TONE

From Big Medical Encyclopedia

TONE (Greek tonos tension) — the constant (background) activity of nerve centers, nek-ry fabrics and bodies providing them readiness for action. T. is one of manifestations of a homeostasis in an organism and at the same time one of mechanisms of its maintenance (see the Homeostasis). So, constancy of T. unstriated muscles of walls of arteries is one of conditions of maintenance of the ABP at the homeostatic level.

In practice usually allocate T. nerve centers, T. skeletal and T. unstriated muscles.

The tone of nerve centers (a nervous tone) is caused by inflow of impulses from the respective receptive fields (see. Receptor zones) and the ascending activating influences of a reticular formation (see). The resistant pulse background activity connected with high chemical sensitivity of a membrane of its cells is inherent to neurons of a reticular formation and convergence to drink collaterals from a huge number of touch ways. Therefore, as a rule, influence of a reticular formation on other structures has tonic character. In maintenance of T. nerve centers, in addition to the afferent impulsation arriving from peripheral receptors and also from a reticular formation, various humoral irritants participate (hormones, level of partial tension of carbonic acid in blood and t1). Constant background activity of nerve centers does possible existence of tonic reflexes (see the Reflex) and maintenance in a complete organism of a condition of wakefulness.

Applies the term «tone of the autonomic nervous system» to assessment of a condition of century of N of page. Disturbance of a tone of century of N of page is the factor promoting disadaptation and development of somatic pathology (see the Autonomic nervous system).

Muscle tone. All muscles never happen completely weakened, in them tension — muscular T remains a nek-swarm. Such condition of muscular tissue is followed by the change of its bioelectric characteristics and insignificant reduction determined by character of mechanical parameters of a muscle (i.e. its viscosity, elasticity, not loaded length, resistance, a cut the muscle renders to the stretching effort, etc.). Often the term «muscle tone» is applied to designation of the processes of muscular contraction connected with poziy reflexes (see the Pose), the long slow reductions of unstriated muscles which are followed by an insignificant metabolic cost.

For determination of T. muscles apply various methods — the straight lines based on determination of passive resistance of a muscle to its ultimate tension and indirect when the size of a tone is estimated on the accompanying changes of mechanical characteristics of muscular tissue. It is possible to carry Mosso's methods to direct methods (1896) and I. N. Filimonova (1925), use to-rykh allows to register not only a curve of resistance of a muscle at the passive movement, but also separate stages of the movement in time. In Broman (1949) method studying of resistance to a passive muscle strain and elektromiografichesky registration of the processes proceeding at the same time is combined. To indirect methods of definition muscular T. measurement of the cross hardness (resistance) of muscles on depth of immersion (cave-in) in a muscle of a load belongs.

The tone of skeletal muscles has the reflex nature. Deaf-ferentation of extremities at «a spinal frog» by section of back roots is followed by disappearance muscular T. (Bron-dgesta tone). In this experience dependence of T is observed. motor nerve centers and T. skeletal muscles from it - a pulsation from proprioceptors of muscles.

To muscular proprioceptors (see) muscle spindles and tendinous bodies belong. Muscle receptors are specialized cells, to-rye react to the changes of length and a muscle tension happening at their stretching or reduction. Spindles are connected to muscle fibers in parallel, and tendinous bodies — is consecutive. Therefore reduction of a muscle is followed by excitement of tendinous bodies and decrease in an impulsation in receptors of spindles. Each muscle spindle consists of several muscle fibers and the connective tissue capsule surrounding them, edges are covered by a spindle not completely; poles of the longest fibers go beyond its limits. Muscle fibers (see. Muscular tissue), the spindles which are a part, received the name intrafusal (intra spindle), and the main fibers of a skeletal muscle — extrafusal (vnevereteppy). The spirals and rings forming primary nerve termination are located ekvatorialno, and each spiral receives a branch from a thick afferent fiber; to each spindle there corresponds one afferent fiber. Secondary endings are located closer to poles of a spindle and are innervated by more fine nerve fibrils (see).

The efferent innervation of spindles is carried out by thin axons of the motor-neurons relating to group A of 7-nerve fibrils (a gamma and efferent innervation). Extrafusal fibers are innervated thick And a-nerve fibrils. A small part of spindles at mammals receives an innervation from branches of a-nerve fibril. But intrafusal fibers are supplied not only a-nerve fibrils: on an intrafa screen fibers of the same spindle also ^-nerve fibrils of fiber terminate. All efferent fibers coming to a spindle are called fusimotor.

