Every nerve signal is a tiny electrical explosion that feeds on itself
A neuron at rest holds its interior at around minus 70 millivolts. Nudge it up to roughly minus 55 and something runaway happens: gates in the membrane swing open, sodium rushes in, which opens still more gates, until the voltage spikes and then crashes back within about a millisecond. That spike is how a nerve fires.
Almost every cell in animals, plants and fungi keeps a voltage across its membrane, set up by pumps and channels embedded in a fatty double layer that otherwise blocks charged particles like an insulator. The special ingredient in excitable cells is the voltage-gated ion channel, a protein that can change shape, forms a passage for particular ions in at least one of those shapes, and flips between them depending on the voltage around it.
That combination makes a feedback loop. Voltage controls the channels, and the channels control the voltage, so a small rise can trigger an explosive one. Once a patch of membrane depolarises, neighbouring patches follow, carrying the signal along the axon toward the synapses that link to other neurons, muscles or glands. Afterward the voltage often dips below its resting level for a while. A neuron's sequence of spikes is called its spike train.
Not all spikes are the same. Those driven by sodium usually finish in under one millisecond, while calcium-based ones can last 100 milliseconds or more. In heart muscle cells, a quick sodium spike acts as a primer that sets off a calcium spike, which in turn produces contraction. Muscle cells, some plant cells and even certain hormone-producing cells are excitable; in beta cells of the pancreas, the spikes prompt insulin release.
The fast sodium channels behind nerve conduction were first characterised by Alan Hodgkin and Andrew Huxley in Nobel Prize-winning work, and are often called NaV channels, V for voltage. Each switches between three states: deactivated, activated and inactivated, letting sodium through only when activated. Individual channels flip at unpredictable moments, so their behaviour is probabilistic rather than clockwork.
Source: Action potential
