MUSCULAR CONTRACTION — set of the processes changing a mechanical condition of a muscle; it is shown in shortening of a muscle and development of mechanical tension by it.
Studying of physiology of muscular system, in particular mechanisms M. of page, began in 18 century. So, D. Bernoulli put forward the principle of muscular mechanics which is that M.'s height of page is with other things being equal proportional to length of muscle fibers. In the middle of 19 century E. Weber developed the idea of emergence of M. of page as a result of a muscle tension. He considered that the muscle is an elastic body, in Krom of force of the elastic resistance (tension) increase with loading. Thus, dependence between length and tension was shown i.e. that change of tension causes change of length of a muscle and vice versa. In 20 century researches of fine structure of muscular elements and biochemical began, the changes happening at M. of page It was established that interaction of proteins of actin and a myosin is the cornerstone of reduction, and the main sokratitelny protein — a myosin — has adenozintrifosfatazny activity. In the 50th Hansson and Huxley (J. Hanson, H. E. Huxley), having formulated a hypothesis of sliding of sokratitelny threads, laid the foundation sovr, understanding of ultrastructure of muscles and the mechanism M. of page.
Basis of sokratitelny activity of a muscle is single M. the page arising in response to nervous impulse. Graphically (fig.) single M. the page has a wave mode with the ascending and descending phases. The first phase is called reduction, the second — relaxation. Relaxation is more long in time, than reduction. The general time of single M. of page makes fractions of a second and depends from funkts, conditions of a muscle. M.'s duration of page decreases during the moderate work and increases at exhaustion. Single M. of page of the isolated fiber in the conditions of constant temperature submits to the law « everything or nothing » (see). Between the action potential of a muscle and single M.'s beginning of page there is a short-term interval — the eclipse period of reduction, in time to-rogo action potential extends on all muscle. At the same time from a sarcoplasmic reticulum there is an allocation of calcium ions in space between sokratitelny protofibrils (threads), elasticity of a muscle changes. At the end of the eclipse period of reduction in the muscle mekhanokhimiche-sky reactions of activation of sokratitelny elements proceed, to-rykh isotonic or isometric M. is result of page. Such M. of page is called isotonic, at Krom the muscle is freely shortened; at isometric M. of page length of a muscle remains to a constant (both of its ends are fixed) and only tension changes. In an organism in normal conditions of purely isotonic and isometric M. of page it is not observed.
If to a muscle, edges did not manage to relax completely from the previous reduction, new nervous impulse arrives, then the second M. is imposed by page on the first (the phenomenon of so-called superposition of M. of page, or mechanical summation). At rhythmic excitement the muscle comes to continuous M.'s condition of page (tetanus). Under a condition when an interval between impulses small, there is conjoint tetanic M. of page, or smooth tetanus (see). Tension during a tetanus is higher, than at single M. than page, elastic components stretch completely, and the muscle reaches the maximum tension; in the course of single M. page energy is spent by hl. obr. on stretching of the elastic components located in a muscle it is consecutive — threads of actin and a myosin, Z-plate (Z-zone), connections of trailer sarcomeres with sinews and sinews.
Cross-striped muscles have two major mechanical characteristics defining M.'s character of page. The first is known as relationship length — force (length — tension) muscles. Its essence is that for each muscle length can be found, at a cut it develops the maximum force (tension).
The second property of muscles is an interdependence of force and M.'s speed of page: the load, the more slowly its rise, and the more a superposed force is heavier, the speed of shortening of a muscle is less. At very big loading of M. of page becomes isometric; in this case the speed of reduction is equal to zero. Without loading M.S.'s speed the greatest. Between these (extreme) values the speed of shortening changes depending on loading. In an organism of a condition for implementation of this or that form of muscle performance are not always optimum in the «length — tension» and «force — speed» parameters. Therefore at implementation of this or that type of M. of page as a result of activity of c. the N of page is selectively selected quantity of bystry or slow neuromotor units. Speed range of M. of page is rather big — from fractions of a second (skeletal muscles) about one minutes (unstriated muscles). It is defined by many factors. So, fibers of cross-striped muscles have short sarcomeres, it is a lot of myofibrils, plentiful sar-kotubulyarny system, one or two nerve terminations; unstriated muscles are characterized by trace amount and the disorder arrangement of myofibrils, underdeveloped sarkotubulyarny system, low activity of miozinovy ATP-ase.
The m of page of skeletal muscles can be caused by one nervous impulse. M.'s emergence by the village of an unstriated muscle requires rhythmic stimulation. Speed of relaxation of skeletal and unstriated muscles considerably differs since depends on quantity of elastic elements in a muscle, lengths of fibers, saturating speeds of calcium ions etc.
Speed of binding of calcium ions an unstriated muscle in ten and more times is less, than in cross-striped. At rest the main part of calcium ions in muscle fiber is deposited in sarcoplasmic a reticulum. It represents protective system of the intracellular tubules and tanks surrounding each myofibril. In mechanisms M. of page a special role is played by that part of a reticulum, edges is located in the field of a Z-plate.
