SMOOTH MUSCLES
SMOOTH MUSCLES(involuntary muscle contractions), one of three types of muscles in vertebrates. Unlike SKELETAL MUSCLES, they are not amenable to conscious control by the brain, but are stimulated by the VEGETATIVE NERVOUS SYSTEM and HORMONES in the blood. Pomi-mo smooth muscles of the digestive tract, blood vessels, Bladder, there is another type of involuntary muscle contraction: heart the muscle (myocardium) that powers the HEART. see alsoHUMAN BODY.
Scientific and technical encyclopedic dictionary.
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Contractile (muscle) tissue, consisting of fusiform mononuclear cells. Unlike striated muscles, they do not have transverse striation. In most invertebrates, they make up the entire musculature of the body; in vertebrates they are part of ... ... Big Encyclopedic Dictionary
Smooth muscle tissue, hematoxylin eosin. Smooth muscle is a contractile tissue that, unlike striated muscles, consists of cells (not syncytium) and does not have ... Wikipedia
Contractile (muscle) tissue, consisting of fusiform mononuclear cells. Unlike striated muscles, they do not have transverse striation. In most invertebrates, they make up the entire musculature of the body; in vertebrates are part of ... ... encyclopedic Dictionary
SMOOTH MUSCLES- muscles internal organs forming the muscular layer of the stomach, intestines, blood vessels, etc. Unlike striated muscles, G.'s contraction of m is slower and longer; they can be in a reduced state for a long time ... Psychomotor: dictionary-reference
SMOOTH MUSCLES (musculi glaberi), contractile tissue, consisting of det. cells and not having a transverse striation. In invertebrates (except for arthropods and some representatives of other groups, for example pteropods) G. m. Form the whole ... ...
Contractile tissue, consisting, in contrast to striated muscles (see. Striated muscles), of cells (and not symplasts) and does not have a transverse striation. In invertebrates (except for all arthropods and individual representatives of others ... Great Soviet Encyclopedia
Contractile (muscle) tissue, consisting of fusiform mononuclear cells. In contrast to the striated muscles, they do not have a transverse striation. In most invertebrates, they make up the entire musculature of the body; in vertebrates are part of ... ... Natural science. encyclopedic Dictionary
MUSCLES- MUSCLES. I. Histology. General morphodogically, the tissue of the contractile substance is characterized by the presence of differentiation in the protoplasm of its specific elements. fibrillar structure; the latter are spatially oriented in the direction of their reduction and ... ...
Muscles (musculi), organs of the body of animals and humans, consisting of muscle tissue that can contract under the influence of nerve impulses. They carry out the movement of the body in space, the displacement of some of its parts relative to others (dynamic function) ... Biological encyclopedic dictionary
HUMAN MUSCLES- “80 №№ The name is Latin and Russian. Synonyms. Forsh, and position. Beginning and attachment. Innervation and relation to the network. Thyreo epiglotticus (thyroid epiglottis M.). Syn .: thyreo epiglotticus inferior, s. major, thyreo membranosus ... Great medical encyclopedia
Gone are the days when from appearance the house required a sense of imposingness and inaccessibility, but the most popular Roman facade decoration to this day is more than in demand when facing country houses. Today we will tell you about the use of rustic stones - the favorite finishing material of Italian architects of the 15th century and Russian masters of Peter's time.
Rusty corners of Chateau de la Bachasse, Rhone, France.
The term "rustic" is used by architects to refer to two things - the finishing stone itself or the dividing lines between stones (including those drawn on plaster). History knows many forms of rustic stones: the outer walls of buildings were usually lined with correctly folded rectangular stone slabs tightly fitted to each other, their front side retained the texture of a "wild" stone, remaining rough (or roughly cut), and around the edges they were surrounded by a narrow smooth strip. Arched openings were decorated with trapezoidal stones. Sometimes the rusts were laid out with bricks or made of planks with a subsequent two-color painting. Today, on the corners of houses, you can more and more often find smooth regular slabs made of artificial materials, and with the advent of rustic plasters, it became possible to simply draw them on the facades of houses.
