TOUCH

TOUCH, one of the functions of the nervous system, consisting in the perception by consciousness of sensations received when touching the skin and mucous membranes with various solid or liquid bodies. Touch gives humans a series of sensations, namely: the feeling of pressure, touch proper—the tactile sense, the feeling of warmth, the feeling of cold, the feeling of pain, tickling, and the hair sense. The main organ of tactile sensations is the skin, but in addition, the conjunctiva and the cornea of the eye, the mucous membranes of the eyelids, lips, mouth, tongue, nasal cavity, vagina, and rectum are not devoid of this sense. The tongue, for example, is endowed with a high tactile sensitivity. According to Goldscheider and Blix, touch is unevenly distributed over the skin; its perceptive apparatuses, not yet fully known histologically, are easily distinguishable on the skin in the form of separate points of warmth, cold, pressure, and pain. Their presence can be easily verified by irritating with a very sharp pencil various places on the back of the hand or forearm while turning one's eyes away. Then it can be easily seen that different places on the skin respond to the same irritation with different sensations. Clinic has not yet made use of the points of Blix and Goldscheider and applies its own methodology for investigating touch (see below). For humans, touch is the sensation with which he willingly verifies the indications of his other sense organs. The sense of touch, like other sensory sensations, is prone to blunting; prolonged, uniform tactile irritations of the same place on the skin eventually cease to be felt. On the other hand, exercise greatly increases the acuity of touch. For a rough examination of the sense of touch (tactile sensitivity), one touches with the eyes closed of the subject to one or another area of the skin with some soft object (cotton, a soft brush, the tip of a finger, etc.) and asks whether he feels it and how clearly, and asks him to localize this sensation. For a more precise examination of the sense of touch, Mochutkovsky proposed his taximeter. It is a quadrangular bar made of hard rubber, divided in the middle by a horizontal groove. Each of the sides of both halves is numbered. Surface No. 1 is perfectly smooth; No. 2 has fine notches, which creates a slight feeling of roughness. Further, on sides from 3 to 8, notches are made with increasing depth. A healthy person very clearly distinguishes the degrees of roughness of each side. With reduced sensitivity, the patient considers only the last numbers as rough, while the first three to four are considered smooth. For the same purpose, mainly for investigating the sense of localization, Weber's compass or Sieveking's esthesiometer is used. The latter consists of a metal plate on which there are two blunt points, one of which is attached immovably, while the other can move freely along the plate. If both points are simultaneously applied to the skin, separate sensations are obtained only at a certain distance between the points. The thinner the perception of a given area of skin, the less distance the points have to be spread apart. Weber found that the minimum distance allowing the perception of two different sensations, even in a normal state, shows considerable variations depending on the place on the skin: from 1.18 mm (tip of the tongue), 2.25 mm (palmar surface of the last phalanx of the finger) to 75.5 mm (midline of the back). Weber's scale of average figures can be used when examining patients. There are also other instruments for investigating tactile sensitivity, for example, a set of the finest metal hairs with known numbering, Hering's instrument (cylinders wound with wire of varying thickness), etc. The second type of tactile sensitivity is the sense of pressure (baresthesia). There is no fundamental difference between touch and light pressure; it is only important that when pressure is applied to the skin, the deeper parts (fasciae, muscles, etc.) are not also irritated. In its simplest form, the sense of pressure is examined by pressing with a finger or some blunt object on a certain area of skin; at the same time, it is determined whether the subject can distinguish pressure from touch. For a more precise examination of this type of sensitivity, weights of different weight are used. Usually, two lead weights of different weight are placed successively on one or another place on the skin; they are asked which weight is heavier; the smallest distinguishable difference is determined, and then it is calculated what part of the original weight this difference constitutes. On the basis of numerous studies, Eulenburg found that in a healthy person, depending on the area of the body, this difference varies from 1/40 to 1/10 of the original weight. For the same purpose, Eulenburg built a special instrument after the type of spring scales, which he named baresthesiometer. In its main features, it consists of a pad, which is pressed against the point under investigation with greater or lesser force by means of a spiral spring wire. The degree of pressure applied is read in grams on a graduated dial. In strictly scientific studies when studying the sense of pressure, Landois's mercury scales or Stratton's scales are used. A special type of tactile sense, little studied from the physiological side, is hair sensitivity. This name denotes the sensations that are obtained when touching the hairs of the skin. Such hairs, often barely noticeable, cover almost the entire surface of human skin, with the exception of the palms, soles, heels, palmar and plantar surfaces of the fingers, and glans penis. The thicker the