Nib asked
How does a neuron send a signal?
A neuron's signal is not electricity racing down a wire: it is a self-renewing wave of sodium and then potassium ions crossing the membrane, passed along the axon and handed to the next cell by chemicals.
8 steps, drawn by Nib. Read it here, or watch him draw it. Updated .
1the neuron
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.
2not a wire
Copper wires carry current almost at the speed of light. A neuron's signal creeps along by comparison, because it is not simply carried: at every patch of the axon, channels in the membrane open and let ions through, and that regenerates the signal.
The signal is a regenerating wave of ions crossing the membrane, not a current in a wire.
a common mix-upA nerve signal is not electrons flowing through a wire. It travels at roughly 1 to 100 meters per second, millions of times slower than a current, because it is rebuilt along the way by ions crossing the membrane.
3resting
The sodium-potassium pump moves three sodium ions out of the cell for every two potassium ions it brings in, and potassium also leaks out through open channels. The result is a resting voltage near minus 70 millivolts: the inside of the neuron is negative compared with outside.
A resting neuron sits near minus 70 millivolts.
What does the sodium-potassium pump do?
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It pushes 3 sodium ions out and brings 2 potassium ions in, helping keep the inside negative.
4the spike
If incoming signals push the voltage up to the threshold, about minus 55 millivolts, voltage-gated sodium channels swing open. Sodium ions rush in, driven by the concentration gradient and the negative interior, and the voltage soars to about plus 30. It is all or nothing: below threshold nothing happens, above it the spike is the same size every time.
Past about −55 mV, sodium gates open and the voltage spikes to about +30 mV, all or nothing.
What happens when the voltage reaches threshold?
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Voltage-gated sodium channels open and sodium rushes in.
5reset
The sodium channels inactivate within a millisecond or so and slower potassium channels open. Potassium leaves the cell, pulling the voltage back down, even slightly below the resting level, before the pump and leak channels restore minus 70. For a short refractory period the neuron cannot fire again, which keeps the signal moving only forward. The whole spike lasts about two milliseconds.
Potassium flowing out resets the voltage; a refractory period keeps the signal moving one way.
6down the axon
The spike at one patch depolarizes the next patch past threshold, so it renews itself as it goes and never fades. In many axons a fatty myelin sheath insulates the cable, leaving gaps called nodes of Ranvier where the channels are packed. The signal jumps from node to node, called saltatory conduction, reaching up to roughly 100 meters per second, while a bare axon manages only around one or two.
The spike renews itself along the axon; myelin lets it jump node to node, up to about 100 m/s.
What does myelin do?
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It insulates the axon so the spike jumps between nodes, much faster.
7the hand-off
When the spike reaches the axon terminal, calcium channels open. The calcium makes vesicles of neurotransmitter fuse with the membrane and spill into the tiny gap, the synapse. The chemical binds receptors on the next cell and may start a new signal there.
At the synapse the electrical spike becomes a chemical message.
a common mix-upNeurons do not touch. In most synapses the signal crosses a tiny gap carried by chemicals, not electricity.
8the whole idea
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.