Sandunovskie Baths on Neglina. Moscow, 1808. Redesign in 1896. Trail of history
The rustic style (from Latin rusticus - "simple, rough, rural" or from rus - "village, village") gained popularity during the Renaissance among Tuscan craftsmen. They drew inspiration from Roman buildings, where those architectural parts that were supposed to give the impression of strength and massiveness (basements of houses, towers, bridges, aqueducts and other more or less significant structures) were faced with stone (still without smooth welts). On the streets Ancient Rome the rustic had a clean practical use: it served as protection from the blows of carts passing through the narrow streets.
Rust corners on the house of V.E. Paisov. Kolyvan region near Novosibirsk. The Italians creatively approached their own heritage and, along with natural rough stone, began to use stucco imitation stone for decoration of facades, stucco imitation of spongy calcareous tuff and just plaster with reproduction of rustic - imitation of breaking the wall into rectangles or stripes. Brilliant examples of rusticism can be found in Florence - Palazzo Vecchio, Palazzo Ricardi - Medici, Palazzo Strozzi. The Pitti Palace, on the other hand, demonstrates new possibilities of rustication: the unsteady and fluid style of mannerism demanded lightness and a whimsical play of light and shadow from architectural forms. This is how the rust diamanti (or brilliant) was born - with "diamond" cut stones (a fine Russian example of style is the facade of the Faceted Chamber in the Kremlin).
Pavilion with rusticated walls near the Chateau de Versailles. Russian architects were carried away by rusticism at the turn of the 18th century in the era of Peter the Great's Baroque and Russian classicism, therefore both the historical center of St. Petersburg and the small merchant mansions of Moscow are often stylized as Florentine Renaissance palazzo, demonstrating elegant examples of French rustic style with deep horizontal incisions.
Bank of Moscow office on Kuznets most. Wint of time
Today, rustic stones are used in decoration only as decorative element, that is, they perform an exclusively aesthetic function. Therefore, the need to use a natural stone disappeared: it loads the load-bearing walls too much, it is difficult to dismantle it, and it is very expensive. It was replaced by an easy one fake diamond made of polyurethane, expanded polystyrene or architectural concrete. Such rusticum can have different shapes and textures; it is used for revealing the corners of buildings, window and door openings and smooth parts of facades. It can be easily combined with almost all types of wall coverings and looks equally elegant with brickwork, rubble, plaster and even siding. To decorate the corners today, rustic panels are used - 3-4 rustic ones, combined into a vertical detail: they allow, when facing the facade with a stone, to significantly simplify the installation of decor. So, having changed the main function from protective to decorative, rust continues to be one of the most noticeable and demanded elements of facade finishes.
Modern Rustas. Fiber cement panels, Metaform Architecture. What does not hurt to know
Rust is a rectangular stone for wall cladding, it can be rectangular, square, trapezoidal with bevelling or right angles. Rusting is a decorative treatment of walls that looks like a masonry of large stones. It can be in the form of horizontal stripes of equal height protruding in relief over the background. Rusty plaster is a modern finishing material, which is stones different shapes separated by rustic seams. The surface of the stones can be smooth or textured, of different colors and shades. The rusts themselves can be wide and narrow, smooth and with elements of architectural fractures. Marble (stone) plaster is a finishing material, which includes aggregate of granite and marble chips, which, when cracked, gives a sparkling chip. Used for finishing plinths and facades.Types of rustic stones
- "Diamond" (diamanty, diamond) rust- processing of protruding stones in the form of tetrahedral pyramids, resembling cut diamonds.
- Wedge rust- processing of an arched opening with large stones in the form of trapeziums with a large lock stone in the center, as well as decoration with the same wedge stones "with a shift" of the horizontal overlap of window or door openings.
- Muffled rust- transverse, "crossing out" the vertical line of the element rust (or clutch), contrary to tectonic logic. Used to decorate columns to create an impression of fragility.
- "French" (ribbon) rust- processing of the facade (usually the lower part) with deep horizontal incisions without vertical seams. Named French, as it was first used on the facade of the Grand Palace at Versailles.
