Smooth muscle structure and function. Smooth muscle

Smooth muscles in the body of higher animals and humans are found in internal organs, blood vessels and skin. Smooth musclecapable of relatively slow movements and prolonged tonic contractions.

Relatively slow, often rhythmic contractions smooth muscles the walls of hollow organs: the stomach, intestines, ducts of the digestive glands, bladder, gall bladder, etc. - ensure the movement and ejection of the contents of these hollow organs. An example is the pendular and peristaltic movements of the intestinal muscles.

Prolonged tonic contractions of smooth muscles are especially pronounced in the sphincters of hollow organs; their tonic contraction prevents the release of organ contents. This ensures the accumulation of bile in the gallbladder and urine in the bladder, the formation of feces in the rectum, etc.

The smooth muscles of the walls of blood vessels, especially arteries and arterioles, also have a pronounced tone. The tone of the muscular layer of the artery walls regulates the size of their lumen, and thereby the level of blood pressure and blood supply to organs.

The tone and motor function of smooth muscles are regulated by impulses arriving through the autonomic nerves and humoral influences.

Basic functions smooth muscles:

in hollow organs (ureter, intestines, etc.) they maintain pressure;
slow contraction of smooth muscles causes wave-like peristalsis of hollow organs,
which ensures the advancement of their contents and emptying of organs;
change the lumen of blood vessels, thereby regulating the pressure in them;
smooth muscles located in the skin at the base of the hair follicles, when contracted, raise the hair and squeeze out fat from the sebaceous glands;
In the eyes, smooth muscles provide constriction and dilation of the pupil and determine the thickness of the lens.

Feature smooth muscle is:

slow contraction and relaxation (tens of seconds);
involuntary nature of the contraction (regardless of the will of the person).

Properties of smooth muscles

Smooth muscle plasticity

Important property of smooth muscle is its great plasticity, i.e. the ability to maintain the length given by stretching without changing the stress. The difference between skeletal muscle, which has little plasticity, and smooth muscle, which has good plasticity, is easily detected if they are first slowly stretched and then the tensile load is removed. Skeletal muscle immediately shortens after removing the load. In contrast, smooth muscle, after removing the load, remains stretched until, under the influence of some irritation, its active contraction occurs.

The property of plasticity is very great importance for the normal activity of the smooth muscles of the walls of hollow organs, for example the bladder: due to the plasticity of the smooth muscles of the walls of the bladder, the pressure inside it changes relatively little when varying degrees filling.

Excitability and arousal

Smooth muscle less excitable than skeletal ones: their irritation thresholds are higher and their chronaxy is longer. The action potentials of most smooth muscle fibers have a small amplitude (about 60 mV instead of 120 in skeletal muscle fibers) and a long duration - up to 1-3 seconds. On rice. 151 The action potential of a single fiber of the uterine muscle is shown.

The refractory period lasts for the entire period of the action potential, i.e. 1-3 seconds. The speed of excitation varies in different fibers from several millimeters to several centimeters per second.

There are a large number of different types of smooth muscles in the body of animals and humans. Most of the hollow organs of the body are lined with smooth muscles of a sensitial type of structure. The individual fibers of such muscles are very closely adjacent to each other and it seems that morphologically they form a single whole.

Smooth muscle irritants . One of the important physiologically adequate stimuli of smooth muscles is their rapid and strong stretching. The latter causes depolarization of the muscle fiber membrane and the occurrence of a spreading action potential.

They perform a very important function in the organisms of living beings - they form and line all organs and their systems. Of particular importance among them is the muscular one, since its importance in the formation of the external and internal cavities of all structural parts of the body is a priority. In this article we will consider what smooth muscle tissue is, its structural features, and properties.

Varieties of these fabrics

There are several types of muscles in the animal body:

transversely striped;
smooth muscle tissue.

Both of them have their own characteristic structural features, functions performed and properties exhibited. In addition, they are easy to distinguish from each other. After all, both have their own unique pattern, formed due to the protein components included in the cells.

Striated is also divided into two main types:

skeletal;
cardiac.

The name itself reflects the main areas of location in the body. Its functions are extremely important, because it is this muscle that ensures the contraction of the heart, the movement of the limbs and all other moving parts of the body. However, smooth muscles are no less important. What are its features, we will consider further.

In general, it can be noted that only the coordinated work performed by smooth and striated muscle tissue allows the entire body to function successfully. Therefore, it is impossible to determine which of them is more or less significant.

Smooth structural features

The main unusual features of the structure in question lie in the structure and composition of its cells - myocytes. Like any other, this tissue is formed by a group of cells that are similar in structure, properties, composition and functions. General Features buildings can be identified at several points.

Each cell is surrounded by a dense plexus of connective tissue fibers that looks like a capsule.
Each structural unit fits tightly to the other, intercellular spaces are practically absent. This allows the entire fabric to be tightly packed, structured and durable.
Unlike its striated counterpart, this structure may include cells of different shapes.

