Concept preview

What you will be able to do

1. 01Label a neuron and state the job of dendrites, cell body, axon, myelin sheath, nodes of Ranvier and axon terminals.
2. 02Explain how the sodium–potassium pump and ion leak set up the resting membrane potential of about $-70$ mV.
3. 03Describe the action potential as a wave of depolarization and repolarization, including threshold and the refractory period.
4. 04Explain the all-or-none principle and how saltatory conduction speeds an impulse along a myelinated axon.

Your nervous system is a living communication network, and its wires are single cells called neurons. A neuron does one astonishing thing: it turns a stimulus into a brief electrical event and passes that event, at high speed, from one end of the cell to the other and on to the next cell. Unlike a copper wire, a neuron does not carry a current of electrons; instead it carries a travelling change in voltage across its own membrane, produced by charged particles called ions flooding in and out through gated channels. A single neuron can be extraordinarily long — a motor neuron running from your spinal cord to a muscle in your foot is a single cell over a metre in length — yet the same tiny electrical trick works reliably along its whole length. Understanding that trick, the action potential, is the key to understanding how you sense, move, think and feel, and it is the foundation for everything else in this chapter.