Myelin

Myelin ( MY-ə-lin) is a lipid-rich material that in most vertebrates surrounds the axons of neurons to insulate them and increase the rate at which electrical impulses (called action potentials) pass along the axon. The myelinated axon can be likened to an electrical wire (the axon) with insulating material (myelin) around it. However, unlike the plastic covering on an electrical wire, myelin does not form a single long sheath over the entire length of the axon. Myelin ensheaths part of an axon known as an internodal segment, in multiple myelin layers of a tightly regulated internodal length. The ensheathed segments are separated at regular short unmyelinated intervals, called nodes of Ranvier. Each node of Ranvier is around one micrometre long. Nodes of Ranvier enable a much faster rate of conduction known as saltatory conduction where the action potential recharges at each node to jump over to the next node, and so on until it reaches the axon terminal. At the terminal the action potential provokes the release of neurotransmitters across the synapse, which bind to receptors on the post-synaptic cell such as another neuron, myocyte or secretory cell. Myelin is made by specialized non-neuronal glial cells, that provide insulation, and nutritional and homeostatic support, along the length of the axon. In the central nervous system, myelination is formed by glial cells called oligodendrocytes, each of which sends out cellular extensions known as foot processes to myelinate multiple nearby axons. In the peripheral nervous system, myelin is formed by Schwann cells, which myelinate only a section of an axon. In the central nervous system (CNS), axons carry electrical signals from one nerve cell body to another. The "insulating" function for myelin is essential for efficient motor function (i.e. movement such as walking), sensory function (e.g. sight, hearing, smell, the feeling of...