The skin is called the largest human sensory organ. And indeed, this is two square meters of surface. Under the layer of the epidermis there are receptors — skin receptors proper, pain receptors. The total number of these receptors is about two million. One and a half million of them are painful, and about half a million are related to the skin sensitivity system. And inside the skin sensitivity, we see subsystems associated with the perception of weak touch, strong pressure, vibrations with different frequencies, heat, and cold. That is, just as, say, there are rods and three types of cones in the visual system, there are also subsystems that eventually combine their efforts in the cerebral cortex so that we can realize a certain holistic skin image, as a whole, perceive this or that skin effect through different subtypes of receptors. These receptors are essentially the endings, dendrites of nerve cells, which are located mainly next to the spinal cord. Inside the spinal column are the so-called spinal ganglia, there are 31 pairs of them. Accordingly, our body is divided into 31 levels, and each pair of spinal ganglia serves its own floor, its own body level. It turns out that such a dendrite reaches, for example, to the tip of a finger and branches here. That is, the length of this process is very decent. If these are so-called naked nerve endings, then most often such endings perceive pain. But for the perception of pressure, touch, temperature, and vibration, there are so-called encapsulated nerve endings: connective tissue cells form multi-layered structures around a neuron branch, and depending on the properties of this structure, this nerve process reacts to heat, cold, pressure, and vibration. That is, a lot is determined by the properties of the capsule. The most well—studied receptors are those that respond to pressure and touch. Some of them are not freely located in the skin, but, for example, they braid the ends of the hair — they are located in the hair follicles. The hairs on our skin don’t really warm us anymore, do they? Once upon a time, we were probably covered with exuberant hair, which carried out thermoregulation. But now the thermoregulatory function of the hairline has practically disappeared, but the sensory function remains. In some animals, this system is very pronounced, they have vibrissae, which, for example, a rat or a cat feels the surrounding space. By the way, not only mammals have vibrissae, but also, for example, some birds. For example, kiwis have beautiful vibrissae.: It is a nocturnal bird, and they are very appropriate for it. There are a large number of such encapsulated receptors. Histologists have come up with many names for them: Pacini corpuscles, Merkel corpuscles, Meissner corpuscles, Krause flasks, and so on. I repeat once again that it’s all about the capsule’s features. Some capsules contain structures that, say, release chemicals when heated or cooled, and, accordingly, the nerve fiber is activated. Or the properties of the capsule allow you to respond to high-frequency and low-frequency vibration, and these will be different receptors. First of all, we need a reaction to vibration in order to determine the roughness of the surface. If you are asked if the table in front of you is smooth, what will you do? You’ll put your fingers down and start driving like that. And if the table is smooth, then the pimples are small, high-frequency receptors will be activated. And if the table is more rough, then the pimples are large, and lower-frequency receptors are activated. Then all this transforms into our perception of smooth or, conversely, rough. It is clear that such receptors are located mainly on the fingertips. Our fingers are about the same for us as vibrissae are for a rat, that is, one of the main areas through which we collect skin sensitivity. Of course, we also have very high skin sensitivity on our lips and tongue. But this is primarily related to the perception of food. But the world around us, of course, is primarily felt with our fingertips. The receptors that deal with heat and cold attract special attention because these receptors are affected by chemicals that are woven into our taste sensations. For example, a menthol sensation is a feeling of such a chill on the tongue, or the effect of spices, pepper or mustard. It turns out that these are not taste sensations at all, but skin sensations. It’s easy to believe, because pepper doesn’t just affect the tongue. If you take a pepper patch and stick it on, after a while it will penetrate even through several layers of the epidermis, the hot receptors will still work. They usually react to heating above 30-35 degrees. But pepper contains a molecule called capsaicin, which activates these receptors and creates the illusion of being hot. And in the case of the taste system, it brings a lot of additional sensory and emotional experiences. The receptors that respond to capsaicin are a separate group of protein―sensitive molecules, the so-called vaniloid receptors. There are several types of them. The main set of spices — pepper, ginger, cloves, cinnamon — goes through the vaniloid receptors of the first type. For example, thyme and basil act through the third type of vanilloid receptors. Interestingly, in addition to vaniloid receptors, there are also so-called ankyrin receptors on the receptors of hot water. Mustard or, for example, wasabi acts through them. It turns out that the perception of hot and spicy goes through different information channels. As a result, there is a wealth of sensory sensations, a holistic perception of taste is formed. The axons of the neurons of the spinal ganglia, through which skin sensitivity is transmitted to the spinal cord, enter the so-called posterior horns of gray matter. The rear horns are those that face towards the back. There, in these posterior horns, the primary analysis takes place, the primary processing of skin sensitivity, two main processes are implemented: first, weak signals are delayed so as not to interfere with the main information processing processes; secondly, what is called habituation, that is, the suppression of constantly acting signals. Some of the effects of addiction are also realized at the level of skin receptors, but a significant part occurs in the gray matter of the spinal cord. And the logic is quite clear: we are interested in new signals, but constant touching is no longer so important. We all know this effect: you feel the moment of touch very well, but if you keep holding your finger, the feeling