Exam 4: Sensation and Perception

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Answer will include that pain carried by large nerve fibers is sharp, bright, and fast, and seems to come from specific body areas. This is called the body's warning system. Although we may dislike this warning pain, it is usually a signal that the body has been, or is about to be, damaged. Without warning pain, we would be unable to detect or prevent injury. A second type of pain message is carried by small nerve fibers. This type of pain is slower, nagging, aching, widespread, and very unpleasant. It gets worse if the pain stimulus is repeated. This is called the body's reminding system because it reminds the brain that the body has been injured. For instance, lower back pain often has this quality. Sadly, the reminding system can cause agony long after an injury has healed, or in terminal illnesses, when the reminder is useless. Sometimes one type of pain will cancel another. Ronald Melzack's gate control theory suggests that pain messages from the different nerve fibers pass through the same neural "gate" in the spinal cord. If the gate is "closed" by one pain message, other messages may not be able to pass through. Messages carried by large, fast nerve fibers seem to close the spinal pain gate directly. Doing so can prevent slower, "reminding system" pain from reaching the brain. Messages from small, slow fibers seem to take a different route. After going through the pain gate, they pass on to a "central biasing system" in the brain. Under some circumstances, the brain then sends a message back down the spinal cord, closing the pain gates. Melzack believes that gate control theory may explain the painkilling effects of acupuncture. Acupuncture is the Chinese medical art of relieving pain and illness by inserting thin needles into the body. As the acupuncturist's needles are twirled, heated, or electrified, they activate small pain fibers. These relay through the biasing system to close the gates to intense or chronic pain. Studies have shown that acupuncture produces short-term pain relief for about half of the patients tested. However, its ability to cure illness is much more debatable. Gate control theory also helps explain counterirritation, one widely used pain control technique. Pain clinics use it by applying a mild electrical current to the skin. This causes only a mild tingling, but it can greatly reduce more agonizing pain. For more extreme pain, the electrical current can be applied directly to the spinal cord. A person can use counterirritation to control his or her own pain. For instance, if a person is having a tooth filled, he or she could pinch him or herself or dig a fingernail into a knuckle, while the dentist is working. The person should focus attention on the pain he or she is creating and increase it anytime the dentist's work becomes more painful. This strategy may seem strange, but it works.