Morphology of muscle tissue. Features of the structure of smooth muscle tissue. Muscle. Striated skeletal muscle tissue

Tissue is a collection of cells of similar structure that are united general functions. Almost all consist of different types of fabrics.

Classification

In animals and humans, the following types of tissues are present in the body:

• epithelial;
• nervous;
• connecting;
• muscular.

These groups combine several varieties. Thus, connective tissue can be fatty, cartilaginous, or bone. This also includes blood and lymph. Epithelial tissue is multilayered and single-layered; depending on the structure of the cells, one can also distinguish flat, cubic, columnar epithelium, etc. Nervous tissue is of only one type. And we will talk about it in more detail in this article.

Types of muscle tissue

In the body of all animals there are three types of it:

• striated muscles;
• cardiac muscle tissue.

The functions of smooth muscle tissue differ from those of striated and cardiac tissue, therefore its structure is different. Let's take a closer look at the structure of each type of muscle.

General characteristics of muscle tissue

Since all three species belong to the same type, they have a lot in common.

Muscle tissue cells are called myocytes, or fibers. Depending on the type of fabric, they may have a different structure.

Another common feature of all types of muscles is that they are capable of contracting, but different types this process occurs individually.

Features of myocytes

Smooth muscle cells, like striated and cardiac tissue, have an elongated shape. In addition, they have special organelles called myofibrils, or myofilaments. They contain (actin, myosin). They are necessary to ensure muscle movement. Required condition muscle functioning, except for the presence contractile proteins, is also the presence of calcium ions in cells. Therefore, insufficient or excessive consumption of foods high in this element can lead to incorrect muscle function - both smooth and striated.

In addition, another specific protein is present in the cells - myoglobin. It is necessary to bind with oxygen and store it.

As for organelles, in addition to the presence of myofibrils, what is special for muscle tissue is the content of a large number of mitochondria in the cell - double-membrane organelles responsible for cellular respiration. And this is not surprising, since muscle fiber needs a large amount of energy to contract, which is produced during respiration by mitochondria.

Some myocytes also have more than one nucleus. This is typical for striated muscles, the cells of which can contain about twenty nuclei, and sometimes this figure reaches one hundred. This is due to the fact that the striated muscle fiber is formed from several cells, subsequently combined into one.

The structure of striated muscles

This type of tissue is also called skeletal muscle. The fibers of this type of muscle are long, collected in bundles. Their cells can reach several centimeters in length (up to 10-12). They contain many nuclei, mitochondria and myofibrils. The basic structural unit of each myofibril of striated tissue is the sarcomere. It consists of contractile protein.

The main feature of this muscle is that it can be controlled consciously, unlike smooth and cardiac muscles.

The fibers of this tissue are attached to the bones using tendons. That is why such muscles are called skeletal.

Structure of smooth muscle tissue

Smooth muscles line some internal organs, such as the intestines, uterus, bladder, and blood vessels. In addition, sphincters and ligaments are formed from them.

Smooth muscle fiber is not as long as striated muscle fiber. But its thickness is greater than in the case of skeletal muscles. Smooth muscle cells have a spindle-like shape, rather than a thread-like shape like striated myocytes.

The structures that mediate smooth muscle contraction are called protofibrils. Unlike myofibrils, they have a simpler structure. But the material from which they are built is the same contractile proteins actin and myosin.

There are also fewer mitochondria in smooth muscle myocytes than in striated and cardiac cells. In addition, they contain only one core.

Features of the heart muscle

Some researchers define it as a subtype of striated muscle tissue. Their fibers are indeed similar in many ways. Heart cells - cardiomyocytes - also contain several nuclei, myofibrils and a large number of mitochondria. This tissue, likewise, is capable of contracting much faster and stronger than smooth muscle.

However, the main feature that distinguishes cardiac muscle from striated muscle is that it cannot be controlled consciously. Its contraction occurs only automatically, as in the case of smooth muscles.

In addition to typical cells, the cardiac tissue also contains secretory cardiomyocytes. They do not contain myofibrils and do not contract. These cells are responsible for producing the hormone atriopeptin, which is necessary for regulating blood pressure and controlling blood volume.

Functions of striated muscles

Their main task is to move the body in space. It is also the movement of body parts relative to each other.

