Structurally, smooth muscle differs from striated skeletal muscle and cardiac muscle. It consists of spindle-shaped cells with a length of 10 to 500 microns, a width of 5-10 microns, containing one nucleus. Smooth muscle cells lie in the form of parallel oriented bundles, the distance between them is filled with collagen and elastic fibers, fibroblasts, feeding highways. The membranes of adjacent cells form nexuses that provide electrical communication between cells and serve to transmit excitation from cell to cell. In addition, the plasma membrane of a smooth muscle cell has special invaginations - caveolae, due to which the membrane area increases by 70%. Outside, the plasma membrane is covered by a basement membrane. The complex of the basement and plasma membranes is called the sarcolemma. Smooth muscle lacks sarcomeres. The contractile apparatus is based on myosin and actin protofibrils. There are much more actin protofibrils in SMC than in striated muscle fiber. Actin/myosin ratio = 5:1.

Thick and thin myofilaments are scattered throughout the sarcoplasm of a smooth myocyte and do not have such a slender organization as in striated skeletal muscle. In this case, thin filaments are attached to dense bodies. Some of these bodies are located on the inner surface of the sarcolemma, but most of them are in the sarcoplasm. Dense bodies are composed of alpha-actinin, a protein found in the Z-membrane structure of striated muscle fibers. Some of the dense bodies located on the inner surface of the membrane are in contact with the dense bodies of the adjacent cell. Thus, the force created by one cell can be transferred to the next. Thick myofilaments of smooth muscle contain myosin, while thin myofilaments contain actin and tropomyosin. At the same time, troponin was not found in the composition of thin myofilaments.

Smooth muscles are found in the walls of blood vessels, skin, and internal organs.

Smooth muscle plays important role in regulation

airway lumen,

vascular tone,

motor activity of the gastrointestinal tract,

uterus, etc.

Classification of smooth muscles:

Multiunitary, they are part of the ciliary muscle, the muscles of the iris of the eye, the muscle that lifts the hair.

Unitary (visceral), located in all internal organs, ducts of the digestive glands, blood and lymphatic vessels, skin.

Multiunit smooth muscle.

consists of separate smooth muscle cells, each of which is located independently of each other;

has a high density of innervation;

as well as striated muscle fibers, outside are covered with a substance resembling a basement membrane, which includes insulating cells from each other, collagen and glycoprotein fibers;

each muscle cell can contract separately and its activity is regulated by nerve impulses;

Unitary smooth muscle (visceral).

is a layer or bundle, and the sarcolemmas of individual myocytes have multiple points of contact. This allows excitation to spread from one cell to another.

membranes of adjacent cells form multiple tight contacts(gap junctions), through which ions are able to move freely from one cell to another

the action potential arising on the membrane of the smooth muscle cell and ion currents can propagate along the muscle fiber, allowing the simultaneous contraction of a large number of individual cells. This type of interaction is known as functional syncytium

An important feature of smooth muscle cells is their ability to self-excitation (automatic), that is, they are able to generate an action potential without exposure to an external stimulus.

There is no constant resting membrane potential in smooth muscles, it constantly drifts and averages -50 mV. Drift occurs spontaneously, without any influence, and when the resting membrane potential reaches a critical level, an action potential arises, which causes muscle contraction. The duration of the action potential reaches several seconds, so the contraction can also last several seconds. The resulting excitation then spreads through the nexus to neighboring areas, causing them to contract.

Spontaneous (independent) activity is associated with stretching of smooth muscle cells, and when they stretch, an action potential occurs. The frequency of occurrence of action potentials depends on the degree of stretching of the fiber. For example, peristaltic contractions of the intestine are enhanced by stretching its walls with chyme.

Unitary muscles mainly contract under the influence of nerve impulses, but spontaneous contractions are sometimes possible. A single nerve impulse is not capable of causing a response. For its occurrence, it is necessary to sum up several impulses.

