The Neuron: Electricity from a Battery of Ions
Nerves run on membrane voltage — about a tenth of a volt, flipped in a millisecond, racing down a wire that isn't metal.
Builds on: 0.2 The Membrane & Getting Things Across0.1 What Is Alive, and What a Cell Is
Same idea, other course:⚡ Spark 2.3 — a charged membrane is a capacitor; same physics, living wire
The battery in every membrane
A neuron is a cell drawn out into a wire: a body that collects inputs, and an axon — sometimes a metre long — that carries the output. The electricity it runs on comes straight from Unit 0’s membrane. A protein pump (the sodium–potassium pump) spends ATP shoving Na⁺ ions out and K⁺ ions in, so charge is stored across the membrane like a stretched spring. At rest the inside sits at about −70 mV relative to the outside.
A charged insulator between two conductors is exactly what Spark calls a capacitor. The difference is the trick the neuron plays with it.
The action potential: flip, then reset
Studded along the axon are voltage-gated channels — pores that open when the local voltage rises. If an input nudges the membrane up past a threshold of about −55 mV, Na⁺ channels snap open, Na⁺ floods in, and the voltage spikes to about +40 mV in under a millisecond. Then Na⁺ channels shut, K⁺ channels open, and the voltage falls back — briefly overshooting below rest. That spike is the action potential.
The spike at one patch of membrane nudges the next patch past threshold, so it regenerates itself down the axon like a flame along a fuse — never fading, because each segment fires fresh.
All or nothing
Below threshold: a local bump that decays and travels nowhere. Above threshold: a full-sized spike, and always the same full size, no matter how strong the stimulus was. A neuron cannot fire half a spike. So how does it report that a touch is hard rather than soft? By rate. A gentle press might trigger 5 spikes a second, a hard one 100. Intensity is encoded in frequency — with a hard ceiling, because after each spike the channels need a refractory period of a few milliseconds before they can fire again.
Myelin: insulation changes everything
A bare axon conducts at walking pace — around 1 m/s. Wrapping it in myelin (fatty insulation laid down by helper cells) lets the signal leap between gaps in the wrapping instead of regenerating at every point, reaching 120 m/s. That hundredfold speed-up is why your reflexes work. It also shows its value by its absence: in multiple sclerosis the immune system — last unit’s friendly fire — attacks myelin, and signals slow, scatter and fail.
Hodgkin and Huxley worked all this out in the 1940s and 50s on the giant axon of a squid — a nerve fibre so wide (a millimetre) that electrodes of the day could fit inside. The equations they wrote for its channels are still the foundation of computational neuroscience.
⚗️ Lab — The Action Potential
Stimulate a patch of membrane and watch the voltage trace.
- Keep the strength below +15 mV: bumps that fade. Cross it: full spikes, instantly.
- Once firing, push the strength higher. The spikes do not get taller — that is all-or-nothing.
- Raise the stimulus rate past ~250/s. The refractory period starts skipping every second one.
Work it by hand — 0 / 2
No multiple choice here: compute the value and type it. Suffixes like 2.5k or 20m are understood; answers within ±2% count.