Other functions of the striated muscles include maintaining posture and storing water and salts. In addition, they play a protective role, especially for muscles abdominals, preventing mechanical damage to internal organs.

The functions of striated muscles can also include temperature regulation, since during active muscle contraction a significant amount of heat is released. This is why, when freezing, the muscles begin to tremble involuntarily.

Functions of smooth muscle tissue

This type of muscle performs an evacuation function. It lies in the fact that smooth muscle The intestines push feces to the point where they are eliminated from the body. This role also manifests itself during childbirth, when the smooth muscles of the uterus push the fetus out of the organ.

The functions of smooth muscle tissue are not limited to this. Their sphincteric role is also important. Special circular muscles are formed from this type of tissue, which can close and open. Sphincters are present in the urinary tract, in the intestines, between the stomach and esophagus, in the gallbladder, and in the pupil.

Another important role played by smooth muscles is the formation of the ligamentous apparatus. It is necessary to maintain correct position internal organs. When the tone of these muscles decreases, prolapse of some organs may occur.

This is where the functions of smooth muscle tissue end.

Purpose of the heart muscle

Here, in principle, there is nothing special to talk about. The main and only function of this tissue is to ensure blood circulation in the body.

Conclusion: differences between the three types of muscle tissue

To clarify this issue, we present a table:

That's all the main characteristics of striated, smooth and cardiac muscle tissue. Now you are familiar with their functions, structure and main differences and similarities.

Muscle tissues are classified into smooth and striated or striated. Striated is divided into skeletal and cardiac. Depending on their origin, muscle tissue is divided into 5 types:

mesenchymal (smooth muscle tissue);

epidermal (smooth muscle tissue);

neural (smooth muscle tissue);

coelomic (cardiac);

somatic or myotome (skeletal striated).

SMOOTH MUSCLE TISSUE DEVELOPING FROM SPLANCHNOTOMIC MESENCHYME

localized in the walls of hollow organs (stomach, blood vessels, respiratory tract, etc.) and non-hollow organs (in the muscle of the ciliary body of the eye of mammals). Smooth muscle cells develop from mesenchymocytes that lose their processes. They develop the Golgi complex, mitochondria, granular ER and myofilaments. At this time, type V collagen is actively synthesized on the granular EPS, due to which a basement membrane is formed around the cell. With further differentiation, organelles of general importance atrophy, the synthesis of collagen molecules in the cell decreases, but the synthesis of contractile myofilament proteins increases.

STRUCTURE OF SMOOTH MUSCLE TISSUE. It consists of smooth myocytes, spindle-shaped, with a length of 20 to 500 microns. with a diameter of 6-8 microns. Externally, myocytes are covered with plasmalemma and basement membrane.

Myocytes are closely adjacent to each other. There are contacts between them - nexuses. In the place where there are nexuses, there are holes in the basement membrane of the myocyte membrane. At this point, the plasmalemma of one myocyte approaches the plasmalemma of another myocyte at a distance of 2-3 nm. Through the nexuses, ions are exchanged, water molecules are transported, and the contractile impulse is transmitted.

On the outside, myocytes are covered with type V collagen, which forms the exocytoskeleton of the cell. The cytoplasm of myocytes is stained oxyphilic. It contains poorly developed organelles of general importance: granular ER, Golgi complex, smooth ER, cell center, lysosomes. These organelles are located at the poles of the nucleus. Well-developed organelles are mitochondria. Cores have a rod-shaped form.

Myocytes have well-developed myofilaments, which are the contractile apparatus of the cells. Among the myofilaments there are

thin, actin, consisting of actin protein;

thick myosin, consisting of the contractile protein myosin, which appear only after an impulse arrives to the cell;

intermediate filaments consisting of connectin and nebulin.

There is no striation in myocytes because all of the above filaments are arranged in a disorderly manner.

ACTIN Filaments connect to each other and to the plasmalemma using dense bodies. In those places where they connect to each other, the bodies contain alpha-actinin; in those places where the filaments connect to the plasmalemma, the bodies contain vinculin. The arrangement of actin filaments is predominantly longitudinal, but they can be located at an angle relative to the longitudinal axis. Myosin filaments are also located predominantly longitudinally. The filaments are arranged so that the ends of the actin filaments are located between the ends of the myosin filaments.