This, of course, is not the whole characteristic that it has. Structural features, as already stated, lie precisely in the myocytes themselves, their functioning and composition. Therefore, this issue will be discussed in more detail below.

Smooth muscle myocytes

Myocytes have different shapes. Depending on the location in a particular organ, they can be:

oval;
fusiform elongated;
rounded;
process.

However, in any case, their general composition is similar. They contain organelles such as:

well defined and functioning mitochondria;
Golgi complex;
core, often elongated in shape;
endoplasmic reticulum;
lysosomes.

Naturally, the cytoplasm with the usual inclusions is also present. An interesting fact is that smooth muscle myocytes are externally covered not only with plasmalemma, but also with a membrane (basal). This provides them with an additional opportunity to contact each other.

These points of contact constitute the features of a smooth muscle tissue. Contact sites are called nexuses. It is through them, as well as through the pores that exist in these places in the membrane, that impulses are transmitted between cells, information, water molecules and other compounds are exchanged.

There is another unusual feature that smooth muscle tissue has. The structural features of its myocytes are that not all of them have nerve endings. This is why nexuses are so important. So that not a single cell is left without innervation, and the impulse can be transmitted through the neighboring structure through the tissue.

There are two main types of myocytes.

Secretory. Their main function is the production and accumulation of glycogen granules, maintaining a variety of mitochondria, polysomes and ribosomal units. These structures got their name because of the proteins they contain. These are actin filaments and contractile fibrin filaments. These cells are most often localized along the periphery of the tissue.
Smooth They look like spindle-shaped elongated structures containing an oval nucleus, displaced towards the middle of the cell. Another name is leiomyocytes. They differ in that they are larger in size. Some particles of the uterine organ reach 500 microns! This is a fairly significant figure compared to all other cells in the body, except perhaps the egg.

The function of smooth myocytes is also that they synthesize the following compounds:

glycoproteins;
procollagen;
elastane;
intercellular substance;
proteoglycans.

The joint interaction and coordinated work of the designated types of myocytes, as well as their organization, ensure the structure of smooth muscle tissue.

Origin of this muscle

There is more than one source of formation of this type of muscle in the body. There are three main variants of origin. This is what explains the differences in the structure of smooth muscle tissue.

Mesenchymal origin. has this most of smooth fibers. It is from mesenchyme that almost all the tissues lining the inside of hollow organs are formed.
Epidermal origin. The name itself speaks about the places of localization - these are all the skin glands and their ducts. They are formed by smooth fibers that have this appearance. Sweat, salivary, mammary, lacrimal glands - all these glands secrete their secretions due to irritation of myoepithelial cells - structural particles of the organ in question.
Neural origin. Such fibers are localized in one specific place - this is the iris, one of the membranes of the eye. The contraction or dilation of the pupil is innervated and controlled by these smooth muscle cells.

Despite their different origins, the internal composition and performance properties of all in the fabric in question remain approximately the same.

Main properties of this fabric

The properties of smooth muscle tissue correspond to those of striated muscle tissue. In this they are united. This:

conductivity;
excitability;
lability;
contractility.

At the same time, there is one rather specific feature. If striated skeletal muscles are capable of contracting quickly (this is well illustrated by tremors in the human body), then smooth muscles can remain in a compressed state for a long time. In addition, its activities are not subject to the will and reason of man. Since it innervates

A very important property is the ability for long-term slow stretching (contraction) and the same relaxation. So, the work of the bladder is based on this. Under the influence biological fluid(by its filling) it is able to stretch and then contract. Its walls are lined with smooth muscles.

Cell proteins

The myocytes of the tissue in question contain many different compounds. However, the most important of them, providing the functions of contraction and relaxation, are protein molecules. Of these, here are:

myosin filaments;
actin;
nebulin;
connectin;
tropomyosin.

These components are usually located in the cytoplasm of cells isolated from each other, without forming clusters. However, in some organs in animals, bundles or cords called myofibrils are formed.

The location of these bundles in the tissue is mainly longitudinal. Moreover, both myosin fibers and actin fibers. As a result, a whole network is formed in which the ends of some are intertwined with the edges of other protein molecules. This is important for fast and correct contraction of the entire tissue.

The contraction itself occurs like this: the internal environment of the cell contains pinocytosis vesicles, which necessarily contain calcium ions. When a nerve impulse arrives indicating the need for contraction, this bubble approaches the fibril. As a result, the calcium ion irritates actin and it moves deeper between the myosin filaments. This leads to the plasmalemma being affected and, as a result, the myocyte contracts.

