![]() ![]() In the CNS, axons carry electrical signals from one nerve cell body to another. Myelination in the central nervous system ( brain, spinal cord and optic nerves), myelination is formed by specialized glial cells called oligodendrocytes, each of which sends out processes (limb-like extensions from the cell body) to myelinate multiple adjacent axons while in the peripheral nervous system, myelination is formed by neurolemmocytes ( Schwann cells), which only myelinate a section of one axon. Myelin is made by glial cells, non-neuronal neural tissue cells that provide nutritional and homeostatic support to the axons, which are too elongated and far away from the soma to be supported by the neurons themselves. ![]() Rather, myelin sheaths occur in segments, separated by short (c. 1 micron) gaps of exposed axonal membrane ( axolemma) called nodes of Ranvier. However, unlike the plastic covering on an electrical wire, myelin does not form a single monolithic sheath over the entire length of the axon. The myelination of axons can be analogized to the shielding of electrical wires with non-conducting insulator covering. A circumferential wrapping of myelin, known as a myelin sheath, increases the conduction speed of electrical impulses ( action potentials) passing along the axon by generating saltatory conductions, which are much faster than conduction along an unmyelinated axon (150 m/s compared to 0.5-10 m/s), and also reduce signal loss due to extrinsic disturbances. Myelin is a lipid-rich extramembranous structure found on the axons and (less commonly) dendrites of neuron in many bilaterian animals, mainly vertebrates, as well as some arthropods and annelids. ![]()
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