Action potential for the MCAT: every step on the graph

Every signal is the same size

In the video at 0:00

A light touch and a hard hit both send action potentials of exactly the same size along a nerve. So the size of the signal can't be what carries the difference. The answer comes from following one action potential across the graph of voltage against time.

Neuron, membrane, ions and channels

In the video at 0:13

An action potential is the electrical signal of a neuron (a nerve cell). The neuron is wrapped in a membrane, and charged particles called ions sit on both sides of it: the concentration of Na+ (sodium) is about 10 times higher outside the cell, and K+ (potassium) is higher inside.

Ions cross only through ion channels, proteins that act as gates. Exams name four kinds by what opens them: voltage-gated channels open when the voltage across the membrane changes (these drive the action potential), ligand-gated channels open when a signaling molecule such as a neurotransmitter binds, mechanically gated channels open when the membrane is stretched or pressed (many touch receptors), and leakage channels open and close at random.

Resting potential and the pump

In the video at 0:34

At rest the inside of the membrane is slightly negative compared with the outside, about -70 mV. That is the resting membrane potential; the exact value varies between cells, but -70 mV is the number most courses use.

The Na+/K+ pump keeps the ions where they are: it uses energy from ATP to move sodium out and potassium in, against their concentration gradients, which is why it is also called an ATPase.

Depolarization and threshold

In the video at 0:49

A stimulus, such as touch on the skin or a neurotransmitter from another neuron, opens a few channels and lets Na+ in. Sodium is positive, so the inside becomes less negative: depolarization.

If the inside reaches about -55 mV, the threshold, voltage-gated Na+ channels open in large numbers, Na+ rushes in and the voltage climbs to a peak of about +30 mV. Below threshold nothing else happens and no action potential starts.

Exam detail: the voltage-gated Na+ channel has two gates. The activation gate opens at -55 mV; the inactivation gate closes a fraction of a millisecond later, which is what stops the sodium flow at the peak.

Repolarization, hyperpolarization and the refractory period

In the video at 1:15

At the peak the Na+ channels shut and voltage-gated K+ channels open. Potassium is positive and flows out, so the inside drops back toward -70 mV: repolarization.

The K+ channels close slowly, so a little too much potassium leaves and the voltage dips below -70 mV: hyperpolarization. Then the channels close and the pump and leakage channels bring the cell back to rest.

The whole action potential takes about 2 milliseconds. While it is in progress another one can't start: the refractory period. In the absolute refractory period no stimulus can start a new signal, because the Na+ inactivation gates are still closed. In the relative refractory period a new signal can start, but only with a stronger stimulus than usual.

Back to the question

In the video at 2:00

Action potentials are all-or-none: once the threshold is reached, every one peaks at the same voltage. A harder hit can't make a bigger signal, so it makes more signals, faster. The nerve codes strength by how often it fires, not by how big each signal is.

How the signal then travels along the axon is called propagation: along an unmyelinated axon it moves continuously, and along a myelinated axon it jumps from one node of Ranvier to the next (saltatory conduction), which is faster. What happens when the signal reaches a muscle is covered in motor unit recruitment and twitch, summation and tetanus.

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