For all smooth muscles, during the generation of excitation, activation of calcium channels is characteristic, therefore, in smooth muscles, all processes are slower than in skeletal ones.

The speed of conduction of excitation along the nerve fibers to smooth muscles is 3-5 cm per second.

One of the important stimuli initiating contraction of smooth muscles is their stretching. Sufficient stretching of the smooth muscle is usually accompanied by the appearance of action potentials. Thus, the appearance of action potentials during smooth muscle stretching is promoted by two factors:

slow wave oscillations of the membrane potential;

depolarization caused by stretching of smooth muscle.

This property of smooth muscle allows it to automatically contract when stretched. For example, during the overflow of the small intestine, a peristaltic wave occurs, which promotes the contents.

Contraction of smooth muscle.

Smooth muscles, like striated muscles, contain cross-bridged myosin that hydrolyzes ATP and interacts with actin to cause contraction. In contrast to striated muscle, smooth muscle thin filaments contain only actin and tropomyosin and no troponin; the regulation of contractile activity in smooth muscles occurs due to the binding of Ca ++ to calmodulin, which activates myosin kinase, which phosphorylates the myosin regulatory chain. This results in ATP hydrolysis and starts the cross-bridge cycle. In smooth muscle, the movement of actomyosin bridges is a slower process. The breakdown of ATP molecules and the release of the energy necessary to ensure the movement of actomyosin bridges does not occur as quickly as in the striated muscle tissue.

Efficiency of energy consumption in smooth muscle is extremely important in the overall energy consumption of the body, since the blood vessels, small intestine, bladder, gallbladder and other internal organs are constantly in good shape.

During contraction, smooth muscle is able to shorten up to 2/3 of its original length ( skeletal muscle 1/4 to 1/3 length). This allows the hollow organs to perform their function by changing their lumen to a significant extent.

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 precisely the muscle, since its importance in the formation of the outer and inner 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, properties.

Varieties of these fabrics

There are several types of muscles in the composition of the animal body:

• striated;
• smooth muscle tissue.

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

Cross-striped 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 provides the contraction of the heart, the movement of the limbs and all other moving parts of the body. However, smooth muscles are no less significant. What are its features, we will consider further.

In general, it can be seen 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 more or less significant of them.

Smooth structural features

The main unusual features of the structure under consideration are 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 of the structure can be identified in several points.

1. Each cell is surrounded by a dense plexus of connective tissue fibers that looks like a capsule.
2. Each structural unit tightly adjoins the other, intercellular spaces are practically absent. This allows the entire fabric to be tightly packed, structured and strong.
3. Unlike the striated colleague, this structure may include cells of unequal shape.

This, of course, is not the whole characteristic that the structural features, as already mentioned, 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 localization in a particular organ, they can be:

• oval;
• spindle-shaped elongated;
• rounded;
• process.

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

• well-defined and functioning mitochondria;
• Golgi complex;
• the 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 covered on the outside not only with a plasma membrane, 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 smooth muscle tissue. The places of contact are called nexuses. It is through them, as well as through the pores that are in these places in the membrane, that the transmission of impulses between cells, the exchange of information, water molecules and other compounds takes place.

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. That's 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.

1. Secretory. Their main function is the production and accumulation of glycogen granules, the preservation of many mitochondria, polysomes and ribosomal units. These structures got their name because of the proteins contained in them. These are actin filaments and contractile fibrin filaments. These cells are most often localized along the periphery of the tissue.
2. Smooth They look like spindle-shaped elongated structures containing an oval nucleus, displaced to the middle of the cell. Another name for leiomyocytes. They differ in that they are larger. Some particles of the uterine organ reach 500 microns! This is a fairly significant figure against the background of 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 well-coordinated work of the indicated types of myocytes, as well as their organization, provide 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 origins. This explains the differences that the structure of smooth muscle tissue has.