Smooth muscle tissue: drawing

If we talk about striated fabric, it is easy to recognize by its striations. But as far as the structure we are considering is concerned, this does not happen. Why does smooth muscle tissue have a completely different pattern than its close neighbor? This is explained by the presence and location of protein components in myocytes. As part of smooth muscles, myofibril threads of different nature are localized chaotically, without a specific ordered state.

That is why the fabric pattern is simply missing. In the striated filament, actin is successively replaced by transverse myosin. The result is a pattern - striations, due to which the fabric got its name.

Under a microscope, smooth tissue looks very smooth and ordered, thanks to the elongated myocytes tightly adjacent to each other.

Areas of spatial location in the body

Smooth muscle tissue forms a fairly large number of important internal organs in the animal body. So, she was educated:

intestines;
genitals;
blood vessels of all types;
glands;
organs of the excretory system;
Airways;
parts of the visual analyzer;
organs of the digestive system.

It is obvious that the localization sites of the tissue in question are extremely diverse and important. In addition, it should be noted that such muscles form mainly those organs that are subject to automatic control.

Recovery methods

Smooth muscle tissue forms structures that are important enough to have the ability to regenerate. Therefore, it is characterized by two main ways of recovery from damage of various kinds.

Mitotic division of myocytes until the required amount of tissue is formed. The most common simple and quick way regeneration. This is how the internal part of any organ formed by smooth muscles is restored.
Myofibroblasts are capable of transforming into smooth tissue myocytes when necessary. This is a more complex and rarely encountered way of regenerating this tissue.

Innervation of smooth muscles

Smooth does its work regardless of the desire or reluctance of a living creature. This occurs because it is innervated by the autonomic nervous system, as well as by the processes of the ganglion (spinal) nerves.

An example and proof of this is the reduction or increase in the size of the stomach, liver, spleen, stretching and contraction of the bladder.

Functions of smooth muscle tissue

What is the significance of this structure? Why do you need the following:

prolonged contraction of organ walls;
production of secrets;
the ability to respond to irritation and influence with excitability.

Smooth muscles in invertebrates and vertebrates

Smooth muscle contractions

Unlike striated muscles, smooth muscles are characterized by slow contraction, the ability to remain in a state of contraction for a long time, expending relatively little energy and without being subject to fatigue. Motor innervation of smooth muscles is carried out by processes of cells of the autonomic nervous system, and sensory innervation is carried out by processes of cells of the spinal ganglia. Not every smooth muscle cell has a specialized nerve ending.

Muscular system

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See what “Smooth muscles” are in other dictionaries:

SMOOTH MUSCLES- (involuntary contracting muscles), one of the three types of muscles in vertebrates. Unlike SKELETAL MUSCLES, they are not subject to conscious control by the brain, but are stimulated by the AUTONOMIC NERVOUS SYSTEM and HORMONES in the blood. Apart from the smooth ones... ... Scientific and technical encyclopedic dictionary

SMOOTH MUSCLES- contractile (muscle) tissue, consisting of spindle-shaped mononuclear cells. Unlike striated muscles, they do not have transverse striations. In most invertebrates they make up the entire musculature of the body; in vertebrates they are part of ... ... Big Encyclopedic Dictionary

smooth muscle- contractile (muscle) tissue, consisting of spindle-shaped mononuclear cells. Unlike striated muscles, they do not have transverse striations. In most invertebrates they make up the entire musculature of the body; in vertebrates they are part of ... ... encyclopedic Dictionary

SMOOTH MUSCLES- muscles of internal organs, forming the muscular layer of the stomach, intestines, blood vessels, etc. Unlike striated muscles, muscle contractions are slower and more prolonged; they can remain in a contracted state for a long time... Psychomotorics: dictionary-reference book

Smooth muscle- SMOOTH MUSCLES (musculi glaberi), contractile tissue, consisting of parts. cells and without transverse striations. In invertebrates (except for arthropods and certain representatives of other groups, for example, pteropods) G. m. form the entire ... ...

Smooth muscle- contractile tissue, consisting, unlike striated muscles (See Striated muscles), of cells (and not symplasts) and does not have transverse striations. In invertebrates (except all arthropods and individual representatives of other... Great Soviet Encyclopedia

MUSCLES- MUSCLES. I. Histology. Generally morphologically, the tissue of the contractile substance is characterized by the presence of differentiation of its specific elements in the protoplasm. fibrillar structure; the latter are spatially oriented in the direction of their reduction and... ...

MUSCLES- muscles (musculi), organs of the body of animals and humans, consisting of muscle tissue capable of contracting under the influence of nerve impulses. They move a body in space, shift some of its parts relative to others (dynamic function) ... Biological encyclopedic dictionary

HUMAN MUSCLES- “80 Nos. Name Latin and Russian. Synonyms. Forsch, and position Beginning and attachment Innervation and relation to network elements Thyreo epiglotticus (thyroid-epiglottic M.). Syn.: thyreo epiglotticus inferior, s. major, thyreo membranosus… Great Medical Encyclopedia

Features of the structure. Smooth muscle is present in almost all tissues and organs: blood vessels, airways, gastrointestinal tract, genitourinary system, etc.

