Neurons myelin sheath

Myelin A protein and phospholipid sheath that surrounds the axons of certain neurons. Myelinated nerves conduct impulses more rapidly than nonmyelinated nerves. 

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.

Oligodendrocytes are present in the CNS as well and wrap around axons to form a myelin sheath. Myelin wraps into concentric layers that spiral around the axon. Gaps in the oligodendrocytes are the nodes of Ranvier, where the membrane maintains contact with extracellular fluid. The nodes serve to propagate the action potential in myelinated axons. Schwann cells perform an analogous function, myelinating axons in the peripheral nervous system. Not all neurons are myelinated, but myelination increases the metabolic efficiency of action potentials. Demyelination of neurons produces deficits in neuronal conduction, as is seen in multiple sclerosis. 

Figure 14.2 A myelinated axon. Some neurones lack the myelin sheath. (See below for description of the myelin sheath.) The cell body may be elsewhere other than at the end of the axon.

Figure 14.4 Different types of glial cells. Astrocytes connect capillaries and neurones. Fibrous astrocytes, with less branching and more filamentous processes, occur mainly in white matter while protoplasmic astrocytes are located principally in the grey matter. Oligodendrocytes form the myelin sheath by wrapping themselves around axons. The connection between the myelin sheath and the oligodendrocyte is permanent and provides material for the myelin sheath. Microgliocytes (microglia) are the phagocytes of the nervous system. The ciliated ependymal cells line the cavities of the central nervous system.

Nerve cells (neurons) are easily excitable cells that produce electrical signals and can react to such signals as well. Their structure is markedly different from that of other types of cell. Numerous branching processes project from their cell body (soma). Neurons are able to receive signals via dendrites and to pass them on via axons. The axons, which can be up to 1 m long, are usually surrounded by Schwann cells, which cover them with a lipid-rich myelin sheath to improve their electrical insulation. 

The myelin sheath forms by extension of the plasma membrane of neurons (Schwann cells) that wraps tightly many times around the extended cytoplasm. 

The lipid nature of the myelin sheath makes it water- and ion-imper-meant, and hence insulates the neuron to permit transfer or propagation of the electrical impulse. 

The neuronal cytoskeleton provides the axon with mechanical support and is directly involved in the transport of materials from the cell body towards the synapse (anterograde transport) and in the opposite direction (retrograde transport). Axons are generally covered (insulated) with a myelin sheath, which is formed by oligodendrocytes (in the CNS) or Schwann cells (PNS). 

It should be noted that the myelin sheath helps to propagate the AP even faster down the neuron. The signal will "jump" from one node of Ranvier to the next (saltatory motion). The distribution of sodium and potassium channels is uneven in the myelinated regions, appropriate to the node and covered areas of the nerve. Should the myelin become stripped from a normally myelinated cell, then the electrical signal cannot... 

The relative proportions of protein and lipid vary with the type of membrane (Table 11-1), reflecting the diversity of biological roles. For example, certain neurons have a myelin sheath, an extended plasma membrane that wraps around the cell many times and acts as a passive electrical insulator. The myelin sheath consists primarily of lipids, whereas the plasma membranes of bacteria and the membranes of mitochondria and... 

The 216-residue hen egg yolk storage protein phosvitin contains 123 serine residues, most of which have been phosphorylated (Eq. 2-16).295 A basic protein of the myelin sheath of neurons contains as many as 6 specific residues of citrulline (Eq. 2-19).296 An adhesive protein from the foot of a marine mollusk contains -80 repeated sequences containing hydroxy-proline 2,3-dihydroxyproline and 3,4-dihydroxyphenylalanine (Dopa) 297/298... 

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... 

The cell body of a mammalian motor neuron has an extended process termed an axon that branches at its tip to make multiple contacts (synapses) with a muscle cell (fig. S1.1). Some axons actually reach lengths of several meters. Much of the axon may be encased in myelin sheaths, which are multilayered membranes formed by other cells that wrap themselves around the neuron. Shorter processes (dendrites) extend from the cell body to make contacts with other neurons. If a motor neuron is stimulated electrically or is triggered by its connecting neurons, an electric signal called an action potential sweeps down the axon and is transmitted to the muscle cell, which then proceeds to contract. Other... 

Genes coding for APO-E are associated with different risks for Alzheimer s disease. There are three alleles (or copies) of the gene coding for this apolipoprotein which are called E2, E3, and E4. For example, a gene on chromosome 19 that codes for APO-E is linked to many cases of late-onset Alzheimer s disease. Moreover, APO-E is associated with cholesterol transport and involved with other neuronal functions, including repair, growth, and maintenance of myelin sheaths and cell membranes. 

Oligodendroglia One of the three types of glia cells responsible for creating the myelin sheath surrounding neurons in the CNS. 

Neurotoxins may selectively attack neurons or even specific kinds of neurons. This can cause injury to the neurons. In severe cases, the neuron cells are killed, leading to irreversible loss of the neuron and associated dentrites, axons, and the insulating myelin sheathing around the axons. 

In addition to neurons and glial cells, the nervous system contains blood vessels, fibroblasts, and other connective tissue elements. In the PNS, processes from Schwann cells that form the multilayered myelin sheaths characteristic of peripheral myelinated nerves surround most neuronal elements. 

Schwann cells Large nucleated cells that wrap around myelinated peripheral neurons to form the myelin sheath. 

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]. 

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