Pain sends a signal that the body needs protection and healing. However, if the physiological changes triggered by pain persist, harm will ensue, and acute pain may become chronic, so pain must be contained and/or relieved. The mechanisms through which pain interacts with the body provide health professionals with various routes of entry and modes of intervention. This article discusses the intricacies of the adaptive response to pain and how they can be used to combat pain.
Citation: Swift A (2018) Understanding pain and the human body’s response to it. Nursing Times[online]; 114: 3, 22-26
Author: Amelia Swift is senior lecturer in nursing, University of Birmingham. She has updated a 2003 article, Understanding the physiological effects of unrelieved pain, written by Carolyn Middleton, clinical nurse specialist at Gwent Healthcare Trust.
Pain, which is caused by an unpleasant (noxious) stimulus, is a stressor that can threaten homoeostasis. The body’s adaptive response to pain involves physiological changes, which are useful and potentially life-saving in the initial stages. If the adaptive response persists, harmful and life-threatening effects may ensue. Pain is noxious, which makes it a powerful protective force: indeed the inability to feel pain is associated with a shortened life expectancy (Shin et al, 2016). After injury, pain encourages us to adopt behaviours that help the healing process; for example, resting the painful part of the body. This article describes the physiological response to pain, its clinical relevance and its wide-ranging effects on the body. It also explains how nurses can provide effective pain relief to their patients.
Transmission of pain
The initial physiological changes taking place in the body after a pain stimulus are concerned with the transmission of pain, which involves four stages: transduction, transmission, perception and modulation.
During transduction, the pain stimulus is transformed into a nerve impulse. Receptors on the surface of the nerve endings, called nociceptors, respond to noxious stimuli, which can be thermal (temperature above 40°C), mechanical (extreme pressure over a small area) or chemical (strong acid or alkali).
The stimulus interacts with receptors, causing chemical changes that lead the nerve to create an electrical signal (action potential). The sensory nerve fibre will only create an action potential if the stimulus is strong enough. A large stimulus creates a higher frequency of action potentials, which is eventually perceived as more severe pain.
The stimulus causes the nerve and nearby mast cells to release chemical pain mediators such as prostaglandin, bradykinin, serotonin, substance P and histamine, which:
- Activate more receptors on the nerve fibre, increasing the likelihood that the threshold for an action potential will be reached – this is called primary sensitisation;
- Provoke changes in the walls of local blood vessels, increasing blood supply and allowing white cells to move into the extracellular fluid – this is the inflammatory response, an essential part of healing.
Pain can be alleviated by reducing the sensitization and activation of nerve endings; for example, non-steroidal anti-inflammatory drugs (NSAIDs) can inhibit the production of prostaglandin, one of the main sensitizing mediators, while opioids can make it harder for the nerve to create an action potential. Precautions must be taken with both NSAIDs and opioids (Box 1)
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