- Faceted rust(seam) - a flat rustic, which has a complicated grainy texture or beveled edges.
They do not have cross striation (hence their name). Secondly, smooth muscles receive innervation not from the somatic, but from the autonomic part of the nervous system, therefore, they are deprived of direct voluntary regulation.
As in skeletal muscle, in smooth muscle, force is generated due to the fact that transverse bridges make their rotational movements between actin and myosin filaments, the activity of which is regulated by Ca2 + ions. However, the organization of contractile filaments and the process of electromechanical coupling are different for these two types of muscles. The mechanism of electromechanical coupling in different smooth muscles varies significantly.
The concentration of myosin in smooth muscle is only about a third of that in striated muscle, while the content of actin can be twice as high. Despite these differences, the maximum stress per unit cross-sectional area developed by smooth muscle is similar to that developed by skeletal muscle.
The relationship between isometric tension and length for smooth muscle cells is quantitatively the same as for fibers skeletal muscle... With the optimal length of a smooth muscle, maximum tension develops, and when it shifts in both directions from the optimal value, it decreases. However, compared to skeletal muscle, smooth muscle is able to develop tension over a wider range of lengths. This is an important adaptive property, considering that most of them are part of the walls of hollow organs, with a change in the volume of which the length of smooth muscle cells also changes. Even with a relatively large increase in volume, as, for example, when filling the bladder, smooth muscle cells in its walls retain to a certain extent the ability to develop tension; in cross-striped fibers, such stretching could lead to separation of thick and thin filaments outside the zone of their overlap.
Smooth muscles are part of the internal organs. Due to contraction, they provide the motor (motor) function of their organs (alimentary canal, genitourinary system, blood vessels, etc.). Unlike skeletal muscle, smooth muscle is involuntary.
Morpho-functional structure of smooth (not striated) muscles. The main structural unit of smooth muscle is a muscle cell, which has a fusiform shape and is covered from the outside by a plasma membrane. Under an electron microscope, numerous depressions - caveolae - can be seen in the membrane, which significantly increase the total surface of the muscle cell. The sarcolemma of the unimplemented muscle cell includes the plasma membrane, together with the basement membrane, which covers it from the outside, and adjacent collagen fibers. The main intracellular elements:
nucleus, mitochondria, lysosomes, microtubules, sarcoplasmic reticulum and contractile proteins.
Muscle cells form muscle bundles and muscle layers. The intercellular space (100 nm or more) is filled with elastic and collagen fibers, capillaries, fibroblasts, etc. In some areas, the membranes of neighboring cells lie very tightly (the gap between the cells is 2-3 nm). It is assumed that these areas (nexus) serve for intercellular communication, transmission of excitement. It has been proven that some smooth muscles contain a large number of nexus (the sphincter of the pupil, circular muscles of the small intestine, etc.), while others have few or none at all (vas deferens, longitudinal muscles of the intestines). There is also an intermediate, or desmopodibny, connection between non-darkened muscle cells (through a thickening of the membrane and with the help of cell processes). Obviously, these connections are important for the mechanical connection of cells and the transfer of mechanical force by cells.
Due to the chaotic distribution of myosin and actin protofibrils, smooth muscle cells are not striated like skeletal and cardiac ones. Unlike skeletal muscles, there is no T-system in smooth muscles, and the sarcoplasmic reticulum is only 2-7% of the volume of myoplasm and has no connection with the external environment of the cell.
Physiological properties of smooth muscles. Smooth muscle cells, like striated ones, contract due to the sliding of actin protofibrils between myosin cells, however, the sliding speed and ATP hydrolysis, and hence the rate of contraction, is 100-1000 times less than in striated muscles. Thanks to this, smooth muscles are well adapted for prolonged sliding with little energy consumption and without fatigue.
Smooth muscles, taking into account the ability to generate AP in response to threshold or supra-horny stimulation, are conventionally divided into phasic and tonic. Phase muscles generate full-fledged AP, tonic - only local, although they also have a mechanism for generating full-fledged potentials. The inability of tonic muscles to PD is explained by the high potassium permeability of the membrane, which prevents the development of regenerative depolarization.