The main structural unit of smooth muscle is the smooth muscle cell (SMC), which usually has an elongated spindle-shaped shape. SMCs are arranged in parallel and sequentially, forming muscle bundles or cords and muscle layers. Their sizes depend on the type and functional state of the smooth muscle: 20-500 µm in length and 5 – 20 µm in thickness in the middle part of the cell.

On the outside, the SMC is covered with a sarcolemma, which, like other muscles, consists of a plasma membrane and a basement membrane. Under electron microscope in the plasma membrane, peculiar flask-shaped invaginations, the so-called caveolae and electron-dense areas, are visible. Some researchers believe that these cords are the site of attachment of actin protofibrils.

Although most of the surface of one muscle cell is separated from neighboring muscle cells by a space of 100 nm or more (intercellular space), which is filled with collagen and elastin fibers, fibroblasts, capillaries, etc., other types of interaction are also characteristic of SMCs:

1. Nexuses: the gap between the contacting membranes of neighboring cells is very narrow - 2 - 3 nm; cluster formations and intramembrane particles measuring 9 nm in size are found in the membranes of the nexuses of contacting cells. These particles are believed to represent intercellular ion channels.

2. Desmosome-like connection. In the areas of these contacts, the presence of areas of electron-dense matter is detected. In visceral muscles, the width of the gap between the contacting membranes with this type of contact can reach 20–60 nm. It is believed that this type of contact serves mainly for the mechanical connection of cells.

3. The third type of communication between cells is communication using processes with which one cell enters the corresponding recess of another. The width of the gap between the membranes of neighboring cells in this case is 10–20 nm. These connections are believed to be important for the transmission of mechanical force between cells.

Passive electrical properties of smooth muscle

Smooth muscle tissue, despite being discrete from a morphological point of view, is a functional syncytium in which the plasma membranes of many muscle cells represent a single continuous membrane of one large muscle cell. Therefore, the main indicators of the SMC can be compared with the cable properties of the axon:

1. time constant (λ) 100-300 ms and length constant (τ) 1-3 mm;

2. membrane resistance and capacitance 0.6 -2.9 GOhm and 30 - 40 pF, respectively;

3. resistivity and membrane capacitance 10–50 kOhm/cm2 and 1.3-3 μF/cm2, respectively;

4. The specific resistance of the myoplasm is about 250 Ohm/cm.

Resting potential (RP) of various SMCs is in the range from –50 to –60 mV. The main ions involved in its formation are K + , Na + and Cl - . A feature of the ionic composition of SMCs is the high intracellular concentration of chlorine and sodium ions.

The fact that the value of SMC PP differs significantly from the equilibrium potassium potential (-55 mV for SMC taenia coli, while E k = -90 mV) is explained primarily by the fact that the SMC membrane also has a relatively high permeability for sodium ions and chlorine. The ratio of the permeability of the SMC membrane for these ions is equal to: P K:P Na:P Cl = 1:0.16:0.61. Calculations of the PP value using the Goldman-Hodgkin-Katz formula, taking into account these permeabilities and equilibrium potentials for the corresponding ions (E K = -89 mV; E Na = +62 mV; E Cl = -22 mV) gave a resting potential value equal to only –37 mV . Thus, the measured PP value turned out to be almost 20 mV higher than the calculated one.

The role of calcium ions in this is small, since they have low permeability through the SMC membrane, but they significantly affect the permeability of the membrane to other ions and, in particular, to Na + ions. The removal of calcium ions from the washing solution is accompanied by cell depolarization and a significant decrease in membrane resistance.

Another reason for this discrepancy may be the participation of the electrogenic component of the sodium pump in the formation of the PP, but the current generated by the sodium pump can only create a potential of about 5 mV. Another reason for the discrepancy between the calculated and theoretical PP values may be the high intracellular concentration of chloride ions.

Action potential (AP) smooth muscles allows us to divide them according to their ability to generate it in response to threshold and suprathreshold stimulation into:

1. Phase – fast-twitch muscles, capable of generating force, have a relatively high rate of shortening and often have spontaneous electrical and contractile activity. Their response to membrane depolarization is relatively rapid but transient. An example is: SMC of the digestive tract, uterus, urinary tract, portal vein.

2. Tonic smooth muscles, as a rule, respond to agonist stimulation with gradual depolarization, do not generate AP and spontaneous contractile activity, have a low shortening rate, but can effectively maintain a contracted (tonic) state for a long time.

APs of various SMCs have the form from simple spike potentials lasting 20–50 ms (myometrium, portal vein, intestine), to complex ones - with plateaus and oscillations on them, lasting up to 1 second or more (ureter, antrum of the stomach).

