Nerve signals leap between gaps in a fatty wrapping named after bone marrow
Many of your nerve fibres are wrapped in myelin, a fatty insulating sheath. Unlike the plastic on a wire, it comes in segments separated by tiny bare gaps about a micrometre long. Electrical impulses jump from gap to gap instead of creeping along, which makes signalling far faster.
Vesalius described the white fibres of the brain in the 16th century, but it was Rudolf Virchow who named myelin in 1854, from the Greek word for bone marrow, which it resembles in colour and texture. Its fat content is what makes white matter white. The brain's corpus callosum, the bridge between its two halves, alone contains more than 200 million axons, many of them sheathed. Every vertebrate has myelin except the jawless fish, and some scientists have suggested it allowed larger bodies by keeping distant parts in quick communication.
The trick is called saltatory conduction, from the Latin for leaping. Along each wrapped stretch there are almost no sodium channels, but at the bare gaps, the nodes of Ranvier, they are packed densely. Sodium rushes in at a node, flows along inside the fibre to the next node and triggers it in turn, so the signal effectively recharges at each stop. The nodes must sit fairly close together, because the flowing charge fades with distance.
Myelin is made by support cells rather than nerve cells. In the brain and spinal cord, each oligodendrocyte reaches out to wrap several nearby axons, while in the rest of the body a Schwann cell covers a single stretch of one axon. By weight myelin is about 40% water, and its dry mass is mostly fat; without cholesterol it fails to form at all. Evidence since 2012 suggests these wrapping cells also act as fuelling stations, feeding the energy-hungry axons they surround.
Myelination begins around week 26 of pregnancy and speeds up in infancy, keeping pace with babies learning to crawl, walk and understand speech. It continues into early adulthood, and some sheaths are added throughout life. When myelin is damaged, as in multiple sclerosis, signals between the brain and body can slow or fail.
Source: Myelin