The magnitude of the membrane potential of smooth muscle cells of non-brained muscles varies from -50 to -60 mV. As in other muscles, including nerve cells, mainly k +, Na +, Cl- are involved in its formation. In the smooth muscle cells of the alimentary canal, uterus, and some vessels, the membrane potential is unstable; spontaneous oscillations are observed in the form of slow depolarization waves, at the top of which PD discharges may appear. The duration of PD for smooth muscles ranges from 20-25 ms to 1 s or more (for example, in the muscles of the bladder), i.e. she
longer than the duration of skeletal muscle AP. Ca2 + plays an important role in the mechanism of AP of smooth muscles next to Na +.
Spontaneous myogenic activity. Unlike skeletal muscles, smooth muscles of the stomach, intestines, uterus, ureters have spontaneous myogenic activity, i.e. develop spontaneous tetanoglyodibny contractions. They are stored under conditions of isolation of these muscles and during pharmacological shutdown of the intrafusal plexus. So, PD occurs in the smooth muscle itself, and is not due to the transmission of nerve impulses to the muscles.
This spontaneous activity is of myogenic origin and occurs in muscle cells that act as pacemakers. In these cells, the local potential reaches a critical level and is transformed into AP. But for membrane repolarization, a new local potential spontaneously arises, which causes another AP, etc. AP, spreading through the nexus to neighboring muscle cells at a speed of 0.05-0.1 m / s, covers the entire muscle, causing it to contract. For example, peristaltic contractions of the stomach occur with a frequency of 3 times in 1 min, segmental and pendulum movements colon-in 20 times in 1 minute in the upper sections and 5-10 times in 1 minute in the lower ones. Thus, the smooth muscle fibers of these internal organs have automaticity, which is manifested by their ability to rhythmically contract in the absence of external stimuli.
What is the reason for the emergence of potential in the smooth muscle cells of the pacemaker? Obviously, it occurs due to a decrease in potassium and an increase in sodium and (or) calcium permeability of the membrane. Regarding the regular occurrence of slow waves of depolarization, most pronounced in the muscles of the gastrointestinal tract, there is no reliable data on their ionic origin. Perhaps a certain role is played by a decrease in the initial inactivating component of the potassium current during depolarization of muscle cells due to inactivation of the corresponding ionic potassium channels. Due to this, the occurrence of repeated G1D becomes possible.
Elasticity and extensibility of smooth muscles. Unlike skeletal muscles, they are smooth when stretched as plastic, elastic structures. Thanks to plasticity, the smooth muscle can be completely relaxed in both contracted and extended states. For example, the plasticity of the smooth muscles of the wall of the stomach or bladder as these organs fill up prevents an increase in intracavitary pressure. Excessive stretching often leads to stimulation of contraction, which is due to depolarization of pacemaker cells, which occurs when the muscle is stretched, and is accompanied by an increase in the frequency of PD, and as a result, an increase in contraction. The contraction, which activates the stretching process, plays an important role in the self-regulation of the basal tone of the blood vessels.
The mechanism of smooth muscle contraction. A prerequisite for the occurrence of contraction of smooth muscles, like skeletal muscles, is an increase in the concentration of Ca2 + in myoplasms (up to 10v-5 M). It is believed that the contraction process is activated mainly by extracellular Ca2 + entering muscle cells through voltage-gated Ca2 + channels.
The peculiarity of neuromuscular transmission in smooth muscles is that innervation is carried out by the autonomic nervous system and it can have both exciting and inhibitory effects. By type, there are cholinergic (acetylcholine mediator) and adrenergic (norepinephrine mediator) mediators. The former are usually found in the muscles of the digestive system, the latter in the muscles of the blood vessels.
One and the same mediator in some synapses can be excitatory, and in others - inhibitory (depending on the properties of cytoreceptors). Adrenergic receptors are divided into a- and B-. Norepinephrine, acting on a-adrenergic receptors, constricts blood vessels and inhibits the motility of the digestive tract, and acting on B-adrenergic receptors, stimulates the activity of the heart and expands the blood vessels of some organs, relaxes the muscles of the bronchi. Described neuromuscular. transmission in smooth muscles for help and other mediators.
