Hockenbury Discovering Psychology 5th txtbk

110 CHAPTER 3 Sensation and PerceptionTable 3.3Sensitivity of Different BodyAreas to PainMost SensitiveBack of the kneeNeck regionBend of the elbowSource: Geldard (1972).Least SensitiveTip of the noseSole of the footBall of the thumbpainThe unpleasant sensation of physicaldiscomfort or suffering that can occur invarying degrees of intensity.nociceptorsSpecialized sensory receptors for pain thatare found in the skin, muscles, and internalorgans.substance PA neurotransmitter that is involved in thetransmission of pain messages to the brain.gate-control theory of painThe theory that pain is a product of bothphysiological and psychological factors thatcause spinal gates to open and relay patternsof intense stimulation to the brain,which perceives them as pain.PainFrom the sharp sting of a paper cut to the dull ache of a throbbing headache, a widevariety of stimuli can trigger pain. Pain can be defined as an unpleasant sensory andemotional experience associated with actual or potential tissue damage. As unpleasantas it can be, pain helps you survive. Pain warns you about potential or actual injury,prompting you to pay attention and stop what you are doing. Sudden pain can triggerthe withdrawal reflex—you jerk back from the object or stimulus that is injuringyou. (We discussed the withdrawal reflex and other spinal reflexes in Chapter 2.)Your body’s pain receptors are called nociceptors. Nociceptors are actuallysmall sensory fibers, called free nerve endings, in the skin, muscles, or internal organs.You have millions of nociceptors throughout your body, mostly in your skin(see Table 3.3). For example, your fingertips may have as many as 1,200 nociceptorsper square inch. Your muscles and joints have fewer nociceptors, and your internalorgans have the smallest number of nociceptors.Fast and Slow Pain Systems To help illustrate pain pathways, imagine this scene:Don was trying to close a stuck window in our old house. As he wrapped his lefthand on the top of the window and used his right hand to push down the loweredge, it suddenly came free and slammed shut, jamming his left fingertips betweenthe upper and lower windows. As pain shot through him, he jerked the windowback up to dislodge his mangled fingers, then headed to the kitchen for ice.Don took little comfort in knowing that his injury had triggered two types of nociceptors:A-delta fibers and C fibers. The myelinated A-delta fibers represent the fastpain system. A-delta fibers transmit the sharp, intense, but short-lived pain of theimmediate injury. The smaller, unmyelinated C fibers represent the slow pain system.As the sharp pain subsides, C fibers transmit the longer-lasting throbbing,burning pain of the injury (Hunt & Mantyh, 2001). The throbbing pain carried bythe C fibers gradually diminishes as a wound heals over a period of days or weeks.As shown in Figure 3.11, both the fast A-delta fibers and the slow C fibers transmittheir messages to the spinal cord. Several neurotransmitters are involved in processingpain signals, but most C fibers produce a pain enhancer called substance P.Substance P stimulates free nerve endings at the site of the injury and also increasespain messages within the spinal cord (Rosenkranz, 2007).Most of these messages from C fibers and A-delta fibers cross to the other sideof the spinal cord, then to the brain. The fast pain messages travel to the thalamus,then to the somatosensory cortex, where the sensory aspects of the pain message areinterpreted, such as the location and intensity of the pain. Interestingly, morphineand other opiates have virtually no effect on the fast pain system.In contrast, slow pain messages follow a different route in the brain. From thespinal cord, the slow pain messages travel first to the hypothalamus and thalamus,and then to limbic system structures, such as the amygdala. Its connections to thelimbic system suggest that the slow pain system is more involved in the emotionalaspects of pain. Morphine and other opiates very effectively block painful sensationsin the slow pain system (Lu & others, 2004).Factors That Influence Pain “Gates” There is considerable individual variation inthe experience of pain. When sensory pain signals reach the brain, the sensory informationis integrated with psychological and situational information. According to the gatecontroltheory of pain, depending on how the brain interprets the pain experience, itregulates pain by sending signals down the spinal cord that either open or close pain“gates,” or pathways (Melzack & Wall, 1965, 1996). If, because of psychological, social,or situational factors, the brain signals the gates to open, pain is experienced or intensified.If for any of the same reasons the brain signals the gates to close, pain is reduced.Anxiety, fear, and a sense of helplessness are just a few of the psychological factorsthat can intensify the experience of pain. Positive emotions, laughter, distraction,and a sense of control can reduce the perception of pain. As one example, considerthe athlete who has conditioned himself to minimize pain during competition.The experience of pain is also influenced by genetic factors, social and situational