Regulation of smooth muscle contraction. Stopping smooth muscle contraction

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

However, electron microscopic studies have shown that there is no membrane and protoplasmic continuity between individual fibers of the muscle 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 propagate unhindered from one fiber to another. Nerve endings are located only on a small number of syncytium fibers. However, due to the unimpeded 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

At great strength A single irritation may cause smooth muscle contraction. The latent period of a single contraction of this muscle is significantly longer than skeletal muscle, reaching, for example, 0.25-1 seconds in the intestinal muscles of a rabbit. The duration of the contraction itself is also long ( rice. 152): in the stomach of a rabbit it reaches 5 seconds, and in the stomach of a frog - 1 minute or more. Relaxation occurs especially slowly 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 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 its cross section.

Smooth muscle tone

Due to the slowness of contraction, smooth muscle, even with rare rhythmic stimulation (for a frog’s stomach, 10-12 stimulations per minute is enough), easily goes into a long-term state of persistent contraction, reminiscent of skeletal muscle tetanus. However, the energy expenditure for such a sustained contraction of smooth muscle is very small, which distinguishes this contraction from tetanus of 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 smooth muscle myofibrils, like those of skeletal muscle, consist of myosin and actin. However, smooth muscles do not have cross-striations, do not have a Z membrane, and are much richer in sarcoplasm. Apparently, these structural features of smooth muscle waves determine the slow pace of the contractile process. This also corresponds to the relatively low level of smooth muscle metabolism.

Automaticity of smooth muscles

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

Automaticity of smooth muscles 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 muscle of the intestinal wall, freed by careful dissection from the nerve plexuses adjacent to it. Such strips, placed in a warm Ringer-Locke solution, which is saturated with oxygen, are capable of automatic contractions. Subsequent histological examination revealed the absence of nerve cells in these muscle strips.

In smooth muscle fibers, the following spontaneous oscillations of 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 fluctuations in potential that precede the occurrence of action potentials; 3) action potentials.

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

Muscle tissue

Efferent innervation smooth muscle tissue is carried out by both the sympathetic (noradrenergic innervation) and parasympathetic (cholinergic innervation) parts of the autonomic nervous system, which have the opposite effect on the contractile activity of muscle tissue. Its serotonergic and peptidergic innervation has also been described. Nerve endings are found only on individual cells and have the appearance of varicose areas of thin branches of axons. Excitation is transmitted to neighboring myocytes via gap junctions.

Afferent innervation is provided by branches of nerve fibers that form free endings in smooth muscle tissue.

Humoral regulation of smooth muscle tissue activity. Hormones and other biologically active substances influence the contractile activity of smooth muscle tissue (different in different organs) due to the presence of corresponding sets of receptors on its cells. These substances include histamine, serotonin, bradykinin, endothelin, nitric oxide, leukotrienes, prostaglandins, neurotensin, substance P, cholecystokinin, vasoactin interstinal peptide (VIP), opioids, etc. Contractions of uterine myocytes at the end of pregnancy and during childbirth are stimulated by the hormone oxytocin ; estrogen increases, and progesterone decreases their tone.

Myogenic activity of smooth muscle tissue. The physiological stimulus of smooth myonites is their stretching, which causes depolarization of the sarcolemma and an influx of Ca 2+ ions into the sarcoplasm. Smooth muscle characterized by spontaneous rhythmic activity (automaticity) due to the cyclically changing activity of calcium pumps in the sarcolemma. Spontaneous activity is most pronounced in the smooth muscle tissue of the intestine, uterus, and urinary tract; it is much weaker in the muscle tissue of blood vessels. For automation, the most typical cycles are contraction and relaxation with an average period of about 1 minute. (from 0.5 to 2 min). Under normal conditions, this myogenic rhythm of activity is influenced by neural and hormonal signals that strengthen, weaken, coordinate and synchronize the contractile activity of myocytes.

Physiological regeneration of smooth muscle tissue carried out constantly at the subcellular level by updating cellular components.

Smooth muscle hypertrophy serves as its reaction to an increase in functional load, usually associated with its stretching.