In response to the action of an excitatory mediator, depolarization of smooth muscle cells occurs, which manifests itself in the form of an excitatory synaptic potential (ERP). When it reaches a critical level, PD occurs. This happens when several impulses come up to the nerve endings one after the other. The emergence of ZSGI is a consequence of an increase in the permeability of the postsynaptic membrane for Na +, Ca2 + and SI ".
The inhibitory mediator causes hyperpolarization of the postsynaptic membrane, which is manifested in the inhibitory synaptic potential (SHP). Hyperpolarization is based on an increase in membrane permeability, mainly for K +. The role of an inhibitory mediator in smooth muscles excited by acetylcholine (for example, the muscles of the intestine, bronchi) is played by norepinephrine, and in smooth muscles, for which norepinephrine is an excitatory mediator (for example, the muscles of the bladder), acetylcholine.
Clinical and physiological aspect. In some diseases, when the innervation of skeletal muscles is disturbed, their passive stretching or displacement is accompanied by a reflex increase in their tone, i.e. resistance to stretching (spasticity or stiffness).
In case of impaired blood circulation, as well as under the influence of certain metabolic products (lactic and phosphoric acids), toxic substances, alcohol, fatigue, a decrease in muscle temperature (for example, during prolonged swimming in cold water) after prolonged active contraction of the muscle, contracture may occur. The more the muscle function is impaired, the more pronounced the contracture aftereffect (for example, the contracture of the masticatory muscles in pathology of the maxillofacial region). What is the origin of contracture? It is believed that the contracture arose due to a decrease in the concentration of ATP in the muscle, which led to the formation of a permanent connection between the transverse bridges and actin protofibrils. In this case, the muscle loses flexibility and becomes hard. The contracture heals and the muscle relaxes when the ATP concentration reaches normal levels.
In diseases such as myotonia, the cell membranes of the muscles are excited so easily that even a slight irritation (for example, the introduction of a needle electrode in electromyography) causes a discharge of muscle impulses. Spontaneous AP (fibrillation potentials) are also recorded at the first stage after muscle denervation (until inactivity leads to muscle atrophy).
Tonic contractions of some smooth muscles, especially the muscles of the vascular walls (basal or myogenic, tone) are activated mainly by extracellular Ca 2 +. Physiologically active substances and mediators can cause a decrease in smooth muscle tone by closing chemosensitive Ca2 + channels (through activation of chemoreceptors) or hyperpolarization, which causes suppression of spontaneous AP and closure of voltage-gated Ca2 + channels.
Smooth muscles are present in hollow organs, blood vessels, and skin. Smooth muscle fibers have no cross-striation. The cells are shortened as a result of the relative sliding of the filaments. The sliding speed and the rate of decomposition of adenosine triphosphate are 100-1000 times less than in. Thanks to this, smooth muscles are well suited for long-term persistent contractions without fatigue, with less energy consumption.
Smooth muscles are an integral part of the walls of a number of hollow internal organs and are involved in ensuring the functions performed by these organs. In particular, they regulate blood flow in various organs and tissues, the patency of the bronchi for air, the movement of fluids and chyme (in the stomach, intestines, ureters, urinary and gallbladder), contraction of the uterus during childbirth, pupil size, skin relief.
Smooth muscle cells are spindle-shaped, 50-400 microns long, 2-10 microns thick (Fig. 5.6).
Smooth muscles refer to involuntary muscles, i.e. their reduction does not depend on the will of the macroorganism. The peculiarities of the motor activity of the stomach, intestines, blood vessels and skin to a certain extent determine the physiological characteristics of the smooth muscles of these organs.
