regulation of smooth muscle contraction. Cessation of smooth muscle contraction

Muscle tissues are tissues that differ in structure and origin, but have a common ability to contract. They consist of myocytes - cells that can perceive nerve impulses and respond to them with a contraction.

Properties and types of muscle tissue

Morphological features:

Elongated form of myocytes;
longitudinally placed myofibrils and myofilaments;
mitochondria are located near contractile elements;
polysaccharides, lipids and myoglobin are present.

Properties of muscle tissue:

contractility;
excitability;
conductivity;
extensibility;
elasticity.

There are the following types muscle tissue depending on morphofunctional features:

Striated: skeletal, cardiac.
Smooth.

Histogenetic classification divides muscle tissue into five types depending on the embryonic source:

Mesenchymal - desmal germ;
epidermal - skin ectoderm;
neural - neural plate;
coelomic - splanchnotomes;
somatic - myotome.

Of 1-3 species, smooth muscle tissues develop, 4, 5 give striated muscles.

The structure and function of smooth muscle tissue

Consists of individual small spindle-shaped cells. These cells have a single nucleus and thin myofibrils that extend from one end of the cell to the other. Smooth muscle cells are combined into bundles, consisting of 10-12 cells. This association arises due to the peculiarities of smooth muscle innervation and facilitates the passage of a nerve impulse to the entire group of smooth muscle cells. Smooth muscle tissue contracts rhythmically, slowly and for a long time, while being able to develop great strength without significant energy expenditure and without fatigue.

In lower multicellular animals, all muscles are composed of smooth muscle tissue, while in vertebrates it is part of the internal organs (except the heart).

The contractions of these muscles do not depend on the will of the person, that is, they occur involuntarily.

Functions of smooth muscle tissue:

Maintaining stable pressure in hollow organs;
regulation of blood pressure;
peristalsis of the digestive tract, movement of contents along it;
emptying Bladder.

The structure and function of skeletal muscle tissue

Consists of long and thick fibers 10-12 cm long. Skeletal muscles are characterized by voluntary contraction (in response to impulses coming from the cerebral cortex). The speed of its contraction is 10-25 times higher than in smooth muscle tissue.

The muscle fiber of the striated tissue is covered with a sheath - the sarcolemma. Under the membrane is the cytoplasm with a large number of nuclei located along the periphery of the cytoplasm, and contractile filaments - myofibrils. The myofibril consists of successively alternating dark and light areas (discs) with different light refractive index. Using an electron microscope, it was found that the myofibril consists of protofibrils. Thin protofibrils are built from a protein - actin, and thicker ones - from myosin.

With the contraction of the fibers, the excitation of contractile proteins occurs, thin protofibrils glide over thick ones. Actin reacts with myosin to form a single actomyosin system.

Functions of skeletal muscle tissue:

Dynamic - movement in space;
static - maintaining a certain position of body parts;
receptor - proprioceptors that perceive irritation;
depositing - liquid, minerals, oxygen, nutrients;
thermoregulation - relaxation of muscles with an increase in temperature to expand blood vessels;
facial expressions - to convey emotions.

The structure and function of cardiac muscle tissue

cardiac muscle tissue

The myocardium is built from cardiac muscle and connective tissue, with vessels and nerves. Muscle tissue refers to striated muscles, the striation of which is also due to the presence of different types of myofilaments. The myocardium is made up of fibers that are interconnected and form a mesh. These fibers include single or binuclear cells that are arranged in a chain. They are called contractile cardiomyocytes.

Contractile cardiomyocytes are 50 to 120 micrometers long and up to 20 microns wide. The nucleus here is located in the center of the cytoplasm, in contrast to the nuclei of striated fibers. Cardiomyocytes have more sarcoplasm and fewer myofibrils than skeletal muscle. There are many mitochondria in the cells of the heart muscle, since continuous heartbeats require a lot of energy.

The second type of myocardial cells are conductive cardiomyocytes, which form the conduction system of the heart. Conductive myocytes provide impulse transmission to contractile muscle cells.

Functions of cardiac muscle tissue:

Pump house;
provides blood flow in the bloodstream.

