Feeling pain occurs when a breach of a normal course of physiological processes in the body that is caused by exposure to harmful factors. Pain signals the need to eliminate or reduce the action of damaging factors. Subjectively, the person perceives the pain as painful feeling, oppressive feeling. Objectively, the pain is accompanied by vegetative reactions (dilated pupils, increased blood pressure, paleness of skin of face and other), the characteristic posture and movement aimed at reducing pain.
Pain occurs when a irritation of sensitive nerve endings, founded in organs and tissues. These include special unencapsulated public receptor education, representing branching endings nerves, excitement from which is transmitted to the Central nervous system with two types of nerve fibers: bezmalinovic nerve fibers carry out pain signals with a speed of 1 to 2 m/s, myelinated nerve fiber - speed 10-45 m/sec. At the level of the spinal cord is the main route of carrying out of painful excitations in the Central nervous system is Spino-thalamic tract, the fibers of which end in the ventral nuclei of the thalamus, which is considered the main structure, forming pain.
In the formation of painful reactions significant role is played by the reticular formation (see) trunk and mid-brain, with wide functional links with other structures of the brain and with extensive zones of its bark. This defines a greater intensity of emotional and physical manifestations of the body when there is pain. The reduction or suppression of activity of the reticular formation using narcotic substances leads to a weakening of the feeling of pain.
The character of the pain depends on the characteristics of the authority and power of the devastating impact. For example pain in a stomach ulcer is significantly different from the pain of renal colic pain at the wound skin is different from a headache. Consequently, differences in the perception of pain is associated not so much with the peculiarities of the distribution of painful excitement on the periphery, but with the features of coverage and its distribution on various structures of the Central nervous system. However, as would be widely painful agitation neither covered nervous system, people can always isolate it to indicate the place, where for the first time, there was pain. This is because any pain inevitably associated with the initiation of various sensitive ways. It is impossible wound in the skin surface without simultaneous excitation tactile receptors in the injury site. The person feels pain in the body, where damaged by these receptors. Accurate localization of pain is difficult in inflammatory lesions of nervous trunks, when the pain can be felt in all organs, innervated from this nerve trunk. When trauma nerve trunks there is a burning painful sensation-causalgia. The pain also has a tendency to wide irradiation in the zone, not innervated by the struck nerve. Many diseases are characterized by pain, which indicates the presence and localization of the process - the so-called symptomatic pain. The place of origin, there are two types of symptomatic pain.
1. Visceral pain appear with the defeat of the pathological process of internal organs (heart, stomach, liver, kidneys and other). These pains are characterized by high intensity wide irradiation; possible so nazwie "reflected pain", when the defeat of internal organ of the pain felt in another part of the body.
2. Somatic pain occur in pathological processes in the skin, bones, muscles, fascia. These precisely localized pain, usually correspond to the location of a pathological focus.
In cases of amputations possible phantom pains in which a person experiences an intense, diverse pain related to an amputated limb.
The reduction or complete elimination of pain is achieved drugs ' effects on pain receptors and nerves, and certain structures of the Central nervous system (see Anesthesia).

Pain is an emotional reaction to the damaging effect. The pain is in the process of evolution has become a powerful means of self-preservation from the damaging effects of the environment. Only on the basis pain as a protective factor organisms can form a defensive reaction and the best way to adapt to conditions of existence. The defensive reaction of the body are based on innate mechanisms of painful emotions, known as the reactions of fear, or reflexes biological caution" (by I. P. Pavlov). On the basis of these congenital conditions and painful reactions that occur in response to irritation, and in the process of individual life are formed conditional defensive reaction that will best help the body to avoid damage effects. The main biological importance pain is that it causes the body to react in a defensive reaction to the harmful effects of much earlier than the localization, shape, type, engine and other features of the damaging effects; the only criterion response are pain that often serves as the main condition of preserving the life of the organism.
However, clinical practice knows of many examples of how the pain of protective reaction becomes pathogenic factor, when the deliverance of the patient from pain is the primary task of the doctor.