FUNCTION OF FILAMENTS- contractile. The reduction process is carried out in the following way: after the arrival of a contractile impulse, pinocytosis vesicles containing calcium ions approach the filaments; Calcium ions trigger the contractile process, which involves the ends of actin filaments moving deeper between the ends of myosin filaments. The traction force is applied to the plasmalemma, to which actin filaments are connected using dense bodies, as a result of which the myocyte contracts.

FUNCTIONS OF MYOCYTES: 1) contractile (ability for long-term contraction); 2) secretory (they secrete type V collagen, elastin, proteoglycans, since they have granular EPS).

REGENERATION smooth muscle tissue is carried out in 2 ways: 1) mitotic division of myocytes; 2) transformation of myofibroblasts into smooth myocytes.

STRUCTURE OF SMOOTH MUSCLE TISSUE AS AN ORGAN. In the wall of hollow organs, smooth myocytes form bundles. These bundles are surrounded by layers of loose connective tissue called perimysium. The layer of connective tissue around the entire layer of muscle tissue is called epimysium. The perimysium and epimysium contain blood and lymphatic vessels and nerve fibers.

INNERVATION OF SMOOTH MUSCLE TISSUE carried out by vegetative nervous system, therefore, contractions of smooth muscles do not obey the will of a person (involuntary). Sensory (afferent) and motor (efferent) nerve fibers approach smooth muscle tissue. Efferent nerve fibers end in motor nerve endings in the connective tissue layer. When an impulse arrives, mediators are released from the endings, which, spreading diffusely, reach the myocytes, causing them to contract.

SMOOTH MUSCLE TISSUE OF EPIDERMAL ORIGIN located in the terminal sections and small ducts of the glands that develop from the skin ectoderm (salivary, sweat, mammary and lacrimal glands). Smooth myocytes (myoepitheliocytes) are located between the basal surface of glandular cells and the basement membrane, covering the basal part of the glandulocytes with their processes. When these processes contract, the basal part of the glandulocytes is compressed, causing secretion to be released from the glandular cells.

SMOOTH MUSCLE TISSUE OF NEURAL ORIGIN develops from optic cups growing from the neural tube. This muscle tissue forms only 2 muscles located in the iris of the eye: the constrictor pupillary muscle and the dilator pupillary muscle. It is believed that the muscles of the iris develop from neuroglia.

STRIPED SKELETAL MUSCLE TISSUE develops from the myotomes of mesodermal somites, and is therefore called somatic. Myotome cells differentiate in two directions: 1) from some, myosatellite cells are formed; 2) myosymplasts are formed from others.

FORMATION OF MYOSYMPLASTS. Myotome cells differentiate into myoblasts, which fuse together to form myotubes. During the process of maturation, myotubes transform into myosymplasts. In this case, the nuclei are shifted to the periphery, and the myofibrils - to the center.

STRUCTURE OF MUSCLE FIBER. Muscle fiber (miofibra) consists of 2 components: 1) myosatellite cells and 2) myosymplast. The muscle fiber is approximately the same length as the muscle itself, with a diameter of 20-50 microns. The fiber is covered on the outside with a sheath - sarcolemma, consisting of 2 membranes. The outer membrane is called the basement membrane, and the inner membrane is called the plasmalemma. Between these two membranes are myosatellite cells.

MUSCLE FIBER NUCLEI are located under the plasmalemma, their number can reach several tens of thousands. They have an elongated shape and do not have the ability for further mitotic division. The CYTOPLASM of a muscle fiber is called SARCOPLASMA. The sarcoplasm contains a large amount of myoglobin, glycogen inclusions and lipids; There are organelles of general importance, some of which are well developed, others less well developed. Organelles such as the Golgi complex, granular ER, and lysosomes are poorly developed and are located at the poles of the nuclei. Mitochondria and smooth ER are well developed.

In muscle fibers, myofibrils are well developed, which are the contractile apparatus of the fiber. Myofibrils have striations because the myofilaments in them are arranged in a strictly defined order (unlike smooth muscle). There are 2 types of myofilaments in myofibrils: 1) thin actin, consisting of actin protein, troponin and tropomyosin; 2) thick myosin consists of the protein myosin. Actin filaments are arranged longitudinally, their ends are at the same level and extend somewhat between the ends of the myosin filaments. Around each myosin filament there are 6 actin filament ends. The muscle fiber has a cytoskeleton, including intermediate filaments, telophragm, mesophragm, and sarcolemma. Thanks to the cytoskeleton, identical myofibril structures (actin, myosin filaments, etc.) are arranged in an orderly manner.

