Myelin sheaths

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

For the pathogenesis of multiple sklerosis, autoimmune T-lymphocy tes play a predominant role, which are directed against components of the neural myelin sheath. T-lymphocy tes by secreting cytokines such as interferon y maintain the chronic inflammation which destructs the myelin sheath. Also cytotoxic T-lymphocytes may participate directly. The cause of multiple sklerosis is unknown. Significantly increased antibody titers against several vitusses, mostly the measles virus, point to a (latent) virus infection initiating the disease. 

Multiple scelerosis is an autoimmune disease mediated by T and B lymphocytes and macrophages. This is characterized by extensive inflammation and demyelination of the myelin sheath that surrounds the nerve fiber. The death of the nerve fiber results in a variety of symptoms that can lead to impairment of movement, paralysis, and death. 

Component of the myelin sheath surrounding the axons of nerve cells. Additional compounds of the myelin sheath are phospholipids, cholesterol, cerebrosides, and specific keratins. The myelin sheath constitutes an isolating barrier during electrophysiological axonal signaling. 

The nodes of Ranvier are the gaps formed between myelin sheath cells along the axons. The sodium channels are densely localized at the nodes of Ranvier. 

Few studies have focused on the pathological changes in the spinal cord associated with HIV neuropathy. Selective degeneration of the gracile tract in patients with sensory neuropathy, characterized by loss of axons and myelin sheaths in the... 

Without ion flux, action potentials cannot be generated in the regions covered with myelin. Instead, they occur only at breaks in the myelin sheath... 

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.

Node of Ranvier The small gap between adjacent segments (internodes) of the myelin sheath. 

Myelinating oligodendrocytes have been studied extensively [5,30] (see Ch. 4). Examination of the CNS during myelinogenesis (Fig. 1-15) reveals connections between the cell body and the myelin sheath [31] however, connections between these elements have never been demonstrated in a normal adult animal, unlike the PNS counterpart, the Schwann cell. In contrast to the Schwann cell (see below), the oligodendrocyte is capable of producing many internodes of myelin simultaneously. It has been estimated that oligodendrocytes in the optic nerve produce... 

FIGURE 1-15 A myelinating oligodendrocyte, nucleus (N), from the spinal cord of a 2-day-old kitten extends cytoplasmic connections to at least two myelin sheaths arrows). Other myelinated and unmyelinated fibers at various stages of development, as well as glial processes, are seen in the surrounding neuropil. Xl2,750. 

FIGURE 1-19 A myelinated PNS axon (A) is surrounded by a Schwann cell nucleus (N). Note the fuzzy basal lamina around the cell, the rich cytoplasm, the inner and outer mesaxons (arrows), the close proximity of the cell to its myelin sheath and the 1 1 (celhmyelin internode) relationship. A process of an endoneurial cell is seen (lower left), and unstained collagen (c) lies in the endoneurial space (white dots). X20,000. 

Bunge, R. P. Glial cells and the central myelin sheath. Physiol. Rev. 48 197-248,1968. 

Myelin sheaths contain other proteins, some of which have only recently been established as myelin-related 65... 

FIGURE 4-2 Light micrograph of a 1 pm Epon section of rabbit peripheral nerve (anterior root), stained with toluidine blue. The myelin sheath appears as a thick black ring around the pale axon. (Courtesy of Dr Cedric Raine.)... 

FIGURE 4-4 Electron micrograph of a single peripheral nerve fiber from rabbit. Note that the myelin sheath has a lamellated structure and is surrounded by Schwann cell cytoplasm. The outer mesaxon (arrowhead) can be seen in lower left. AX, axon. (Courtesy of Dr Cedric Raine.)... 

FIGURE 4-7 A typical CNS myelinated fiber from the spinal cord of an adult dog. Contrast this figure with the PNS fiber in Figure 4-4. The course of the flattened oligodendrocytic process, beginning at the outer tongue (arrow), can be traced. Note that the fiber lacks investing cell cytoplasm and a basal lamina-as is the case in the PNS. The major dense line and the paler, double intraperiod line of the myelin sheath can be discerned. The axon contains microtubules and neurofilaments. 

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). 

Before myelination the axon lies in an invagination of the Schwann cell (Fig. 4-10A). The plasmalemma of the cell then surrounds the axon and joins to form a double membrane structure that communicates with the cell surface. This structure, called the mesaxon, then elongates around the axon in a spiral fashion (Fig. 4-10). Thus, formation of myelin topologically resembles rolling up a sleeping bag the mesaxon winds about the axon, and the cytoplasmic surfaces condense into a compact myelin sheath and form the major dense line. The two external surfaces form the myelin intraperiod line. 

FIGURE 4-10 Myelin formation in the peripheral nervous system. (A) The Schwann cell has surrounded the axon but the external surfaces of the plasma membrane have not yet fused in the mesaxon. (B) The mesaxon has fused into a five-layered structure and spiraled once around the axon. (C) A few layers of myelin have formed but are not completely compacted. Note the cytoplasm trapped in zones where the cytoplasmic membrane surfaces have not yet fused. (D) Compact myelin showing only a few layers for the sake of clarity. Note that Schwann cell cytoplasm forms a ring both inside and outside of the sheath. (Adapted with permission from Norton, W. T. The myelin sheath. In E. S. Goldensohn and S. H. Appel (eds), Scientific Approaches to Clinical Neurology. Philadelphia Lea Febiger, 1977, pp. 259-298.)... 

Myelin affects axonal structure. The presence of a myelin sheath affects the structure of the axon that it surrounds [5], presumably optimizing its properties for transmission of action potentials by saltatory conduction. Generally, one of the effects of myelin is to increase axonal diameter by inducing biochemical changes in components of the axonal cytoskeleton such as neurofilaments (see Ch. 8). The effects of myelin on axonal structure imply... 

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