Neuron resting and action potential
BiologyHuman & Animal PhysiologyAges 16–17
Use it with your class: students open it from a link or a QR code, with no account needed.
Next in Human & Animal Physiology
A cross-section of the neuron membrane shows Na⁺ and K⁺ ions on both sides, Na⁺ channels, K⁺ channels and the Na⁺/K⁺ pump; click Stimulate (or tap the axon) to depolarize the membrane. A subthreshold stimulus only causes a small graded change that fades out; once the −55 mV threshold is reached, Na⁺ channels open in a rush, the potential spikes to +30 mV, then K⁺ channels open to repolarize and hyperpolarize back toward −70 mV. The membrane potential vs. time (ms) graph scrolls continuously, with the refractory period marked.
Lesson: Animal responsiveness: resting potential, action potential, nerve impulse propagation
What it shows
At rest, the inside of a neuron's membrane is about −70 mV relative to the outside, because K⁺ leaks out through open channels and the Na⁺/K⁺ pump moves 3 Na⁺ out for every 2 K⁺ in. A stimulus depolarizes the membrane; below threshold the change fades, but at threshold voltage-gated Na⁺ channels open, Na⁺ floods in and the potential spikes to about +30 mV. Na⁺ channels then close and K⁺ channels open, repolarizing and briefly hyperpolarizing the membrane. This all-or-none response and the refractory period that follows underlie nerve impulse transmission.
How to use
Press Weak stimulus (5 mV), then ⚡ Stimulate, and compare both traces with the threshold line. Move the Strength slider in small steps to find the weakest stimulus that fires, and check that stronger stimuli do not raise the peak. Shift the Threshold slider to see how excitability changes. Turn on Auto-stimulate every 12 ms and lower the speed to catch the refractory period, keeping ions and channels shown to link each phase of the graph to ion movement.
Parameters you can change
- Stimulus strength (depolarization) 0–40 mV
- Firing threshold -65–-45 mV
- Auto-stimulate every 12 ms
- Speed (simulated ms per real second) 2–20 ms/s
- Show ions and channels on the membrane
Questions to explore
- Why does increasing the stimulus above threshold not make the action potential peak any higher?
- What happens if you stimulate the membrane again during the absolute refractory period, and why?
- Which ion movement causes the rising phase of the action potential, and which causes the falling phase?