Components of the contractile system

The structural features of muscle tissue are determined by the functions performed, the ability to receive and conduct impulses, and the ability to contract. The contraction mechanism consists in the coordinated work of a number of elements: myofibrils, contractile proteins, mitochondria, myoglobin.

In the cytoplasm of muscle cells there are special contractile filaments - myofibrils, the contraction of which is possible with the friendly work of proteins - actin and myosin, as well as with the participation of Ca ions. Mitochondria supply all processes with energy. Also, energy reserves form glycogen and lipids. Myoglobin is necessary for the binding of O 2 and the formation of its reserve for the period of muscle contraction, since during contraction there is compression of the blood vessels and the supply of O 2 to the muscles is sharply reduced.

Table. Correspondence between the characteristics of muscle tissue and its type

Type of fabric	Characteristic
smooth muscle	Included in the walls of blood vessels
	Structural unit - smooth myocyte
	Decreases slowly, unconsciously
	There is no transverse striation
Skeletal	Structural unit - multinuclear muscle fiber
	Characterized by transverse striation
	Decreases quickly, consciously

Where is muscle tissue located?

Smooth muscles are an integral part of the walls of internal organs: gastrointestinal tract, genitourinary system, blood vessels. They are part of the capsule of the spleen, skin, sphincter of the pupil.

Skeletal muscles occupy about 40% of the human body weight, they are attached to the bones with the help of tendons. This tissue consists of skeletal muscles, muscles of the mouth, tongue, pharynx, larynx, upper esophagus, diaphragm, mimic muscles. Also, striated muscle is located in the myocardium.

How is skeletal muscle fiber different from smooth muscle tissue?

The fibers of striated muscles are much longer (up to 12 cm) than the cellular elements of smooth muscle tissue (0.05-0.4 mm). Also, skeletal fibers have transverse striation due to the special arrangement of actin and myosin filaments. For smooth muscles this is not typical.

IN muscle fibers there are many nuclei, and the contraction of the fibers is strong, fast and conscious. Unlike smooth muscles, smooth muscle tissue cells are mononuclear, able to contract at a slow pace and unconsciously.

Smooth muscles are present in the walls of the digestive canal, bronchi, blood and lymphatic vessels, the bladder, in the uterus, as well as in the iris, in the ciliary muscle, skin and glands. Unlike striated muscles, they are not separate muscles, but only part of the organs. Smooth muscle cells have an elongated fusiform or ribbon-like shape with pointed ends. Their length in humans is usually about 20 microns. The greatest length (up to 500 microns) is reached by smooth muscle cells in the wall of the human pregnant uterus. In the middle part of the cell there is a rod-shaped nucleus, and in the cytoplasm along the entire cell, the thinnest, completely homogeneous myofibrils run parallel to each other. Therefore, the cell does not have a transverse striation. Thicker myofibrils are located in the outer layers of the cell. They are called boundary and have uniaxial birefringence. In an electron microscope, it can be seen that myofibrils are bundles of protofibrils and have a transverse striation that is not visible in a light microscope. Smooth muscle cells can regenerate by dividing (mitosis). They contain a variety of actomyosin - tonoactomyosin. Between smooth muscle cells there are the same areas of membrane contact, or nexuses, as between cardiac cells, 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 stimuli than skeletal ones. The latent period of its contraction lasts several seconds. Smooth muscles contract much more slowly than skeletal ones. Thus, the period of smooth muscle contraction in the frog's stomach 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, gallbladder, 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 that occur 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 the stresses of non-simultaneously tensing 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 are automatic. They contract under the influence of local reflex mechanisms, such as the Meisner and Auerbach plexuses in the alimentary canal, or chemicals entering the bloodstream, such as acetylcholine, norepinephrine, and adrenaline. Automatic contractions of smooth muscles are amplified 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 smooth muscle relaxation. Some smooth muscles that have a large number of nerve endings do not have automatism, for example, the sphincter of the pupil, the nictitating membrane of a cat.

