How do nerves send signals?
By a travelling electrical pulse generated by ions moving across the cell membrane, then converted to a chemical signal to cross the gap to the next cell — an electrical-chemical relay rather than a wire.
The resting state. A neuron maintains a voltage difference across its membrane, around −70 millivolts inside relative to outside, by actively pumping sodium out and potassium in. This stored imbalance is the potential energy the signal draws on.
The action potential:
A stimulus makes a patch of membrane slightly less negative. If it crosses a threshold, voltage-gated sodium channels open.
Sodium rushes in, driven by both concentration and charge, and the voltage swings sharply positive.
Sodium channels close and potassium channels open, potassium leaves, and the voltage falls back — briefly overshooting.
A refractory period follows, during which that patch cannot fire again. This is what makes the signal travel in one direction rather than spreading backwards.
It is all-or-nothing. An action potential does not vary in size — a stronger stimulus produces more frequent firing, not bigger pulses. Intensity is encoded as rate.
Why myelin matters. Many axons are wrapped in insulating myelin with gaps. The signal jumps between gaps rather than propagating continuously, which raises conduction speed enormously — from around a metre per second to over a hundred. Damage to myelin, as in multiple sclerosis, slows or blocks conduction, which is why the consequences are so varied.
Crossing the synapse. The electrical signal cannot jump the gap. It triggers release of neurotransmitters, which diffuse across and bind receptors on the next cell, making it more or less likely to fire. The receiving neuron sums thousands of such inputs, excitatory and inhibitory, and fires only if the total crosses threshold — which is where computation happens.
Why this matters medically: most drugs acting on the nervous system, and many toxins and anaesthetics, work on ion channels, neurotransmitter release, or receptors.