fades. Addiction is provided by both receptors and the gray matter of the spinal cord, the posterior horns. There is a classic psychophysiological experiment where you are given three basins, one with hot water, the second with warm water, and the third with cold water. You dip your right and left hands into basins of hot and cold water, and after a while the sensations disappear, as if they have become accustomed. And then you put both hands in the middle basin, and there’s what’s called cognitive dissonance: one hand tells you that the water is cold, the other tells you that it’s hot, and it’s the same water in the same basin. That’s when the addictive effect shows up. If the signal is strong enough and new, it passes through the posterior horns of the gray matter of the spinal cord and is then able to trigger the reflexes of the spinal cord, and sometimes the brain, if it is the sensitivity of the head. Such reflexes include, for example, finger grabbing, which is realized in young children.: you touch the baby’s palm, and he grabs your finger. This is an ancient reflex of holding onto our mother, on our mother’s fur, which we inherited, of course, from our monkey ancestors. And if you start touching the baby’s lips, the sucking reflex starts. It is triggered not only by taste signals, but also by skin sensitivity. The head is no longer serviced by the spinal cord, but by the trigeminal nerve. We have 31 segments in the spinal cord that work with 31 floors of the body, and the head, more precisely, the front of the head ― There are three floors: forehead, upper jaw, lower jaw. And they are served by the trigeminal nerve, the fifth cranial nerve, which is called the trigeminal nerve because it works with these three floors. Touching, warmth, or even pain from different parts of the face are reactions of the trigeminal nerve. In addition to triggering reflex reactions, skin sensitivity must also rise up in order to eventually reach the cerebral cortex. And in the spinal cord, this conduction is carried out by the so-called dorsal (or posterior) columns of white matter. They are really located in the very back of the spinal cord and are divided into a so-called thin bundle and a wedge-shaped bundle. A thin beam picks up information from the legs and lower half of the torso, a wedge—shaped beam picks up information from the arms and upper half of the torso. And the first synapse is in the medulla oblongata. There, the signal switches, which has risen from the spinal cord to the neurons of the brain. After this switch, all the fibers cross, creating a so-called medial loop. Because of this intersection, as you know, our right hemisphere feels the left half of the body, and the left hemisphere feels the right half of the body, that is, the right arm, and so on. At the level of the thalamus, the signals that rise from the spinal cord are joined by signals from the trigeminal nerve. As a result, a more or less complete map of the human skin surface appears in the posterior part of the thalamus (where the thalamic nucleus is located, it is called the VPL — ventral posterolateral nucleus). There is a head zone, a trunk zone, an arm zone, and a leg zone. And then all this finally rises into the cerebral cortex, where the area that analyzes and evaluates skin sensations is located just behind the central furrow in the anterior part of the parietal lobe. If you put your hands on the top of your head, leading down, this is the central furrow, and immediately behind it there are maps of skin sensitivity. And here there is a kind of reflection of our surface. Neurophysiologists call this a somatosensory homunculus, meaning such a little man is lying here. Moreover, he lies upside down: the area of our leg and foot is located at the top, then the shin, thigh, torso, then the huge area of the hand, each finger is separately prescribed. The hand in this area lies like this: the little finger is on top and then all the fingers, and the thumb is the lowest. Behind the area of the hand, very large, very large, is the area of our face. And although the whole man is upside down, the face is positioned correctly: first comes the forehead area, then the upper jaw, then the lower jaw. Finally, at the very transition to the lateral sulcus, there is a zone of skin sensitivity of the tongue and pharynx, and this zone passes into the zone of taste sensitivity, which is at the bottom of the lateral sulcus. As a result, it turns out that the skin sensitivity of the tongue is adjacent to the taste sensitivity of the tongue. And this helps to form the most holistic taste sensation, which includes not only the sensations of spicy or menthol, but also the sensations of heat, cold, touch, and consistency of food: whether there are bubbles or not, large or small pieces, sticky, viscous… All this is an important component of the perception of the food we eat. The map of our skin surface, represented in the anterior parietal cortex, has distorted proportions. Those areas where the signal is read most accurately — say, fingertips, lips, tongue — they are very large, and a large number of nerve cells analyze information from these areas. But, for example, the back — there are much fewer neurons involved. Accordingly, this homunculus has really such distorted proportions. And in a rat, for example, the vibrissus area is very large, and therefore neurophysiologists say that it is a ratunculus (from rat — rat). He has such proportions. In addition to the information flow and information load, skin sensitivity also has an emotional component. But the hypothalamus is already mainly working here. It can produce positive and negative emotions, and skin sensitivity is characterized by a very high complexity of emotional perception, because, say, a disgusting smell is disgusting to everyone, a sweet taste is pleasant to everyone, but the same touch can cause positive emotions, negative emotions, and a defensive reaction in different situations. and some kind of reaction related to liking someone who touched you. I must say that biology Homo sapiens is associated with the increased importance of mutual touching. Grooming reactions, which make it possible to cleanse the body and remove parasites, have become a means of communication in monkeys. To touch a packmate is to convey to him some kind of positive emotional signal. Therefore, when a pleasant touch occurs on the skin surface, the tomogram shows how the centers of positive emotions are ignited in the brain. And, for example, massage can give us positive emotional experiences.