Characteristics of smooth muscles
- Possesses automatism (the influence of intramural nervous system is corrective)
- Plasticity - the ability to maintain length for a long time without changing tone
- Functional synthesium - individual fibers are separated, but there are special contact areas - nexuses
- The value of the resting potential is 30-50 mV, the amplitude of the action potential is less than that of skeletal muscle cells
- Minimum "critical zone" (excitement occurs if a certain minimum number of muscle elements is excited)
- For the interaction of actin and myosin, the Ca 2+ ion is required, which comes from outside
- The duration of a single contraction is long
Feature of smooth muscles- their ability to exhibit slow rhythmic and long tonic contractions. Slow rhythmic contractions of the smooth muscles of the stomach, intestines, ureters and other hollow organs facilitate the movement of their contents. Prolonged tonic contractions of the smooth muscles of the sphincters of the hollow organs prevent the voluntary release of their contents. The smooth muscles of the walls of blood vessels are also in a state of constant tonic contraction and affect the level of blood pressure and blood supply to the body.
An important property of smooth muscles is their mysticism, those. the ability to maintain shape caused by stretching or deformation. The high plasticity of smooth muscles is of great importance for the normal functioning of organs. For example, the plasticity of the bladder allows, when it is filled with urine, to prevent an increase in pressure in it without disrupting the process of urination.
Excessive stretching of smooth muscles causes them to contract. This occurs as a result of depolarization of cell membranes caused by their stretching, i.e. smooth muscles have automatism.
The contraction caused by stretching plays an important role in the autoregulation of the tone of the blood vessels, the movement of contents gastrointestinal tract and other processes.
Rice. 1. A. Skeletal muscle fiber, cardiac muscle cell, smooth muscle cell. B. Sarcomere of skeletal muscle. B. The structure of smooth muscle. D. Mechanogram of skeletal muscle and heart muscle.
Automatism in smooth muscles is due to the presence of special pacemaker (rhythm-setting) cells in them. In their structure, they are identical to other smooth muscle cells, but they have special electrophysiological properties. In these cells, pacemaker potentials arise that depolarize the membrane to a critical level.
Excitation of smooth muscle cells causes an increase in the entry of calcium ions into the cell and the release of these ions from the sarcoplasmic reticulum. As a result of an increase in the concentration of calcium ions in the sarcoplasm, contractile structures are activated, but their activation mechanism in smooth fiber differs from the activation mechanism in striated muscles. In a smooth cell, calcium interacts with the protein calmodulin, which activates the light chains of myosin. They connect to the active centers of actin in protofibrils and perform a "stroke". The smooth muscles relax passively.
Smooth muscles are involuntary, and they do not depend on the will of the animal.
Physiological properties and features of smooth muscles
Smooth muscles, like skeletal muscles, have excitability, conductivity and contractility. Unlike skeletal muscles, which have elasticity, smooth muscles have plasticity - the ability long time maintain the length given to them when stretched without increasing stress. This property is important for the function of depositing food in the stomach or fluids in the gallbladder and bladder.
The peculiarities of the excitability of smooth muscle cells are to a certain extent associated with a low potential difference across the membrane at rest (E 0 = (-30) - (-70) mV). Smooth myocytes can be automatic and generate an action potential spontaneously. Such cells - pacemakers of smooth muscle contraction are found in the walls of the intestine, venous and lymphatic vessels.
Rice. 2. The structure of smooth muscle cells (A. Guyton, J. Hall, 2006)
The duration of AP in smooth myocytes can reach tens of milliseconds, since AP in them develops mainly due to the entry of Ca 2+ ions into the sarcoplasm from the intercellular fluid through slow calcium channels.
The rate of conduction of PD along the membrane of smooth myocytes is low - 2-10 cm / s. Unlike skeletal muscles, excitation can be transmitted from one smooth myocyte to others lying nearby. This transmission occurs due to the presence of nexus between smooth muscle cells, which have low resistance electric current and providing the exchange between the cells of Ca 2+ ions and other molecules. As a result, the smooth muscle exhibits the properties of functional synthesia.
The contractility of smooth muscle cells is characterized by a long latency period (0.25-1.00 s) and a long duration (up to 1 min) of a single contraction. Smooth muscles develop a small force of contraction, but they are able to stay in tonic contraction for a long time without the development of fatigue. This is due to the fact that smooth muscle consumes 100-500 times less energy than skeletal muscle to maintain tonic contraction. Therefore, the reserves of ATP consumed by the smooth muscle have time to recover even during contraction, and the smooth muscles of some structures of the body are almost constantly in a state of tonic contraction. The absolute strength of a smooth muscle is about 1 kg / cm 2.
