In the process of long-term adaptation of living beings to the external environment in the body, a special sensitive nerve endings formed, which convert various types of energy coming from external and internal stimuli into nerve impulses. They were called receptors.
The receptors differ in structure and function. They are present in almost all tissues and organs. Some of them perceive tactile stimuli (feeling of touch, pressure, weight, etc.), others - thermal (feeling of heat, cold, their combination), the third - chemical (effect of various chemicals), etc..
The simplest device has pain receptors. Painful feelings are perceived through the free ends of the sensitive nerve fibers. The pain receptors of the head are no different from the pain receptors in other parts of the body.
Pain receptors are not evenly distributed in different tissues and organs. Most of them are located in the fingertips, face and mucous membranes. Considerably equipped with pain receptors of the vascular walls, tendons, brain membranes, periosteum (surface shell of the bone).
Everyone knows how painful it is to hit the periosteum, especially in areas where it is not covered with soft tissue, such as the front of the tibia. At the same time, bone surgery is painless as the bone does not contain any pain receptors. There are only a few pain receptors in subcutaneous fat.
There is no pain receptor in the brain and neurosurgeons know that the brain can be cut without the use of painkillers. Since the membranes of the brain are equipped with sufficient pain receptors, compressing or stretching the membranes causes considerable pain sensations.
The activity of the cerebral cortex depends strongly on the specific formation of the nervous system, the so-called mesh formation of the brain stem, which can activate and inhibit the activity of the cerebral cortex of the large hemispheres.
Pain sensitivity to super-strong and destructive stimuli is associated with the development of painful sensations, which have a strongly negative emotional coloration, as well as vegetative reactions (increased respiratory rate, pupil dilation, narrowing of peripheral vessels, etc.). Painful feelings of various kinds can be caused by harmful stimuli (temperature, mechanical, chemical, radiation energy, electric current).
Painful feelings are an incentive for various defensive reactions, the main purpose of which is to eliminate external or internal factors that caused the pain. Sensitivity to pain is therefore of great biological importance.
Some people believe that any irritation or destruction of a body's receptor can lead to pain. On the skin surface, the total number of pain points corresponding to the location of pain receptors in the skin is 900000 - 1 000000000000 (up to 100-200 per 1 cm³).
Painful feelings are easily caused by the conditionally reflective path. So if you combine the bell with painful skin irritation, after several combinations the isolated effect of the bell begins to cause pain and characteristic vegetative reactions.
Sensitivity to pain is the most primitive, undifferentiated form of sensitivity. Painful feelings are very difficult to locate. Their localization is made possible by the accompanying tactile and other sensations.
Sensitivity to pain depends not only on the number of pain receptors, but also on age and sex. There is a dependence on the state of mind.
Anything that distracts attention from painful stimuli reduces the feeling of pain. This explains the weakening or cessation of pain in the time of affects, anger and anxiety. A passionate person does not feel pain. For example, in the heat of battle, he may not notice any injuries. Conversely, depression, physical fatigue, nervous exhaustion and pain increase.
Waiting and anxiety increase painful feelings; the same happens when there are no distractions. This can also explain the increase in all kinds of pain at night.
Painful impulses received by the receptors are then difficultly directed through special sensitive fibers to various parts of the brain and eventually reach the cortical cells of the cerebral home spheres.
Brain stem pain centers are located in different parts of the central nervous system. Much of the activity of the cerebral cortex depends on the particular formation of the nervous system, the network structure of the brain stem, which can activate and inhibit the activity of the cerebral cortex of the major hemispheres.