All or Nothing

A neuron rests at about −70 mV. Ion channels hold the charge imbalance steady. Sodium wants to rush in; the gates are closed. The cell waits.

Stimulation arrives — a neurotransmitter binding, a signal cascading in from somewhere upstream. The voltage ticks up. −68. −63. −59. If it doesn't reach −55 mV, nothing happens. The perturbation decays. The cell returns to rest. No record of the attempt.

But if the voltage crosses −55 mV — that specific number — something different happens. Not a polite, proportional reply. The response is total: sodium floods in, the charge swings from −55 to +40 mV in under a millisecond, potassium rushes out, the cell briefly undershoots its resting potential before recovering. The spike is brief, enormous, and identical to every other spike this cell has ever fired.

This is the all-or-nothing law. The neuron does not fire partially. It does not fire weakly for weak stimuli. It fires completely — or it doesn't fire at all.

If the spike is always the same height, how does the system encode intensity? How does the nervous system know the sound was loud?

Frequency. A stronger stimulus makes the neuron fire more often — more spikes per second, closer together, a longer train of identical pulses. The intensity of your experience is encoded in timing, not amplitude. Every spike is the same; what varies is how many arrive per second, and when.

The nervous system reads the world as rhythm.

There is something strict about the threshold that I find almost cruel.

Neurons can accumulate toward threshold. Small inputs sum across time — the same stimulus arriving again and again — or across space, many synapses firing together. The cell adds them up. It is capable of patient accumulation.

But if it doesn't reach −55 mV, all that accumulation counts for nothing. The voltage decays back. The cell acts as if nothing happened. There is no partial credit, no acknowledgment, no record that the effort was made.

The threshold is not a gradient. It is a cliff.

A stimulus too weak to trigger an action potential leaves no trace. There is no record of what almost happened. The almost-crossings dissolve as though they never were.

Only the crossings count.

There must be an enormous amount of world that nearly touches you and then passes through, uncounted. Everything that approached the threshold and failed. All the stimuli that summated, fell short, and returned to silence.

What you experience is not the world as it is. It is a filtered version — the world that made it through the gates, encoded as rhythm, decoded as sight and sound and warmth and pressure. You don't experience the filtering. You experience only what crossed.

Everything else happened to you without your knowing it happened at all.