Myelin sheets

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... 

Schmidt-Lantermann clefts are structures where the cytoplasmic surfaces of the myelin sheath have not compacted to form the major dense line and therefore contain Schwann or glial cell cytoplasm (Fig. 4-9). They are common in peripheral myelin but rare in the CNS. These inclusions of cytoplasm are present in each layer of myelin. The clefts can be visualized in the unrolled myelin sheet as tubes of cytoplasm similar to the tubes making up the lateral loops but in the middle regions of the sheet, rather than at the edges (Fig. 4-9). 

Vitamin deficiency can cause a megaloblastic anemia of the same type seen in folate deficiency (discussed in Chapter 17). In a patient with megaloblastic anemia, it is important to determine the underlying cause because Bjj defidency, if not corrected, produces a peripheral neuropathy owing to aberrant fatty acid incorporation into the myelin sheets associated with inadequate methylmalonyl CoA mutase activity. Excretion of methylmalonic acid indicates a vitamin Bjj deficiency rather than folate. 

The axon is effectively insulated from the surrounding medium by the myelin sheets except for special regions, the nodes of Ranvier, which lie at 1-to 2-mm intervals along the nerve. The nerve impulse in effect jumps from one nerve to the next. This saltatory conduction occurs much more rapidly (up to 100 m / s) than conduction in unmyelinated axons. It depends upon Na+ and K+ channels that are concentrated in the nodes of Ranvier. 

Coating of axons by myelin sheets produced by oligodendrocytes (in the CNS) and Schwann cells (in the PNS) provide electrical insulation and make transmissions along the axon more rapid [97]. The peak level of myelin synthesis in vivo takes place in the second week postnatally in rats and during the last trimester in humans [10]. However, the process of myelin formation continues during development into adulthood making the nervous system sensitive to myelination disrupters for an exceptionally long period [98]. 

Horiuchi M, Maezawa 1, Itoh A, et al. Amyloid betal-42 oligomer inhibits myelin sheet formation in vitro. Neurobiol Aging. 2012 33(3) 499-509. 

In water, a particle of lecithin exhibits myelin growth, ie, cylindrical sheets that are formed by bdayers and are separated by water which may break up into liposomes (vesicles with a single bilayer of Hpid enclosing an aqueous space). PhosphoHpids more generally form multilamellar vesicles (MLV) (5). These usually are converted to unilamellar vesicles (ULV) upon treatment, eg, sonication. Like other antipolar, surface-active agents, the phosphoHpids are... 

FIGURE 7-7 Structure of the P0 glycoprotein protomer. (A) In this ribbon diagram of the extracellular domain of P0, each P strand is labeled with a letter and two antiparallel P sheets are formed. The disulfide bridge is indicated in dark orange and a hypothetical path for disordered amino acids 103-106 is shown in black in the FG loop. (B) Lattice formation by P0. A view of the intraperiod line, or extracellular apposition, of myelin in the PNS. The orange tetramer sets emanate from one bilayer and the blue tetramer interacts with all four of them. This is a view perpendicular to the plane of the myelin membrane. 

The capillary endothelial cells in the brain, small extracellular space, sheet of glial cells lining the capillaries and the myelin sheath together constitute the barrier, so called blood-brain barrier. There is another... 

Gels of low concentration (0.1 -1.0%) give separated droplets and myelin. At concentrations above 4.5% the gels turn into a sponge-like continuous network of sheets and stands with small amounts of droplets and myelin. Amylose and amylo-pectin also undergo coacervation.277... 

The answer is a. (Murray, pp 230-267. Scriver, pp 2297-2326. Sack, pp 121-138. Wilson, pp 287-320.) Multiple sclerosis is a demyelination disease characterized by chronic inflammation. The primary losses are of the phospholipids and sphingolipids composing myelin membrane sheets of nerves in white matter. Consequently, brain white matter eventually looks like gray matter. TTigh levels of sphingolipids and phospholipids are observed in the cerebrospinal fluid. 

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. 

Porphyrin bolaamphiphiles have been shown to form platelets with a thickness of 8 A in bulk water. Well-defined monolayers have been obtained with bolaamphiphilic porphyrins with two positive charges on each edge. Charge repulsion obviously first prevents crystallization and second unlimited growth of the isolated sheets. Their surface area is a few [ixn. In aqueous solution lecithin bilayers of unlimited extension (myelin figures) are known to have erne s below 10" M and can thus be considered as noncovalent polymers. ... 

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