What it does
Nerve cells (neurons) carry messages as tiny electrical impulses. At rest, the inside of the membrane is negative compared with the outside, about −70 mV in most textbooks (−65 mV in the model used here). When a stimulus pushes it past a threshold, sodium channels open, Na⁺ rushes in and the inside turns positive for a moment: a spike (action potential). Then potassium channels open, K⁺ flows out and the membrane goes back to rest. The spike sets off the next patch of membrane, so the impulse travels along the axon.
This lab has three parts:
- One spike. Slide the stimulus strength. Below threshold nothing happens; above it you get a full spike every time: the all-or-none law. At Class 11 level a second graph shows the Na⁺ and K⁺ channels opening.
- Along the axon. Send an impulse along 10 cm of an unmyelinated axon and a myelinated one of the same thickness. On the myelinated axon the impulse jumps from gap to gap (the nodes of Ranvier): saltatory conduction.
- How fast are you? A reaction-time test, compared with how long the nerve part takes.
It fits Class 10 Science: control and coordination (neurons, how impulses travel, reflexes) and Class 11 Biology: neural control and coordination (resting potential, depolarisation, Na⁺ and K⁺, myelinated fibres). It is a teaching model. To run the full equations with your own currents, use hodgkin-Huxley simulator; for proper reaction-time experiments, use reaction time experiments; and see electrical timing in the heart with cardiac cycle.
How to use it
- Pick the level. Class 10 keeps to the spike, the speed sum and reaction time. Class 11 adds the after-spike dip (hyperpolarisation), the Na⁺ current and a graph of Na⁺ and K⁺ channels.
- Slide the stimulus. Watch for the sudden jump from no spike to a full spike between 6 and 7.
- Slide the axon thickness and press Send an impulse. The race is shown 50 times slower than real life. The sum below gives the speeds and times.
- Reaction test. Tap the box, wait until it turns dark, tap again. Five goes give an average, a middle value and your best.
- Check yourself gives fresh questions every time.
Worked example
With the stimulus at 10 the page shows Spike! It peaks at +39 mV. Set it to 6 and it says Too weak: no impulse; 7 gives a full spike again. For a 4 µm axon, the speed without myelin is about √4 = 2 m/s, so 10 cm takes 0.1 ÷ 2 = 50 ms; with myelin it is about 6 × 4 = 24 m/s, so 10 cm takes 4.17 ms, 12 times faster. If your median reaction time is 250 ms, the nerve travel (about 1.5 m at 60 m/s) is only 25 ms of it; the rest is synapses, the brain deciding and muscles getting going.
FAQ
Why does a stronger stimulus not make a bigger spike? Once threshold is passed, the Na⁺ channels open themselves in a chain reaction, so every spike is the same size. Stronger stimuli give more spikes per second instead.
Why is the resting potential −65 mV here? The model is fitted to the squid giant axon; many textbooks quote about −70 mV for a typical neuron. The idea is the same.
Does tapping on a phone measure my true reaction time? Roughly. Screens and touch sensors add a few tens of milliseconds, so compare your own goes rather than other people’s.
Where the numbers come from
- The spike is the Hodgkin and Huxley (1952) squid giant axon model (fibub's hhsim.ts), checked in the tests against a separate Python version.
- Speeds use rules of thumb: about √d m/s for an unmyelinated axon of diameter d µm and about 6 × d m/s for a myelinated one (Hursh 1939). They are rounded teaching values; real nerves vary.
- The reaction-time box measures taps in your browser only; nothing is stored or sent anywhere. Drawings are fibub's own.
Standard scientific values, such as constants, atomic weights and melting points, are facts found in every textbook; we name where ours were checked. If you are an author or publisher and want a value, a credit or a table changed or removed, tell us and we will act quickly.
Last updated 2026-11-09.