These muscles form the muscular layers of the walls of the stomach, intestines, ureters, bronchi, blood vessels and other internal organs. They are built from spindle-shaped mononuclear muscle cells. Smooth muscles are divided into two main groups: multiunitary and unitary. Multiunitary muscles function independently of each other, and each fiber can be innervated by a separate nerve ending. Such fibers are found in the ciliary muscle of the eye, the nictitating membrane and the muscle layers of some large vessels, these include the muscles that raise the hair. U unitary muscles the fibers are so tightly intertwined that their membranes can fuse to form electrical contacts (nexuses). When one fiber is irritated by these contacts, APs rapidly propagate to neighboring fibers. Therefore, despite the fact that the motor nerve endings are located on a small number of muscle fibers, the entire muscle is involved in the reaction. Such muscles are found in most organs: the digestive tract, uterus, and ureters.

A feature of smooth muscles is their ability to carry out slow and prolonged tonic contractions. Slow, rhythmic contractions of the smooth muscles of the stomach, intestines, ureters and other organs ensure the movement of the contents of these organs. Prolonged tonic contractions of smooth muscles ensure the functioning of the sphincters of hollow organs, which prevent the release of their contents.

The smooth muscles of the walls of blood vessels, especially arteries and arterioles, are also in a state of constant tonic contraction. Changes in muscle tone in the walls of arterial vessels affect the size of their lumen and, consequently, the level of blood pressure and blood supply to organs. An important property of smooth muscles is their plasticity, i.e. the ability to maintain the length given to them when stretched. Normal skeletal muscle has almost no plasticity. When the tensile load is removed, the skeletal muscle quickly shortens, but the smooth muscle remains stretched. The high plasticity of smooth muscles is of great importance for the normal functioning of hollow organs. For example, the plasticity of the bladder muscles as it fills prevents excessive pressure build-up.

A strong and sharp stretch of smooth muscles causes their contraction, which is due to the depolarization of cells that increases with stretching, which ensures the automaticity of the smooth muscle. This contraction plays an important role in the autoregulation of blood vessel tone, and also contributes to the involuntary emptying of a full bladder in cases where neural regulation is absent as a result of spinal cord damage.

In smooth muscle, tetanic contraction occurs at low stimulation frequencies. Unlike skeletal muscles, smooth muscles are capable of developing spontaneous thetan-like contractions under conditions of denervation and even after blockade of the intramural ganglia. Such contractions occur due to the activity of cells with automaticity (pacemaker cells), which differ in electrophysiological properties from other muscle cells. Pacemaker potentials appear in them, depolarizing the membrane to a critical level, which causes the occurrence of an action potential.

A feature of smooth muscles is their high sensitivity to mediators, which have a modulating effect on the spontaneous activity of pacemakers. When acetylcholine is applied to the colon muscle preparation, the frequency of PD increases. The contractions they cause merge, forming an almost smooth tetanus. The higher the AP frequency, the stronger the contraction. Norepinephrine, on the contrary, hyperpolarizes the membrane, reducing the frequency of AP and the magnitude of tetanus.

Excitation of smooth muscle cells causes an increase in calcium concentration in the sarcoplasm, which activates contractile structures. Like cardiac and skeletal muscle, smooth muscle relaxes when the concentration of calcium ions decreases. Smooth muscle relaxation occurs more slowly because the removal of calcium ions is slower.

Gone are the days when the appearance of the house required a sense of imposingness and impregnability, but the most popular Roman facade decoration is still more than in demand when facing country houses. Today we will talk about the use of rustic stones - the favorite finishing material of Italian architects of the 15th century and Russian masters of Peter the Great's time.

Rusticated corners of Chateau de la Bachasse, Rhone, France.

The term "rust" is used by architects to refer to two things - the finishing stone itself or the dividing strips between the stones (including those drawn on the plaster). History knows many forms of rusticated stones: the outer walls of buildings were usually lined with properly folded quadrangular stone slabs tightly fitted to each other, their front side retained the texture of the “wild” stone, remaining unhewn (or roughly hewn), and along the edges they were surrounded by a narrow smooth strip. Arched openings were decorated with trapezoidal stones. Sometimes rustication was laid out with bricks or made of boards with subsequent two-tone coloring. Today, on the corners of houses, you can increasingly find smooth regular slabs made of artificial materials, and with the advent of rustic plasters, it has become possible to simply draw them on the facades of houses.

