same size.
• Mixing up threshold and resting potential. Rest is −70 mV; threshold is about −55 mV; the peak is about +40 mV.
• Saying the impulse jumps the synapse. It is carried across by neurotransmitter diffusing over the gap.

Frequently asked questions

How fast do nerve impulses travel?

Nerve impulses travel at anything from about 0.5 m/s to 120 m/s. The fastest are in wide, myelinated axons, such as the motor neurons to your leg muscles, where the impulse jumps between the nodes of Ranvier. The slowest are thin unmyelinated fibres, like the ones carrying the dull ache after an injury, which is why that ache arrives a moment after the sharp first pain.

What is the difference between a neuron and a nerve?

A neuron is a single cell. A nerve is a bundle of hundreds or thousands of axons from many neurons, wrapped together in connective tissue, like a cable holding many wires. The sciatic nerve in your leg, for example, contains the axons of many sensory and motor neurons, some of them over a metre long.

Why is the resting potential negative?

The inside of the neuron is negative because the sodium–potassium pump moves three positive sodium ions out for every two positive potassium ions it moves in, and potassium also leaks out through open channels. More positive charge ends up outside than inside, so the inside reads about −70 mV compared with the outside.

Can neurons grow back?

Most neurons in the brain and spinal cord cannot divide, which is why spinal injuries are so serious. Neurons in the peripheral nerves, like those in your arms, can regrow a cut axon slowly, about 1 mm a day, along the path of their old myelin sheath. The hippocampus, which forms memories, also keeps making some new neurons in adults.

Try it yourself

Open the Brain & Neurons lab and switch to A neuron. Set the stimulus below −55 mV and press Stimulate: nothing travels. Turn it above threshold and watch the impulse jump between the nodes and release neurotransmitter across the synapse, while the probe draws the action potential. Then switch the myelin off and compare the speed. For the electrical side of the story, see Ohm's law explained, and for another membrane process, what happens to a red blood cell in salt water.