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

Smooth muscle tissue also develops from the mesoderm (originates from the 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 µm, and their width varies from 4 to 7 µm. 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 run in the longitudinal direction, which do not have transverse striations and are completely invisible without special treatment. Each smooth muscle cell is dressed in the thinnest connective tissue sheath. These membranes are adjacent cells are interconnected. In contrast to the striated fibers located almost along 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 the connective tissue, or associated in muscle complexes of various sizes.

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

Smooth muscle cells of organs are combined into muscle bundles. In many cases (urinary tract, uterus, etc.), these bundles branch and merge with other bundles, forming surface networks of varying density. If a large number of bundles are closely located, then a dense muscular membrane is formed (for example, the gastrointestinal tract). The blood supply to smooth muscles is carried out through the vessels that pass in 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 take hours, develop such strength that is inaccessible to the striated muscles. The activity of smooth muscles, as a rule, is not subject to our will (vegetative innervation, see below) - they are involuntary.

Smooth muscles in their development (phylogenesis) are more ancient than striated muscles, and are more common in the lower forms of the animal world.

Classification of smooth muscles

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

Functions and properties of smooth muscles

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

Connection of excitation with 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 registered. The contraction of both skeletal and smooth muscles is based on the sliding of actin in relation 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. Phosphorylation and dephosphorylation of myosin is also observed in skeletal muscle, but in it the process of phosphorylation is not necessary for the activation of the ATPase activity of myosin. 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. It should be noted that the starting point for smooth muscle contraction is the attachment of the Ca2+ ion to calmodulin, while in the skeletal and cardiac muscle the starting point is the attachment 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. If epinephrine or norepinephrine is added to an intestinal smooth muscle preparation, the membrane potential increases, the frequency of AP decreases, and the muscle relaxes, i.e., the same effect is observed as with excitation of sympathetic nerves.

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+.

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 during the removal of the ovaries are relatively unexcitable. During estrus or in animals deprived of ovaries, which were injected with estrogen, the excitability of smooth muscles increases. Progesterone increases the membrane potential even more than estrogen, but in this case, the electrical and contractile activity of the muscles of the uterus is inhibited.

Smooth muscles are part of the internal organs. Due to the contraction, they provide the motor (motor) function of their organs (alimentary 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 muscles is the muscle cell, which has a spindle 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 an unfazed muscle cell includes the plasma membrane, together with the basement membrane that covers it from the outside, and adjacent collagen fibers. The main intracellular elements are: 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, transmission of excitation. It has been proven that some smooth muscles contain a large number of nexus (sphincter of the pupil, circular muscles of the small intestine, etc.), while others have few or none at all (vas deferens, longitudinal muscles of the intestines). There is also an intermediate, or desmotic, connection between non-smoking muscle cells (through a thickening of the membrane and with the help of cell processes). Obviously, these connections are important for the mechanical connection of cells and the 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, there is no T-system in smooth muscles, and the sarcoplasmic reticulum makes up only 2-7% of the volume of the myoplasm and has no connections with the external environment of the cell.

Physiological properties of smooth muscles .

Smooth muscle cells, like striated, contract due to the sliding of actin protofibrils between myosin, however, the speed of sliding and ATP hydrolysis, and hence the rate of contraction, is 100-1000 times less than in striated muscles. Thanks to this, smooth muscles are well adapted for long-term sliding with little energy and without fatigue.

Smooth muscles, taking into account the ability to generate AP in response to threshold or supra-horn stimulation, are conditionally divided into phasic and tonic. Phasic muscles generate a full-fledged AP, tonic muscles - only local ones, although they also have a mechanism for generating full-fledged potentials. The inability of tonic muscles to 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-frightening muscles varies from -50 to -60 mV. As in other muscles, including nerve cells, it is formed mainly to +, Na +, Cl-. In the smooth muscle cells of the alimentary canal, uterus, and some vessels, the membrane potential is unstable, spontaneous fluctuations are observed in the form of slow depolarization waves, at the top of which AP discharges may appear. The duration of AP of smooth muscles varies 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 AP of skeletal muscles. In the mechanism of AP of smooth muscles, Ca2+ plays an important role next to Na+.