That part of the myofibril in which only actin filaments are located is called disk I (isotropic or light disk). A Z-stripe, or telophragm, about 100 nm thick and consisting of alpha-actinin, passes through the center of disk I. Actin filaments are attached to the telophragm (the zone of attachment of thin filaments).

Myosin filaments are also arranged in a strictly defined order. Their ends are also at the same level. Myosin filaments, together with the ends of actin filaments extending between them, form disk A (an anisotropic disk with birefringence). Disc A is also divided by the mesophragm, which is similar to the telophragm and consists of M protein (myomysin).

In the middle part of disk A there is an H-stripe, bounded by the ends of actin filaments that extend between the ends of the myosin filaments. Therefore, the closer the ends of the actin filaments are located to each other, the narrower the H-band.

SARCOMER is a structural and functional unit of myofibrils, which is a section located between two telophragms. Sarcomere formula: 1.5 disks I + disk A + 1.5 disks I. Myofibrils are surrounded by well-developed mitochondria and well-developed smooth ER.

SMOOTH EPS forms a system of L-tubules that form complex structures in each disc. These structures consist of L-tubules located along the myofibrils and connecting to transversely directed L-tubules (lateral cisterns). FUNCTIONS of smooth ER (L-tubule system): 1) transport; 2) synthesis of lipids and glycogen; 3) deposition of calcium ions.

T-CHANNELS- these are invaginations of the plasmalemma. At the border of the disks from the plasma membrane deep into the fiber, an invagination occurs in the form of a tube located between two lateral cisterns.

TRIAD includes: 1) T-canal and 2) 2 lateral cisterns of smooth EPS. THE FUNCTION OF TRIADS is that in the relaxed state of myofibrils, calcium ions accumulate in the lateral cisterns; at the moment when an impulse (action potential) moves along the plasmalemma, it passes to the T-channels. When an impulse moves along the T-channel, calcium ions come out of the lateral cisterns. Without calcium ions, contraction of myofibrils is impossible, because in actin filaments the centers of interaction with myosin filaments are blocked by tropomyosin. Calcium ions unblock these centers, after which the interaction of actin filaments with myosin filaments begins and contraction begins.

MECHANISM OF MYOFIBRILL CONTRACTION. When actin filaments interact with myosin filaments, Ca ions unblock the adhesion centers of actin filaments with the heads of myosin molecules, after which these outgrowths attach to the adhesion centers on the actin filaments and, like a paddle, carry out the movement of actin filaments between the ends of the myosin filaments. At this time, the telophragm approaches the ends of the myosin filaments, since the ends of the actin filaments also approach the mesophragm and each other, and the H-stripe narrows. Thus, during myofibril contraction, disc I and the H-stripe narrow. After the termination of the action potential, calcium ions return to the L-tubules of the smooth ER, and tropomyosin again blocks the centers of interaction with myosin filaments in actin filaments. This leads to the cessation of contraction of myofibrils, their relaxation occurs, i.e. actin filaments return to their original position, the width of disk I and the H-band is restored.

MYOSATELLITOCYTES muscle fibers are located between the basement membrane and the plasmalemma of the sarcolemma. These cells are oval in shape, their oval nucleus is surrounded by a thin layer of organelle-poor and weakly stained cytoplasm. FUNCTION of myosatellite cells- these are cambial cells involved in the regeneration of muscle fibers when they are damaged.

STRUCTURE OF MUSCLE AS AN ORGAN . Each muscle of the human body is a unique organ with its own structure. Each muscle is made up of muscle fibers. Each fiber is surrounded by a thin layer of loose connective tissue - endomysium. Blood and lymphatic vessels and nerve fibers pass through the endomysium. The muscle fiber together with blood vessels and nerve fibers is called "myon". Several muscle fibers form a bundle surrounded by a layer of loose connective tissue called perimysium. The entire muscle is surrounded by a layer of connective tissue called the epimysium.

CONNECTION OF MUSCLE FIBERS WITH COLLAGEN FIBERS OF TENDON.

