Watch a nerve impulse travel from stimulus to synapse

How to use this simulation

1. Drag stimulus down to 10 mV, below threshold, and press play: the potential nudges up and settles back — no signal ever leaves the cell body.
2. Set stimulus to 20 mV, above threshold, and play again: the graph on the right shows a full spike from −70 to +30 mV, and the signal then races down the axon.
3. Turn the myelin sheath off: the signal now advances continuously along the whole axon instead of hopping node to node, and the conduction speed reading drops sharply.
4. Turn myelin back on and drag the diameter slider: watch the conduction speed and the time-to-terminal reading change together.
5. Once the signal reaches the terminal, watch the synapse: vesicles release neurotransmitter that crosses the cleft and reaches the receptors on the next neuron.

How your hand moves before you even feel the burn

Touch a hot pan and your hand is already pulling back before the pain even registers. The whole round trip — skin to spinal cord, spinal cord back to muscle — happens in a few milliseconds. How does one message cross that whole distance so fast?

The answer is built into the neuron's own membrane. It is not a passive wrapper; it can generate and carry an electrical signal itself. That signal is the nerve impulse, or action potential, and this simulation lets you fire one yourself and watch it race down an axon.

A vivid real fact: pufferfish toxin (tetrodotoxin) works by jamming exactly this signal — it blocks Na⁺ channels, so depolarisation can never start, muscles never receive the message, and paralysis follows. A dentist's local anaesthetic uses the very same trick, just safely and temporarily.

Starting from zero: the structure of a neuron

A neuron, the structural and functional unit of the nervous system, has four main parts. Dendrites are short branches off the cell body that receive messages from other neurons or from a sense organ. The cell body (soma) holds the nucleus and other organelles — it is the cell's life-support centre.

A single long projection, the axon, leaves the cell body and carries the signal away. Many axons are wrapped in fatty myelin, laid down by Schwann cells in the peripheral nervous system, with regular gaps called nodes of Ranvier where the myelin is absent. At the far end the axon branches into the axon terminal, which forms a synapse with the next cell.

Some human axons are only a few millimetres long (interneurons deep inside the brain); others run over a metre — a motor neuron from the spinal cord to a toe, for instance. The axon-length slider here spans that real range, 2 cm to 120 cm.

Key terms in nerve conduction

Get the vocabulary straight before the formulas; definition questions in exams come straight from this table.

The resting potential: why the inside is negative

Even with no stimulus at all, a neuron's membrane is not electrically neutral — the inside sits about -70 mV lower than the outside. Two things build this resting potential. First, the Na⁺–K⁺ pump spends ATP to push 3 Na⁺ out and 2 K⁺ in on every cycle, so Na⁺ piles up outside and K⁺ piles up inside.

Second, at rest the membrane is far more permeable to K⁺ than to Na⁺ (K⁺ leak channels stay open), so K⁺ drifts back out carrying positive charge with it, leaving the inside even more negative. The balance between these two settles at roughly −70 mV.

The pump runs continuously, but the fast rise and fall of an action potential is driven mainly by voltage-gated channels opening and closing — the pump's job is to keep restoring the underlying ion gradients over the longer term, not to generate the spike itself.

Threshold and the all-or-none law

A stimulus depolarises the membrane a little. If that depolarisation never reaches the threshold of -55 mV, the voltage-gated Na⁺ channels never open — the potential rises a little and drifts back to rest by itself. This is a local (graded) potential, and it does not travel.

Cross the threshold, though, and everything changes: a flood of Na⁺ channels opens and a full action potential fires, whatever the stimulus was beyond threshold — the peak is always about the same, +30 mV. This is the all-or-none law: the spike either fires completely, or not at all, with nothing in between.

So why does a harder pinch feel more painful, if every spike looks the same? Intensity is coded by how often spikes fire and how many fibres join in, not by how tall any one spike is. A stronger stimulus means a higher firing rate, not a bigger spike.

The four named phases of an action potential

On the graph beside the simulation, this whole sequence takes only about two to three milliseconds. Here is each phase in turn.

Conduction: continuous versus saltatory

Once a patch of membrane fires, it depolarises the patch right next to it past threshold, so the wave keeps moving forward down the axon — in one direction only, because the patch just behind is still in its refractory period.

In an unmyelinated axon, ion channels are spread all along the membrane, so the signal advances step by step, continuously. In a myelinated axon, the myelin insulates the membrane so channels can only cluster at the nodes of Ranvier — the signal effectively hops from node to node, saltatory conduction, which is both faster and more energy-efficient (far less membrane needs active ion pumping).

