All or nothing
A $16$ mV stimulus fires a spike that peaks at $+30$ mV. A $30$ mV stimulus fires a spike peaking at…
At $14$ mV the trace bumps to $-56$ mV and sags back. At $16$ mV the cell crosses the dashed $-55$ mV line and runs the full event — $+30$ mV peak, undershoot to $-80$ mV, recovery, about $6$ ms. At $30$ mV: identical trace, peak reached $+30.0$ mV again. Fire twice at a $4$ ms gap: one spike plus a REFRACTORY tag at the second stimulus; at $8$ ms, two identical spikes.
Below the $-55$ mV threshold, an input is a passive bump that decays away ($\tau = 10$ ms). At or above it — $15$ mV or more from rest — the model cell runs a stereotyped sequence: depolarize, repolarize, undershoot. The cell manufactures the waveform, not the stimulus — and in this model the waveform is perfectly identical every time. Real spikes are close to stereotyped rather than perfectly so: their shape varies across neuron types and can change during rapid firing. The working approximation stands — within one cell, a spike behaves like a one-bit event.
What does a stronger stimulus buy? Not a taller spike — an earlier, more frequent one. Here intensity is coded by rate — the f–I curve of lesson 2 — with the $5$ ms refractory period capping how fast that code can run. Rate is one code real brains use; the precise timing of spikes carries information too.
About this model
What it represents: the all-or-none logic of the action potential — a fixed threshold, a stereotyped waveform, a refractory period — drawn here as a stylized template.
What it leaves out: the mechanism. In a real cell the waveform is produced by voltage-gated sodium and potassium channels opening and closing, which Hodgkin and Huxley captured in a system of differential equations; here it is a canned shape. Real spike shapes also vary across neuron types, and real thresholds drift with a cell's recent history rather than sitting on one fixed line.
Source: Gerstner, Kistler, Naud and Paninski, Neuronal Dynamics, Ch. 2.2 (EPFL, free online) — the Hodgkin-Huxley model this stylized spike stands in for.
What does all-or-none mean?
A $14$ mV stimulus arrives at rest. What does the trace do?
Transfer: if spike height carries no information, where does stimulus intensity live?
In this model a spike is a fixed event triggered by crossing a threshold. Intensity lives in when and how often spikes happen, not in how big — and the refractory period sets that code's speed limit.