The Cell That Fires
The neuron is the basic signalling cell of the nervous system. Three parts do the work. The dendrites are branching fibers that receive incoming signals from other neurons. The soma (cell body) integrates those signals and houses the nucleus. The axon is the long fiber that carries the outgoing signal away to the next cell; many axons are wrapped in a fatty myelin sheath whose gaps (nodes of Ranvier) let the signal leap forward far faster.
At rest, the neuron holds a resting potential of about millivolts: the inside is negatively charged relative to the outside, a battery waiting to discharge. When incoming signals push the membrane past a threshold, voltage-gated channels snap open and positive ions rush in — the membrane briefly reverses to positive. This spike is the action potential, and it sweeps down the axon.
The action potential obeys the all-or-none principle: it either fires completely or not at all. A stronger stimulus does not produce a bigger spike — every action potential in a given neuron is the same size. What a strong stimulus changes is the frequency of firing (spikes per second), not their amplitude. After firing, a brief refractory period must pass before the neuron can fire again.
Common pitfall: believing a more intense stimulus makes a "bigger" action potential. It cannot. Because of the all-or-none principle, intensity is coded by how OFTEN the neuron fires, not by how large each spike is.