Myelination of nerve fibres. Coloured transmission electron micrograph (TEM) of Schwann cells (red, with blue nuclei) insulating nerve fibres (axons, orange) with a myelin sheath. The axons are seen in cross-section. Myelin (not seen) is an insulating fatty layer that surrounds axons, 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.