One of the most common reasons an individual seeks the advice of a physician is because he or she is in pain. The pain was called by Sherrington, “the physical adjunct of an imperative protective reflex.” Painful stimuli generally initiate potent withdrawal and avoidance responses.
Nociceptors & thermo receptors
Pain and temperature sensations arise from unmyelinated dendrites of sensory neurons located around hair follicles throughout the glabrous and hairy skin as well as deep tissue. Impulses from nociceptors (pain) are transmitted via two fiber types. One system comprises thinly militated Aδ fibers (2–5 μm in diameter) which conduct at rates of 12–30 m/s.
Classification of pain
For scientific and clinical purposes, pain is defined by the International Association for the Study of Pain (IASP) as, “an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage.” This is to be distinguished from the term nociception which the IASP defines as the unconscious activity induced by a harmful stimulus applied to sense receptors.
Pain is sometimes classified as fast and slow pain. A painful stimulus causes a “bright,” sharp, localized sensation (fast pain) followed by a dull, intense, diffuse, and unpleasant feeling (slow pain).
The main difference between superficial and deep sensibility is the different nature of the pain evoked by noxious stimuli. This is probably due to a relative deficiency of Aδ nerve fibers in deep structures, so there is little rapid, bright pain. In addition, deep pain and visceral pain are poorly localized, nauseating, and frequently associated with sweating and changes in blood pressure. Pain can be elicited experimentally from the periosteal and ligaments by injecting hypertonic saline into them. The pain produced in this fashion initiates reflex contraction of nearby skeletal muscles. This reflex contraction is similar to the muscle spasm associated with injuries to bones, tendons, and joints.
Irritation of a visceral organ frequently produces pain that is felt not at that site but in some somatic structure that may be a considerable distance away. Such pain is said to be referred to as the somatic structure. Obviously, knowledge of referred pain and the common sites of pain referral from each of the viscera is of great importance to the physician. Perhaps the best-known example is the referral of cardiac pain to the inner aspect of the left arm.
Other examples include pain in the tip of the shoulder caused by irritation of the central portion of the diaphragm and pain in the testicle due to distention of the ureter. Additional instances abound in the practices of medicine, surgery, and dentistry. However, sites of reference are not stereotyped, and unusual reference sites occur with considerable frequency. Cardiac pain, for instance, may be referred to the right arm, the abdominal region, or even the back and neck.
Pain impulses are transmitted via lightly myelinated Aδ and unmyelinated C fibers. Cold receptors are on dendritic endings of Aδ fibers and C fibers, whereas heat receptors are on C fibers.
Pain is an unpleasant sensory and emotional experience associated with actual or potential tissue damage, or described in terms of such damage, whereas nociception is the unconscious activity induced by a harmful stimulus applied to sense receptors.