Spontaneous myogenic activity. Unlike skeletal muscles, smooth muscles of the stomach, intestines, uterus, and ureters have spontaneous myogenic activity, i.e. develop spontaneous tetanohyodibne contractions. They are stored under conditions of isolation of these muscles and with pharmacological shutdown of the intrafusal nerve plexuses. So, PD occurs in the smooth muscles themselves, and is not due to the transmission of nerve impulses to the muscles.

This spontaneous activity is of myogenic origin and occurs in muscle cells that act as a pacemaker. In these cells, the local potential reaches a critical level and transforms into AP. But after the repolarization of the membrane, a new local potential spontaneously arises, which causes another AP, and so on. AP, propagating 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 min, segmental and pendulum movements of the colon - 20 times per 1 min in the upper sections and 5-10 per 1 min - in the lower. Thus, the smooth muscle fibers of these internal organs have automatism, 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 the cells of the smooth muscles of the pacemaker? 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. It is possible that a decrease in the initial inactivating component of the potassium current during depolarization of muscle cells due to the inactivation of the corresponding potassium ion channels plays a certain role.

Elasticity and extensibility of smooth muscles. Unlike skeletal muscles, they are smooth when stretched themselves as plastic, elastic structures. Due to plasticity, smooth muscle can be completely relaxed both in a contracted and stretched state. For example, the plasticity of the smooth muscles of the wall of the stomach or bladder, as these organs are filled, prevents an increase in intracavitary pressure. Excessive stretch often leads to stimulation of contraction, which is due to the depolarization of the pacemaker cells that occurs when the muscle is stretched, and is accompanied by an increase in the frequency of AP, and as a result, an increase in contraction. The contraction, which activates the stretching process, plays a large role in the self-regulation of the basal tone of the blood vessels.

mechanism of smooth muscle contraction. A prerequisite for the occurrence of contraction of smooth muscles, as well as skeletal ones, is an increase in the concentration of Ca2 + in myoplasm (up to 10v-5 M). It is believed that the contraction process is activated mainly by extracellular Ca2 +, which enters muscle cells through voltage-dependent Ca2 + channels.

A feature of neuromuscular transmission in smooth muscles is that innervation is carried out by the autonomic nervous system and it can have both excitatory and inhibitory effects. By type, cholinergic (mediator acetylcholine) and adrenergic (mediator norepinephrine) mediators are distinguished. The former are usually found in the muscles of the digestive system, the latter in the muscles of the blood vessels.

The same mediator can be excitatory in some synapses, and inhibitory in others (depending on the properties of cytoreceptors). Adrenoreceptors are divided into a- and B-. Norepinephrine, acting on a-adrenergic receptors, constricts blood vessels and inhibits the motility of the digestive tract, and acting on B-adrenergic receptors, stimulates the activity of the heart and dilates the blood vessels of some organs, relaxes the muscles of the bronchi. Described neuromuscular. ny transfer in smooth muscles for the help and other mediators.

In response to the action of an excitatory mediator, depolarization of smooth muscle cells occurs, which manifests itself in the form of an excitatory synaptic potential (SSP). When it reaches a critical level, PD occurs. This happens when several impulses come one after another to the nerve ending. The emergence of ISGI is a consequence of an increase in the permeability of the postsynaptic membrane for Na +, Ca2 + and SI ".

The inhibitory neurotransmitter causes hyperpolarization of the postsynaptic membrane, which is manifested in the inhibitory synaptic potential (GSP). Hyperpolarization is based on an increase in membrane permeability mainly for K +. The role of an inhibitory mediator in smooth muscles excited by acetylcholine (for example, muscles of the intestine, bronchi) is played by norepinephrine, and in smooth muscles for which norepinephrine is an excitatory mediator (for example, bladder muscles) - acetylcholine.