NERVOUS TISSUE

Nervous tissue consists of neurons (neurocytes, nerve cells themselves), which have the ability to produce and conduct nerve impulses, and neuroglial cells, which perform a number of auxiliary functions (supportive, trophic, barrier, protective, etc.) and ensure the activity of neurons. Neurons and neuroglia (with the exception of one of its varieties, microglia) are derivatives of the neuronal rudiment.

NEURONS

Neurons (neurocytes, nerve cells themselves) are cells of various sizes (which vary from the smallest in the body - neurons with a body diameter of 4-5 microns - to the largest with a body diameter of about 140 microns). Their total number in nervous system number of people exceeds 100 billion (10 11), and according to some estimates reaches one trillion (10 12). By birth, neurons lose the ability to divide, so during postnatal life their number does not increase, but, on the contrary, due to the natural loss of cells, gradually decreases.

In vertebrates and humans there are three different muscle groups:

striated muscles of the skeleton;
striated muscle of the heart;
smooth muscles of internal organs, blood vessels and skin.

Rice. 1. Types of human muscles

Smooth muscle

Of the two types of muscle tissue (striated and smooth), smooth muscle tissue is at a lower stage of development and is characteristic of lower animals.

They form the muscular layer of the walls of the stomach, intestines, ureters, bronchi, blood vessels and other hollow organs. They consist of spindle-shaped muscle fibers and do not have transverse striations, since the myofibrils in them are located less orderly. In smooth muscles, individual cells are connected to each other by special sections of outer membranes - nexuses. Due to these contacts, action potentials propagate from one muscle fiber to another. Therefore, the entire muscle is quickly involved in the excitation reaction.

Smooth muscles carry out movements of internal organs, blood and lymphatic vessels. In the walls of internal organs, they are usually located in the form of two layers: the inner annular and the outer longitudinal. They form spiral-shaped structures in the walls of the artery.

A characteristic feature of smooth muscles is their ability to spontaneous automatic activity (muscles of the stomach, intestines, gallbladder, ureters). This property is regulated by nerve endings. Smooth muscles are plastic, i.e. are able to maintain the length given by stretching without changing the tension. Skeletal muscle, on the contrary, has low plasticity and this difference can be easily established in the following experiment: if you stretch both smooth and striated muscles with the help of weights and remove the load, then the skeletal muscle immediately shortens to its original length, and the smooth muscle for a long time may be in a stretched state.

This property of smooth muscles is of great importance for the functioning of internal organs. It is the plasticity of smooth muscles that ensures only a small change in pressure inside the bladder when it is filled.

Rice. 2. A. Skeletal muscle fiber, cardiac muscle cell, smooth muscle cell. B. Skeletal muscle sarcomere. B. The structure of smooth muscle. D. Mechanogram of skeletal muscle and cardiac muscle.

Smooth muscle has the same basic properties as striated skeletal muscle, but also some special properties:

automation, i.e. the ability to contract and relax without external irritation, but due to excitations that arise within themselves;
high sensitivity to chemical irritants;
pronounced plasticity;
contraction in response to rapid stretch.

The contraction and relaxation of smooth muscles occurs slowly. This contributes to the onset of peristaltic and pendulum-like movements of the digestive tract organs, which leads to the movement of the food bolus. Prolonged contraction of smooth muscles is necessary in the sphincters of hollow organs and prevents the release of contents: bile in the gallbladder, urine in the bladder. The contraction of smooth muscle fibers occurs regardless of our desire, under the influence of internal reasons not subordinate to consciousness.

Striated muscles

Striated muscles are located on the bones of the skeleton and contraction sets individual joints and the entire body in motion. They form a body, or soma, which is why they are also called somatic, and the system that innervates them is the somatic nervous system.

Thanks to the activity of skeletal muscles, the body moves in space, the varied work of the limbs, and the expansion of chest when breathing, movement of the head and spine, chewing, facial expressions. There are more than 400 muscles. The total muscle mass makes up 40% of the weight. Typically, the middle part of the muscle consists of muscle tissue and forms the belly. The ends of the muscles - tendons are made of dense connective tissue; they are connected to the bones using the periosteum, but can also attach to other muscles and to the connective layer of the skin. In a muscle, muscle and tendon fibers are combined into bundles using loose connective tissue. Nerves and blood vessels are located between the bundles. proportional to the number of fibers making up the muscle belly.

