Painful, tactile, cold, thermal …
Our skin is one of the main senses. It contains a dense network of nerve fibers, and millions of the thinnest branches branch off from them, ending on the surface of the skin with sensitive nerve devices – receptors. On average, one square centimeter of skin accounts for 100 – 200 pain receptors, 12 – 15 cold receptors, 1 – 2 heat receptors, and about 25 tactile ones. They also translate various stimuli – mechanical, physical, chemical – into the universal electroimpulsive language of the nervous system.
The most common type of skin receptors are free nerve endings (13); they perceive mainly pain. There is practically no area on our skin where pain receptors are completely absent, but they are distributed unevenly. For example, axillary and inguinal areas are richly equipped with them, and the most “painless” ones are soles, palms, and auricles.
Unlike bare pain receptors, tactile ones are dressed in “overalls”. In Meisner’s bodies (14), the nerve endings are hidden in a connective tissue capsule. and for Merkel’s little bodies (15), to the disks. These receptors perceive touch and carry information about the nature of the surface of the object. In a multilayer connective tissue capsule, Pacini bodies are also dressed (16), which perceive pressure and vibration. The nerve plexuses of the hair follicles also respond to touch. When the hairs covering our body deviate by only 5 degrees, this mechanical irritation is transmitted to the nerve spiral, which is braided to the base of the hair, and we feel even the lightest cobweb that has landed on the hand or face.
Receptors, called Krause flasks (17), react to cold exposure, and Ruffini bodies to heat; there are much fewer than Krause flasks. In total, there are about 250,000 cold and 30,000 thermal receptors.
Recently, data on skin receptors obtained in research laboratories force scientists to rethink the idea that each type of sensation arises from irritation of strictly specific receptors. Researchers have found that most receptors, specializing in any one type of irritation, “master” and adjacent. For example. among the free nerve endings there are receptors that are sensitive not only to pain stimuli, but also to mechanical, temperature ones, and Ruffini bodies, specializing in thermal perception, under certain conditions begin to perceive pain irritations. In general, the system of skin sensitivity is very mobile: depending on various factors of the external and internal environment, the number of functioning receptors and the degree of their sensitivity change. I will give an interesting experience.
In the morning, the test volunteer noted pain points in different areas of the skin with different ink, cold, thermal, and tactile – a peculiar mosaic was obtained. But when in the evening each of these points was checked for a specific stimulus (pain – for pain, cold – for cold, etc.), it turned out. that they all … shifted a few millimeters down, up, to the sides. This means: receptors that were highly sensitive in the morning turned off by the evening, but neighboring ones became more active, which the experimenter recorded in a repeated experiment. Maps of skin sensitivity. drawn with a gap of a few hours, are never the same.
In addition to the aforementioned, skin receptors are also subject to a sense of place, stereognostic, two-dimensional spatial feeling. Finally, they show a capacity for perception, it would seem. they are not characteristic of irritants. Separate areas of the skin. for example, very sensitive to light. If the eyes are completely closed from the light with a dark bandage, and a certain area of the skin, say hands, brightly illuminate, then in the photosensitive structures of the eye, experimenters observe such rearrangements as if the eyes were illuminated, and not the hands. Researchers have not yet found an explanation for such an amazing ability of skin receptors. Nevertheless, the very fact of its existence suggests that the receptor apparatus of the skin is fraught with many unknown mechanisms, unsolved mysteries, which have yet to be studied and known.