Nerve fiber myelinated

In the CNS, myelin is formed by oligodendrocytes. This has many similarities but also points of difference with respect to myelination in the PNS. CNS nerve fibers are not separated by connective tissue nor are they surrounded by cell cytoplasm, and specific glial nuclei are not obviously associated with particular myelinated fibers. CNS myelin is a spiral structure similar to PNS myelin it has an inner mesaxon and an outer mesaxon that ends in a loop, or tongue, of glial cytoplasm (Fig. 4-3). Unlike peripheral nerve, where the sheath is surrounded by Schwann cell cytoplasm on the inside and outside (Fig. 4-10), the cytoplasmic tongue in the CNS is restricted to a small... 

A number of OPC are capable of rendering a delayed neurotoxic effect (DNE). This effect becomes apparent gradually, after a certain latent period (usually 14 to 21 days, sometimes 1 to 5 years after the acute poisoning survived) and is characterized clinically by the development of ataxia, muscular weakness, paresis and paralysis of the extremities. Morphologically, it is characterized by fiber de-myelinization of spinal pathways and peripheral nerves. Till present time, near 40,000 cases have been described, when paresis and paralysis developed in human beings as a result of their exposure to OPC (TOCP, mipaphox, chloropyrophos, trichlorfon, etc.) [30-34],... 

Normally, visible brain white matter consists of large tracts of many thousands of myelinated nerve fibers—nerve fibers individually covered by a periodically interrupted fatty sheath that makes rapid conduction of electrical impulses over long distances possible. Rapid conduction of nerve activity to peripheral muscles is necessary for effective muscle movements, especially for movements such as walking. 

Communications mechanisms must be fast enough to enable the control system to respond in a timely manner. Humoral communications cannot be used for fast responses, and many unmyelinated nerve fibers are also too slow for many purposes. The organism that can transmit information and process it the fastest often has a survival and reproductive advantage therefore, some specialized communication means (such as nerve fibers in myelin sheathes) have developed. 

R = / -C H ), in low doses, exhibits the former behavior and is used primarily as an extradural agent in obstetrics. The lowest effective extradural concentration of etidocaine (21, X = CH, R = R = 2H, R = / -C H ), however, shows both adequate sensory and profound motor blockade so that it is useful in surgical situations where maximum neuromuscular blockade is necessary. In an isolated nerve preparation, bupivacaine blocks unmyelinated C fibers which are mainly responsible for pain perception at a much greater extent than the myelinated A fibers which carry motor impulses. It is postulated that absorption of bupivacaine by the vasculature at the site of injection, combined with the slow diffusion of this agent, results in an insufficient amount of the drug penetrating the large A fibers to cause motor conduction blockade. Clinically, motor block can be observed in some procedures. 

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. 

Vitamin B12 is essential to growth, cell reproduction, the manufacture of myelin (which surrounds some nerve fibers), and blood cell manufacture. The intrinsic factor, which is produced by cells in the stomach, is necessary for the absorption of vitamin B12 in the intestine A deficiency of the intrinsic factor results in abnormal formation of erythrocytes because of the body s failure to absorb vitamin B12, a necessary component for blood cell formation. The resulting anemia is a type of megaloblastic anemia called pernicious anemia. 

Figure 41-1. Ratio of protein to lipid in different membranes. Proteins equal or exceed the quantity of lipid in nearly all membranes. The outstanding exception is myelin, an electrical insulator found on many nerve fibers.

When large areas of the membrane are depolarized in this manner, the electrochemical disturbance propagates in wave-like form down the membrane, generating a nerve impulse. Myelin sheets, formed by Schwann cells, wrap around nerve fibers and provide an electrical insulator that surrounds most of the nerve and greatly speeds up the propagation of the wave (signal) by allowing ions to flow in and out of the membrane... 

Exposures of 10 weeks (5 days/week) to 2,500 mg/kg/day trichloroethylene in com oil by gavage resulted in altered myelin thickness in the rat mental nerve, a branch of the trigeminal nerve (Barret et al. 1991). Effects of similar exposures on the rat trigeminal nerve included decreased fiber diameter and altered fatty acid composition in total lipid extracts, indicative of demyelination (Barret et al. 1992). Stronger effects were seen with the trichloroethylene decomposition product dichloroacetylene. 

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.

Ritchie, J. M. Physiological basis of conduction in myelinated nerve fibers. In P. Morell (ed.), Myelin. New York Plenum Press, 1984, pp. 117-146. 

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.)... 

Animals continually exposed to concentrations between 100 and 600 ppm developed signs of peripheral neuropathy after 4-8 weeks in cats, the conduction velocity of the ulnar nerve was less than one-half of normal after exposure for 7-9 weeks. In these animals, histologic examination revealed focal denudation of myelin from nerve fibers with or without axonal swelling. In rats and monkeys, adverse effects on neurophysiological indicators of nervous system integrity were found with 9-month exposures to 100 ppm, 6 hours/day, 5 days/week. MBK neuropathies, however, occurred only after 4-month exposure at 1000 ppm. Four months of intermittent respiratory exposure of rats to 13 00 ppm caused severe symmetric weakness in the hind limbs. ... 

---