Rice. 3. Functions of muscle tissue

Some muscles pass through only one joint and, when contracted, cause it to move—single-joint muscles. Other muscles pass through two or more joints - multi-joint muscles, they produce movement in several joints.

As the ends of the muscles attached to the bones move closer to each other, the size of the muscle (length) decreases. Bones connected by joints act as levers.

By changing the position of the bone levers, the muscles act on the joints. In this case, each muscle affects the joint in only one direction. A uniaxial joint (cylindrical, trochlear) has two muscles or groups of muscles acting on it, which are antagonists: one muscle is a flexor, the other is an extensor. At the same time, each joint is acted in one direction, as a rule, by two or more muscles, which are synergists (synergism is a joint action).

In a biaxial joint (ellipsoidal, condyle, saddle-shaped) the muscles are grouped according to its two axes around which movements are performed. To a ball-and-socket joint, which has three axes of movement (multi-axial joint), muscles are adjacent on all sides. So, for example, in shoulder joint there are flexor and extensor muscles (movements around the frontal axis), abductors and adductors (sagittal axis) and rotators around the longitudinal axis, inward and outward. There are three types of muscle work: overcoming, yielding and holding.

If, due to muscle contraction, the position of a body part changes, then the resistance force is overcome, i.e. overcoming work is performed. Work in which the muscle force yields to the action of gravity and the load being held is called yielding. In this case, the muscle functions, but it does not shorten, but lengthens, for example, when it is impossible to lift or hold a weighted body large mass. With great muscle effort, you have to lower this body onto some surface.

Holding work is performed due to muscle contraction; the body or load is held in a certain position without moving in space, for example, a person holds a load without moving. In this case, the muscles contract without changing length. The force of muscle contraction balances the weight of the body and the load.

When a muscle, contracting, moves the body or its parts in space, they perform overcoming or yielding work, which is dynamic. Statistical work is holding work, in which there is no movement of the whole body or part of it. The mode in which the muscle can freely shorten is called isotonic(there is no change in muscle tension and only its length changes). The condition in which the muscle cannot shorten is called isometric- only the tension of the muscle fibers changes.

Rice. 4. Human muscles

The structure of striated muscles

Skeletal muscles consist of a large number of muscle fibers, which are combined into muscle bundles.

One bundle contains 20-60 fibers. Muscle fibers are cylindrical cells 10-12 cm long and 10-100 microns in diameter.

Each muscle fiber has a membrane (sarcolemma) and cytoplasm (sarcoplasm). Sarcoplasm contains all the components of an animal cell and thin filaments are located along the axis of the muscle fiber - myofibrils, Each myofibril consists of protofibrils, which include threads of the proteins myosin and actin, which are the contractile apparatus of muscle fiber. Myofibrils are separated from each other by partitions called Z-membranes into sections - sarcomeres. At both ends of the sarcomeres, thin actin filaments are attached to the Z-membrane, and thick myosin filaments are located in the middle. The ends of the actin filaments partially fit between the myosin filaments. In a light microscope, myosin filaments appear as a light stripe in a dark disk. Under electron microscopy, skeletal muscles appear striated (cross-striped).

Rice. 5. Cross bridges: Ak - actin; Mz - myosin; Gl - head; Ш - neck

On the sides of the myosin filament there are projections called cross bridges(Fig. 5), which are located at an angle of 120° relative to the axis of the myosin filament. Actin filaments appear as a double filament twisted into a double helix. In the longitudinal grooves of the actin helix there are filaments of the protein tropomyosin, to which the protein troponin is attached. In the resting state, tropomyosin protein molecules are arranged in such a way as to prevent the attachment of myosin cross bridges to actin filaments.

Rice. 6. A - organization of cylindrical fibers in skeletal muscle attached to bones by tendons. B - structural organization of filaments in a skeletal muscle fiber, creating a pattern of transverse stripes.

