Brain myelin sheath

Cerebrosides are found in the brain and in the myelin sheath of nerve tissue The structure of the cerebroside phrenosine is... 

Figure 1.1 Neurons (nerve cells) transmit information throughout the brain and the body. A typical neuron is shown here. Electrical impulses are received by the dendrites and transmitted to the next neuron via the axon. The myelin sheath insulates the axon and increases the speed at which electrical impulses can travel.

The biochemical basis nnderlying the normal fnnctioning of the brain is discussed above, where it is emphasised that of the two forms of communication, electrical and chemical, it is the latter that provides most flexibility. Not snrprisingly, therefore, there are more disorders dne to disturbances in neurotransmitter metabolism than there are due to disturbances in electrical commnnication. Nonetheless, damage to the structure of the myelin sheath aronnd the axon which interferes with electrical commnnication can lead to the severe disorder mnltiple sclerosis. 

Another pathway of some importance occurs in the brain this is the cholesterol 24-hydroxylase pathway. About 25% of the body s cholesterol exists in the plasma membranes of myelin sheaths. Here, the blood-brain barrier prevents cholesterol exchanges with the circulating lipoproteins, which makes it difficult for cholesterol to leave the brain. The cytochrome P-450 enzymes (CYP 46), expressed almost exclusively in the endoplasmic reticula of the brain, allows formation of 24-hydroxycholesterol. 

Multiple sclerosis (MS) is a devastating disease of the CNS that produces demyelination and inflammation. It is believed that its pathogenesis involves an immune reaction against various components of the myelin sheath. In a study by Lock et al. (2002), the gene expression patterns of brain lesions obtained during autopsies of MS patients were examined by microarray cluster analysis. A total of 1080 genes (from 7026 represented... 

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... 

Glycolipids are found predominantly in cell membranes of the brain and peripheral nervous tissue, with high concentrations in the myelin sheath. Glycolipids are very antigenic. 

The most hydrophobic integral membrane proteins can be extracted into organic solvents such as mixtures of chloroform and methanol. One such proteolipid protein, the 23.5-kDa lipophilin, accounts for over half the protein of myelin.57 182 The purified protein from rat brain contains 66% of nonpolar amino acids and six molecules of covalently bound palmitic acid and other fatty acids per peptide chain in thioester linkage to cysteine side chains. This protein evidently has four transmembrane helical segments with the six fatty acid chains incorporated into the membrane bilayer. It also has cytoplasmic and extracellular loops, one of which binds inositol hexakisphosphate (Ins P-6). (Fig. 11-9).183 The myelin proteolipid is an essential component of the myelin sheath and defects in this protein are associated with some demyelinating diseases57 which are discussed in Chapter 30. 

Most cells of the immune system are ordinarily kept apart from those of the nervous system by means of the blood-brain barrier. However, allergic encephalomyelitis, in which T cells attack the myelin sheath of brain neurons, can easily be induced in mice.506 A similar autoimmune process is thought to be involved in human multiple sclerosis (see Chapter 30, pp. 1769, 1808, and Fig. 30-9).507,508 High levels of circulating IgM are found in some demyelinating diseases of peripheral neurons.508 In Rasmussen s encephalitis, which causes brain inflammation and epilepsy, serum antibodies attack a glutamate receptor subunit GluR3.509... 

Volatile solvents are useful in industry and in homes because of their ability to dissolve fat. When inhaled, however, this property poses problems to the brain and the network of nerves that connect the brain and spinal cord to the rest of the body, ...thus, because the brain is a lipid-rich organ, chronic solvent abuse dissolves brain cells, the American Academy of Pediatrics wrote in a 1996 policy paper about inhalants. The chemical vapors also damage the myelin sheath, the fatty wrapper that insulates the fibers of many nerve cells that carry signals. 

Multiple sclerosis is a disorder of the brain and the spinal cord that attacks the myelin sheath of nerve fibres. The cause is unknown but it is presumably an autoimmune disease triggered by an environmental factor. There is a ge-... 

The nervous system is particularly susceptible to chemical abuse. Damage to the peripheral nervous system can cause numbness, weakness, and muscle paralysis. Many inhalants have been shown to damage the myelin sheath (Figure 6.1), the protective coat around nerve cells. This causes symptoms like those seen in individuals with multiple sclerosis, including abnormal fatigue, vision problems, loss of muscle coordination, slurred speech, and tremors. Direct damage to the brain... 

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