Myelinated nerves. Coloured scanning electron micrograph (SEM) of a section through myelinated nerve fibres and Schwann cells. Myelin (dark brown) is an insulating fatty layer that surrounds the nerve fibre (axon, black), increasing the speed at which nerve impulses travel. It is formed when Schwann cells (light brown) wrap around the fibre, depositing layers of myelin between each coil. The outermost layer consists of the Schwann cell’s cytoplasm and is known as the neurolemma or sheath of Schwann.