Physiology and pain characteristics In most cases, pain is elicited by the activation of numerous free nerve endings in the body called nociceptors , which are stimulated by thermal, mechanical, or chemical factors (Basbaum, Bautista, Scherrer, & Julius, 2009). These nerve endings are located within the skin, joint surfaces, and bone periosteum. When the nociceptors are stimulated, nerve impulses carry their signal to the spinal cord and up to the brain, which may interpret the impulses as pain. Figure 1 shows the pathway that pain takes from the site of injury up to and through the brain. However, not all pain is localized to the area of injury, and the pain a client feels may seem to be in a location that is different from the site of the actual tissue damage; this phenomenon is called referred pain . For example, a heart attack can produce pain in the jaw, neck, or left arm, or may manifest as a toothache or even sinusitis. Also, referred pain elicited from myofascial trigger points in head and neck/shoulder muscles can be reproduced and felt in the orofacial region in women with myofascial temporomandibular pain or fibromyalgia syndrome (Alonso-Blanco et al., 2012). Many factors can alter the transmission of nerve impulses to the brain, including the actions of neurotransmitters, neuromodulators, and opioid receptors (McGuire, 2006). Neurotransmitters are the chemical messengers that send a message from one nerve cell to another. The neurotransmitters include norepinephrine, epinephrine, serotonin, and acetylcholine. Neurotransmitters are able to produce pain on their own. Neuromodulators act to enhance or dampen neuronal activities; examples of neuromodulators are endorphins and enkephalins, which act to depress pain by interfering with neural transmission (McGuire, 2006). Opioid receptors are usually located in the brain and the superficial dorsal horn of the spinal cord regions, where the sensory nerve fibers are found. Certain types of opioid receptors have a high affinity for natural enkephalins and endorphins, and they also bind with medications such as morphine and codeine to diminish the pain response. Processes in the brain trigger the personal response to recognizing pain. Three areas of the brain are prominently involved in processing pain signals: the thalamus, the midbrain, and the cerebral cortex. The thalamus, where nociception is perceived in the brain, acts as a relay station and sends the nerve signal to the cerebral cortex. The midbrain sends messages to the cerebral cortex concerning awareness of the stimuli, and the cortex interprets the pain experience and determines if it is localized (McGuire, 2006). Pain perception The sensation of pain is not the same for everyone, and it is not the same from hour to hour or from day to day. In order for pain to be perceived, nerve signals must reach the brain for interpretation and integration. Many factors affect how clients feel pain and the degree to which they perceive its intensity and duration. Sometimes this has to do with environmental and emotional circumstances. In cases of severe battle injuries or sports mishaps, the person may not feel pain until much later. In these cases, descending inhibitory systems modulate or limit nociceptive input. This response is thought to have an evolutionary advantage because it enables the person to ignore pain in critical situations. There is an adrenalin response (fight or flight), which overrides the perception of pain in that moment (Kuner, 2010; Merck Sharp & Dohme Corp., 2011). Louw (2013) illustrates this phenomenon with the example of an individual who sprains his ankle while stepping from a curb into the street. The person would feel the pain immediately; however, if he or she simultaneously saw a bus coming at a high speed, the person would spring to safety before feeling the discomfort in the injured ankle. Gender differences may exist with respect to the perception of pain (Aufiero, Stankewicz, Quazi, Jacoby, & Stoltzfus, 2017).
Figure 1: The Pain Pathway
Note. From Berger, T. (2013). Feeling pretty remarkable: Preventing chronic pain . http://www.feelingprettyremarkable.com/blog/ preventing-chronic-pain. Reprinted with permission.
The reasons for these gender differences may include genetic factors, hormonal effects, differences in anatomy and physiology, psychiatric comorbidities such as depression, psychosocial factors (roles, coping strategies), and differences in the response to drugs and other treatments (Schopper, Fleckenstein, & Irnich, 2013). For some pain problems, however, the clinician’s gender stereotypes, as well as the clinician’s gender, seem to have some influence on how he or she perceives and treats an individual’s pain (Bartley et al., 2015; Fjellman-Wiklund, Wiklund, Hammarström, Stålnacke, & Lehti, 2016). Sociocultural factors affecting pain include ethnicity, family history, and cultural factors (Al-Harthy, Ohrbach, Michelotti, & List, 2016; Shavers et al., 2010). Culture has much to do with self-image and the desire and ability to express emotions. Pride, embarrassment, or shame may prevent a client from acknowledging or expressing pain. Self-image plays a role in a client’s pain perception and expression; a person who thinks of himself or herself as being proud and tough or having a “stiff upper lip” may mask verbalizations and postures that signify pain. To this type of person, pain may imply vulnerability and a loss of personal control.
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