The Action Potential: How a Cell Fires

We have the battery and the switches. The pump loaded the spring; the channels stand ready to release it. Now watch them work together in a single millisecond — the moment a cell fires. It is the most important event in the nervous system, and also the simplest: a sharp electrical spike that either happens completely or not at all, no half-measures, no dimmer. Cross a threshold and the membrane throws its voltage from negative to positive and back in about a thousandth of a second, then sends that spike racing down the axon without losing an ounce of strength. Every sensation you have ever felt, every thought you have ever had, every command your brain has ever sent to a muscle, was written in this one word, repeated: the action potential.

All or nothing

The strangest and most beautiful thing about the action potential is that it is all-or-none. A cell does not fire a little bit. Push the membrane gently and nothing propagates; push it just past a critical threshold — roughly −55 millivolts, up from a resting value near −70 mV — and the cell commits to a full spike, every time, identical in size whether the stimulus barely crossed the line or blew far past it.^[4] This is why the nervous system's information lives not in the height of any spike — they are all the same height — but in their timing and rate. A bright light and a dim one are not a big spike and a small one; they are a fast train of identical spikes and a slow one. The action potential is a digital pulse in an analog body.

And it is fast. The whole event lasts about one millisecond, and the spike travels down the axon at speeds ranging from under a metre per second in thin, bare fibres to more than a hundred metres per second in the thick, insulated ones — the difference between a stroll and a bullet, in the same body, depending on the wire.

The upstroke: a runaway loop

Here is the mechanism, and it is a masterpiece of positive feedback. When a stimulus nudges the membrane to threshold, a few voltage-gated sodium channels — the fast gates we met on Day 60 — snap open. Sodium, held in vast excess outside the cell by the pump, pours in. But sodium entering depolarizes the membrane further, and that further depolarization opens more sodium channels, which admits more sodium, which opens more channels still. This runaway loop — sometimes called the Hodgkin cycle — is why the spike is all-or-none: once threshold is crossed, the feedback takes over and drives the voltage upward explosively, with nothing to stop it until the inside of the cell has not merely reached zero but overshot into positive territory, to around +40 mV. For a fraction of a millisecond the cell's polarity flips: the inside, always negative at rest, becomes briefly positive. That overshoot is the signature of the whole affair — and, historically, the clue that overturned an older theory, as we will see.

1. Step 1 · Stimulus & thresholdCross ~−55 mV, or nothing happensAn input depolarizes the resting membrane. Below threshold it fades; at threshold, the cell commits to a full, all-or-none spike.
2. Step 2 · The sodium upstrokeA runaway positive-feedback loopVoltage-gated Na⁺ channels open; Na⁺ floods in; the depolarization opens still more channels (the Hodgkin cycle) — an explosive upstroke.^[1]
3. Step 3 · Peak & overshootThe inside briefly goes positiveThe voltage overshoots past zero to about +40 mV — the cell's polarity momentarily reverses.
4. Step 4 · RepolarizationSodium inactivates, potassium leavesNa⁺ channels auto-inactivate; slower voltage-gated K⁺ channels open and K⁺ flows out, driving the voltage back down (often undershooting).
5. Step 5 · Refractory & propagationOne spike, one directionThe inactivated patch cannot refire briefly (refractory period), so the spike travels one way; the pump restores gradients over many spikes.

The downstroke, and the pause

A spike that only went up would be useless; the cell has to reset. Two things end the upstroke almost as fast as it began. First, the sodium channels inactivate — a subtle but crucial point. They do not simply close; a part of the channel swings in and plugs it from the inside, so that even though the membrane is still depolarized, sodium can no longer flow. (Inactivated is not the same as closed: a closed channel can reopen; an inactivated one must first reset.) Second, the slower voltage-gated potassium channels, which have been lumbering open all along, now let potassium rush out of the cell, carrying positive charge away and driving the voltage back down. Often it overshoots the other way, dipping briefly below the resting value — the after-hyperpolarization — before settling home.^[4]

That inactivation of the sodium channels creates the refractory period, and it is one of the quiet geniuses of the design. For a brief window after a spike, the sodium channels are inactivated and cannot reopen no matter how hard they are pushed — the absolute refractory period — followed by a relative refractory period in which a stronger-than-usual stimulus is needed. This does two things at once. It caps how fast a neuron can fire, and — because the stretch of membrane just behind the advancing spike is refractory — it forces the action potential to travel in one direction only, forward down the axon, never doubling back. A single ion channel's habit of plugging itself is what makes nerve signals one-way streets.