Smooth muscle contraction mechanism
The most important feature of smooth muscle cells is that they are excited under the influence of numerous stimuli. in vivo is initiated only by a nerve impulse coming to. Contraction of smooth muscle can be caused both by the influence of nerve impulses and by the action of hormones, neurotransmitters, prostaglandins, some metabolites, as well as by physical factors such as stretching. In addition, the excitation and contraction of smooth myocytes can occur spontaneously - due to automatics.
The ability of smooth muscles to respond by contraction to the action of various factors will create significant difficulties for correcting violations of the tone of these muscles in medical practice. This can be seen in the examples of the difficulties in the treatment of bronchial asthma, arterial hypertension, spastic colitis and other diseases requiring correction of the contractile activity of smooth muscles.
The molecular mechanism of smooth muscle contraction also differs from the mechanism of skeletal muscle contraction. The actin and myosin filaments in smooth muscle cells are less ordered than in skeletal cells, and therefore smooth muscle does not have a transverse striation. There is no troponin protein in the actin filaments of smooth muscle, and actin centers are always open for interaction with the myosin heads. At the same time, the myosin heads at rest are not energized. In order for the interaction of actin and myosin to occur, it is necessary to phosphorylate the myosin heads and give them an excess of energy. The interaction of actin and myosin is accompanied by a turn of the myosin heads, in which actin filaments are drawn between myosin filaments and the smooth myocyte contracts.
Phosphorylation of myosin heads is carried out with the participation of the enzyme kinase of myosin light chains, and dephosphorylation - with the help of phosphatase. If the activity of myosin phosphatase prevails over the activity of kinase, then the myosin heads are dephosphorylated, the connection between myosin and actin is broken, and the muscle relaxes.
Therefore, in order for smooth myocyte to contract, it is necessary to increase the activity of myosin light chain kinase. Its activity is regulated by the level of Ca 2+ ions in the sarcoplasm. Neurotransmitters (acetylcholine, noradrsnalin) or hormones (vasopressin, oxytocin, adrenaline) stimulate their specific receptor, causing the dissociation of the G-protein, the a-subunit of which further activates the enzyme phospholipase C. cell membranes. IPZ diffuses to the endoplasmic reticulum and, after interacting with its receptors, causes the opening of calcium channels and the release of Ca 2+ ions from the depot into the cytoplasm. An increase in the content of Ca 2+ ions in the cytoplasm is a key event for the initiation of smooth myocyte contraction. An increase in the content of Ca 2+ ions in the sarcoplasm is also achieved due to its entry into the myocyte from the extracellular environment (Fig. 3).
Ca 2+ ions form a complex with the protein calmodulin, and the Ca 2+ -calmodulin complex increases the kinase activity of myosin light chains.
The sequence of processes leading to the development of smooth muscle contraction can be described as follows: entry of Ca 2+ ions into the sarcoplasm - activation of calmodulin (by the formation of a 4Ca 2 -calmodulin complex) - activation of myosin light chain kinase - phosphorylation of myosin heads - binding of myosin heads with actin and the rotation of the heads, in which the actin filaments are drawn between the myosin filaments - contraction.
Rice. 3. Ways of entry of Ca 2+ ions into the sarcoplasm of a smooth muscle cell (a) and their removal from the sarcoplasm (b)
Conditions for smooth muscle relaxation:
- decrease (up to 10-7 M / l and less) the content of Ca 2+ ions in the sarcoplasm;
- decomposition of the 4Ca 2+ -calmodulin complex, leading to a decrease in the activity of myosin light chain kinase - dephosphorylation of myosin heads under the influence of phosphatase, leading to the rupture of the bonds of actin and myosin filaments.
Under these conditions, elastic forces cause a relatively slow recovery of the original length of the smooth muscle fiber and its relaxation.
