Some single nerve cells stretch from your spine all the way to your toes
A nerve cell's axon is usually about a micrometre wide, yet the longest ones in your body, in the sciatic nerve, reach from the bottom of the spinal cord down to each big toe. Others span barely a millimetre. The squid, needing speed, evolved a giant axon nearly as thick as a pencil lead.
An axon is the slender output cable of a neuron, carrying electrical impulses away from the cell body to other neurons, muscles or glands. Its partners, the dendrites, receive signals, usually taper, and stay close to the cell body; axons keep a constant width and can be enormously long. Most neurons have just one axon, though it often branches profusely, and a single axon with all its branches can reach several brain regions and form thousands of connections. Some make contact in passing, with hundreds or even thousands of swellings along their length.
Speed depends on insulation. Many axons are wrapped in myelin, a fatty sheath made by support cells: Schwann cells in the body, each wrapping a single axon, and oligodendrocytes in the brain and spinal cord, each able to wrap up to 50. Short bare gaps called nodes of Ranvier interrupt the sheath at regular intervals. The impulse effectively jumps from node to node, regenerating at each gap, which lets it travel far faster than along even the quickest uninsulated fibre. Myelin also gives white matter its colour, in contrast to the grey matter that holds the cell bodies.
Every impulse begins in a short unmyelinated stretch next to the cell body, the axon initial segment, only about 20 to 60 micrometres long and packed with sodium channels. It can shift position and change length, and a longer segment makes the neuron more excitable, a way of fine-tuning output.
Because the protein-making machinery stays in the cell body, axons depend on an internal railway. Microtubules all point toward the terminals, and motor proteins haul cargo along them: kinesin carries mitochondria and building materials outward, while dynein brings waste back. Research on this traffic is what led to kinesin being named.
Source: Axon