1. mesenchymal origin. most of the smooth fibers have this. It is from the mesenchyme that almost all the tissues lining the inner part hollow organs.
2. epidermal origin. The name itself speaks of the places of localization - these are all skin glands and their ducts. It is they that are formed by smooth fibers that have this variant of appearance. Sweat, salivary, milk, lacrimal - all these glands secrete their secret due to irritation of the cells of myoepitheliocytes - the structural particles of the organ in question.
3. neural origin. Such fibers are localized in one specific place - this is the iris, one of the membranes of the eye. The contraction or expansion of the pupil is innervated and controlled by these smooth muscle cells.

Despite the different origins, the internal composition and performance properties of all in the tissue under consideration remain approximately the same.

The 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 the striated skeletal muscles are able to contract rapidly (this is a good illustration of the trembling in the human body), then the smooth one can be held in a compressed state for a long time. In addition, its activities are not subject to the will and mind of man. Because it innervates her

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

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 precisely 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 strands called myofibrils are formed.

The location in the tissue of these bundles 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 rapid and correct contraction of the entire tissue.

The contraction itself occurs as follows: in the composition of the internal environment of the cell there are pinocytic 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 involvement of the plasmalemma and as a result, the myocyte is reduced.

Smooth muscle tissue: drawing

If we talk about striated tissue, then it is easy to recognize it by its striation. But with regard to the structure we are considering, this does not happen. Why does smooth muscle tissue have a completely different pattern than its close neighbor? This is due to the presence and location of protein components in myocytes. In the composition of smooth muscles, the filaments of myofibrils of different nature are localized chaotically, without a definite ordered state.

That is why the fabric pattern is simply absent. In the striated filament, actin is successively replaced by transverse myosin. As a result, a pattern arises - striation, thanks to which the fabric got its name.

Under the microscope, the smooth tissue looks very even and ordered, due to the longitudinally located elongated myocytes tightly adjacent to each other.

Areas of spatial arrangement in the body

Smooth muscle tissue produces enough a 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.

Obviously, 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.

1. Mitotic division of myocytes until the required amount of tissue is formed. The most common simple and fast way regeneration. This is how the restoration of the internal part of any organ formed by smooth muscles occurs.
2. Myofibroblasts are able to transform into myocytes smooth fabric if necessary. This is a more complex and rare way of regeneration of this tissue.

Smooth muscle innervation

Smooth performs its own regardless of the desire or unwillingness of a living being. This is due to the fact that its innervation is carried out by the autonomic nervous system, as well as the processes of the nerves of the ganglia (spinal).

An example of this and proof 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 meaning of this structure? Why do you need the following:

• prolonged contraction of the walls of organs;
• development of secrets;
• the ability to respond to stimuli and exposure with excitability.

electrical activity. Visceral smooth muscles are characterized by unstable membrane potential. Fluctuations in membrane potential, regardless of nerve 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 gallbladder, bladder, at the junction of the stomach into the duodenum and the small intestine into the colon, as well as in the smooth muscles of small arteries and arterioles.

In some smooth muscles, such as the ureter, stomach, and lymphatics, APs have a long plateau during repolarization. Plateau-like APs ensure the entry into the cytoplasm of myocytes of a significant amount of extracellular calcium, which subsequently participates in the activation of contractile proteins of smooth muscle cells. The ionic nature of smooth muscle AP is determined by the features of the channels of the smooth muscle cell membrane. Ca2+ ions play the main role in the mechanism of AP occurrence. Calcium channels of the membrane of smooth muscle cells pass not only Ca2+ ions, but also other doubly charged ions (Ba 2+, Mg2+), as well as Na+. The entry of Ca2+ into the cell during PD is necessary to maintain tone and develop contraction; therefore, blocking the calcium channels of the smooth muscle membrane, which leads to a restriction of the entry of Ca2+ ions into the cytoplasm of myocytes of internal organs and vessels, is widely used in practical medicine for the correction of motility of the digestive tract and vascular tone in the treatment of patients with hypertension.

