Nib asked, and Nib — askNib's tutor — drew the answer live at a whiteboard. This is the spoken transcript; enable JavaScript to watch it drawn.
how does a neuron send a signal
A neuron takes in signals at its branching dendrites, adds them up at the cell body, and sends the result down one long cable, the axon.
The signal is not electricity racing along a wire. It's a wave of ions rushing through tiny gates in the membrane, renewed at every step.
At rest, the inside of a neuron is negative, about minus seventy millivolts. A pump pushes sodium out and potassium in, and potassium leaks back out.
A strong enough input lifts the voltage to about minus fifty-five, the threshold. Sodium gates fly open, sodium floods in, and the inside shoots up to about plus thirty.
Then the sodium gates shut and potassium gates open. Potassium flows out, the voltage crashes back below rest, then recovers. All in about two milliseconds.
The spike opens sodium gates in the next patch of axon, and the next, a wave renewed all the way along. Myelin insulation lets it leap between gaps, fast.
At the far end, the spike triggers a chemical hand-off. Calcium enters, vesicles release neurotransmitter, and it crosses the synapse to the next cell.
So the whole journey: rest, a sodium spike, a potassium reset, a leap down the axon, and a chemical hand-off to the next cell.
A neuron's signal is a wave of ions, renewed along the axon and handed on by chemicals.