hairs, the weaker this sensitivity is expressed; the more hairs per 1 cm², the stronger it is. To investigate hair sensitivity, an ordinary thin hair taken from a human head is used, with which the hairs covering a given area of skin are gently moved. Novishevsky proposed his trichoesthesiometer, consisting of a pincer and spiral clock springs of varying fineness. For the same purpose, Bekhterev's electrical trichoesthesiometer is also suitable. For the perception of various types of sensitivity in the skin and mucous membranes, there is a huge number of sensory nerves, ending both as "free endings" in the epithelium of the skin and with various receptive apparatuses (see Nerve endings and Skin, histology). Pathways of conducting tactile sensitivity. Excitation of the receptive organs is transmitted along the nerves to the sensory cells of the intervertebral ganglia, from where along the central processes of these cells (posterior roots) it enters the spinal cord. In the spinal cord, the fibers conducting tactile sensitivity, according to some authors, go without interruption along the same side in the bundles of Goll (fibers from the lower extremities and the lower half of the trunk) and Burdach (fibers from the upper extremities and the upper half of the trunk). This is the first neuron, which ends in the corresponding nuclei nuclei funiculi gracilis et cuneati. From the cells of these nuclei, fibers in the composition of the second neuron go after the decussation in the loop to the ventral nuclei of thalami optici. The fibers of the third neuron, having their origin in the cells of the thalamus opticus, ascend through the internal capsule to the cerebral cortex, where the arriving excitations are perceived as touch to the skin or its hairs or as pressure. According to other scientists, the first neuron ends in the cells of the posterior horn of the spinal cord, and the second neuron, starting from these last cells, ascends in the composition of the posterior bundles to their nuclei, with part of the fibers going along the same side, and the other part making a decussation in the spinal cord and going to the opposite side to the cells of the nuclei funiculi gracilis et cuneati. The further course of the fibers (third and fourth neuron) is similar to the one just described. In addition to the described pathway, the tactile sense is also conducted in part by the funiculus spino-thalamicus and funiculus spino-tectalis (parts of Govers' bundle), lying in the lateral columns of the spinal cord. (For the course of these fibers, see Govers' bundle). According to some authors, the conducting pathways of tactile sensitivity are also in the anterior columns of the spinal cord, and according to Förster, the conductors of the sense of pressure lie closer to the anterior groove of the spinal cord, and the sense of touch lies outward from them. The pathway for tactile sensations of the skin of the head, with the exception of the occipital region and part of the outer ear (nn. occipitalis major et minor and auricularis magnus), and the mucous membranes of the eye, nasal cavity, oral cavity, tongue, palate, and pharynx lies in the nn. trigeminus, glosso-pharyngeus or vagus. The place of termination of the last neuron conducting the sense of touch is mainly the gyrus centralis posterior and the anterior parts of the parietal lobe adjacent to this gyrus, and possibly also partly the gyrus centralis anterior. As for the conductors of hair sensitivity, they are connected with the autonomic nervous system. This is proven by numerous experiments of Novishevsky and others.

The excitation received by receptive cells is transmitted along sympathetic fibers to the spinal cord through its posterior roots, where these fibers are directed to cells located in the tractus intermedio-lateralis of the lateral columns of its own and the opposite side. The further path of the conductors of this type of sensitivity is unknown. The spread of nerve excitation from skin receptors to the brain. Lazarev proposed the ionic theory of excitation, according to which the cause of touch is the biochemical decomposition of substances contained in the terminal nerve apparatus of the skin. Although the question of the effect of touch or pressure on the decomposition of substances in the tactile nerves has not yet been finally studied, nevertheless by analogy with other sense organs (vision, taste, smell) it can be accepted that when touching or pressing on the terminal cells of nerves conducting tactile sensation, a change in ion concentration occurs in them. This change in the nerve thus transmits excitation from one point to another up to the cortical centers of the brain, the irritation of which converts a certain physiological process into consciousness. (Pathology of tactile sensitivity - see Sensitivity.) Wherever the interruption of the conducting pathways of touch occurs on the periphery, in the spinal or brain, there is always a loss of tactile sensations. For example, when the parietal gyrus is damaged, patients with their eyes closed do not recognize objects and refuse to determine their shape. When the tractus spino-thalamicus and tractus spino-tectalis in the lateral columns of the spinal cord are damaged, tactile sensations usually remain unchanged, since there are still fibers in the posterior columns of the spinal cord for conducting this sensitivity. Complete disappearance of touch is observed when the posterior roots are damaged, as well as with simultaneous destruction of the lateral and posterior columns. Regarding hair sensitivity, Noishchevsky noted its loss in various diseases of the nervous system, most often in tabes dorsalis.