The Neuron

The nerve cell is the basic signalling unit of the nervous system: dendrites gather inputs, the cell body integrates them, and the axon sends an electrical pulse to other cells.

Neurons come in thousands of shapes and sizes, but almost all share the same basic parts — and the same job: receive signals, integrate them, and pass a signal on.

The parts

Dendrites are the branching, tree-like extensions that receive signals from other neurons. Many are covered in tiny protrusions called dendritic spines, each typically the site of one synapse. The cell body (or soma) contains the nucleus and most of the cell's machinery for making proteins. It sums up the incoming signals.

The axon is a single, often long fibre that carries the neuron's output. It begins at the axon hillock and initial segment, the trigger zone where the decision to fire is made. Many axons are wrapped in myelin, a fatty insulating sheath interrupted by gaps called nodes of Ranvier. The axon branches at its end into axon terminals, which form synapses with other cells.

How long can an axon be?

Some axons are a fraction of a millimetre long. Others are astonishing: the motor neurons that move your toes have cell bodies in the lower spinal cord and axons reaching all the way to the foot — around a metre in a tall adult. For a cell body about 0.1 mm across, that's like a basketball with a garden hose kilometres long.

A cell that lasts a lifetime

Most neurons are not replaced. The ones in your cortex now were mostly made before you were born, and they must keep themselves running for decades — transporting proteins along their axons on molecular "motors", maintaining their electrical charge, and constantly remodelling their synapses.