How a Neuron Fires: The Action Potential
A patch of membrane with its Na⁺ and K⁺ channels, shown above a voltage trace that draws in step with it: rest at −70 mV, threshold at −55, spike to about +30, repolarisation and undershoot. Then all-or-none firing, where a stronger stimulus gives more spikes rather than bigger ones, and the spike travelling one way down the axon with a refractory region behind it.
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The prompt
Made in the educationalaivideos Studio. Topic: How a neuron fires: the action potential. At rest the inside of the membrane sits near −70 mV. When a stimulus pushes it to threshold (about −55 mV), voltage-gated sodium channels open and Na⁺ rushes in, spiking the voltage to about +30 mV. Then the sodium channels inactivate and potassium channels open; K⁺ flows out, the membrane repolarises and briefly overshoots below rest before settling. The spike travels down the axon because each patch triggers the next, and the refractory period behind it keeps it moving one way. Audience: First-year biology or psychology students Length: 2m, square Notes: Use the square frame in two parts: a membrane cross-section with channels and ions above, and a voltage-vs-time trace below that draws in sync with it. Then show the spike travelling along an axon as a wave with the refractory region behind it. Stress all-or-none: a stronger stimulus gives more spikes per second, not bigger spikes. Keep ion numbers qualitative. The sodium–potassium pump maintains the gradients over time but is not what ends each spike; don't say it is. This is one of the library's launch videos, so hold it to a very high bar: smooth transitions, something meaningful moving on screen at all times, a clean uncluttered layout, and nothing cropped at the frame edges. Before finishing, snapshot the final video at least every 10 seconds and fix anything cropped, overlapping or too small to read.
Transcript
Every thought you have rides on tiny electrical pulses called action potentials. Let's watch one happen. On top is a small patch of a neuron's membrane, with sodium ions in gold and potassium in blue. Below, we'll track the voltage across it. At rest, the inside sits at about minus seventy millivolts. There's more sodium outside the cell, more potassium inside, and these channels are closed. Now a stimulus nudges the inside a little more positive. If the voltage reaches threshold, about minus fifty-five millivolts, voltage-gated sodium channels snap open. Sodium rushes in, carrying positive charge, which opens even more sodium channels. The voltage shoots up to about plus thirty. Almost at once, the sodium channels inactivate, plugged from the inside. Meanwhile, slower potassium channels open, and potassium flows out. Positive charge leaves, so the voltage drops. It even dips below minus seventy, because the potassium channels are slow to close. Once they do, the membrane settles back at rest. Here's the crucial part: the spike is all or none. A weak stimulus that misses threshold fades away. A stronger stimulus doesn't make a bigger spike; every spike reaches the same peak. Instead, a stronger stimulus makes more spikes per second. Intensity is coded by how often spikes come, not by how big they are. So how does the signal travel? Zoom out to the axon. When one patch fires, the positive charge that rushed in spreads along the inside, pushing the next patch to threshold, so it fires too. Patch by patch, the spike moves along. Behind it, the membrane is refractory: its sodium channels are still inactivated and can't reopen yet. So the spike can't turn back. It only moves forward. Over the long run, the sodium-potassium pump keeps these gradients topped up, but it isn't what ends each spike; the channels do that. Threshold, sodium in, potassium out, reset. That's how a neuron fires.
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