A feature of SMC electrogenesis is that Ca 2+ ions play the main role in AP generation. These ions are responsible for generating a depolarizing incoming current, which consists of two components: 1. initial inactivating – having reached its maximum, it does not remain at a constant level, but slowly decreases;

2.subsequent non-inactivating, which is not inactivated by large depolarizing shifts in membrane potential.

Inactivation of incoming calcium current depends not so much on the magnitude of the membrane potential as on the concentration of calcium ions inside the smooth muscle cell. The functional significance of this phenomenon is, apparently, that calcium ions entering the SMC through negative feedback regulate the intensity of their excitation, and consequently the entry of calcium ions into the cell.

Potassium ions, responsible for the generation of the outgoing hyperpolarizing current, also have an effect on the amplitude and duration of AP depending on the concentration of calcium ions inside the SMC. Although the potassium current continues to increase with ever increasing positive displacements of the membrane potential.

All influences leading to inhibition of potassium conductivity contribute to the emergence of AP in those SMCs that in the initial state are not capable of generating AP. This explains the lack of PD in the tonic muscles. Under normal conditions, the membrane of these SMCs has a high membrane potassium conductivity, which prevents the development of regenerative depolarization.

SMC PDs, consisting of an initial fast peak component and a subsequent plateau, have a more complex ionic nature. For example, in ureteral SMCs, the initial peak component is predominantly calcium in nature, whereas the subsequent slow plateau component is predominantly sodium in nature.

Spontaneous smooth muscle activity , which is myogenic in nature, has two main types:

1. Repeatedly occurring PDs of varying frequency and degree of regularity , not accompanied by long-term persistent depolarization of the SMC. It is based on the ability of a certain group of MMCs to generate so-called generator potentials (prepotentials). They are detected with intracellular microelectrode leads in the form of a small slow depolarization, which, having reached the excitation threshold, goes into a rapidly increasing phase of AP depolarization.

2. Slow waves of depolarization can be different in shape, amplitude (10 – 30 mV), duration (2 – 10 s), frequency (1 – 18 vibrations per minute), propagation speed (up to 8 cm/sec). It is assumed that these waves primarily arise in special pacemaker muscle cells. When the slow wave reaches the excitatory threshold, action potentials can occur, the frequency of which depends on the amplitude of the wave.

Features of the contractile apparatus of the SMC are due to the following:

1. Lack of T-system;

2. Insignificant volume of SPR (2 – 7% of the cytoplasm volume).

The contractile apparatus of the SMC is represented by myosin and actin protofibrils, as well as a number of regulatory proteins: myosin light chain kinase, myosin light chain phosphatase, tropomyosin, caldesmon, calponin. The ratio of actin and myosin filaments in SMCs ranges from 1:5 to 1:27, which is noticeably higher than in skeletal cells.

The smooth muscle myosin molecule consists of two heavy chains and two pairs of light chains – regulatory with a mass of 20 kDa (RLC) and essential with a mass of 17 kDa (LC).

Myosin SMC is different from myosin skeletal muscles size (thickness 12–15 nm, length 2.2 µm), shape, amino acid composition, solubility, sensitivity to enzymes, salts and denaturation, lower (10 times) ATPase activity.

GM actin protofibrils are almost indistinguishable from striated ones. They have a simple elongated shape, their diameter is 6–8 nm. In a cross section, actin protofibrils have a round shape. Sometimes a hesagonal arrangement of thin protofibrils relative to thick ones is found, as in striated muscle fibers.

The actin protofibrils of SMCs include actin, tropomyosin and caldesmon. The protein leotonin was isolated from tropomyosin of SMCs, which, apparently, performs functions similar to skeletal muscle troponin C. Actin protofibrils also contain a number of additional minor and modulating proteins: filamin and vinculin, which are involved in the attachment of thin protofibrils to dense membrane bodies, and, in addition, are involved in the activation of actomyosin ATPase and in a number of other processes.

In SMCs, in addition to myosin and actin protofibrils, there are so-called intermediate protofibrils, which form a kind of intracellular network and connect dense bodies of the plasma membrane and myoplasm with each other.

It is assumed that actin and myosin protofibrils are combined into myofibrils that extend over a relatively short distance at an angle to the long axis of the muscle cell. At their ends, myofibrils are attached to dense bodies of the plasma membrane (which include the α-actin protein), which are analogues of the z-plates of skeletal muscle fibers.

Myosin light chain kinase is an enzyme containing:

A) catalytic domain, which contains binding sites for ATP and myosin regulatory light chains.

B) a regulatory domain containing a binding site for the calcium-calmodulin complex.

C) an autoinhibitory pseudosubstrate sequence, which, in the absence of the calcium-calmodulin complex, interacts with the catalytic center and blocks the phosphotransferase reaction.

Myosin light chain phosphatase is an enzyme belonging to type 1 phosphatases, consisting of catalytic and regulatory subunits.

