Glial cells, plasma membrane

Myelin is a spiral structure made up of extensions of the plasma membrane from the myelinating glial cells, namely the oligodendrocytes in the CNS and the Schwann cells in the PNS. These cells send out sail-like extensions of their cytoplasmic membrane, each of which forms a segment of sheathing around an axon, which then constitutes the myelin sheath. 

There are different types of cholinesterases in the human body, which differ in their location in tissues, substrate affinity, and physiological function. The principal ones are acetylcholinesterase (AchE, EC 3.1.1.7), found in the nervous system and also present in the outer membrane of red blood cells, and plasma cholinesterases (EC 3.1.1.8, ChE), which are a group of enzymes present in plasma, liver, cerebrospinal fluid, and glial cells. Under normal physiological conditions, AChE performs the breakdown of acetylcholine (ACh), which is the chemical mediator... 

FIGURE 5-3 Variation in biomembranes in different cell types, (a) A smooth, flexible membrane covers the surface of the discoid erythrocyte cell, (b) Tufts of cilia (Cl) project from the ependymal cells that line the brain ventricles, (c) Many nerve axons are enveloped in a myelin sheath composed of multiple layers of modified plasma membrane. The individual myelin layers can be seen in this electron micrograph of a cross section of an axon (AX). The myelin sheath is formed by an adjacent supportive (glial) cell (SC). [Parts (a) and (b) from R. G. Kessel and R. H. Kardon, 1979, Tissues and Organs A Text-Atlas oT Scanning Electron Microscopy, W. H. Freeman and Company. Part (c) from P C. Cross and K. L. Mercer, 1993, Cell and Tissue Ultrastructure A Functional Perspective, W. FI. Freeman and Company, p. 137]... 

The myelin sheath Is a stack of specialized plasma membrane sheets produced by a glial cell that wraps Itself around the axon (Figure 7-39). In the peripheral nervous system. 

As the plasma membrane of the glial cell is converted into myelin, the lipid composition of the brain changes (Table 48.2). The lipid-to-protein ratio is greatly increased, as is the content of sphingolipids. The myelin is a tightly packed structure. 

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