At reduction of intrafusal fibers the equatorial area of a spindle therefore there is a shift of spiral turns of a receptor, and as a result — emergence of rhythmic bioelectric activity stretches. The isolated irritation of 7 fibers against the background of blockade of carrying out excitement on and - to efferent fibers is not followed by a gain of muscular tension, but leads to strengthening of an impulsation in afferent fibers.

Impulsation at - motor-neurons increases activity of a muscle spindle. The irritation of primary endings activates and - motor-neurons of a muscle and its synergists. At the same time activity and - motor-neurons of antagonistic muscles is braked. As primary endings can be stretched as a result of activity of fusimotor nerves, there was an assumption of a possibility of excitement and - motor-neurons through at - a loop.

Along with activation and - motor-neurons through system of muscle spindles also their direct excitement is possible. In extensor muscles excitement of secondary endings causes braking of activity and - motor-neurons. At implementation of myotatic reflexes (i.e. reflexes on stretching) the irritation causing excitement of primary endings is counteracted by activation of tendinous bodies, secondary receptors and unloading of spindles (see the Reflex).

In skeletal muscles there is a functional differentiation of motive units on «bystry» and «slow». «Slow» motive units are characterized by the smaller force of single tetanic reduction, create a smooth tetanus (see) with a smaller frequency of the irritating incentives and in comparison with «bystry» motor units are less tired. These features of muscle fibers were closely connected with type of the innervating motor-neuron — «tonic» or «phasic».

Various motor units have different thresholds of excitement («tonic» low thresholds, «phasic» high) that provides reduction of various motor units depending on a situation.

The system tonic small and - mo-toneyronov is sensitive to the impulses from spindles coming on thick afferent fibers and almost does not create the active muscle tension capable to weaken effect of these impulses.

Joint activation and - and 7 motor-neurons, characteristic of many movements, allows to control length of muscle fibers; insignificant increase in loading leads to lengthening of a muscle, Krom is counteracted by an afferent impulsation from at the same time activated spindles. Conditions for constant correction of zhzkhmeneniye of an active pose are created.

Thus, the tone of skeletal muscles is supported by system tonic and - motor-neurons, the impulsation to-rykh depends on the level of activity of primary endings of muscle spindles. Primary endings can be stretched at excitement

of 7 motor-neurons. Therefore, T. skeletal muscles it is mediated through at - a loop. Alternation of reductions of various motor units within one muscle in the course of maintenance of T. interferes with development of exhaustion of a muscle in general and provides its constant activity, and also preservation of a pose without considerable energy costs.

Muscle spindles can perform functions inherent to them in that are a case if intrafusal fibers are reduced together with extra - • faugh screen, preventing excessive unloading of muscular «sensors of length», otherwise motor-neurons would appear under rigid control of receptors of tension (i.e. • tendinous receptors), reactions to-rykh are more stereotypic. There has to 'be a central control link providing joint activation and - and 7 motor-neurons. Disturbances in

the oc ii 7 mechanism of interface can be the cornerstone of pathology muscular T. (both a hyper tone, and its decrease — hypotonia). It is established that 'the cerebrate rigidity (see) is connected with strengthening of activity

of 7 motor-neurons of extensor muscles. Blocking carrying out excitement on 7 fibers, it is possible to achieve disappearance of rigidity. The increase in spinal reflexes during removal of a cerebellum and also arising at a detserebellyation strengthening anti-gravitational T. are caused by increase in excitability of a-, a not of 7 motor-neurons. The fact that removal of all cerebellum or its front share causes recovery of rigidity in the decerebrated cats and dogs (so-called and - rigidity) demonstrates to it. Disturbance muscular T., arising during removal of a cerebellum, it is connected with disturbance in mechanisms and - and 7 interfaces making an integral part of regulation of activity of muscles, including and tonic.

The tone of unstriated muscles is provided with two mechanisms: myogenetic and neurohumoral. Tonic contractions of unstriated muscles can proceed a long time at low energy level, differing in that from phase reductions of skeletal muscles that is explained by unequal structure of myofibrils of different types of muscular tissue.