Rice. 7. Structure of actin and myosin

In many places, the surface membrane deepens in the form of microtubes inside the fiber, perpendicular to its longitudinal axis, forming a system transverse tubules(T-system). Parallel to the myofibrils and perpendicular to the transverse tubules between the myofibrils there is a system longitudinal tubules(sarcoplasmic reticulum). The terminal extensions of these tubes are terminal tanks - come very close to the transverse tubules, forming together with them so-called triads. The bulk of intracellular calcium is concentrated in the cisterns.

Mechanism of skeletal muscle contraction

Muscle consists of cells called muscle fibers. Outside, the fiber is surrounded by a sheath - the sarcolemma. Inside the sarcolemma is the cytoplasm (sarcoplasm), which contains nuclei and mitochondria. It contains a huge number of contractile elements called myofibrils. Myofibrils run from one end of the muscle fiber to the other. They exist for a relatively short period of time - about 30 days, after which they are completely replaced. Intense protein synthesis occurs in the muscles, which is necessary for the formation of new myofibrils.

Muscle fiber contains a large number of nuclei, which are located directly under the sarcolemma, since the main part of the muscle fiber is occupied by myofibrils. It is the presence of a large number of nuclei that ensures the synthesis of new myofibrils. Such a rapid change of myofibrils ensures high reliability of the physiological functions of muscle tissue.

Rice. 7. A - diagram of the organization of the sarcoplasmic reticulum, transverse tubules and myofibrils. B - diagram of the anatomical structure of the transverse tubules and sarcoplasmic reticulum in an individual skeletal muscle fiber. B - the role of the sarcoplasmic reticulum in the mechanism of skeletal muscle contraction

Each myofibril consists of regularly alternating light and dark areas. These areas, having different optical properties, create transverse striations of muscle tissue.

In skeletal muscle, contraction is caused by the arrival of an impulse along a nerve. The transmission of a nerve impulse from a nerve to a muscle occurs through the neuromuscular synapse (contact).

A single nerve impulse, or single irritation, leads to an elementary contractile act - a single contraction. The onset of contraction does not coincide with the moment of application of irritation, since there is a hidden, or latent, period (the interval between the application of irritation and the beginning of muscle contraction). During this period, the development of the action potential, the activation of enzymatic processes and the breakdown of ATP occur. After this the contraction begins. The breakdown of ATP in muscle leads to the conversion of chemical energy into mechanical energy. Energy processes are always accompanied by the release of heat and thermal energy is usually intermediate between chemical and mechanical energies. In muscle, chemical energy is converted directly into mechanical energy. But heat in the muscle is formed both due to the shortening of the muscle and during its relaxation. The heat generated in the muscles plays a large role in maintaining body temperature.

Unlike the heart muscle, which has the property of automation, i.e. it is capable of contracting under the influence of impulses arising within itself, and unlike smooth muscles, which are also capable of contracting without receiving signals from the outside, skeletal muscle contracts only when signals from outside are received by it. Signals to muscle fibers are directly transmitted through the axons of motor cells located in the anterior horns of the gray matter of the spinal cord (motoneurons).

Reflex nature of muscle activity and coordination of muscle contractions

Skeletal muscles, unlike smooth muscles, are capable of performing voluntary rapid contractions and thereby producing significant work. The working element of a muscle is muscle fiber. A typical muscle fiber is a structure with several nuclei, pushed to the periphery by a mass of contractile myofibrils.

Muscle fibers have three main properties:

excitability - the ability to respond to the actions of a stimulus by generating an action potential;
conductivity - the ability to conduct an excitation wave along the entire fiber in both directions from the point of irritation;
contractility - the ability to contract or change tension when excited.

In physiology, there is the concept of a motor unit, which means one motor neuron and all the muscle fibers that this neuron innervates. Motor units vary in size: from 10 muscle fibers per unit for muscles performing precise movements, to 1000 or more fibers per unit. motor unit for “power-oriented” muscles. The nature of the work of skeletal muscles can be different: static work (maintaining a posture, holding a load) and dynamic work (moving the body or load in space). Muscles are also involved in the movement of blood and lymph in the body, the production of heat, the acts of inhalation and exhalation, they are a kind of depot for water and salts, and they protect internal organs, for example, the muscles of the abdominal wall.

