Michigan: Pain and Its ManagementPage 4 of 11

3. The Physiology of Pain

The International Association for the Study of Pain (IASP) describes pain as “an unpleasant sensory and emotional experience associated with actual or potential tissue damage or described in terms of such damage.” Pain can be acute or chronic and, if left untreated, chronic pain can develop into what is referred to as a chronic pain syndrome.

Pain occurs when sensory nerve endings called nociceptors (also referred to as pain receptors) come into contact with a painful or noxious stimulus. The resulting painful impulse travels from the sensory nerve ending, enters the dorsal spinal cord, and travels to diverse parts of the brain via nerve tracts in the spinal cord and brainstem. The brain processes the pain sensation and quickly makes a motor response in an attempt to cease the action causing the pain (IASP, 2017; NCI, 2017). The sensory process of detecting the “actual or potential tissue damage” is called nociception (Kyranou & Puntillo, 2012).


Sensitization is a neurophysiologic response in which the pain pathways become more sensitive. This can include a drop in the threshold for activating nociceptors and an increase in the frequency of firing for all stimuli (IASP, 2017).

Hyperalgesia (exaggerated responses to stimuli) and allodynia (in which a stimulus not normally painful is perceived as painful) are clinical markers used to detect the presence of sensitization (IASP, 2017; Zouikr et al., 2016). There are two types of sensitization.

Peripheral sensitization occurs in response to the release of inflammatory molecules such as histamine, prostaglandins, and pro-inflammatory cytokines. These substances sensitize nociceptors by creating an “inflammatory soup” that enhances pain sensitivity by reducing the threshold of nociceptor activation (IASP, 2017; Zouikr et al., 2016). Under normal circumstances, peripheral hypersensitivity returns to normal when inflammation subsides or the source of the injury is removed (Kyranou & Puntillo, 2012).

In central sensitization, nociceptive-specific neurons progressively increase their response to repeated non-painful stimuli, develop spontaneous activity, and increase the area of the body that is involved with the pain. The hyperalgesia of central sensitization usually develops as part of ongoing pathology, such as damage to peripheral or central nerve fibers, cancer, or rheumatoid arthritis, and is considered maladaptive (IASP, 2017; Kyranou & Puntillo, 2012).

Acute Pain

Acute pain comes on quickly and, although it can be severe, lasts a relatively short time (IOM, 2011). Its location is usually well-defined and there is usually an identifiable painful stimulus related to an injury, brief disease process, surgical procedure, or dysfunction of muscle or viscera. Acute pain is often successfully treated with patient education, mild pain medications, environmental changes, and stress reduction.

The Institute of Medicine (IOM) has targeted improved treatment of acute pain as an area of significant healthcare savings. Better treatment of acute pain, through education about self-management and better clinical treatment, can avoid its progression to chronic pain, which is more difficult and more expensive to treat (IOM, 2011).

Chronic Pain

Chronic pain refers to pain that exists for three or more months and does not resolve with treatment. The three-month time frame is not absolute and some conditions may become chronic in as little as a month. When pain becomes chronic, sensory pathways continue to transmit the sensation of pain even though the underlying condition or injury that originally caused the pain has healed. In such situations, the pain itself may need to be managed separately from the underlying condition.

Chronic pain can be difficult to distinguish from acute pain and can be difficult to treat. Chronic pain does not resolve quickly and opioids or sedatives are often needed for treatment. Because medical practitioners frequently approach chronic pain management from a medication perspective, other modalities are sometimes overlooked. Chronic pain is a silent epidemic that reduces quality of life, negatively impacts relationships and jobs, and increases rates of depression (Sessle, 2012).

Chronic pain affects 1 in 5 adults, is more prevalent among women and older adults, and is associated with physically demanding work and a lower level of education (King & Fraser, 2013). Chronic pain is also a symptom of many diseases. Up to 70% of cancer patients suffer from chronic pain, and among individuals living with HIV/AIDS, pain has been reported at all stages of infection (Lohman et al., 2010).

Musculoskeletal pain, especially joint and back pain, is the most common type of chronic pain (IOM, 2011). Although musculoskeletal pain may not correspond exactly to the area of injury, it is nevertheless commonly classified according to pain location. However, most people with chronic pain have pain at multiple sites (Lillie et al., 2013).

What Is Chronic Pain? [2:31]