At assessment of T. various vessels use the terms «arterial tone», «venous tone», «lymphatic tone».

Feature of nek-ry smooth muscle cells of walls of vessels is their ability to spontaneous activity and spread of activation from a cell to a cell. This feature of unstriated muscles provides basal T. vessels, supporting them in a condition of nek-ry tension in the absence of external (nervous or humoral) influences. The influences reducing the level of membrane potential (e.g., mechanical stretching or swelling of cells), increase the frequency of spontaneous categories and amplitude of reduction of unstriated muscles. On the contrary, hyperpolarization of cellular membranes leads to disappearance of spontaneous excitement and muscular contractions. The metabolites developed by fabrics make active impact on smooth muscle cells by the principle of a negative feed-back. So, at increase in T. precapillary sphincters the capillary blood stream decreases and concentration of metabolites increases that has vasodilating effect (see Mikrotsirkulyation). The low tension of oxygen and high — carbonic acids, strengthening of hydrogen ions, milk to - you, etc. has vasodilating effect.

Activity of unstriated muscles of arteries and veins, and also periblasts of large precapillary vessels, as well as skeletal, is regulated by generally external reflex and humoral influences. At the same time the role of the central nervous and humoral mechanisms of regulation vascular T increases. Excitement of a sikhmpatichesky nervous system promotes reduction of periblasts of a vascular wall, and inner layers at the expense of spread of activation from a cell to a cell are involved in response (it reaches centralization of control over vessels). In the absence of sympathetic influences and relaxation of periblasts of a vascular wall control over a local blood-groove is exercised by unstriated muscles of its interlayers, to-rye possess to own basal T. Because veins play a role of capacity vessels, sensitivity of their muscle cells to stretching is low. In unstriated muscles of veins (except for portal and mesenteric veins) signs of spontaneous activity, and in providing them T are not revealed. the big role belongs reflex furs-nizmakhm of regulation. Though level basal T. vascular system is defined by interaction of local mechanical and chemical factors, reflex mechanisms exercise control over all situation in general that creates conditions for redistribution of a blood-groove according to requirements of an organism. E.g., sympathetic adrenergic nerves have vasoconstrictive effect under the influence of change of a condition of nerve centers (e.g., a hypothalamus) or at the reflex reactions which are carried out at the bulbar and spinal levels as a result of change of activity of the receptor device of vascular reflexogenic zones. Humoral influence on vascular T. renders a number of substances (hormones cortical and marrow of adrenal glands,

angiotensin II, vasopressin, prostaglandins, a histamine, serotonin etc.).

Existence of two mechanisms (myogenetic and neurohumoral) causing T. vessels, creates conditions for maintenance of a certain level of the system ABP, and also for bystry local redistribution of a blood-groove in connection with the changing requirements of an organism.

Pathology of a muscle tone can be shown or in the form of its decrease (an atony, a hypomyotonia), or in the form of increase (a muscular hypertension). For objective registration of a muscle tone in clinic the palpation of muscles or assessment of degree of resistance of a muscle to compression by means of special devices is used. They allow to register also resistance of a muscle at its passive stretching. In this case determine a state so-called kontraktil-ny by T., under the Crimea understand the reflex muscle tension caused at its stretching owing to irritation of proprioceptors (see). At the same time by means of these devices often happens it is impossible or it is difficult to determine synergy changes muscular by T. in various muscular groups. These methods are more suitable for assessment of various therapeutic influences directed to normalization muscular to T. According to Gepfert's data (H. Gopfert, 1960), the ideal possibility of quantitative definition of a muscle tone would be reached if it is possible to determine number of muscle fibers excitable at present. Most closer elektromno-graphic registration of activity of muscle fibers approaches the specified purpose (see the Electromyography). Registration of electric activity of muscles at various pozotonichesky reactions was possible by means of the highly sensitive electrographic devices allowing to catch low-voltage fluctuations of biopotential, characteristic of tonic tension. Elektromiogra-fichesky researches T. also pathologies at the passive movements are made normal and by registration of bioelectric activity in a condition of muscular rest, at various reflex tonic reactions, and also at autokinesias.

In the clip, practice investigate resistance of muscles to stretching * at the same time inspected ask to relax muscles more often, not to show active resistance and make the passive movement, defining at the same time degree of tonic resistance of muscles.