Tropomyosin in SMC it is contained in an amount of 1:14 relative to actomyosin; it prevents the interaction of myosin with actin.

Caldesmon is a regulatory protein associated with actin filaments, located directly along tropomyosin in a groove formed by dumbbell-shaped actin molecules. The function of caldesmon is to hold tropomyosin in a position that prevents the interaction of myosin with the active center of actin, as well as to prevent the movement of actin filaments along myosin.

Kalponin – actin- and calmodulin-binding protein, relatively specific for smooth muscle. It is assumed that calponin is involved in the calcium-dependent regulation of contraction, and its direct phosphorylation by protein kinase C contributes to the increase in calcium sensitivity of the brain. It is located on actin filaments, inhibits actomyosin ATPase and the mobility of actin filaments along myosin.

Electromechanical interface in mining and metallurgical complex represents the chain of events leading to the activation of contraction. As in skeletal muscles, it is triggered by an increase in the concentration of ionized calcium in the myoplasm above 10 -7 M. The maximum contraction of SMCs is observed at a concentration of -10 -5 M.

Peculiarities. Since when Ca 2+ ions were removed from the external environment or calcium current blockers were added, both the electrical and contractile activity of the SMC was inhibited, which means that the development of excitation-contraction coupling is ensured by extracellular Ca 2+ ions involved in the generation of AP.

The main routes of calcium ions entering the SMC:

1. Plasma membrane calcium channels:

A. Voltage-dependent inactivating calcium channels responsible for generating action potentials.

B. Voltage-dependent non-inactivating calcium channels that provide a stationary current of calcium ions across a depolarized membrane.

IN. Chemosensitive(receptor-gated) calcium channels that open when membrane receptors are activated.

2. Non-mitochondrial depot:

A. Sarcoplasmic reticulum (SRR).

B.Primembrane layers.

B. Intracellular vesicles-calcisomes.

The main ways to remove calcium ions from SMCs are:

1. Plasma membrane calcium pumps and SPR.

2. Sodium-calcium metabolism.

Molecular mechanisms of SMC contraction.

The main Ca 2+ acceptor in the cytoplasm of SMCs is calmodulin, which, after binding 4 calcium ions, interacts with regulatory proteins - myosin light chain kinase and caldesmon. Myosin light chain kinase activated in this way phosphorylates the regulatory myosin light chains and thereby activates the Mg 2+ -dependent myosin ATPase, thereby causing contraction in an actin-dependent manner .

However, in resting muscle, the sites of interaction with myosin are shielded by the complex of tropomyosin with caldesmon lying along the actin cable. Therefore the second a necessary condition activation of actomyosin is a change in the conformation of caldesmon, which, apparently, releases tropomyosin, resulting in the exposure of myosin-binding sites on actin. This occurs when caldesmon interacts with the calcium-calmodulin complex, or another calcium-binding protein similar to it.

Thus, the development of smooth muscle contraction requires the simultaneous activation of both myosin by direct phosphorylation and actin by eliminating the inhibitory effect of caldesmon. That is, at a high degree of myosin activation, caldesmon can only inhibit, but is not able to completely block its cooperative binding with actin.

A decrease in intracellular calcium concentration is accompanied by dissociation of calmodulin complexes with myosin light chain kinase and caldesmon, its inactivation and restoration of the inhibitory effect of caldesmon. Subsequent dephosphorylation of myosin light chains by a specific, calcium-independent myosin light chain phosphatase and the transition of thin filaments to an inactive state determines the relaxation of SMCs. As in the case of contractile activation, the main condition for relaxation is the dephosphorylation of myosin, while caldesmon-dependent inactivation of thin filaments can accelerate relaxation.

However, it is well known that the force of contraction of SMCs is not always directly proportional to the intracellular concentration of calcium ions. By changing the sensitivity of the contractile apparatus of the SMC to calcium ions, with its actual constancy, it is possible to modulate changes in the intracellular calcium level. Currently, several mechanisms are being considered to ensure an increase in calcium sensitivity of the contractile apparatus.

1. The mechanism is associated with the activation of protein kinase C by diacylglycerol. The targets of protein kinase C can be all the main proteins that regulate smooth muscle contraction - myosin light chain kinase and phosphorylase, caldesmon and myosin regulatory chains:

2. Activation of monomeric G-proteins of the Rho family and inhibitory phosphorylation of myosin light chain phosphorylase by Rho protein kinase.

3. Latch phenomenon. This mechanism postulates the SMC-specific formation of non-cycling dephosphorylated actomyosin bridges. Moreover, myosin is dephosphorylated within bridges that have already been formed and are in a state of strong binding, which leads to a significant decrease in the rate constant for the dissociation of myosin heads and the formation of so-called latched bridges.

However, in vivo, the tonic contractile response of SMCs is achieved by a combination of all mechanisms.

