How Pain Works

Pain can accompany injury or disease, but its intensity does not measure damage directly. Clinicians respect the experience while assessing timing, function, associated symptoms and possible mechanisms.

Pain is a personal experience Pain is an unpleasant sensory and emotional experience linked with actual or possible tissue damage, or resembling that experience. Pain is always personal. Biological, psychological and social factors can influence it, which is called a biopsychosocial view. This does not make pain imagined or voluntary. A person's report of pain should be respected. People who cannot communicate verbally may also experience pain. Nociception is not the same as pain A noxious stimulus is an event that damages or threatens normal tissue. Heat, pressure, chemicals and inflammation can provide this type of input. A nociceptor is a specialised sensory receptor that detects and encodes noxious stimuli. The nervous system processing of this information is called nociception. Nociception can trigger withdrawal, changes in heart rate or other protective responses without a conscious pain experience. Pain cannot be inferred from nerve activity alone. Pain can also persist when clear ongoing tissue damage is not found. This does not mean that the experience is unreal. Pain involves a distributed nervous system network Nociceptive information can travel through peripheral nerves into the spinal cord and then through several brain pathways. There is no single pain centre. Processing occurs in spinal, brainstem, thalamic, cortical and deeper brain networks. These networks contribute to location, intensity, unpleasantness, attention and behavioural responses. Pain modulation means that nervous system activity can increase or reduce the experience. Modulation can occur in tissues, the spinal cord and descending pathways from the brain. Attention, expectation, past experience, sleep, mood and perceived threat can influence pain modulation. These influences are genuine parts of physiology, not evidence against physical illness. Gate control is a useful but limited model Gate control theory proposed that spinal circuits regulate how nociceptive information is transmitted. It helped replace the idea of a fixed pain wire. The gate is a metaphor rather than a literal door. Modern research describes interacting excitatory, inhibitory and descending circuits rather than one simple switch. Touch or rubbing can reduce pain for some people through spinal and brain mechanisms. The effect varies and does not prove that the underlying injury is minor. Heat and massage may help selected pain problems through several mechanisms. Their benefit cannot be explained or guaranteed by gate control alone. Different pain mechanisms can overlap Nociceptive pain arises when actual or threatened damage to non neural tissue activates nociceptors. Examples can include a burn, sprain or inflamed joint. Neuropathic pain is caused by a lesion or disease of the somatosensory nervous system. Diabetes, shingles, stroke or nerve injury can sometimes produce it. Burning, electric, shooting or tingling descriptions can support neuropathic pain assessment. These words do not confirm a nerve lesion or disease by themselves. Nociplastic pain arises from altered nociception when clear tissue damage or a somatosensory lesion does not adequately explain the pain. The term describes a mechanism, not imaginary pain. Nociceptive, neuropathic and nociplastic mechanisms can coexist and change over time. A person does not always fit into one permanent category. Sensitivity can change Sensitisation means increased responsiveness within nociceptive pathways. It can develop in peripheral tissues, the spinal cord or higher nervous system networks. Hyperalgesia means increased pain from a stimulus that normally causes pain. Allodynia means pain from a stimulus that does not usually cause pain. These findings describe what happens during testing. They do not identify one mechanism or diagnosis without the wider clinical pattern. Pain intensity is not a damage meter Pain often accompanies injury, inflammation or disease. However, the amount of pain does not provide a direct measurement of tissue damage. A small visible injury can feel extremely painful. A serious problem can sometimes begin with mild, vague or delayed pain. Clinicians therefore consider onset, location, progression, function and associated symptoms. They do not decide safety from a pain score or appearance alone. Pain descriptions provide clues Clinicians ask where pain is felt, when it began, how it behaves and what affects it. They also ask about sleep, activity, work and daily function. Words such as sharp, dull, burning or aching can guide possibilities. No pain quality reliably identifies one tissue, mechanism or diagnosis alone. Questions about stress, beliefs, mood and social circumstances help understand the complete experience. They should never be used to dismiss pain or stop appropriate investigation. Examination and tests investigate causes Examination looks for tenderness, swelling, movement change, weakness, sensory loss, circulation problems and other features. The focus depends on the body area and what the person describes. Blood tests, X rays, scans and nerve tests answer specific questions. They do not measure how much pain a person feels. A normal test does not make pain unreal. An abnormal scan can also show a change that does not fully explain the pain. Clinicians combine the person's report, examination and test limitations. They reassess when the pattern changes or remains unexplained. Acute and chronic describe time, not severity Acute pain begins recently and often accompanies injury, illness or treatment. It can be mild or severe. Chronic pain persists or recurs for more than three months. It can be primary, secondary to another condition, or a combination of both. Chronic pain does not always mean that tissue has fully healed or that sensitisation is the only mechanism. The next lesson develops this distinction. Treatment follows the cause, mechanism and person Pain care may include treating an underlying cause, medicines, movement, rehabilitation, psychological therapies and practical support. The useful combination varies between people. Psychological treatment can change pain related distress, function and coping. Its use does not mean that pain is psychological or imagined. Pain relief and improved function may not change together. Clinicians agree goals and review benefits, harms and uncertainty over time. When to get help Call 999 for pain with severe breathing difficulty, collapse, sudden confusion, possible stroke signs or a serious injury. Do not drive yourself. Call 999 for chest pressure or heaviness that persists, spreads or occurs with sweating, nausea or breathlessness. Call 999 or go to A&E for sudden severe abdominal pain, a very painful abdomen, vomiting blood or black sticky stool. Go to A&E for back pain with new bladder or bowel problems, numbness in the back passage or rapidly worsening limb weakness. Call 999 if safe travel is unavailable. Contact NHS 111 or request urgent GP assessment for severe pain that starts suddenly, worsens quickly or occurs with fever and feeling very unwell. Arrange a GP review when pain persists, repeatedly returns or limits sleep, mobility, work or daily life. New or changed pain still needs reassessment during chronic pain. This lesson explains pain mechanisms and clinical assessment. It cannot identify the cause of an individual pain or replace urgent medical care.

Pain and nociception are different. Pain emerges from interacting biological, psychological and social influences, while its intensity cannot confirm the amount or cause of tissue damage.