Clinical and physiological aspect. In some diseases, when the innervation of skeletal muscles is disturbed, their passive stretching or displacement is accompanied by a reflex increase in their tone, i.e. resistance to stretching (spasticity or 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 contraction of the muscle, contracture may occur. The more the muscle function is disturbed, the more pronounced the contracture aftereffect (for example, the contracture of the masticatory muscles in the pathology of the maxillofacial region). What is the origin of contracture? It is believed that the contracture arose due to a decrease in the concentration of ATP in the muscle, which led to the formation of a permanent connection between the transverse bridges and actin protofibrils. In this case, the muscle loses flexibility and becomes hard. The contracture subsides, the muscle relaxes when the ATP concentration reaches a normal level.

In diseases such as myotonia, muscle cell membranes are excited so easily that even slight stimulation (for example, the introduction of a needle electrode during electromyography) causes a discharge of muscle impulses. Spontaneous AP (fibrillation potentials) are also recorded at the first stage after muscle denervation (until inactivity leads to its atrophy).

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 an important role in the 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;

like striated muscle fibers, they are covered on the outside 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 intensified when its walls are stretched 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 excitation according to 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 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 energy necessary to ensure the movement of actomyosin bridges does not occur as quickly as in 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 of its length). This allows the hollow organs to perform their function by changing their lumen to a significant extent.

important properties 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, with well-defined plasticity, is easily detected if they are first slowly stretched, and then the tensile load is removed. immediately shortened after the load is removed. In contrast, the smooth muscle after the removal of the load remains stretched until, under the influence of some kind of irritation, its active contraction occurs.

The property of plasticity is very important for the normal activity of the smooth muscles of the walls of hollow organs, such as the bladder: due to the plasticity of the smooth muscles of the walls of the bladder, the pressure inside it changes relatively little with different degrees of filling.

Excitability and arousal

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

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

Exists big number various types smooth muscles in the body of animals and humans. Most of the hollow organs of the body are lined with smooth muscles that have 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.

However, electron microscopic studies have shown that there is no membrane and protoplasmic continuity between the individual fibers of the muscular syncytium: they are separated from each other by thin (200-500 Å) slits. The concept of "syncytial structure" is currently more physiological than morphological.

syncytium- this is a functional formation that ensures that action potentials and slow waves of depolarization can freely propagate from one fiber to another. Nerve endings are located only on a small number of syncytium fibers. However, due to the unhindered spread of excitation from one fiber to another, the involvement of the entire muscle in the reaction can occur if the nerve impulse arrives at a small number of muscle fibers.

Smooth muscle contraction

With a large force of a single irritation, smooth muscle contraction may occur. The latent period of a single contraction of this muscle is much longer than that of a skeletal muscle, reaching, for example, in the intestinal muscles of a rabbit 0.25-1 second. The duration of the contraction itself is also large ( rice. 152): in the stomach of a rabbit, it reaches 5 seconds, and in the stomach of a frog - 1 minute or more. Relaxation is especially slow after contraction. The wave of contraction propagates through the smooth muscles also very slowly, it travels only about 3 cm per second. But this slowness of the contractile activity of smooth muscles is combined with their great strength. Thus, the muscles of the stomach of birds are capable of lifting 1 kg per 1 cm2 of their cross section.

Smooth muscle tone

Due to the slowness of contraction, a smooth muscle, even with rare rhythmic stimuli (for a frog's stomach, 10-12 stimuli per minute is enough), easily passes into a long-term state of persistent contraction, reminiscent of tetanus of skeletal muscles. However, the energy expenditure during such a persistent contraction of the smooth muscle is very small, which distinguishes this contraction from the tetanus of the striated muscle.

The reasons why smooth muscles contract and relax much more slowly than skeletal muscles have not yet been fully elucidated. It is known that myofibrils of smooth muscle, like those of skeletal muscle, consist of myosin and actin. However, smooth muscles do not have striation, no Z membrane, and are much richer in sarcoplasm. Apparently, these features of the structure of smooth muscle waves determine slow pace contraction process. This corresponds to a relatively low level of smooth muscle metabolism.

Smooth muscle automation

A characteristic feature of smooth muscles, which distinguishes them from skeletal muscles, is the ability for spontaneous automatic activity. Spontaneous contractions can be observed in the study of the smooth muscles of the stomach, intestines, gallbladder, ureters and a number of other smooth muscle organs.

