Answer
Myelination is the wrapping of lengths of an axon by glial cells. In the central nervous system (CNS) myelination is carried out by oligodendrocytes. In the peripheral nervous system (PNS) the myelination is done by Schwann cells.
An oligodendrocytes grows many processes and can simultaneously wrap up to 60 axons.
Each Schwan cell can form only one segment of an axon's myelin shearh. It does this by rolling around the axon and wrapping it with many concentric layers of Schwann cell membrane. Unmyelinated gaps between adjacent Schwann cells are known as Nodes of Ranvier. Impulse conduction in myelinated fibers is thirty times faster than in nonmyelinated fibers. In the former the impulse travels from from node to node (of Ranvier) by what is called saltatory (jumping) conduction.
Work Step by Step
A Schwann cell myelinates a segment of an axon by tightly wrapping layers of membrane around it. This membrane is deficient in carrier and channel proteins ,which deficiency enhances its insulating capacity. The wrapping process squeezes the Schwann cell nucleus and cytoplasm to a peripheral location. This outer collar of perinuclear cytoplasm used to be called the neurilemma.--this structure is absent in the CNS. Sometimes a Schwann cells only partially myelinates portions of several axons (as many as 15). Such nonmyelinated axons are also found in the CNS.
Myelinated axons or nerve fibers constitute the white matter of the nervous sysrem, but in CNS as in PNS only axons are myelinated. Dendrites are never wrapped by glial cells.