At the ends of the muscle fibers there are invaginations of the sarcolemma. These invaginations include collagen and reticular fibers of the tendons. Reticular fibers pierce the basement membrane and, using molecular linkages, connect to the plasmalemma. Then these fibers return to the lumen of the invagination and braid the collagen fibers of the tendon, as if tying them to the muscle fiber. Collagen fibers form tendons that attach to the bone skeleton.

TYPES OF MUSCLE FIBERS. There are 2 main types of muscle fibers:

Type I (red fibers) and type II (white fibers). They differ mainly in the speed of contraction, the content of myoglobin, glycogen and enzyme activity.

TYPE 1 (red fibers) are characterized by a high myoglobin content (that's why they are red), high succinate dehydrogenase activity, slow type ATPase, not so rich in glycogen content, duration of contraction and low fatigue.

TYPE 2 (white fibers) are characterized by low myoglobin content, low succinate dehydrogenase activity, fast-type ATPase, rich glycogen content, rapid contraction and high fatigue.

The slow (red) and fast (white) types of muscle fibers are innervated by different types of motor neurons: slow and fast. In addition to the 1st and 2nd types of muscle fibers, there are intermediate ones that have the properties of both.

Each muscle contains all types of muscle fibers. Their number may vary and depends on physical activity.

REGENERATION OF STRIPED SKELETAL MUSCLE TISSUE . When muscle fibers are damaged (ruptured), their ends at the site of injury undergo necrosis. After rupture, macrophages arrive at the fragments of fibers, which phagocytose the necrotic areas, clearing them of dead tissue. After this, the regeneration process is carried out in 2 ways: 1) due to increased reactivity in muscle fibers and the formation of muscle buds at the sites of rupture; 2) due to myosatellite cells.

The 1st PATH is characterized by the fact that at the ends of broken fibers the granular ER is hypertrophied, on the surface of which the proteins of myofibrils, membrane structures inside the fiber and sarcolemma are synthesized. As a result, the ends of the muscle fibers thicken and transform into muscle buds. These buds, as they grow, move closer to each other from one torn end to the other, and finally the buds connect and grow together. Meanwhile, due to the endomysium cells, new formation of connective tissue occurs between the muscle buds growing towards each other. Therefore, by the time the muscle buds join, a connective tissue layer is formed, which will become part of the muscle fiber. Consequently, a connective tissue scar is formed.

The 2nd WAY of regeneration is that myosatellite cells leave their habitats and undergo differentiation, as a result of which they turn into myoblasts. Some myoblasts join the muscle buds, some join into myotubes, which differentiate into new muscle fibers.

Thus, during reparative muscle regeneration, old muscle fibers are restored and new ones are formed.

INNERVATION OF SKELETAL MUSCLE TISSUE carried out by motor and sensory nerve fibers ending in nerve endings. MOTOR (motor) nerve endings are the terminal devices of the axons of motor nerve cells of the anterior horns of the spinal cord. The end of the axon, approaching the muscle fiber, is divided into several branches (terminals). The terminals pierce the basement membrane of the sarcolemma and then plunge deep into the muscle fiber, dragging the plasmalemma with them. As a result, a neuromuscular ending (motor plaque) is formed.

STRUCTURE OF THE NEUROMUSCULAR endings The neuromuscular ending has two parts (poles): nervous and muscular. There is a synaptic gap between the nerve and muscle parts. The nerve part (axon terminals of the motor neuron) contains mitochondria and synaptic vesicles filled with the neurotransmitter acetylcholine. In the muscular part of the neuromuscular ending there are mitochondria, an accumulation of nuclei, and there are no myofibrils. The synaptic cleft, 50 nm wide, is bounded by a presynaptic membrane (axon plasmalemma) and a postsynaptic membrane (muscle fiber plasmalemma). The postsynaptic membrane forms folds (secondary synaptic clefts), it contains receptors for acetylcholine and the enzyme acetylcholinesterase.

FUNCTION of neuromuscular endings. The impulse moves along the axon plasmalemma (presynaptic membrane). At this time, synaptic vesicles with acetylcholine approach the plasmalemma, from the vesicles acetylcholine flows into the synaptic cleft and is captured by receptors of the postsynaptic membrane. This increases the permeability of this membrane (muscle fiber plasma membrane), as a result of which sodium ions move from the outer surface of the plasma membrane to the inner surface, and potassium ions move to the outer surface - this is a depolarization wave or a nerve impulse (action potential). After the occurrence of an action potential, acetylcholinesterase of the postsynaptic membrane destroys acetylcholine and the transmission of the impulse through the synaptic cleft stops.