And the pump? It is not driving any of this. A common misconception is that the sodium–potassium pump powers the spike. It does not — the spike is a fast, passive collapse of gradients that were already there. The pump's job is slower and humbler: over many spikes, it quietly bails out the sodium that leaked in and reclaims the potassium that leaked out, keeping the battery charged for next time. The firing is the channels' work; the pump merely pays the electric bill afterward.

The equations, and an honest word from their authors

All of this was made quantitative in one of the towering achievements of twentieth-century biology. In 1952, Alan Hodgkin and Andrew Huxley, working on the enormous giant axon of the squid — big enough to thread a wire down — used the newly invented voltage clamp to hold the membrane at fixed voltages and measure the sodium and potassium currents separately.^[1] From those measurements they wrote a set of equations that, hand-cranked on a mechanical calculator, reproduced the action potential in full: its threshold, its shape, its overshoot, its conduction speed. Their series of 1952 papers is the foundation of modern neurophysiology, and it earned Hodgkin and Huxley, with John Eccles, the 1963 Nobel Prize in Physiology or Medicine "for their discoveries concerning the ionic mechanisms involved in excitation and inhibition in the peripheral and central portions of the nerve cell membrane." The mechanism had been glimpsed earlier — Kenneth Cole and Howard Curtis had shown in 1939 that the membrane's electrical conductance shoots up during activity^[2] — but Hodgkin and Huxley made it a law.

The agreement must not be taken as evidence that our equations are anything more than an empirical description of the time-course of the changes in permeability to sodium and potassium. — Hodgkin & Huxley, 1952

Read that again, because it is the most honest sentence in all of electrophysiology. Having just built the most successful quantitative model biology had ever seen — a model that predicted the spike to the millivolt — its own authors declined to overclaim it. They had not, they insisted, revealed the molecular machinery; they had written an empirical description. They were right to be cautious, and right in a way that made them stronger: the molecular channels whose existence they could only infer were later seen and heard directly (the patch clamp, the crystal structures), and every one of those discoveries deepened Hodgkin and Huxley rather than overturning them. That is what a real theory does under scrutiny. It also refuted an older idea gracefully: Julius Bernstein had proposed that the spike was the membrane briefly losing its selectivity and falling to zero — but a fall to zero cannot explain an overshoot into positive voltage, and the overshoot is exactly what Hodgkin and Huxley measured. The truth was findable because the earlier guess had been clean enough to be wrong.

Where the honesty lives

The action potential is a superb place to hold this Journal's line, precisely because it is so stereotyped. A spike is not a tunable waveform you can coax toward health; it is a threshold-gated, all-or-none, gene-encoded event that fires in full or not at all, its message carried in when and how often — not in some "frequency" an external field could retune. Consider what a real intervention at this level actually looks like: a local anesthetic works by blocking the voltage-gated sodium channels in a defined patch of tissue, so spikes cannot start there — a specific molecule, at a specific site, with a defined region of effect and a known risk profile. That is the standard of specificity the machinery demands. So when wellness marketing claims that a device or a "frequency" will retune, reset, or optimize your nerve firing to heal you, the claim has no mechanism to stand on. "Your nerves are electrical" is true and wonderful; "therefore this gadget heals them" simply does not follow. The spike's magnificent, stubborn uniformity is exactly what the overclaim ignores.

Quick answers

Bioelectric Mechanisms · The resting potential · The pump · The channels · The spike · Hodgkin & Huxley · Biofield Hub →

Tomorrow on the Journal

Day 62 — The Synapse: How One Neuron Speaks to the Next. The spike races to the end of the axon and stops at a cliff — a gap it cannot cross. What happens there is the strangest twist in the whole story: an electrical nervous system that, at its junctions, turns to chemistry.