TACTILE ANALYZER (Latin tactilis tangible) — set of the peripheral and central nervous educations providing perception and processing of information on action on outside covers of an organism of various not painful mechanical irritants.
Tactile sensitivity is a component of tactile feelings (see Touch), and T. and. — part of the somatosensory analyzer. The mechanical influences causing emergence of tactile feelings usually subdivide into a touch and pressure. Often as an independent type of tactile irritants allocate the vibration which is actually representing a kind of a rhythmic touch.
Peripheral department of T. and. form different types mechanioreceptors (see) and the related afferent nerve fibrils (see). A main type of mechanioreceptors in a pilar part of skin (90% of all surface of skin) free nerve terminations make (see), nerve terminations around hair follicles, disks (little bodies) of Merkel (tactile disks), tactile corpuscles of Iggo. In the skin deprived of indumentum, the main receptor structures are free nerve terminations, Merkel's disks, Meyssner (touch corpuscles) and Pachini's little bodies (lamellar little bodies). Tangoreceptors on a body surface are distributed very unevenly that causes unequal tactile sensitivity of its different sites. The thinnest tactile sensitivity is noted on finger-tips of hands and language, the smallest — on skin of belly and buttock areas. Very rich afferent innervation and respectively tactile sensitivity vibrissas — the special sensitive hairs which are found at many primacies and other mammals, but absent at the person possess. Activity of tangoreceptors together with receptors of the musculoskeletal device is the cornerstone of difficult tactile feelings.
Perception of a touch (and vibrations) is carried out by means of phase (quickly adaptive) receptors, and pressure — thanks to excitement of tonic (slowly adaptive) receptors. Pachini's little bodies, a number of receptor structures of a hair bag concern to the first (so-called. T - G-, D-receptors which are the terminations of afferent fibers of Aa and Aa group), to the second — tactile corpuscles, Ruffini's little bodies (the termination of fibers of Aa of group), free nerve terminations (branchings of fibers of group C). The absolute threshold of tactile sensitivity is defined by activity of the most sensitive mechanioreceptors, to-ry-mi Pachini's little bodies capable to be excited already at shifts about 10 are - 4 — 10 - 5 mm. Vibration irritations are perceived in the range from shares to 1 000 — 1 200 Hz, but the greatest sensitivity to vibrations is observed in the range from 200 to 400 Hz (see. Vibration sensitivity ). In addition to an absolute threshold tactile sensitivity is characterized by a differential threshold and a threshold of space. A differential threshold — size, on to-ruyu it is necessary to change the operating irritant to cause the minimum change of feeling. The differential threshold characterizes process of adaptation of tangoreceptors to action of an irritant and is used for identification of disturbances of tactile sensitivity. The threshold of space — the smallest distance between two points of skin, at simultaneous irritation to-rykh arises feeling of action of two separate incentives. Thresholds of space depend on intensity of branching of nerve fibril on peripheries and numbers of the receptors transferring excitement to one nerve fibril. Thresholds of space considerably differ on different sites of skin: they are minimum (1 — 2,5 mm) on finger-tips, lips, language and are maximum (more than 60 mm) on skin of hips, shoulders, the centerline of a back. For definition of differential thresholds and thresholds of space use the same methodical receptions, as for a research of touch (see) and sensitivity of skin (see. Skin ), applying only tactile irritants.
The analysis of mechanisms of excitement of the tangoreceptors making a biophysical basis of tactile reception is carried out for the present in small degree that is connected with big technical difficulties of a research of the single receptors located in dense tissues of a body. Therefore almost all data on activity of these receptors are received during the studying of flows of the impulses registered from single afferent nerve fibrils. The only exception are large lamellar little bodies of Pachini, a research to-rykh allowed to find out appointment and mechanisms of activity of all structures forming them: capsules, not - the myelinized bombway and pulpy nerve fibril. The capsule of a receptor (as well as the fabric surrounding it) carries out a role of the auxiliary structure which is carrying out mechanical irritation from the outside to a nervous bombway. It is the mechanical filter determining the speed of adaptation of a receptor, and also its frequency characteristics (at action of vibrations). Thanks to a special structure of the interior of the capsule Pachini's little body, as well as some other tangoreceptors, has sensitivity to the direction of action of an irritant. Actually process of reception is carried out by a membrane of a nervous bombway. At one direction of an incentive there is stretching of a superficial membrane of a bombway that leads to the excitement of a receptor which is shown emergence of depolyarizatsionny receptor potential and after this generation of pulse categories in the first interception of Ranvye of nerve fibril. At other direction of an incentive the membrane of a bombway is exposed to compression that leads to emergence of brake hyper polarizing receptor potentials. Receptor potentials represent local answers of a superficial membrane of a nervous bombway. It is known that depolyarizatsionny receptor potentials are caused by increase in permeability of a membrane preferential to ions of sodium, to-rye transfer the main (to 90%) a part of charges.