The main sokratitelny unit of a skeletal muscle is the sarcomere — the site of muscle fiber located between Z-plates. In a sarcomere it is arranged also fine (octynic) ends are located thick (miozino-Vyya), there is a system of longitudinal and cross tubules (tubules). The system of tubules consists of the embolies of a sarcolemma called cross or T-tubules, networks of longitudinal tubules of a sarcoplasmic reticulum and bubbles (trailer tanks) between myofibrils of amp. The complex of educations including T-tubules and adjacent two trailer tanks is called triad system of muscle fiber. Tolstoy and fine ends have zones of overlapping and unite crossbridges. Fine ends are attached to Z-plates.
At M. excitement of a membrane of muscle fiber is transferred to page inside on tubules of T-system, walls to-rykh contain charged particles and carry out an electric signal (process of transition from excitement to reduction is called electromechanical communication). The sarcoplasmic reticulum under the influence of the electric signal which came to T-system leaves calcium ions and immediately enter interaction with tropo-niny owing to what system troponin — tropomyosine loses ability to slow down interaction of actin with a myosin (troponin and tropomyosine is the proteins regulators interfering interaction of actin and a myosin). ATP-ase of a myosin is activated by actin with the participation of ions of magnesium. Energy for M. page is released by the activated ATP-ase of actomyosin hydrolyzing ATP on ADF and phosphate. At the same time a large amount of energy is released (to 10 kcal on 1 mol of ATP). Resynthesis of ATP comes from ADF and phosphate at the expense of creatine phosphate, processes of a glycogenolysis and glycolysis (see. Muscular tissue, biochemistry ). The termination of excitement leads to decrease in concentration of calcium in mezhfibrillyar-number space owing to active absorption of calcium membranes of a sarcoplasmic reticulum. Energy for active absorption of calcium ions is released during the splitting of ATP. This process on time matches heat production in a phase of relaxation of muscle fiber. Binding of calcium membranes of a reticulum reactivates system troponin — tropomyosine, interaction of actin and a myosin begins to brake edges again (a role of a factor of relaxation, or Marsh's factor — the substance found in homogenate of a fresh muscle, lowering ATF-aznuyu activity of a myosin and promoting relaxation the identical role of membranes of a sarcoplasmic reticulum should consider).
The majority of the theories explaining the mechanism M. of page is based on Huxley's idea about mutual sliding of thick and fine sokratitelny ends. In skeletal muscles the maximum tension develops at full mutual overlapping of miozi-new and octynic threads in the fields of education of bridges. When the muscle is stretched to such an extent that overlapping is absent, tension reduces to zero. At compression of a muscle fine ends, coming the friend for the friend, break process of interaction from a tolstbma threads, and tension of a muscle falls. Sliding of threads without change of their length, in addition to formation of crossbridges and splitting of ATP in a molecule of a myosin, arises at the expense of some conformational changes. On a hypothesis of R. J. Podolsky hydrolysis of ATP changes a tilt angle of bridges that leads to rotation of heads of a myosin or twisting of a miozinovy chain. In favor of this hypothesis (i.e. hypotheses of sliding without change of length of threads) the fact that tension in the reduced muscle if suddenly to relax her, is recovered in two stages says. Assume that each bridge consists of an elastic shoulder, to-ry, being in one of several power situations of stability, enters interaction with akti-new thread. Tension is recovered owing to elasticity of the basis of the bridge and turn of a miozinovy head in the situation having more low level of a potential energy. With physical. - chemical positions M. of page it is possible to present as process of transition of liquid state (a complex of molecules of actin, a myosin, ATP, calcium ions) in high-elastic (formation of the actomyosin having high elasticity and elasticity).
M.'s disturbances by the village can arise at defeats of various elements of neuromotor unit. Damage of bodies of motor-neurons or their axons, napr, at poliomyelitis (see) pl of progressive muscular dystrophy (see. Myopathy ), causes falling of a tone, an atrophy or dystrophy of muscles, muscular fibrillation, disturbance of sensitivity of muscles to influence of acetylcholine, increase in thresholds to electric irritation. Defeat of a motor plate is expressed by weakness and extreme fatigue of muscles. Dysfunction of a muscular membrane brings to myatonias (see), edges it is characterized by disturbance of normal process of relaxation: miotonichesky muscles are well reduced, but cannot normally relax. Defeat of function of actually sokratitelny device of muscles is observed at the progressing muscular dystrophy and contractures. Disturbance of the sokratitelny device is observed as well at the muscular hypertrophy caused by excessive load of any group of muscles.
M of page study by methods submicroscopy (see), X-ray crystallographic analysis (see), bystry stretching and bystry relaxation, etc. For this purpose use an intact muscle, the isolated sarcomere, the fibers processed by glycerin, trypsin, the isolated actomyosin. See also Muscular tissue , Muscles .
Bibliography: Arronet N. I. Muscular and cellular sokratitelny (motive) models, JI., 1971, bibliogr.; B of en-dollars of J. Muscles, molecules and the movement, the lane with English, M., 1970, bibliogr.; Deshcherevsky V. I. Mathematical models of muscular contraction, M., 1977, bibliogr.; Development of sokratitelny function of muscles of the motive device, under the editorship of JI. G. Magazanika and, A. Nasledo-va, JI., 1974, bibliogr.; Hill A. V. Mechanics of muscular contraction, the lane with English, M., 1972; Close R.I. Dynamic properties of mammalian skeletal muscles, Physiol. Rev., v. 52, p. 129, 1972, bibliogr.; E b a s h i S. Excitation-contraction coupling, Ann. Rev. Physiol., v. 38, p. 293, 1976, bibliogr.
P. S. Orlov.