Automation. APs of smooth muscle cells have an autorhythmic (pacemaker) character, similar to the potentials of the conduction system of the heart. Pacemaker potentials are recorded in various parts of the smooth muscle. This indicates that any visceral smooth muscle cells are capable of spontaneous automatic activity. Smooth muscle automation, i.e. the ability for automatic (spontaneous) activity is inherent in many internal organs and vessels.

Stretch response. Smooth muscle contracts in response to stretch. 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 full, its wall is stretched. 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. Dr. etc., stretching the muscles of the uterus by a growing fetus is one of the reasons for the onset of labor.

Plastic. If the visceral smooth muscle is stretched, its tension will increase, but if the muscle is held in a state of lengthening caused by the stretch, the tension will gradually decrease, sometimes not only to the level that existed before the stretch, but even below this level. The plasticity of smooth muscles contributes to the normal functioning of the internal hollow organs.

Connection of excitation with contraction. Under conditions of relative rest, a single AP can be registered. Smooth muscle contraction, as in skeletal muscle, is based on the sliding of actin relative to myosin, where the Ca2+ ion performs a trigger function.

The mechanism of smooth muscle contraction has a feature that distinguishes it from the mechanism of skeletal muscle contraction. This feature is that before smooth muscle myosin can exhibit its ATPase activity, it must be phosphorylated. The mechanism of smooth muscle myosin phosphorylation is carried out as follows: the Ca2+ ion combines with calmodulin (calmodulin is a receptor protein for the Ca2+ ion). The resulting complex activates the enzyme - myosin light chain kinase, which in turn catalyzes the process of myosin phosphorylation. Then actin slides in relation to myosin, which forms the basis of contraction. That. the starting point for smooth muscle contraction is the addition of the Ca2+ ion to calmodulin, while in skeletal and cardiac muscle the starting point 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 membrane of smooth muscle cells.

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

ACh has an effect on the membrane potential and contraction of the smooth muscles of the intestine, opposite to the action of norepinephrine. The addition of ACh to an intestinal smooth muscle preparation reduces the 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 with excitation of the parasympathetic nerves. ACh depolarizes the membrane, increases its permeability to Na+ and Ca++.

Similar information.

PHYSIOLOGY OF SMOOTH MUSCLE

Smooth muscles are built from muscle fibers that have a diameter of 2 to 5 microns and a length of only 20 to 500 microns, which is much smaller than in skeletal muscles, the fibers of which are 20 times larger in diameter and thousands of times longer. They do not have transverse striations. The mechanism of contraction of smooth muscle fibers is fundamentally the same as in the lopere-swallowing. It is built on the interaction between the contractile proteins actin and myosin, although there are some differences - they are not characterized by an ordered arrangement of filaments. An analogue of Z-lines in smooth muscles is dense bodies, which are contained in the myoplasm and are connected to the cell membrane and actin filaments. The contraction of various smooth muscles lasts from 0.2 s to 30 s. Their absolute strength is 4-6 kg/cm2, in skeletal muscles - 3-17 kg/cm2.

Types of Smooth Muscles: smooth muscles are divided into visceral, or unitary, polyelemental, or multiunitary, And vascular smooth muscle, having properties of both previous types.

Visceral, or unitary muscles are contained in the walls of hollow organs - the digestive canal, uterus, ureters, gallbladder and bladder. their feature is that they transmit excitation from cell to cell with low-resistance gap junctions, which allows the muscles to respond as a functional syncytium, that is, as one cell, hence the term unitary muscles. They are spontaneously active, have pacemakers (pacemakers), which are modulated under the influence of hormones or neurotransmitters. The resting potential for these muscle fibers is not typical, since in the active state of the cell it is low, during its inhibition it is high, and at rest it is about -55 mV. They are characterized by the so-called sinusoidal slow waves of depolarization, on which peak APs are superimposed, lasting from 10 to 50 ms (Fig. 2.34).