Contractile and electrical activity of SMCs is regulated many physiologically and biologically active substances. The implementation of their effects on smooth muscle cells is carried out with the participation of secondary messenger systems.

Activation of the cAMP-dependent signaling system inhibits SMC contractions due to:

1. Increased potassium conductivity of the membrane - its hyperpolarization.

2. Stimulation of the calcium pumps of the plasma membrane and SPR.

3. Decreased affinity of phosphorylated myosin light chain kinase for calmodulin.

4. Reduced sensitivity of the contractile apparatus of the SMC to calcium ions.

5. Activation of sodium-potassium ATPase.

Activation of the calcium signaling system :

1. Stimulates the functioning of the plasma membrane calcium pump and SPR.

2. The calcium-calmodulin complex is capable of potentiating calcium-dependent potassium conductance of the SMC membrane

3. The calcium-calmodulin complex is involved in calcium-dependent inactivation of calcium channels.

Signaling system associated with the metabolism of membrane phosphoinositides.

1. Ionositol 1,4,5,-triphosphate induces the release of Ca 2+ from the SPR.

2. Stimulates the activity of the calcium pump, ensuring calcium reabsorption.

3. Activation of protein kinase C has an inhibitory effect on calcium channels, the metabolism of membrane phosphoinositides, and reduces the affinity of receptors for receptor agonists.

4. Activation of protein kinase C increases membrane potassium conductance due to activation of sodium-proton exchange.

Activation of the cGMP-dependent signaling system is associated with nitric oxide metabolism and causes:

1. Modulating effect on calcium conductivity of the membrane

2. Reduces the affinity of myosin light chain kinase for calmodulin.

3. Increases membrane potassium conductance

4. Inhibits the activity of some isoforms of protein kinase C

5. Reduces phospholipase C activity

6. modulates the activity of the sodium-potassium pump

Features of the biomechanics of muscle contraction.

ATP consumption by smooth muscle cells (in warm-blooded animals) in a contracted state is almost 1000 times less than in skeletal muscles.

The force developed by smooth muscle is determined by the following factors

1. agent that causes activity

2. concentration of this agent

3. initial long muscle.

There is an optimal length L 0 of the muscle at which the force it develops reaches its maximum when the agonist acts at a given concentration.

Unlike skeletal muscle, at lengths shorter than L 0 the GM generates greater force than skeletal muscle, and at lengths greater than L 0 , the active force of the GM decreases more gradually than that of the skeletal muscle.

Smooth muscle are presented in the walls of the digestive canal, bronchi, blood and lymphatic vessels, bladder, uterus, as well as in the iris, ciliary muscle, skin and glands. Unlike striated muscles, they are not separate muscles, but form only part of the organs. Smooth muscle cells have an elongated spindle- or ribbon-like shape with pointed ends. Their length in humans is usually about 20 microns. Smooth muscle cells reach the greatest length (up to 500 microns) in the wall of the pregnant human uterus. In the middle part of the cell there is a rod-shaped nucleus, and in the cytoplasm along the entire cell, thin, completely homogeneous myofibrils run parallel to each other. Therefore, the cell does not have transverse striations. Thicker myofibrils are located in the outer layers of the cell. They are called boundary and have uniaxial birefringence. An electron microscope shows that myofibrils are bundles of protofibrils and have cross-striations that are not visible in a light microscope. Smooth muscle cells can regenerate by division (mitosis). They contain a type of actomyosin - tonoactomyosin. Between smooth muscle cells there are the same areas of membrane contact, or nexuses, as between cardiac ones, along which excitation and inhibition are supposed to spread from one smooth muscle cell to another.

In smooth muscles, excitation spreads slowly. Contractions of smooth muscles are caused by stronger and longer-lasting stimuli than skeletal muscles. The latent period of its contraction lasts several seconds. Smooth muscles contract much slower than skeletal muscles. Thus, the period of contraction of smooth muscle in the stomach of a frog is 15-20 s. Smooth muscle contractions can last for many minutes or even hours. Unlike skeletal muscles, smooth muscle contractions are tonic. Smooth muscles are capable of being in a state of tonic tension for a long time with an extremely low expenditure of substances and energy. For example, the smooth muscles of the sphincters of the digestive canal, bladder, gall bladder, uterus and other organs are in good shape for tens of minutes and many hours. The smooth muscles of the walls of the blood vessels of higher vertebrates remain in good shape throughout life.

There is a direct relationship between the frequency of impulses arising in the muscle and the level of its tension. The higher the frequency, the greater the tone up to a certain limit due to the summation of stresses of non-simultaneously tense muscle fibers.

Smooth muscles have tasticity - the ability to maintain their length when stretched without changing tension, unlike skeletal muscles, which are tense when stretched.

