Nociception is a term that describes the sensation of noxious stimulation that may be interpreted as pain. Nociceptors are nerve cells that reside in the dorsal root ganglion with free nerve endings in the skin, muscles, tendons, ligaments and viscera that relay noxious input to the spinal cord. The predominant nociceptive neurones are known as A delta fibres and C fibres. Nociceptors are sensitive to noxious chemicals such as substance P, cytokines and prostaglandins. Any such irritant excites the nociceptor by binding to a protein receptor on its membrane. This causes positive ions to move into the nociceptor producing an impulse to travel along the nociceptor to the spinal cord, much like dialling the operator will send information along the telephone wire. Therefore the nociceptors job is to pass on information from the peripheries to the central nervous system.
It is important to know that this type of pain, however unpleasant, is a normal and important piece of sensory information. It allows us to remove ourselves form dangerous situations, such as moving our hand away from a flame for example, it allows us to protect the injured area by immobilising it and allows the injured area to heal. This would much more likely have been of benefit during our evolutional development as no sensation of pain or not paying attention to pain might have meant death for our ancestors.
Once the nociceptive information arrives at the dorsal horn of the spinal cord the nociceptor communicates with a second order neurone or central nerve, whose cell body resides in the brain. The peripheral nerve releases a chemical that binds to a receptor on the second order neurone’s membrane. This causes an impulse to travel up to the brain in much the same way the impulse was triggered in the peripheral nerve. This information is then processed in the brain and the perception of pain is the result.
It is safe to say that most tissue damage will heal in 8 – 16 weeks (broken bones, muscle tears, ligament sprains etc…). Some tissues may take longer to fully reach maturation such as intervertebral discs or become as effective as the original tissue was due to poor nutritional status. Similarly if there is chronic inflammation within the tissues such as in rheumatoid arthritis, tissue healing will be of poor quality. However, pain that persists from an injury longer than known tissue healing times is probably not due to tissue damage.
Explain Pain Spiral-bound – 1 Jun 2003 by