Sandunovskie baths on Neglina. Moscow, 1808. Redesign in 1896.

Trail of history

Rustic style (from Latin rusticus - "simple, rough, rural" or from rus - "village, village") gained popularity during the Renaissance among Tuscan masters. They drew inspiration from Roman buildings, where those architectural parts that were supposed to give the impression of strength and massiveness (the plinths of houses, towers, bridges, aqueducts and other more or less significant structures) were faced with stone (still without smooth welts). On the streets of ancient Rome, rustication also had a purely practical use: it served as protection against the blows of carts passing through the narrow streets.

Rustic corners on the house of V.E. Paisova. Kolyvansky district near Novosibirsk.

The Italians creatively approached their own heritage and, along with natural unhewn stone, began to use stucco imitation stone for finishing facades, a stucco imitation of spongy calcareous tuff, and simply plaster with a reproduction of rust - an 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 demonstrates new possibilities for rustication: the unsteady and fluid style of Mannerism required lightness and a bizarre play of light and shadow from architectural forms. This is how diamanti (or diamond) rustication was born - with “diamond” cut stones (an excellent Russian example of the style is the facade of the Faceted Chamber in the Kremlin).

Pavilion with rusticated walls near the Château de Versailles.

Russian architects became interested in rusticism at the turn of the 18th century during the era of Peter the Great’s Baroque and Russian classicism, which is why both the historical center of St. Petersburg and the small merchant mansions of Moscow are often stylized as Florentine palazzos of the Renaissance, demonstrating elegant examples of French rustication with deep horizontal cuts.

Office of the Bank of Moscow on the Kuznets bridge.

The spirit of the times

Today, rusticated stones are used in decoration only as a decorative element, that is, they perform an exclusively aesthetic function. Therefore, there is no need to use natural stone: it puts too much load on load-bearing walls, is difficult to dismantle, and is very expensive. It was replaced by lightweight artificial stone made of polyurethane, polystyrene foam or architectural concrete. Such rustication can have different shapes and textures; it is used to cover 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. Today, to decorate corners, rusticated panels are used - 3-4 rusticated panels combined into a vertical part: they make it possible to significantly simplify the installation of decor when cladding the facade with stone. So, having changed the main function from protective to decorative, rust continues to be one of the most noticeable and sought-after elements of facade finishing.

Modern rustics. Fiber cement panels, Metaform Architecture.

What doesn't hurt to know

Rust is a quadrangular stone for wall cladding, it can be rectangular, square, trapezoidal with a chamfer or right angles. Rustication is a decorative treatment of walls that looks like masonry made of large stones. It may look like horizontal stripes of equal height protruding above the background. Rusticized plasters are a modern finishing material made up of stones of different shapes, separated by rusticated seams. The surface of the stones is smooth or textured, different colors and shades. The rustics themselves can be wide and narrow, smooth and with elements of architectural breakage. Marble (stone) plasters are finishing materials that contain a filler of granite and marble chips, which, when split, produce sparkling chips. Used for finishing plinths and facades.

Types of rustics

• "Brilliant" (diamanti, diamond) rust- processing of protruding stones in the form of tetrahedral pyramids, reminiscent of faceted diamonds.
• Wedge rust- processing the arched opening with large stones in the shape of trapezoids with a large keystone in the center, as well as decorating the horizontal overlap of window or door openings with the same wedge-shaped stones “with a shift”.
• Coupled rust- transverse rust (or coupling) that “crosses out” the vertical line of the element, contradicting tectonic logic. Used to decorate columns to give the impression of fluctuation.
• "French" (tape) rust- processing of the facade (usually the lower part) with deep horizontal cuts without vertical seams. It was called French because it was first used on the façade of the Grand Palace at Versailles.
• Faceted rustication(seam) - a flat rustication that has a complicated granular texture or beveled edges.

Smooth muscle is a contractile tissue consisting of individual cells and without transverse striations (Fig. 1.). The smooth muscle cell has a spindle-shaped shape, approximately 50 - 400 µm in length and 2-10 µm in thickness. Individual threads are connected by special intercellular contacts - desmosomes and form a network with collagen fibers woven into it. The lack of cross-striations characteristic of cardiac and skeletal muscles is explained by the irregular distribution of myosin and actin filaments. Smooth muscles also shorten due to the sliding of myofilaments relative to each other, but the speed of sliding and the breakdown of ATP here is 100 - 1000 times lower than that of striated muscles. In this regard, smooth muscles are especially well adapted for long-term sustainable contraction, which does not lead to fatigue and significant energy consumption.

