Neurons demyelination diseases

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... 

Conduction velocity of an action potential can be increased dramatically by the presence of myelin, a fatty sheath that surrounds the axon and that is interrupted into gaps every millimeter or so at the nodes of Ranvier. Myelin is elaborated by Schwann cells in the peripheral nervous system and by oligodendrocytes in the central nervous system (the biochemistry of myelin will be discussed later in the article). The presence of myelin will dramatically alter the mode and velocity of conduction of the action potential in the axon. As in unmyelinated nerves, the action potential is still transmitted from one section of the axon to another by the presence of local circuit currents. However, the fatty sheath of myelin has poor conduction properties and therefore acts as an insulator. Hence, the local circuit currents jump from one gap to another at the nodes of Ranvier and the rate of conduction is enhanced as local circuit currents travel faster than the action potential itself This process of discontinuous conduction is known as saltatory conduction. Numerous diseases involving myelin deficiency have been described clinically. As one might predict, demyelinating diseases have profound effects on neuronal conduction and on the well-being of the patient. A few of these conditions will be described briefly in the upcoming section on myelin biochemistry. 

One prevalent neurological disease among human adults is multiple sclerosis (MS), usually characterized by spasms and weakness in one or more limbs, bladder dysfunction, local sensory losses, and visual disturbances. This disorder—the prototype demyelinating disease—is caused by patchy loss of myelin in areas of the brain and spinal cord. In MS patients, conduction of action potentials by the demyellnated neurons is slowed, and the... 

JC virus is an interesting case of a neurotropic virus that may bind to cell surface glyco-sphingolipids to enter specific brain cell t5q>es. This polyomavirus is responsible for a fatal demyelinating disease of the central nervous system called progressive multifocal leukoen-cephalopathy. In vivo, JC virus infection has been detected in neurons, astrocytes, and oligodendrocytes. Two main types of cell surface receptors for JC virus have been described ... 

Multiple sclerosis A disease characterised by the progressive demyelination of CNS neurons and the loss of oligodendrocytes. Cannabinoids may be of benefit in treating MS. 

MS is an autoimmune disease that attacks the myelin sheath of oligodendrocytes around the neuronal axons. This allows the axonal cytoskeleton to be damaged, bringing about secondary axonal loss and persisting neurological dysfunction. The characteristic pathology is of a lesion or plaque in the CNS white matter, formed by inflammation and demyelination and these can be classified into active, chronic active, or chronic silent plaques [86]. 

In addition to the effect of increased VLCFA on membrane and possibly cellular function, the rapid cerebral form of X-ALD is characterized by an inflammatory response that is believed to contribute to the demyelination that characterizes this phenotype and which is similar to that seen in multiple sclerosis. These cerebral lesions are characterized by breakdown in myelin with sparing of the axons accompanied by the accumulation of cholesterol ester in the neurons. A perivascular inflammatory response with infiltration of T cells, B cells, and macrophages also is present. Therefore, it is believed that the rapid cerebral disease has an im-munologically-mediated component. It has been suggested that the inflammatory response occurs in response to the elevated levels of VLCFA in lipids, which elicits an inflammatory cascade that may be mediated in part by cytokines. Once this cascade begins, it may be more difficult to intervene in the disease process, and in general therapeutic interventions studied to date have been most effective when initiated early. Therefore, prevention of the initiation of the immune response is important for improving outcome. 

MS is a chronic neurologic disease associated w ith demyelin-ation and death of neuronal axons. Multiple plaques or scares form w ithin the CNS resulting in a w ide range of neurological deficits. Because of the role of demyelination in the disease, it is frequently classified w ith autoimmune disorders. Information on the neuroimmunologic aspects of the various types of MS are covered in a previous chapter. 

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