The mechanism of AP generation in smooth muscles and their contraction is largely initiated by Ca2 ions. The contraction occurs 100–200 ms after excitation, and the maximum develops only 500 ms after the onset of the peak. Therefore, smooth muscle contraction is a slow process. However visceral muscles have a high degree of electrical conjugation between cells, provides a high coordination of their contraction.

Polyelement, or multiunitary smooth muscles are composed of individual units without connecting bridges, and the response of the whole muscle to stimulation consists of the response of individual muscle fibers. Each muscle fiber is innervated by one nerve ending, as in skeletal muscle. These include the muscles of the iris of the eye, the ciliary muscle of the eye, the piloerectoral muscles of the hair of the skin. They do not have voluntary regulation, they are reduced due to nerve impulses that are transmitted through the neuromuscular synapses of the autonomic nervous system, whose neurotransmitters can cause both excitation and inhibition.

Mechanisms of contraction and relaxation of smooth muscles

The mechanism of conjugation of excitation and contraction differs from a similar process occurring in skeletal muscles, since smooth muscles do not contain troponin.

The sequence of processes in smooth muscles that leads to contraction and relaxation has the following steps:

1. When the cell membrane is depolarized, potential deposits of calcium channels and ions open

RICE. 2.34.

Ca 2+ enter the cell with an electrochemical gradient, the concentration of Ca 2+ ions in the cell increases.

2. The entry of Ca 2+ ions through the cell membrane can cause an additional exit of Ca 2+ ions from the sarcoplasmic reticulum (SPR) through the Ca 2+ dependent gate of calcium channels. Hormones and neurotransmitters also stimulate the release of Ca 2+ ions from SBP through inositol triphosphatide (ISP) dependent calcium channel gates.

3. intracellular concentration of Ca 2+ ions increases.

4. Ca 2+ ions bind to calmodulin, a regulatory protein that has 4 Ca 2+ bindings and plays an important role in the activation of enzymes. Calcium calmodulin complex activates the enzyme kinase myosin light chain, resulting in phosphorylation of myosin head molecules. Myosin hydrolyzes ATP, energy is generated, and the cycle of formation of transverse actin-myosin bridges, sliding of actin along myosin chains begins. Phosphorylated myosin bridges repeat their cycle until they are dephosphorylated. myosinphosphatase.

5. Dephosphorylation of myosin leads to relaxation of the muscle fiber, or a state of residual tension due to the formed cross bridges, until the final dissociation of the calcium-calmodulin complex occurs.

AGE CHANGES IN EXCITIVE STRUCTURES

In the process of ontogenesis, the properties of excitable structures change in connection with the development of the musculoskeletal system and its regulation.

Muscle mass increases - from 23.3% of body weight in a newborn to 44.2% at the age of 17-18 years. Muscle tissue grows due to the lengthening and thickening of muscle fibers, and not an increase in their number.

In a newborn child, the activity of sodium-potassium pumps located in myocyte membranes is still low, and therefore the concentration of K + ions in the cell is almost half that in an adult, and only at 3 months begins to increase. APs are already generated after birth, but they have a lower amplitude and a longer duration. The generation of PD of muscle fibers in newborns is not blocked by tetrodotoxin.

After birth, the length and diameter of the axial cylinders in the nerve fibers increase from 1-3 microns to 7 microns at 4 years, and their formation is completed at 5-9 years. Until the age of 9, myelination of nerve fibers ends. The rate of excitation conduction after birth does not exceed 50% of the rate in adults and increases within 5 years. The increase in conduction velocity is due to: an increase in the diameter of nerve fibers, their myelination, the formation of ion channels and an increase in the amplitude of AP. A decrease in the duration of AP and, accordingly, the phase of absolute refractoriness leads to an increase in the number of AP that a nerve fiber can generate.