Smooth muscles are part of internal organs, blood vessels and skin. They are distinguished by the presence of interesting functional features: the ability to carry out relatively slow movements and prolonged tonic contractions. Slow movements (contractions), often having a rhythmic contraction of the smooth muscles of the walls of hollow organs: the stomach, intestines, ducts of the digestive glands, bladder, gall bladder, ensure the movement of the contents of these organs. An example is the pendular and peristaltic movements of the intestines. Prolonged tonic contractions of smooth muscles are especially pronounced in the sphincters of hollow organs; their tonic contractions prevent the release of contents. This ensures the presence of bile in the gallbladder and urine in the bladder, and the formation of feces in the large intestine.

Shows the structure (left) of striated and smooth muscles in vertebrates and the relationship between electrical (solid lines) and mechanical (dashed lines) activity (right). A. Striated muscles are multinucleated cylindrical cells. They generate fast action potentials and fast contractions. B. Smooth muscle fibers have one core, small size and fusiform shape. They are connected to each other by their lateral surfaces through gap junctions and form electrically united groups of cells.

The innervation is diffuse, the activation of the fibers is carried out due to the release of the mediator from the extensions located along the autonomic nerve. Although smooth muscle cell action potentials are fast, the resulting contractions are slow and long-lasting.

The thin smooth muscles of the walls of blood vessels, especially arteries and arterioles, are in a state of constant tonic contraction. The tone of the muscle layer of the artery walls regulates blood pressure and blood supply to organs.

Motor innervation of smooth muscles is carried out by processes of cells of the autonomic nervous system, sensitive - by processes of cells of sympathetic ganglia. The tone and motor function of smooth muscles are also regulated by humoral influences.

All smooth muscles can be divided into two groups:

1. Smooth muscles with myogenic activity. In many intestinal smooth muscles (eg, the cecum), a single contraction caused by an action potential lasts several seconds. Consequently, contractions that follow with an interval of less than 2 s overlap each other, and at a frequency above 1 Hz they merge into a more or less smooth tetanus (tetan-like tone) (Fig. 2). The nature of such a tetanus is myogenic; Unlike skeletal muscle, smooth muscles of the intestine, ureter, stomach and uterus are capable of spontaneous thetan-like contractions after isolation and denervation and even with blockade of intramural ganglion neurons. Consequently, their action potentials are not caused by the transmission of nerve impulses to the muscle, but are of myogenic origin.

Myogenic excitation occurs in pacemaker cells, which are identical to other muscle cells in structure, but differ in electrophysiological properties. Pacemaker potentials depolarize the membrane to a threshold level, causing an action potential. Due to the entry of cations into the cell (mainly Ca2+), the membrane depolarizes to zero level and even changes polarity to +20 mV for a few milliseconds. After repolarization, a new pacemaker potential follows, ensuring the generation of the next action potential. When a colon preparation is exposed to acetylcholine, pacemaker cells depolarize to a near-threshold level, and the frequency of action potentials increases. The contractions they cause merge to an almost smooth tetanus. The higher the frequency of action potentials, the more united the tetanus and the stronger the contraction resulting from the summation of single contractions. Conversely, application of norepinephrine to the same preparation forms a hyperpolar membrane and, as a result, reduces the frequency of action potentials and the magnitude of tetanus. These are the mechanisms of modulation of the spontaneous activity of pacemakers by the autonomic nervous system and its mediators.

Fig.2.

Treatment with acetylcholine (arrow) increases the frequency of action potentials so that single beats coalesce into tetanus. The bottom record is the time course of muscle tension.

2. Smooth muscles without myogenic activity. Unlike the intestinal muscles, the smooth muscles of the arteries, seminal ducts, iris, and ciliary muscles usually have little or no spontaneous activity. Their contraction occurs under the influence of impulses supplied to these muscles via the autonomic nerves. Such features are due to the structural organization of their tissue. Although the cells in it are electrically connected by nexuses, many of them form direct synaptic contacts with the axons innervating them, but do not form the usual neuromuscular synapses in smooth muscle tissue. The release of the transmitter occurs from numerous thickenings (extensions) located along the length of the autonomic axons (Fig. 1).

Mediators reach muscle cells through diffusion and activate them. At the same time, excitatory potentials arise in the cells, turning into action potentials that cause a tetanic contraction.