Unlike skeletal muscles, many smooth muscles exhibit automaticity. They contract under the influence of local reflex mechanisms, such as the Meissner and Auerbach plexuses in the alimentary canal, or chemicals entering the blood, such as acetylcholine, norepinephrine and adrenaline. Automatic contractions of smooth muscles are enhanced or inhibited under the influence of nerve impulses coming from the nervous system. Therefore, unlike skeletal muscles, there are special inhibitory nerves that stop contraction and cause relaxation of smooth muscles. Some smooth muscles that have a large number of nerve endings do not have automaticity, for example, the sphincter of the pupil, the nictitating membrane of a cat.

Smooth muscles can shorten greatly, much more than skeletal muscles. A single stimulation can cause smooth muscle contraction by 45%, and the maximum contraction with a frequent rhythm of stimulation can reach 60-75%.

Smooth muscle tissue also develops from mesoderm (arises from mesenchyme); it consists of individual, highly elongated spindle-shaped cells, much smaller in size compared to the fibers of striated muscles. Their length ranges from 20 to 500 μ, and their width from 4 to 7 μ. As a rule, these cells have one elongated nucleus lying in the center of the cell. In the protoplasm of the cell, numerous and very thin myofibrils pass in the longitudinal direction, which do not have transverse striations and are completely invisible without special treatment. Each smooth muscle cell is covered with a thin connective tissue membrane. These membranes connect neighboring cells to each other. In contrast to striated fibers, located almost the entire length of the skeletal muscle, throughout any smooth muscle complex there is a significant number of cells located in one line.

Smooth muscle cells are found in the body either scattered singly in connective tissue, or linked into muscle complexes of varying sizes.

In the latter case, each muscle cell is also surrounded on all sides by intercellular substance, penetrated by the finest fibrils, the number of which can be very different. The finest networks of elastic fibers are also found in the intercellular substance.

Smooth muscle cells of organs are united into muscle bundles. In many cases (urinary tract, uterus, etc.), these bundles branch and merge with other bundles, forming surface networks of varying densities. If a large number of bundles are located closely, then a dense muscular layer is formed (for example, the gastrointestinal tract). The blood supply to smooth muscles is carried out through vessels that pass through large connective tissue layers between the bundles; capillaries penetrate between the fibers of each bundle and, branching along it, form a dense capillary network. Smooth muscle tissue also contains lymphatic vessels. Smooth muscles are innervated by fibers of the autonomic nervous system. Smooth muscle cells, unlike striated muscle fibers, produce slow, sustained contractions. They are able to work for a long time and with great strength. For example, the muscular walls of the uterus during childbirth, which lasts for hours, develop a force that is inaccessible to striated muscles. The activity of smooth muscles, as a rule, is not subject to our will (vegetative innervation, see below) - they are involuntary.

Smooth muscle in its development (phylogeny) is more ancient than striated muscle, and is more common in the lower forms of the animal world.

Classification of smooth muscles

Smooth muscles are divided into visceral (unitary) and multiunitary. Visceral smooth muscles are found in all internal organs, ducts of the digestive glands, blood and lymphatic vessels, and skin. Mulipunitary muscles include the ciliary muscle and the iris muscle. The division of smooth muscles into visceral and multiunitary is based on the different densities of their motor innervation. In visceral smooth muscle, motor nerve endings are present on a small number of smooth muscle cells. Despite this, excitation from the nerve endings is transmitted to all smooth muscle cells of the bundle due to tight contacts between neighboring myocytes - nexuses. Nexes allow action potentials and slow waves of depolarization to propagate from one muscle cell to another, so visceral smooth muscles contract simultaneously with the arrival of a nerve impulse.

Functions and properties of smooth muscles

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. The mechanism of phosphorylation of smooth muscle myosin is as follows: the Ca2+ ion combines with calmodulin (calmodulin is a receptive protein for the Ca2+ ion). The resulting complex activates the enzyme, myosin light chain kinase, which in turn catalyzes the process of myosin phosphorylation. Actin then slides against myosin, which forms the basis of contraction. Note that the trigger for smooth muscle contraction is the addition of Ca2+ ion to calmodulin, while in skeletal and cardiac muscle the trigger is the addition of Ca2+ to troponin.

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 α- and β-adrenergic receptors on the smooth muscle cell membrane. 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 α-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 ovarian 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.

Smooth muscles are part of the internal organs. Thanks to contraction, they provide the motor function of their organs (digestive canal, genitourinary system, blood vessels, etc.). Unlike skeletal muscles, smooth muscles are involuntary.

Morpho-functional structure of smooth 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. 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 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. it is 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 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.

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).

In case of circulatory disorders, as well as under the influence of certain metabolic products (lactic and phosphoric acids), toxic substances, alcohol, fatigue, decreased muscle temperature (for example, during prolonged swimming in cold water) after prolonged active muscle contraction, contracture may occur. 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).