Speed also depends on diameter. In a myelinated fibre, speed is roughly proportional to diameter — Hursh's rule: v ≈ 6 × d (m/s, with d in micrometres). In an unmyelinated fibre, speed grows only as the square root of diameter, so at the same diameter a myelinated fibre conducts far faster.

Try this in the simulation

Predict each outcome before you press play, then compare.

• Set stimulus to 5 mV: far below threshold, nothing travels, and the graph shows only a small bump that decays away.
• Set stimulus to exactly 15 mV: right at threshold, small changes either side flip the outcome completely — the all-or-none law in action.
• Fire the spike at 20 mV and again at 30 mV: the graph's peak stays the same both times, only the time to reach threshold shifts slightly — the potential itself never gets "bigger".
• Toggle myelin on and off at the same diameter and length, and compare the conduction speed and time-to-terminal readings.
• Drag the diameter slider from one end to the other and watch speed change linearly with myelin on, but only as the square root of diameter with it off.

Solved problems

Each solution states the formula first, then substitutes the numbers — write it the same way in an exam.

The synapse: from electrical signal to chemical message

An electrical signal cannot leap across to the next neuron on its own — a synaptic cleft, roughly 20 nanometres wide, sits in the way. So the message crosses in chemical form instead.

The steps run in order: the signal reaches the presynaptic terminal, voltage-gated Ca²⁺ channels open, Ca²⁺ flowing in triggers vesicles to fuse with the presynaptic membrane and release neurotransmitter (acetylcholine is the classic teaching example) into the cleft. The neurotransmitter diffuses across and binds receptors on the postsynaptic membrane, opening new channels there and creating a fresh change in potential in the next cell.

Finally, an enzyme breaks the neurotransmitter down, or the presynaptic cell reabsorbs it — otherwise the signal would just linger. Because only the presynaptic side has vesicles and only the postsynaptic side has receptors, a chemical synapse always passes the signal in one direction.

• The signal reaches the axon terminal
• Ca²⁺ channels open and Ca²⁺ enters
• Vesicles fuse and release neurotransmitter
• Neurotransmitter diffuses across the cleft
• It binds receptors and creates a new potential
• Enzymes or reuptake clear the neurotransmitter away

Common mistakes

Clearing these up is worth marks in every exam that touches this topic.

• Thinking a stronger stimulus makes a bigger spike. The peak is always the same (all-or-none); only the firing frequency changes.
• Picturing depolarisation as the membrane tearing open. Specific protein channels open and close; the membrane itself stays intact.
• Assuming saltatory conduction is the slow one. It is the fast, efficient one — continuous conduction along a bare axon is slower.
• Thinking nothing at all happens during the refractory period. Nothing can happen in the absolute part; a stronger stimulus can still fire a spike in the relative part.
• Assuming the electrical signal jumps straight across the synapse. Most synapses convert it into a chemical (neurotransmitter) message first.

Real life and medicine

One simple ion-movement principle explains most of the nervous system, and a good deal of medicine besides.

• Local anaesthetics (like a dentist's injection) briefly block Na⁺ channels, so pain signals never reach the brain.
• In multiple sclerosis, the immune system damages myelin, and conduction becomes slow and unreliable.
• A nerve conduction study measures exactly this conduction speed to diagnose nerve damage in a clinic.
• A knee-jerk reflex feels instant partly because the signal crosses only one synapse in the spinal cord and returns straight to the muscle, without a trip to the brain.
• The heart's own pacemaker cells and every skeletal-muscle contraction run on the very same Na⁺/K⁺-driven change in potential.

Exam corner

The neuron and the nerve impulse show up across school biology: structure and function questions on the neuron, definition and short-answer questions on threshold and the all-or-none law, and longer questions asking you to explain the resting-threshold-peak sequence with reasons.

Revision: the one-screen summary

This list plus the speed formulas above are all you need the night before an exam.

• Resting potential ≈ -70 mV, threshold ≈ -55 mV, peak ≈ +30 mV.
• Below threshold: only a local graded potential, no propagation. Above threshold: a full, identical action potential (all-or-none).
• Four phases: depolarisation (Na⁺ in) → repolarisation (K⁺ out) → hyperpolarisation (undershoot) → refractory period.
• Speed: myelinated v ≈ 6 × d (linear in diameter); unmyelinated v ∝ √d.
• Myelinated axons conduct by saltatory jumps between nodes of Ranvier; unmyelinated axons conduct continuously.
• At the synapse, the signal crosses as a one-way chemical message carried by neurotransmitter.

Frequently asked questions