The receptor apparatus of the muscles develops faster than the motor nerve endings are formed. The duration of neuromuscular transmission after birth is 4.5 ms, in an adult it is 0.5 ms. In the process of ontogenesis, the synthesis of acetylcholine, acetylcholinesterase, and the density of cholinergic receptors of the end plate increase.

In the process of aging, the duration of AP in excitable structures increases, and the number of AP generated by muscle fibers per unit time (lability) decreases. Muscle mass decreases due to a decrease in metabolic rate.

Smooth muscles that form the walls (muscle layers) of the internal organs are divided into two types - visceral(i.e. internal) smooth muscles lining the walls gastrointestinal tract and urinary tract, and unitary - smooth muscles located in the walls of blood vessels, in the pupil and lens of the eye and at the hair roots of the skin (muscles that ruffle the hair in animals). These muscles are built from spindle-shaped mononuclear cells that do not have transverse striation, which is due to the chaotic arrangement of contractile proteins in their fibers. Muscle fibers are relatively short (from 50 to 200 microns), they have branches at both ends and fit tightly to each other, forming long and thin cylindrical bundles with a diameter of 0.05-0.01 mm, which branch and connect with other bundles. Their network forms either layers (layers) or even thicker bundles in the internal organs.

Neighboring cells in smooth muscles are functionally interconnected by low-resistance electrical contacts - nexuses. Due to these contacts, action potentials and slow waves of depolarization spread freely from one muscle fiber to another. 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 contractile reaction. Consequently, smooth muscles are not only morphological, but also functional syncytium.

As in skeletal muscle, smooth muscle contractile proteins are activated as a result of an increase in the concentration of calcium ions in the sarcoplasm. However, calcium does not come from the cisterns of the sarcoplasmic reticulum, as in skeletal muscles, but from the extracellular environment, along a concentration gradient, through the plasma membrane of the cell, through slow, potential-sensitive calcium channels that are activated as a result of membrane depolarization when it is excited. This significantly affects the development of the action potential of smooth muscle cells, which is clearly reflected in the PD curve (Fig. 12. 1).

Fig.12. Action potential (1) and curve

contraction (2) of a smooth muscle cell.

A - depolarization phase (Na + - input);

B - "calcium plateau" (Ca 2+ - input);

B - phase of repolarization (K + - output);

(dashed line indicates PD of skeletal muscle)

A slow but significant incoming calcium current forms a characteristic “calcium plateau” on the AP curve, which does not allow the membrane to be quickly depolarized, which leads to a significant increase in the duration of the refractory period. Calcium is removed from the cell even more slowly, through the Ca 2+ - ATPase of the plasma membrane. All this significantly affects both the characteristics of excitability and the contractility of smooth muscles. Smooth muscles are much less excitable than striated ones and excitation spreads through them at a very low speed - 2-15 cm / s. In addition, they contract and relax very slowly, and the time of a single contraction can last several seconds.

Due to the long refractory period, the duration of the action potential of a smooth muscle fiber practically coincides with the time of entry and removal of calcium ions from the cell, that is, the time of development of AP and the duration of contraction practically coincide (Fig. 12. 2) As a result, smooth muscles are practically not capable of forming a classic tetanus. Due to the very slow relaxation, the fusion of single contractions (“smooth muscle tetanus”) occurs even at a low frequency of stimulation and is, to a greater extent, the result of a slow undulating involvement in a prolonged contraction of cells adjacent to the irritated one.

Smooth muscles are capable of relatively slow and prolonged tonic abbreviations. 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 are especially pronounced in the sphincters of hollow organs, which prevent the release of the contents of these organs.

The smooth muscles of the walls of blood vessels, especially arteries and arterioles, are also in a state of constant tonic contraction. A change in muscle tone of the walls of arterial vessels affects 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. Skeletal muscle normally has almost no plasticity. These differences are well observed in slow stretching of smooth and skeletal muscle. When the tensile load is removed, the skeletal muscle shortens rapidly, while the smooth muscle remains stretched. High plasticity of smooth muscles is of great importance for the normal functioning of hollow organs. Due to its high plasticity, smooth muscle can be completely relaxed both in a shortened and in a stretched state. So, for example, the plasticity of the muscles of the bladder as it fills prevents an excessive increase in pressure inside it.