Functions and properties of smooth muscles

Electrical activity. Visceral smooth muscles are characterized by unstable membrane potential. Fluctuations in membrane potential, regardless of nervous influences, cause irregular contractions that maintain the muscle in a state of constant partial contraction - tone. The tone of smooth muscles is clearly expressed in the sphincters of hollow organs: the gall bladder, bladder, at the junction of the stomach into the duodenum and the small intestine into the large intestine, as well as in the smooth muscles of small arteries and arterioles. The membrane potential of smooth muscle cells does not reflect the true value of the resting potential. When the membrane potential decreases, the muscle contracts, and when the membrane potential increases, it relaxes.

Automation. The action potentials of smooth muscle cells are autorhythmic (pacemaker) in nature, similar to the potentials of the conduction system of the heart. Pacemaker potentials are recorded in various areas of smooth muscle. This indicates that any visceral smooth muscle cells are capable of spontaneous automatic activity. Automaticity of smooth muscles, i.e. the ability for automatic (spontaneous) activity is inherent in many internal organs and vessels.

Tensile response. A unique feature of visceral smooth muscle is its response to stretch. In response to stretch, smooth muscle contracts. This is due to the fact that stretching reduces the membrane potential of cells, increases the frequency of AP and, ultimately, the tone of smooth muscles. In the human body, this property of smooth muscles is one of the ways to regulate the motor activity of internal organs. For example, when the stomach is filled, its wall stretches. An increase in the tone of the stomach wall in response to its stretching contributes to the preservation of the volume of the organ and better contact of its walls with the incoming food. In blood vessels, the stretch created by fluctuations in blood pressure is the main factor in myogenic self-regulation of vascular tone. Finally, stretching of the uterine muscles by the growing fetus is one of the reasons for the onset of labor.

Plastic. Another important specific characteristic of smooth muscle is the variability of tension without a regular connection with its length. Thus, if visceral smooth muscle is stretched, its tension will increase, but if the muscle is held in the state of elongation caused by stretching, then the tension will gradually decrease, sometimes not only to the level that existed before the stretch, but also below this level. This property is called smooth muscle plasticity. Thus, smooth muscle is more similar to a viscous plastic mass than to a poorly pliable structured tissue. The plasticity of smooth muscles contributes to the normal functioning of internal hollow organs.

Relationship between excitation and contraction. It is more difficult to study the relationship between electrical and mechanical manifestations in visceral smooth muscle than in skeletal or cardiac muscle, since visceral smooth muscle is in a state of continuous activity. Under conditions of relative rest, a single AP can be recorded. The contraction of both skeletal and smooth muscle is based on the sliding of actin in relation to myosin, where the Ca2+ ion performs a trigger function.

The mechanism of contraction of smooth muscle has a feature that distinguishes it from the mechanism of contraction of skeletal muscle. This feature is that before smooth muscle myosin can exhibit its ATPase activity, it must be phosphorylated. Phosphorylation and dephosphorylation of myosin is also observed in skeletal muscle, but in it the phosphorylation process is not necessary to activate the ATPase activity of myosin.

Chemical sensitivity. Smooth muscles are highly sensitive to various physiologically active substances: adrenaline, norepinephrine, ACh, histamine, etc. This is due to the presence of specific receptors on the smooth muscle cell membrane. If you add adrenaline or norepinephrine to a preparation of intestinal smooth muscle, the membrane potential increases, the frequency of AP decreases and the muscle relaxes, i.e., the same effect is observed as when the sympathetic nerves are excited.

Norepinephrine acts on b- and b-adrenergic receptors of the membrane of smooth muscle cells. The interaction of norepinephrine with β-receptors reduces muscle tone as a result of activation of adenylate cyclase and the formation of cyclic AMP and a subsequent increase in the binding of intracellular Ca2+. The effect of norepinephrine on β-receptors inhibits contraction by increasing the release of Ca2+ ions from muscle cells.

ACh has an effect on membrane potential and contraction of intestinal smooth muscle that is opposite to the effect of norepinephrine. The addition of ACh to an intestinal smooth muscle preparation reduces membrane potential and increases the frequency of spontaneous APs. As a result, the tone increases and the frequency of rhythmic contractions increases, i.e., the same effect is observed as when the parasympathetic nerves are excited. ACh depolarizes the membrane and increases its permeability to Na+ and Ca+.

