Pain can be categorised into the following groups:
- Nociceptive pain
This occurs when specific pain receptors are stimulated, which can sense temperature, vibration, stretch and chemicals released by damaged cells.
This type of pain can be further classified as somatic or visceral. Somatic, or musculoskeletal pain is felt on the skin, in the muscles, joints, bones and ligaments - either as the result of extreme temperature, a stretch or sprain, or a lack of oxygen causing ischemic muscle cramp. Visceral pain is felt internally in the organs and main body cavities such as the thorax (lungs and heart), abdomen (bowels, spleen, liver and kidneys), or the pelvis (ovaries, womb and bladder). This type of pain is much more difficult to isolate than somatic pain as the sensation felt is often more vague.
- Non-nociceptive pain
There are no specific pain receptors in non-nociceptive pain. When a nerve is damaged it becomes unstable and the signals it sends out become confused. The brain then interprets these abnormal signals as pain.
There are two classifications of non-nociceptive pain: neuropathic, or nerve pain and sympathetic pain. Neuropathic pain comes from within the nervous system itself and is commonly referred to as a 'pinched' or 'trapped' nerve. The actual pain may originate from either between the tissues and the spinal cord (in the peripheral nervous system), or between the spinal cord and the brain (central nervous system). Sympathetic pain usually occurs after a fracture or a soft tissue injury of the limbs. The skin and area around the injury usually becomes extremely sensitive.
'Referred pain' happens when the pain is felt a distance from the origin of an injury - such as when pain in the shoulders or neck while experiencing a heart attack. We have known about referred pain for centuries, but we have little solid evidence explaining what actually causes it.
UK/DIF/15/0002d Date of preparation: January 2016