Peripheral nerves. Coloured scanning electron micrograph (SEM) of a freeze-fracture through peripheral nerves to show Schwann cells (yellow) and a myelinated nerve fibre (axon, grey-green). A Schwann cell wraps its cell membrane around a nerve (axon) a number of times. In between each layer of membrane is a layer of myelin, an electrically insulating fatty layer. A Schwann cell effectively thickens a nerve's membrane and increases the speed at which nerve impulses are relayed.