Decrease or loss of T. it is caused in most cases by disturbance of integrity of a reflex arc; it can be a consequence of defeat as efferent link of a spinal arch (cells of front horns of a spinal cord, ventral roots, motor peripheral nerves), and an afferent link (sensory nerves, back roots). The hypomyotonia is characterized by increase in volume of passive movements in these or those joints, decrease in resistance of muscles to stretching, a cut in a certain measure is inherent in the stretched muscle normal; at an atony of muscles these signs inherent to hypotonia are expressed in sharper degree up to full «looseness» in joints. The hypomyotonia is observed usually and owing to the progressing atrophic defeat of muscular system (see the Atrophy muscular).

Quite often the hypomyotonia arises and in the absence of direct having rummagedand a spinal reflex arc (e.g., at damage of a cerebellum and its bonds). The hypomyotonia can arise at sharply arising cerebral paralyzes, napr, in the early postinsultny period, and is connected with temporary or more permanent oppression of function of efferent department of a spinal arch (cells of a front horn) owing to a diaschisis (see), weakening of the facilitating suprasni-nalny influences. In most cases this diaskhizalny hypotonia is replaced by the muscular hypertension which is a consequence of permanent defeat of depressor (brake) systems of a brain and gradual strengthening of the facilitating descending influences on spinal motor-neurons.

Sometimes the hypomyotonia at postinsultny paralyzes remains resistant (see Paralyses, paresis). It is observed, in particular, at defeat of nizhnestvolovy and .mozzhechkovy systems, and also at a bad general somatic condition of patients with secondary change of function T. nerve centers.

Increase in a muscle tone is connected with defeat of pyramidal and extrapyramidal system, and the main role in genesis of a muscular hypertension is played by defeat of extrapyramidal system (see).

Distinguish two types of a muscular hypertension — spasticity and rigidity. The spastic hypertension is characterized by irregularity of the distribution in mytptsakh-agonists and antagonists («selective type»), existence of a phenomenon of «penknife», to-ry consists in the uneven resistance of a muscle to passive stretching and sharp falling of resistance after a phase of the maximum resistance (lengthening reaction), the springing nature of a hypertension with a phenomenon of return upon termination of passive stretching, existence of direct dependence at most and speeds of a muscle strain: the quicker the N

is stronger stretching, the tension is stronger.

A muscular hypertension like rigidity or a plastic tone (see) unlike the spasticity which is characterized by selective type of distribution of a muscular hypertension usually evenly covers agonist muscles and antagonists. At the passive movements the uniform «wax-like» strength of muscles to stretching (the «wax» nature of a hypertension) or the rhythmic tolchkoobrazny resistance (a phenomenon of «cogwheel») is defined. The repeated passive movements lead quite often to strengthening of T. At rigidity there is no springing character of a hypomyotonia inherent to spasticity, and also direct dependence on the speed of a muscle strain. Muscles, on the contrary, find ability to adapt to passive rapprochement of points of their attachment and to stiffen tonic in this situation; extremities can is long to keep the pose (a cataleptic state) given them. These features come to light in clinic by means of a number of the tests reflecting dynamics of manifestation of postural reflexes (see).

Disturbances muscular T. are observed at many diseases of a nervous system. The hypomyotonia and an atony is characteristic of the diseases connected with defeat of perednerogovy motor-neurons and a peripheral nervous system (see Poliomyelitis, the Polyneuritis, the Plexitis, Neuritis); it is observed and at widespread defeat of back roots and back columns of a spinal cord, napr, at back to tabes (see), at diseases of a brain (e.g., at damage of a cerebellum and its bonds, striatal system). The spasticity accompanies many cerebral and spinal diseases proceeding with defeat of the nerve centers and motive ways transferring depressor influences on perednerogovy motor-neurons. Muscular rigidity can develop at many focal damages of a brain, but is most characteristic of the diseases connected with defeat of extrapyramidal system (see Parkinsonism, epatotsereb-

raljny dystrophy). At early children's age of change muscular T. can be noted at cerebral children's paralyzes (see); at nek-ry inborn diseases, napr, at a myatonia of Oppen-geyma (see the Myatonia). Quite often elements of a plastic tone are observed owing to age changes of subcrustal formations of a brain.

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