Pain receptors
Pain is a signal that is transmitted from the injured part of the body to the brain. Most illness causes pain; therefore the pain signal cannot be ignored. Noxious stimuli elicit tissue damage and activated pain receptors. Activation of pain receptors initiates the process that pain is experienced. Pain receptors are free nerve endings presented in the skin, muscles, joints, bone and viscera. They are sensory receptors that sense and detect tissue damage and then transmit the signal through nerves to the brain. There are three types of pain receptors. First type is skin pain receptors which are in the skin. They can sense pinching, cutting, stretching, temperature, chemicals and other mechanical. If the stimulation is prolonged, these receptors are sensitized and respond more widely. Second type is joint receptors in the joint capsules and ligaments. Third type is visceral pain receptors located in internal organs. Silent pain receptors or sleep pain receptors are these receptors that are normally not responding to noxious mechanical stimulation, however they can become responding to mechanical stimulation (They are awakened and no longer sleep any more) during inflammation and after tissue injury. This is because continuous stimulation from the damaged tissue reduces the threshold of these receptors and causes them to be responding again.
Pain receptors respond as soon as the tissues are damaged regardless the causes. Damaged tissues release substances from lysed cells and newly synthesized inflammatory substances at the site of injury such as globulin and protein kinase, arachidonic acid, histamine, nerve growth factors, substance P, potassium, 5HT and acetylcholine, lactic acid etc. Release of these substances reduces the threshold of the pain receptors and this is so called sensitization. If the receptors are sensitised, low level stimulation which was harmless in origin and did not cause any response before tissue damage can activate the pain receptors causing pain sensation. The response is rapid within 15-30 seconds after injury. The redness appears around the injury site caused by vasodilation and this inflammatory response becomes maximal after 5-10 minutes. Lowered pain threshold is seen in the area.
