Illustration of a section through a myelinated nerve fibre and Schwann cells (foreground). Myelin (yellow) is an insulating fatty layer that surrounds the nerve fibre (axon, protruding from Schwann cell at left centre), increasing the speed at which nerve impulses travel. It is formed when Schwann cells 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.