It seems intuitive that a stronger stimulus should produce a stronger nerve signal — a harder poke should mean a bigger electrical spike travelling up the nerve. That intuition is wrong at the level of an individual neuron's action potential. Once a stimulus is strong enough to trigger a neuron to fire at all, the resulting electrical spike is always the same fixed size and shape, regardless of how much stronger the triggering stimulus was. Neuroscientists call this the all-or-none principle, and it's one of the more counterintuitive facts about how individual neurons actually communicate.
A threshold that either gets crossed, or doesn't
A neuron's cell membrane maintains an electrical voltage difference across it, and incoming signals from other neurons or sensory receptors shift that voltage up or down. If those shifts push the voltage past a specific threshold, voltage-gated ion channels in the membrane spring open in a fast, self-reinforcing cascade, producing the rapid electrical spike known as an action potential. Crucially, whether the stimulus that crossed the threshold did so by just barely enough or by a huge margin makes no difference to the resulting spike — the same voltage-gated channels open the same way, producing an action potential of the same fixed amplitude every single time, as long as the threshold gets crossed at all. Fall short of the threshold, and no action potential fires whatsoever; there's no partial, weaker firing available as an intermediate option.
So how does the nervous system encode stimulus strength at all?
If every action potential is identical regardless of triggering stimulus strength, an obvious question follows: how does the nervous system communicate that a stimulus was strong versus weak at all, given that individual action potentials themselves carry no such information? The answer lies not in the size of any single spike, but in the frequency and pattern of spikes over time: a stronger stimulus typically causes a neuron to fire action potentials more rapidly and for longer, while a weaker stimulus, if strong enough to trigger firing at all, produces a slower rate of firing. Stimulus intensity is encoded in this firing rate — the rate of identical all-or-none spikes — rather than in the size or shape of any individual spike, which stays constant regardless of how far past threshold the triggering stimulus actually was.
What we're still unsure about
The all-or-none principle governing individual action potentials, and the rate-coding mechanism that lets neurons still communicate stimulus intensity despite it, are both thoroughly established, foundational neuroscience, confirmed through extensive direct electrophysiological measurement. What remains a more genuinely active area of research is understanding precisely how the brain decodes complex, rapidly changing patterns of firing across large populations of neurons simultaneously, since real sensory and cognitive processing generally isn't well captured by looking at the firing rate of a single neuron in isolation, and how populations of neurons collectively encode and process more complex information remains a substantial ongoing area of neuroscience research.
This sits inside Neuronal Structure & Action Potentials, one of seven topics in Neuroscience, one of four domains in Psychology, one of seventeen subjects the app can quiz you on.