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Unformatted text preview: 2010 NPB 12, Note 10, Page: 1 LECTURE 10: Pain, Temperature, and Vision (Part I) Pain and temperature pathways: At this point it is important to define two key terms, pain and nociception. Pain is a subjective quality that will differ among different people. You can imagine that the same injury may cause excruciating pain in one individual, whereas the exact same injury will cause minimal pain in a second individual. Examples are athletes, such as football players, that will experience pain from their injuries, yet are able to endure it and continue to practice and play. Others seem to fall apart from the simplest injuries. Therefore, in order to study pain, scientists have to throw out this subjective definition and use a more quantitative and reliable term. Nociception: This is the perception of a stimulus that, if continued, will cause tissue damage. It doesn't have to cause tissue damage to be nociceptive, for example one can put a heat lamp on the tail of a rat, and if it is left there, it will burn the skin. If the heat lamp (or the tail) is removed, then no burn will occur. This is different than if you just put a light on the tail, which would never burn the tail. It is also different than ultraviolet radiation, or exposure to radioactivity, which is not perceived at the time, but can and does cause sever tissue damage (and hurts a lot afterwards). As we saw earlier, these afferent fibers, again with their cell bodies in the dorsal root ganglion, project their axons to neurons in the dorsal horn of the spinal cord. At the other end of these neurons is usually just a free nerve ending, meaning that there is no obvious structure that could be receiving mechanical stimuli. There are two classes of nociceptors: those with small, myelinated axons and those with small, unmyelinated axons. The small myelinated axons are called delta-fibers. These axons mediate the "fast pain" perceptions and are responsive to temperature and mechanical stimuli. Grabbing a cigarette lighter the wrong way, stepping on a tack, etc., will cause these neurons to fire like crazy. Once the stimulus is removed, they stop firing. The small, unmyelinated axons are called C fibers. These axons mediate the "slow pain" perceptions and their activity result in the perception of slow, aching, burning pain. They are providing the sensation of the toothache, muscle soreness, sunburn, etc. These nociceptor fibers project their axons into the dorsal horn of the spinal cord, where they make synapses of neurons in that region. These dorsal horn neurons, in turn, project to each other, to ventral horn neurons, and some send their axons across the spinal cord to the contralateral (opposite) side, and project up the white matter to the brain. This fiber tract is referred to as the spinothalamic tract....
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This note was uploaded on 11/18/2010 for the course NPB 72121 taught by Professor Hwai-jongcheng during the Spring '10 term at UC Davis.
- Spring '10