Skeletal muscle is characterized by two main modes of contraction - isometric and isotonic.

The isometric mode manifests itself in the fact that tension increases in the muscle during its activity (force is generated), but due to the fact that both ends of the muscle are fixed (for example, when trying to lift a very large load), it does not shorten.

The isotonic regime is manifested in the fact that the muscle initially develops tension (force) capable of lifting a given load, and then the muscle shortens - changes its length, maintaining tension equal to the weight of the load being held. It is practically impossible to observe a purely isometric or isotonic contraction, but there are techniques for the so-called isometric gymnastics when an athlete tenses his muscles without changing length. These exercises develop muscle strength to a greater extent than exercises with isotonic elements.

The contractile apparatus of skeletal muscle is represented by myofibrils. Each myofibril with a diameter of 1 micron consists of several thousand protofibrils - thin, elongated polymerized molecules of the proteins myosin and actin. Myosin filaments are twice as thin as actin filaments, and in the resting state of the muscle fiber, actin filaments fit in loose rings between the myosin filaments.

In the transmission of excitation, calcium ions play an important role, which enter the interfibrillar space and trigger the contraction mechanism: mutual retraction of actin and myosin filaments relative to each other. Retraction of the threads occurs with the obligatory participation of ATP. In active centers located at one end of the myosin filaments, ATP is broken down. The energy released during the breakdown of ATP is converted into movement. In skeletal muscles, the ATP reserve is small - only enough for 10 single contractions. Therefore, constant re-synthesis of ATP is necessary, which occurs in three ways: first, through creatine phosphate reserves, which are limited; the second is the glycolytic pathway during the anaerobic breakdown of glucose, when two molecules of ATP are formed for one molecule of glucose, but at the same time lactic acid is formed, which inhibits the activity of glycolytic enzymes, and finally the third is the aerobic oxidation of glucose and fatty acids in the Krebs cycle, which occurs in mitochondria and forms 38 ATP molecules per 1 glucose molecule. The last process is the most economical, but very slow. Constant training activates the third oxidation pathway, resulting in increased muscle endurance for long-term stress.

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 contraction.

Properties and types of muscle tissue

Morphological characteristics:

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

Properties of muscle tissue:

Contractility;
excitability;
conductivity;
extensibility;
elasticity.

The following types are distinguished 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 rudiment;
epidermal - skin ectoderm;
neural - neural plate;
coelomic - splanchnotomes;
somatic - myotome.

Of the 1-3 types, smooth muscle tissues develop, 4, 5 give rise to striated muscles.

Structure and functions of smooth muscle tissue

Consists of individual small spindle-shaped cells. These cells have one nucleus and thin myofibrils that extend from one end of the cell to the other. Smooth muscle cells are united into bundles consisting of 10-12 cells. This association occurs due to the peculiarities of the innervation of smooth muscles and facilitates the passage of a nerve impulse to the entire group of smooth muscle cells. Smooth muscle tissue contracts rhythmically, slowly and over a long period of time, and is capable of developing great strength without significant energy expenditure and without fatigue.

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

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

Functions of smooth muscle tissue:

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

Structure and functions of skeletal muscle tissue

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

When fibers contract, the contractile proteins are excited, and thin protofibrils slide over thick ones. Actin reacts with myosin, and a single actomyosin system arises.

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 - muscle relaxation when temperature rises to dilate blood vessels;
facial expressions - to convey emotions.

Structure and functions of cardiac muscle tissue

Cardiac muscle tissue

The myocardium is made up of cardiac muscle and connective tissue, with blood vessels and nerves. Muscle tissue refers to striated muscles, the striations of which are also due to the presence different types myofilaments. The myocardium consists 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 from 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 compared to skeletal muscles. Heart muscle cells contain many mitochondria, since continuous heart contractions require a lot of energy.

