Schwann cell

A Schwann cell is a helper cell that lives in your peripheral nerves — the wiring that runs out beyond your brain and spinal cord into your limbs, skin, and organs. Neurons send their signals down long, thread-like tails called axons, and a Schwann cell's main job is to wrap itself tightly around one short stretch of a single axon, coiling around it many times like electrical tape spun around a wire. That fatty wrapping is called myelin, and it insulates the axon so the nerve signal can leap quickly from one bare gap to the next instead of crawling along every inch. The gaps between neighboring Schwann cells are the spots where the signal jumps, which is what lets a stubbed toe register almost the instant it happens.

Because each Schwann cell covers only one small segment, a long axon is sheathed by a whole chain of them lined up end to end, each tending its own patch. (This is the peripheral counterpart of the oligodendrocyte, the cell that does the same insulating work inside the brain and spinal cord — but one oligodendrocyte wraps many axons at once, while a Schwann cell devotes itself to just one.) Beyond insulating, Schwann cells nourish and protect the axon, and they shine when a nerve is injured: after a cut or crush, they clear away the debris of the broken fiber and form living tunnels that guide the regrowing axon back toward its target. This is a big reason peripheral nerves can heal and reconnect, while nerves inside the brain and spinal cord usually cannot.