Coloured transmission electron micrograph (TEM) of a longitudinal section through a myelinated nerve fibre. Myelin (dark blue) is an insulating fatty layer that surrounds nerve fibres (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. At varying intervals along the length of the fibre are gaps, called nodes of Ranvier (centre). Nerve impulses, travelling along the axon as electrical action potentials, jump from node to node in a process known as saltatory conduction.