The smooth muscles of some organs respond to various hormones. Thus, the smooth muscles of the uterus in animals during the periods between ovulation and when the ovaries are removed are relatively inexcitable. During estrus or in ovariectomized animals that have been given estrogen, smooth muscle excitability increases. Progesterone increases membrane potential even more than estrogen, but in this case the electrical and contractile activity of the uterine muscles is inhibited.

They do not have transverse striations (hence their name). Secondly, smooth muscles receive innervation not from the somatic, but from the autonomic nervous system, and therefore are deprived of direct voluntary regulation.

As in skeletal muscle, in smooth muscle force is generated due to the fact that cross bridges, the activity of which is regulated by Ca2+ ions, perform their rotational movements between actin and myosin filaments. 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 varies significantly between smooth muscles.

The myosin concentration in smooth muscle is only about a third of that in striated muscle, while the actin content can be twice as high. Despite these differences, the maximum tension 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 skeletal muscle fibers. At the optimal length of the smooth muscle, maximum tension develops, and when it shifts to either side of the optimal value, it decreases. However, compared to skeletal muscle, smooth muscle is capable of developing 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, when the volume of which changes, the length of the smooth muscle cells also changes. Even with a relatively large increase in volume, as, for example, when the bladder is filled, the smooth muscle cells in its walls retain a certain ability to develop tension; in striated fibers, such stretching could cause the thick and thin filaments to diverge beyond their overlap zone.

