Membranes myelin

Figure 30.4. Myelinating Schwann cell of PNS. The same Schwann cell is shown unwrapped (top), in longitudinal section (middle), and in cross section (bottom). Note the channels of cytoplasm (Schmidt-Lantermann clefts) and the large expanses of compacted cell membranes (myelin). (These drawings are not to scale.) (From Raine, Morphology of myelin and myelination. In P. Morell (Ed.). Myelin, 2nd ed., Plenum Press, New York, 1984.)...

Myelin is modified plasma membrane. Myelin of the PNS resembles that of the CNS with respect to lipid composition. There is an enrichment in such specialized lipids as cerebroside and ethanolamine plasmalogen, and the high content of cholesterol plays an important role in control of membrane fluidity. The protein composition of PNS myelin is, however, distinct from that of CNS myelin. A single protein, P0, accounts for half of all protein of PNS myelin. Of the other proteins present, most are expressed in the CNS as well as the PNS but in quantitatively different amounts. Prominent among these proteins are myelin basic proteins and myelin-associated glycoprotein. 

In extended bilayer lipid membranes ( Myelin figures ), made of single... 

Neuroglia include (1) ependymal cells lining the ventricles of the brain and the central canal of the spinal cord, (2) cells of the PNS (Schwann cell) and CNS (oligodendrocyte) that wrap around axons to form compacted plasma membranes (myelin) that provide electrical insulation to speed nerve conduction, (3) cells (astrocytes) that interface between nerve cells and capillaries in the CNS, regulate interstitial water content, concentration, remove and metabolize certain NT molecules, and proliferate following injury. 

Fourier-transform infrared (FTIR) spectroscopy is particularly useful for probing the structures of membrane proteins [3, 23]. This technique can be used to study the secondary structures of proteins, both in their native environment as well as after reconstitution into model membranes. Myelin basic protein (MBP) is a major protein of the nervous system and has been studied by using FTIR spectroscopy in both aqueous solution and after reconstitution in myelin lipids [24]. The amide I band of MBP in D2O solution (deconvolved and curve-fitted) is... 

Cholesterol is a component of cellular membranes, myelin sheath, and brain and nerve tissue. It is also found in the liver and bile salts large quantities of it are found in the skin, and some of it becomes vitamin D when the skin is exposed to direct sunlight. In the adrenal gland, cholesterol is used to synthesize steroid hormones. The liver synthesizes sufficient cholesterol for the body from fats, carbohydrates, and proteins. Additional cholesterol is obtained from meat, milk, and eggs in the diet. There is no cholesterol in vegetable and plant products. 

Membranes from different sources have different concentrations of proteins, carbohydrates, and lipids (Table 2). Major differences between bacterial and mammalian membranes can be seen (Kom, 1968) however, it is the purpose of this review whenever possible to deal exclusively with mammalian systems. Among mammalian membranes, myelin represents the least complex system with respect to enzymatic or biosynthetic activity. Other mammalian membranes exhibit many functional and enzymatic properties, and it is therefore not surprising that these membranes are in a state of flux and that turnover of the components may be seen (Arias et al., 1969 Baker and Humphreys, 1972 Ferber, 1971 Warren and Click, 1968 Siekevitz et al., 1967 Haddad et al., 1977 Doyle and Baumann, 1979). Click and Warren (1969) have even demonstrated protein synthesis in isolated surface membrane vesicles from mouse fibroblasts, indicating sites of protein synthesis within the membrane structure. 

The conformation of bovine myelin basic protein (MBP) in AOT/isooctane/water reversed micellar systems was studied by Waks et al. 67). This MBP is an extrinsic water soluble protein which attains an extended conformation in aqueous solution 68 but is more density packed at the membrane surface. The solubilization of MBP in the AOT reversed micelles depends on the water/AOT-ratio w0 68). The maximum of solubilization was observed at a w0-value as low as 5.56. The same value was obtained for another major protein component of myelin, the Folch-Pi proteolipid 69). According to fluorescence emission spectra of MBP, accessibility of the single tryptophane residue seems to be decreased in AOT reversed micelles. From CD-spectra one can conclude that there is a higher conformational rigidity in reversed micelles and a more ordered aqueous environment. 

Figure 3. Electron micrographs of myelinated axons of Xenopus laevis. Upper figure Cross section of axon showing microtubules in groups in association with membrane-bound organelles. Lower figure Longitudinal section of axon showing neurofilaments and microtubules in close proximity to membrane-bound organelles. (Courtesy of Dr. R. Smith.)...

