Myelin and Oligodendrocytes

Myelin, the fatty insulation wrapped around axons by oligodendrocytes and Schwann cells, speeds nerve signals up to around a hundred times — and its loss causes diseases such as multiple sclerosis.

Myelin is a multi-layered sheath of membrane wrapped tightly around an axon, made mostly of fats (about 70–80%) and some special proteins. Its high fat content gives the brain's white matter its pale colour; the grey matter is grey because it is dominated by cell bodies and dendrites with less myelin.

Who makes it

In the brain and spinal cord, myelin is made by oligodendrocytes. Each sends out several flat processes that wrap around segments of different axons — one cell may myelinate segments of dozens of axons. In the peripheral nervous system, Schwann cells do the job, each wrapping a single segment of one axon. That difference matters for repair: Schwann cells help peripheral nerves regrow after injury, while the central nervous system regenerates poorly.

Why insulation speeds things up

Myelin isn't continuous. Every millimetre or so there's a tiny gap, a node of Ranvier, where the axon membrane is exposed and packed with sodium channels. Because myelin prevents current from leaking out between nodes, the electrical signal effectively jumps from node to node — saltatory conduction. The fastest myelinated axons conduct at around 100–120 metres per second; thin unmyelinated fibres manage around 0.5–2 metres per second.

Myelin over a lifetime

Myelination continues long after birth. Different brain regions myelinate at different times, with parts of the prefrontal cortex continuing into the twenties. Myelin also changes with experience: learning a new motor skill can increase myelination in relevant pathways, a newer form of brain plasticity.

When myelin fails

In multiple sclerosis, the immune system attacks central myelin. In Guillain–Barré syndrome and some inherited neuropathies, peripheral myelin is damaged. Without myelin, signals slow, become unreliable or fail altogether.