Nerve cell myelin

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 transport of information from sensors to the central nervous system and of instructions from the central nervous system to the various organs occurs through electric impulses transported by nerve cells (see Fig. 6.17). These cells consist of a body with star-like projections and a long fibrous tail called an axon. While in some molluscs the whole membrane is in contact with the intercellular liquid, in other animals it is covered with a multiple myeline layer which is interrupted in definite segments (nodes of Ranvier). The Na+,K+-ATPase located in the membrane maintains marked ionic concentration differences in the nerve cell and in the intercellular liquid. For example, the squid axon contains 0.05 MNa+, 0.4 mK+, 0.04-0.1 m Cl-, 0.27 m isethionate anion and 0.075 m aspartic acid anion, while the intercellular liquid contains 0.46 m Na+, 0.01 m K+ and 0.054 m Cl-. 

Figure 7.1 Cross-sectional view of the spinal cord. In contrast to the brain, the gray matter of the spinal cord is located internally, surrounded by the white matter. The gray matter consists of nerve cell bodies and unmyelinated intemeuron fibers. This component of the spinal cord is divided into three regions the dorsal, lateral, and ventral horns. The white matter consists of bundles of myelinated axons of neurons, or tracts. Each segment of the spinal cord gives rise to a pair of spinal nerves containing afferent and efferent neurons. Afferent neurons enter the spinal cord through the dorsal root and efferent neurons exit it through the ventral root.

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 anaemia in B deficiency is caused by an inability to produce sufficient of the methylating agent S-adenosyhnethionine. This is required by proliferating cells for methyl group transfer, needed for synthesis of the deoxythymidine nucleotide for DNA synthesis (see below and Chapter 20). This leads to failure of the development of the nucleus in the precursor cells for erythrocytes. The neuropathy, which affects peripheral nerves as well as those in the brain, is probably due to lack of methionine for methyl transfer to form choline from ethanolamine, which is required for synthesis of phosphoglycerides and sphingomyelin which are required for formation of myelin and cell membranes. Hence, the neuropathy results from a... 

Biological membranes consist of lipids, proteins, and carbohydrates (see p. 214). These components occur in varying proportions (left). Proteins usually account for the largest proportion, at around half. By contrast, carbohydrates, which are only found on the side facing away from the cytoplasm, make up only a few percent. An extreme composition is seen in myelin, the insulating material in nerve cells, three-quarters of which consists of lipids. By contrast, the inner mitochondrial membrane is characterized by a very low proportion of lipids and a particularly high proportion of proteins. 

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. 

Handlers of methyl methacrylate cement have developed paresthesia of the fingers. Dental technicians who use bare hands to mold methyl methacrylate putty had significantly slower distal sensory conduction velocities from the digits, implicating mild axonal degeneration in the area of contact with methyl methacrylate. The toxic effect on the nervous tissues may be due to diffusion into the nerve cells causing lysis of the membrane lipids and destruction of the myelin sheath. 

Assists in growth and repair of body tissues, and to maintain myelin sheaths that protect nerve cells... 

One problem with long-term inhalant abuse is that it can break down myelin in the body, according to a NIDA report. Myelin is a fatty tissue that surrounds many of the body s nerve cells called neurons. The nerve cells in the brain and spinal cord are like a command central for the body. They transmit messages that control just about everything the body does. If the myelin breaks down, the nerve cells may not be able to transmit messages. 

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. 

D. F., et al. (2004) Restricted growth of Schwann cells lacking Cajal bands slows conduction in myelinated nerves. Nature 431, 191-195. 

Nerve cells Nerve cells, or neurons, consist of a cell body from which the dendrites and axon extend. The dendrites receive information from other cells the axon passes this information on to another cell, the post-synaptic cell. The axon is covered in a myelin membranous sheath except at the nodes of Ranvier. The axon ends at the nerve terminal where chemical neurotransmitters are stored in synaptic vesicles for release into the synaptic cleft. 

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. 

The structure of a nerve is not simple. In the following account, the stress is upon a single aspect of the mechanism of the action of a nerve, the origin of the spike potential in sections of the nerve called nodes in which the axon is in contact on the outside with the extracellular fluids. The relevant properties of a nerve cell free of a myelin sheath can be seen in Table 14.1. 

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