Smooth muscles are part of the internal organs. Thanks to contraction, they provide the motor (motor) function of their organs (digestive canal, genitourinary system, blood vessels, etc.). Unlike skeletal muscles, smooth muscles are involuntary.
Morpho-functional structure of smooth (non-striated) muscles. The main structural unit of smooth muscle is the muscle cell, which has a spindle-shaped shape and is covered on the outside with a plasma membrane. Under an electron microscope, numerous depressions can be seen in the membrane - caveolae, which significantly increase the total surface of the muscle cell. The sarcolemma of a muscle cell includes a plasma membrane along with the basement membrane, which covers it from the outside, and adjacent collagen fibers. 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 cells is 2-3 nm). It is assumed that these areas (nexus) serve for intercellular communication and transmission of excitation. It has been proven that some smooth muscles contain a large number of nexus (pupillary sphincter, circular muscles of the small intestine, etc.), while others have little or no nexus (vas deferens, longitudinal muscles of the intestines). There is also an intermediate, or desmopodibny, connection between non-skinned muscle cells (through thickening of the membrane and with the help of cell processes). Obviously, these connections are important for the mechanical connection of cells and the transmission 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 cells. Unlike skeletal muscles, smooth muscles do not have a T-system, and the sarcoplasmic reticulum makes up only 2-7% of the myoplasm volume and has no connections 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 protofibrils, but the speed of sliding and hydrolysis of ATP, and therefore the speed of contraction, is 100-1000 times less than in striated muscles. Thanks to this, smooth muscles are well adapted for long-term gliding with little energy expenditure and without fatigue.
Smooth muscles, taking into account the ability to generate AP in response to threshold or supra-horn stimulation, are conventionally divided into phasic and tonic. Phasic muscles generate a full-fledged potential action, while tonic muscles generate only a local one, although they also have a mechanism for generating full-fledged potentials. The inability of tonic muscles to perform AP is explained by the high potassium permeability of the membrane, which prevents the development of regenerative depolarization.
The value of the membrane potential of smooth muscle cells of non-skinned muscles varies from -50 to -60 mV. As in other muscles, including nerve cells, mainly +, Na +, Cl- take part in its formation. In the smooth muscle cells of the digestive canal, uterus, and some vessels, the membrane potential is unstable; spontaneous fluctuations are observed in the form of slow waves of depolarization, at the top of which AP discharges may appear. The duration of smooth muscle action potential 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. In the mechanism of action of smooth muscles, next to Na +, Ca2 + plays an important role.
Spontaneous myogenic activity. Unlike skeletal muscles, smooth muscles of the stomach, intestines, uterus, and ureters have spontaneous myogenic activity, i.e. develop spontaneous tetanohyodine contractions. They are stored under conditions of isolation of these muscles and with pharmacological switching off of the intrafusal nerve plexuses. So, AP occurs in the smooth muscles themselves, and is not caused by the transmission of nerve impulses to the muscles.
This spontaneous activity is of myogenic origin and occurs in muscle cells that function as a pacemaker. In these cells, the local potential reaches a critical level and passes into AP. But after membrane repolarization, a new local potential spontaneously arises, which causes another AP, etc. The AP, spreading through the nexus to neighboring muscle cells at a speed of 0.05-0.1 m/s, covers the entire muscle, causing its contraction. For example, peristaltic contractions of the stomach occur with a frequency of 3 times per 1 minute, segmental and pendulum-like movements of the colon - 20 times per 1 minute in the upper sections and 5-10 per 1 minute in the lower sections. Thus, the smooth muscle fibers of these internal organs have automaticity, which is manifested by their ability to contract rhythmically in the absence of external stimuli.
What is the reason for the appearance of potential in pacemaker smooth muscle cells? Obviously, it occurs due to a decrease in potassium and an increase in sodium and (or) calcium permeability of the membrane. As for 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 potassium ion channels. Thanks to this, the occurrence of repeated G1D becomes possible.
Elasticity and extensibility of smooth muscles. Unlike skeletal muscles, smooth muscles act as plastic, elastic structures when stretched. Thanks to plasticity, smooth muscle can be completely relaxed in both contracted and stretched states. For example, the plasticity of the smooth muscles of the wall of the stomach or bladder as these organs fill prevents an increase in intracavitary pressure. Excessive stretching often leads to stimulation of contraction, which is caused by the depolarization of pacemaker cells that occurs when the muscle is stretched, and is accompanied by an increase in the frequency of action potential, and as a result, an increase in contraction. Contraction, which activates the stretching process, plays a large role in the self-regulation of the basal tone of blood vessels.
The mechanism of smooth muscle contraction. A prerequisite for the occurrence is a contraction of smooth muscles, as well as skeletal muscles, and an increase in the concentration of Ca2 + in the myoplasm (up to 10-5 M). It is believed that the contraction process is activated primarily by extracellular Ca2+, which enters 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 an excitatory and an inhibitory effect. By type, there are cholinergic (mediator acetylcholine) and adrenergic (mediator norepinephrine) mediators. The former are usually found in the muscles of the digestive system, the latter in the muscles of the blood vessels.
The same transmitter in some synapses can be excitatory, and in others - inhibitory (depending on the properties of the cytoreceptors). Adrenergic receptors are divided into a- and b-. Norepinephrine, acting on α-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 dilates the blood vessels of some organs, relaxes the muscles of the bronchi. Described neuromuscular. transmission in smooth muscles for the help of other mediators.
In response to the action of an excitatory transmitter, depolarization of smooth muscle cells occurs, which manifests itself in the form of an excitatory synaptic potential (ESP). When it reaches a critical level, PD occurs. This happens when several impulses approach the nerve ending one after another. The occurrence of PGI is a consequence of an increase in the permeability of the postsynaptic membrane for Na +, Ca2 + and SI."
The inhibitory transmitter causes hyperpolarization of the postsynaptic membrane, which is manifested in the inhibitory synaptic potential (ISP). Hyperpolarization is based on an increase in membrane permeability, mainly for K +. The role of inhibitory mediator in smooth muscles excited by acetylcholine (for example, muscles of the intestine, bronchi) is played by norepinephrine, and in smooth muscles for which norepinephrine is an excitatory mediator (for example, muscles of the bladder), acetylcholine plays the role.
Clinical and physiological aspect. In some diseases, when the innervation of skeletal muscles is disrupted, their passive stretching or displacement is accompanied by a reflex increase in their tone, i.e. resistance to stretching (spasticity or rigidity).
If blood circulation is impaired, as well as under the influence of certain metabolic products (lactic and phosphoric acids), toxic substances, alcohol, fatigue, or a decrease in muscle temperature (for example, during prolonged swimming in cold water), contracture may occur after prolonged active muscle contraction. The more the muscle function is impaired, the more pronounced the contracture aftereffect is (for example, 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 cross bridges and actin protofibrils. In this case, the muscle loses flexibility and becomes hard. The contracture goes away and the muscle relaxes when the ATP concentration reaches normal levels.
In diseases such as myotonia, muscle cell membranes are excited so easily that even a slight irritation (for example, the introduction of a needle electrode during electromyography) causes the discharge of muscle impulses. Spontaneous APs (fibrillation potentials) are also recorded at the first stage after denervation of the muscle (until inaction leads to its atrophy).
Tonic contractions of some smooth muscles, especially the muscles of the vascular walls (basal or myogenic tone) are activated predominantly 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 APs and closing voltage-dependent Ca2 + channels.