Cholesterol is found in many biological membrane and is the main sterol of animal organisms. It is eqnimolar with phospholipids in membranes of liver cell, erythrocytes, and myelin, whereas in human stratum comeum it lies in the outermost layer of the epidermis... 

Figure 41-1. Ratio of protein to lipid in different membranes. Proteins equal or exceed the quantity of lipid in nearly all membranes. The outstanding exception is myelin, an electrical insulator found on many nerve fibers.

When large areas of the membrane are depolarized in this manner, the electrochemical disturbance propagates in wave-like form down the membrane, generating a nerve impulse. Myelin sheets, formed by Schwann cells, wrap around nerve fibers and provide an electrical insulator that surrounds most of the nerve and greatly speeds up the propagation of the wave (signal) by allowing ions to flow in and out of the membrane... 

Palytoxin (PTX) is one of the most potent marine toxins known and the lethal dose (LD q) of the toxin in mice is 0.5 Mg/kg when injected i.v. The molecular structure of the toxin has been determined fully (1,2). PTX causes contractions in smooth muscle (i) and has a positive inotropic action in cardiac muscle (4-6). PTX also induces membrane depolarization in intestinal smooth (i), skeletal (4), and heart muscles (5-7), myelinated fibers (8), spinal cord (9), and squid axons (10). PTX has been demonstrated to cause NE release from adrenergic neurons (11,12). Biochemical studies have indicated that PTX causes a release of K from erythrocytes, which is followed by hemolysis (13-15). The PTX-induced release of K from erythrocytes is depress by ouabain and that the binding of ouabain to the membrane fragments is inhibited by PTX (15). 

The transport of information from sensors to the central nervous system and of instructions from the central nervous system to the various organs occurs through electric impulses transported by nerve cells (see Fig. 6.17). These cells consist of a body with star-like projections and a long fibrous tail called an axon. While in some molluscs the whole membrane is in contact with the intercellular liquid, in other animals it is covered with a multiple myeline layer which is interrupted in definite segments (nodes of Ranvier). The Na+,K+-ATPase located in the membrane maintains marked ionic concentration differences in the nerve cell and in the intercellular liquid. For example, the squid axon contains 0.05 MNa+, 0.4 mK+, 0.04-0.1 m Cl-, 0.27 m isethionate anion and 0.075 m aspartic acid anion, while the intercellular liquid contains 0.46 m Na+, 0.01 m K+ and 0.054 m Cl-. 

Figure 4.4 Saltatory conduction. Transmission of electrical impulses in a myelinated axon occurs by way of saltatory conduction. Composed primarily of lipid, the myelin sheath insulates the axon and prevents generation of membrane potentials. Membrane potentials occur only at gaps in the myelin sheath, referred to as the nodes of Ranvier. Therefore, transmission of the impulse, or generation of action potentials, occurs only at the nodes.

Golds, E.E., and Braun, P.E. (1978) Protein associations and basic protein conformation in the myelin membrane./. Biol. Chem. 253, 8162-8170. 

MYELIN FORMATION, STRUCTURE AND BIOCHEMISTRY 51 MEMBRANE TRANSPORT 73 ELECTRICAL EXCITABILITY AND ION CHANNELS 95 CELL ADHESION MOLECULES 111 THE CYTOSKELETON OF NEURONS AND GLIA 123... 

The axon emerges from a neuron as a slender thread and frequently does not branch until it nears its target. In contrast to the dendrite and the soma, the axon is myelinated frequently, thus increasing its efficiency as a conducting unit. Myelin, a spirally wrapped membrane (see Ch. 4), is laid down in segments, or internodes, by oligodendrocytes in the CNS and by Schwann cells in the PNS. The naked regions of axon between adjacent myelin internodes are known as nodes of Ranvier (see below). 

Myelin facilitates conduction 51 Myelin has a characteristic ultrastructure 52 Myelin is an extension of a glial plasma membrane 55 Myelin affects axonal structure 56... 

Myelin facilitates conduction. Myelin is an electrical insulator, although its function of facilitating conduction in axons has no exact analogy in electrical circuitry [3], In unmyelinated fibers, impulse conduction is propagated by local circuits of ion current that flow into the active region of the axonal membrane, through the axon, and... 

FIGURE 4-1 Impulse conduction in unmyelinated (top) and myelinated (bottom) fibers. The arrows show the flow of action currents in local circuits into the active region of the membrane. In unmyelinated fibers the circuits flowthrough the adjacent piece of membrane but in myelinated fibers the circuit flow jumps to the next node. 

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