Afferent neurons of the first order from tangoreceptors of a trunk and extremities are located in spinal nodes, and from receptors of the person and the head — in semi-lunar (gasserovy) nodes of trifacials (trigeminal nodes, T.). Data on the ascending ways of a tactile afferentation are received by hl. obr. in experiments on animals, preferential cats. The main part of the fibers which are fast-carrying out (Aa) occupying a medial part of back roots after entry into a spinal cord goes as a part of back columns of white matter to nuci, gracilis and nuci, cuneatus of a myelencephalon. The bodies of afferent neurons of the second order located in these kernels give rise to the fibers of a medial loop forming decussation at the level of olives of a myelencephalon and which are reaching a limit in specific ventrobazalny kernels of a thalamus. Information from tangoreceptors of the head also reaches these structures. Here neurons of the third order are localized, to-rye send the fibers to the first somatosensory area of bark of hemicerebrums (the post-center a lny crinkle). On quickly - to the carrying-out fibers from tangoreceptors of a body also information on a lateral path of Morin comes to a thalamus. Having begun from cells of back horns of a spinal cord, this spinotalamichesky way switches in a side cervical kernel of a myelencephalon, and then after decussation merges with a medial loop. Slowly carrying out fibers from tangoreceptors of a body pass in a lateral part of back roots and reach a limit on large funicular cells of back horns of a spinal cord. From here spinotalamichesky ways begin (front and side). At the spinal level these ways carry out semidecussation and give a diffusion projection in a complex of ventrobazalny kernels of a thalamus (more ventrally, than a medial loop), in nonspecific kernels of a thalamus and in macrocellular department of a medial cranked body. From here the fibers going to the second somatosensory area of bark begin (the cat — in a front ekto-silviyeva a crinkle, at the person and monkeys has kzad and ventralny the first somatosensory area). There the fibers connected with Morin's path arrive. Thanks to these fibers signals reach the second somatosensory area of bark with short stage of latency. In bark allocate sometimes and the third somatosensory zone, edges at animals is located between a lobby of an ekto-sa of l with viyevy and ansatny crinkles. Answers of its neurons have big stages of latency, than answers of neurons of other two zones, and are caused by specific irritants from small receptive fields.
The system of a medial loop provides information transfer about specific tactile stimulations of small intensity (a touch, easy pressure); the lateral path of Morin transfers data on stronger deformations of skin, and the spinotalamichesky system is way, on Krom signals of considerable mechanical influences arrive, and this way is nonspecific, providing coding of information and on temperature influences. At neurons of system of a medial loop receptive fields are small, and the strict topographical organization of projections of areas of a surface of skin is characteristic of all system. Neurons of spinotalamichesky system receptive fields very big, covering sometimes all body, but in its kernels have no accurate topical representation of a body surface. In general the system back columns — a medial loop carries out specific forms of tactile distinguishing, and the spinotalamichesky system gives much more generalized forms of feelings and is connected with information transfer about the qualitative nature of influences from the periphery, but not about localization and a form of an irritant, its time history.
Together with other types of sensitivity of skin (see Skin) tactile sensitivity can compensate to some extent absence or insufficiency of function of other sense bodys (sight, hearing). Blind people have a quantity of little bodies of Pachini in skin of fingers and palms sharply increases, and tactile sensitivity of these areas considerably exceeds that at sighted people. Disturbances of tactile sensitivity (see Touch) are closely connected with disturbances of other types of sensitivity of skin and in clinic are investigated together with them.
Bibliography: Esakov A. I. and Dmitriyeva T. M. Neurophysiological bases of tactile perception, M., 1971, bibliogr.; Ilyinsky O. B. Physiology of mechanioreceptors, JI., 1975; Physiology of touch systems, under the editorship of A.S. Batuyev, L., 1976.
O. B. Ilyinsky.