An adequate irritant for smooth muscles is their rapid and strong stretching, which causes their contraction, due to the increasing depolarization of cells during stretching. The frequency of action potentials (and, accordingly, the frequency of contractions.) The greater, the more and faster the smooth muscle is stretched. Thanks to this mechanism, in particular, the promotion of the food bolus along the digestive tract is ensured. The muscular wall of the intestine, stretched by a lump of food, responds with a contraction and thus pushes the lump into the next section of the intestine. Stretch-induced contraction plays an important role in autoregulation of vascular tone, and also provides involuntary (automatic) emptying of a full bladder in cases where neural regulation is absent as a result of spinal cord injury.

The nervous regulation of smooth muscles is carried out through the sympathetic and parasympathetic fibers of the autonomic nervous system.

A feature of visceral smooth muscle cells is that they are able to contract even in the absence of direct nerve influences under conditions of their isolation and denervation, and even after blockade of neurons of the autonomic ganglia.

In this case, contractions do not occur as a result of the transmission of nerve impulses from the nerve, but due to the activity of its own cells ( pacemakers), which are identical in structure to other muscle cells, but differ in electrophysiological properties - have automaticity. In these cells, the activity of membrane ion channels is regulated in such a way that their membrane potential does not balance, but constantly “drifts”. As a result, the membrane regularly prepotentials or pacemaker potentials, with a certain frequency depolarizing the membrane to a critical level. When an action potential occurs in a pacemaker, excitation spreads from them to neighboring ones, which leads to their excitation and contraction. As a result, one section of the muscle layer after another is consistently reduced.

It follows from this that visceral smooth muscles are controlled by the autonomic nervous system, which performs in relation to these muscles not a starting, but a tuning, regulating function. This means that the activity of visceral smooth muscles itself occurs spontaneously, without nervous influences, but the level of this activity (strength and frequency of contractions) changes under the influence of the autonomic nervous system. In particular, by changing the rate of "drift" of the membrane potential, nerve impulses of vegetative fibers affect the frequency of contractions of visceral smooth muscle fibers.

Unitary smooth muscles can also be spontaneously active, but they contract mainly under the influence of nerve impulses from autonomic fibers. Their peculiarity lies in the fact that a single nerve impulse coming to them is not able to cause a contraction; in response, only a temporary subthreshold depolarization of the muscle cell membrane occurs. Only when a series of impulses follows along the autonomic nerve fiber with a frequency of about 1 impulse per 1 sec. and more, it is possible to develop the action potential of the muscle fiber and its contraction. That is, unitary muscle fibers "sum up" nerve impulses and respond to irritation when the frequency of impulses reaches a certain value.

In unitary smooth muscle, as in visceral smooth muscle, excited muscle cells affect neighboring cells. As a result, excitation captures many cells (hence the name of these muscles - unitary, that is, consisting of units - "units" with a large number of muscle fibers in each of them).

Two mediators involved in the nervous regulation of smooth muscle contraction are acetylcholine (ACh) and epinephrine (norepinephrine). The mode of action of ACh in smooth muscle is the same as in skeletal muscle: ACh increases the ion permeability of the membrane, causing its depolarization. The mechanism of action of adrenaline is unknown. Skeletal muscle fibers respond to the action of the mediator only in the region of the end plate (neuromuscular synapse), while smooth muscle fibers respond to the action of the mediator, regardless of the place of its application. Therefore, smooth muscles can be affected by mediators contained in the blood (for example, adrenaline, which has a long-term effect on smooth muscles, causes them to contract).

From all of the above, one more characteristic feature of smooth muscles follows - their contraction does not require large energy expenditures.