F PGs and the Pain Mechanism

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The pain mechanism is essential for survival, since acute pain is a warning mechanism for threatening conditions. Chronic pain is a more complicated subject, but both conditions may be grouped together for the purposes of our discussion. Relatively little is known about the pain mechanism.

The nociceptors, when excited by potentially harm figure 16-12 Schematic diagram of the three-dimensional structure of soybean lipoxygenase-1. The a-helices are represented by cylinders, the strands in the /3-sheets by arrows, the coils by narrow rods, and the iron by a ball. Only three of the four iron ligands are shown. Domain I consisting of a single ¡6-barrel is on the left. Domain II contains the iron, all of the helices in the structure, and two small /3-sheet structures that lie on the surface of the enzyme. Reproduced as in black and white from a colored figure from Boyington, J. C., Gaffney, B. J., and Amzel, L. M. (1993). The three-dimensional structure of an arachidonic acid 15-lipoxygenase. Science 260, 1482-1486.

figure 16-12 Schematic diagram of the three-dimensional structure of soybean lipoxygenase-1. The a-helices are represented by cylinders, the strands in the /3-sheets by arrows, the coils by narrow rods, and the iron by a ball. Only three of the four iron ligands are shown. Domain I consisting of a single ¡6-barrel is on the left. Domain II contains the iron, all of the helices in the structure, and two small /3-sheet structures that lie on the surface of the enzyme. Reproduced as in black and white from a colored figure from Boyington, J. C., Gaffney, B. J., and Amzel, L. M. (1993). The three-dimensional structure of an arachidonic acid 15-lipoxygenase. Science 260, 1482-1486.

ful stimuli, cause pain by way of their afferent nerve fibers (Figure 16-16). Probably every organ in the body contains these receptors. There are two classes of nociceptive afferents: those found among thin myelinated AS fibers and those among the nonmyelinated C fibers. The AS fibers are associated with sharp, focused pain, whereas the C fibers are associated with dull, burning, diffuse pain.

Chemical substances excite nociceptors or sensitize them to other stimuli (Figure 16-16A), resulting in the generation of pain. Endogenous substances in this category are PGE and bradykinin. These are referred to as "algesic substances" or agents producing pain. Substance P is considered in this category as it is thought to be a transmitter of the pain signal being released from the nociceptor (afferent) nerve terminal. Nociceptor firing occurs at levels of stimuli below those producing a frank sensation of pain; thus, these impulses probably summate spatially and temporally in the central ner vous system before pain is perceived. Nociceptor firing rate and perception of pain both increase with the strength of the stimulus. PGs and other algesics apparently can cause pain when applied to the skin of humans. PGE2 can lower the threshold of nociceptors to other stimuli, such as heat. It seems clear that PGEs operating through a receptor to stimulate adenylate cyclase and the level of cyclic AMP have a direct relation to the generation of pain. A speculative overall scheme tying together a number of observations is presented in Figure 16-17.

Pain is produced by an irritant, trauma, or nervous stimulation causing the release of fatty acids from membrane phospholipids, and PGEs are formed by the PG synthetase system. These dissolve in a cellular membrane and bind to the PGE receptor, which causes the stimulation of adenylate cyclase to form elevated amounts of cyclic AMP. It is not clear whether cyclic AMP is actually required for the production of pain or

Arachidonic acid

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