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

Functions of cardiac muscle tissue:

Pumping station;
ensures 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 cooperative work of proteins - actin and myosin, as well as with the participation of Ca ions. Mitochondria supply energy to all processes. Glycogen and lipids also form energy reserves. Myoglobin is necessary for binding O 2 and forming its reserve for the period of muscle contraction, since during contraction the blood vessels are compressed 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	Part of the walls of blood vessels
	Structural unit – smooth myocyte
	Contracts slowly, unconsciously
	There is no transverse striation
Skeletal	Structural unit – multinucleate muscle fiber
	Characterized by transverse striations
	Contracts quickly, consciously

Where is muscle tissue located?

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

Skeletal muscles occupy about 40% of the human body weight and 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, and facial muscles. Also, striated muscles are located in the myocardium.

How does skeletal muscle muscle fiber differ 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 striations due to the special arrangement of actin and myosin filaments. This is not typical for smooth muscles.

There are many nuclei in the muscle fibers, and the contraction of the fibers is strong, fast and conscious. Unlike smooth muscles, smooth muscle cells are mononuclear and can contract at a slow pace and unconsciously.

The structure of 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, and feeding highways. The membranes of adjacent cells form nexuses, which provide electrical communication between cells and serve to transmit excitation from cell to cell. In addition, the plasma membrane of the smooth muscle cell has special invaginations - caveolae, due to which the area of the membrane increases by 70%. The outside of the plasma membrane is covered by the basement membrane. The complex of the basal membrane and plasma membrane is called the sarcolemma. Smooth muscle lacks sarcomeres. The basis of the contractile apparatus is made up of myosin and actin protofibrils. There are much more actin protofibrils in SMCs than in striated muscle fibers. Actin/myosin ratio = 5:1.

Thick and thin myofilaments are scattered throughout the sarcoplasm of the smooth myocyte and do not have such a harmonious 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 found in the sarcoplasm. Dense bodies are composed of alpha-actinin, a protein found in the structure of the Z-membrane of striated muscle fibers. Some of the dense bodies located on inner surface the membranes 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 smooth muscle myofilaments contain myosin, and thin ones contain actin and tropomyosin. At the same time, troponin was not found in 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

lumen of the airways,

tone of blood vessels,

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, and the levator pili muscle.

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

Multiunitary smooth muscle.

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

has a high innervation density;

like striated muscle fibers, they are covered on the outside with a substance resembling a basement membrane, which includes collagen and glycoprotein fibers that insulate cells from each other;

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 junctions(gap junctions), through which ions are able to move freely from one cell to another

action potential arising on the membrane of the smooth muscle cell, and ion currents can spread along muscle fiber, allowing the simultaneous reduction 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 (automation), that is, they are able to generate an action potential without the influence of an external stimulus.

There is no constant resting membrane potential in smooth muscles; it constantly drifts and averages -50 mV. The drift occurs spontaneously, without any influence, and when the resting membrane potential reaches a critical level, an action potential occurs, 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 action potentials depends on the degree of fiber stretch. For example, peristaltic contractions of the intestine are enhanced when its walls are stretched by 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 it to occur, it is necessary to sum up several impulses.

All smooth muscles, when generating excitation, are characterized by activation of calcium channels, therefore, in smooth muscles all processes proceed more slowly compared to skeletal muscles.

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

One of the important stimuli that initiates contraction of smooth muscles is their stretching. Sufficient stretching of smooth muscle is usually accompanied by the appearance of action potentials. Thus, two factors contribute to the appearance of action potentials when smooth muscle is stretched:

slow wave oscillations of membrane potential;

depolarization caused by stretching of smooth muscle.

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

Contraction of smooth muscle.

Smooth muscles, like striated muscles, contain cross-bridged myosin, which 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; 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 leads to ATP hydrolysis and starts the cycle of cross-bridge formation. 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.

The efficiency of energy expenditure in smooth muscle is extremely important in the body’s overall energy consumption, since blood vessels, small intestines, bladder, gall bladder and other internal organs are constantly in good shape.

During contraction, smooth muscle can shorten up to 2/3 of its original length (skeletal muscle from 1/4 to 1/3 length). This allows hollow organs to perform their function by changing their lumen within significant limits.