Smooth muscle automation is of myogenic origin. It is inherent in the muscle fibers themselves and is regulated by nerve elements that are located in the walls of smooth muscle organs. The myogenic nature of automaticity has been proven by experiments on strips of muscles of the intestinal wall, freed by careful dissection from the adjacent nerve plexuses. Such strips, placed in a warm Ringer-Locke solution, which is saturated with oxygen, are capable of making automatic contractions. Subsequent histological examination revealed the absence of nerve cells in these muscle strips.

In smooth muscle fibers, the following spontaneous oscillations of the membrane potential are distinguished: 1) slow waves of depolarization with a cycle duration of the order of several minutes and an amplitude of about 20 mV; 2) small rapid potential fluctuations preceding the emergence of action potentials; 3) action potentials.

Smooth muscle responds to all external influences by changing the frequency of spontaneous rhythm, which results in contraction and relaxation of the muscle. The effect of irritation of the smooth muscles of the intestine depends on the ratio between the frequency of stimulation and the natural frequency of spontaneous rhythm: with a low tone - with rare spontaneous action potentials - the applied irritation enhances the tone; with a high tone, relaxation occurs in response to irritation, since an excessive increase in impulses leads to that each next impulse falls into the refractory phase from the previous one.

As in skeletal muscle, the trigger stimulus for most smooth muscle contraction is an increase in the amount of intracellular calcium ions. IN different types In smooth muscle, this increase can be caused by nerve stimulation, hormonal stimulation, stretching of the fiber, or even a change in the chemical composition of the environment surrounding the fiber.

However, in smooth muscle lacks troponin(a regulatory protein that is activated by calcium). Smooth muscle contraction is activated by a completely different mechanism, described below.

The connection of calcium ions with calmodulin. Myosin kinase activation and phosphorylation of the myosin head.

Instead of troponin smooth muscle cells contain large amounts of another regulatory protein called calmodulin. Although this protein is similar to troponin, it differs in the way the contraction is triggered. Calmodulin does this by activating myosin cross-bridges. Activation and reduction are carried out in the following sequence.

1. Calcium ions bind to calmodulin.
2. The calmodulin-calcium complex binds to the phosphorylating enzyme myosin kinase and activates it.
3. One of the light chains of each myosin head, called the regulatory chain, is phosphorylated by the action of myosin kinase. When this strand is not phosphorylated, there is no cyclic attachment and detachment of the myosin head relative to the actin filament. But when the regulatory chain is phosphorylated, the head acquires the ability to re-bind to the actin filament and carry out the entire cyclic process of periodic "pull-ups" that underlie contraction, as in skeletal muscle.

Termination of contraction. The role of myosinphosphatase. When the concentration of calcium ions falls below a critical level, the above processes automatically develop in the opposite direction, except for the phosphorylation of the myosin head. For the reverse development of this state, another enzyme, myosinphosphatase, is needed, which is localized in the fluids of the smooth muscle cell and cleaves the phosphatase from the regulatory light chain. After that, the cyclic activity, and hence the contraction, stops.
Therefore, the time necessary for muscle relaxation, is largely determined by the amount of active myosinphosphatase in the cell.

Possible mechanism for regulating the "latch" mechanism. Due to the importance of the “latch” mechanism in smooth muscle function, attempts are being made to explain this phenomenon, since it makes it possible to maintain long-term smooth muscle tone in many organs without significant energy costs. Among the many proposed mechanisms, we present one of the simplest.

When strongly activated and myosin kinase, and myosinphosphatase, the frequency of cycles of myosin heads and the rate of contraction are high. Then, as enzyme activation decreases, the frequency of cycles decreases, but at the same time, deactivation of these enzymes allows myosin heads to remain attached to actin filaments for an increasingly long part of the cycle. Therefore, the number of heads attached to an actin filament in any given this moment time remains large.

Since the number heads attached to actin determines the static strength of the contraction, the tension is held, or "snaps". However, little energy is used, since there is no splitting of ATP to ADP, except in those rare cases when some head is disconnected.