The Resting Potential and the Action Potential
A neurone holds its inside negative by pumping sodium out and potassium in, and an impulse is that difference briefly reversing. Voltage-gated channels open in sequence and the membrane resets behind the impulse.
What a learner can do afterwards
- Explains what maintains the resting potential and what it costs the cell.
- Orders depolarisation, repolarisation and hyperpolarisation on a voltage trace.
- Says which ion moves at each stage and through which kind of channel.
1 · Read
A resting neurone sits near minus 70 millivolts, negative inside. The sodium potassium pump builds this charge by pushing three sodium ions out and pulling two potassium ions in for every ATP it burns. Potassium leak channels let some potassium drift back out, which keeps the inside negative. This standing voltage is stored energy waiting for a signal.
A stimulus past threshold fires a fixed sequence. First voltage-gated sodium channels snap open and sodium rushes in, so the trace spikes upward in depolarisation. Then sodium channels shut, voltage-gated potassium channels open, and potassium streams out to drag the voltage down in repolarisation. A brief hyperpolarisation dips below resting before the pump restores the start point. The whole spike takes a couple of milliseconds.
Read a voltage trace like a story in three beats. The sharp rise is sodium rushing in, the fall is potassium streaming out, and the dip below the line is potassium still leaving. Match each beat to its ion and its gate, and every trace question falls into place.
Small graded wobbles fade with distance, but a wobble past threshold triggers the all or nothing spike. Chemically gated channels start the signal, voltage-gated channels drive it, and myelin plus channel density set its speed.
The pump holds the membrane negative, then gated sodium and potassium flows fire and reset the spike.
2 · Watch
Take it off screen
Where it sits
Where this leads
Jobs that lean on this skill. Follow one to see everything it is built on.
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.