SENSITIVE NERVE ENDINGS(neuromuscular spindles - fusi neuro-muscularis) end the dendrites of the sensory neurons of the spinal ganglia. Neuromuscular spindles are covered with a connective tissue capsule, inside which there are 2 types of intrafusal (intraspindle) muscle fibers: 1) with a nuclear bursa (in the center of the fiber there is a thickening in which there is an accumulation of nuclei), they are longer and thicker; 2) with a nuclear chain (the nuclei in the form of a chain are located in the center of the fiber), they are thinner and shorter.

Thick nerve fibers penetrate into the endings, which entwine both types of intrafusal muscle fibers in a ring and thin nerve fibers ending in grape-shaped endings on muscle fibers with a nuclear chain. At the ends of the intrafusal fibers there are myofibrils and motor nerve endings approach them. Contractions of intrafusal fibers do not have great strength and do not add up to the rest (extrafusal) muscle fibers.

FUNCTION of neuromuscular spindles consists in the perception of the speed and force of muscle stretching. If the tensile force is such that it threatens to rupture the muscle, then the contracting antagonist muscles from these endings reflexively receive inhibitory impulses.

CARDIAC MUSCLE TISSUE develops from the anterior section of the visceral layers of the splanchnotome. From these sheets, 2 myoepicardial plates stand out: right and left. The cells of the myoepicardial plates differentiate in two directions: from some the mesothelium covering the epicardium develops, from others - cardiomyocytes of five varieties;

contractile

pacemaker

conductive

intermediate

secretory or endocrine

STRUCTURE OF CARDIOMYOCYTES . Cardiomyocytes have a cylindrical shape, 50-120 µm long, 10-20 µm in diameter. Cardiomyocytes connect their ends to each other and form functional cardiac muscle fibers. The junction of cardiomyocytes is called intercalated discs (discus intercalatus). The discs contain interdigitations, desmosomes, attachment sites for actin filaments, and nexuses. Metabolism between cardiomyocytes occurs through nexuses.

On the outside, cardiomyocytes are covered with a sarcolemma, consisting of an outer (basal) membrane and a plasmalemma. Processes extend from the lateral surfaces of the cardiomyocytes and intertwine into the lateral surfaces of the cardiomyocytes of the adjacent fiber. These are muscle anastomoses.

CORE cardiomyocytes (one or two), oval in shape, usually polyploid, located in the center of the cell. MYOFIBRILLS are localized along the periphery. ORGANELLES - some are poorly developed (granular ER, Golgi complex, lysosomes), others are well developed (mitochondria, smooth ER, myofibrils). The oxyphilic CYTOPLASMA contains inclusions of myoglobin, glycogen and lipids.

STRUCTURE OF MYOFIBRILLS the same as in skeletal muscle tissue. Actin filaments form a light disk (I), separated by a telophragm; due to myosin filaments and actin ends, disk A (anisotropic) is formed, separated by a mesophragm. In the middle part of disk A there is an H-stripe bounded by the ends of actin filaments.

Cardiac muscle fibers differ from skeletal muscle fibers in that they consist of individual cells - cardiomyocytes, the presence of muscle anastomoses, the central location of the nuclei (in the skeletal muscle fiber - under the sarcolemma), the increased thickness of the diameter of T-channels, since they include plasmalemma and basement membrane (in skeletal muscle fibers - only plasmalemma).

REDUCTION PROCESS in the fibers of the heart muscle is carried out according to the same principle as in the fibers of skeletal muscle tissue.

CONDUCTING CARDIOMYOCYTES characterized by a thicker diameter (up to 50 μm), lighter cytoplasm, central or eccentric arrangement of nuclei, low content of myofibrils, and a simpler arrangement of intercalary discs. The discs have fewer desmosomes, interdigitations, nexuses, and actin filament attachment sites.

Conducting cardiomyocytes lack T channels. Conducting cardiomyocytes can connect to each other not only with their ends, but also with their lateral surfaces. The FUNCTION of conductive cardiomyocytes is to produce and transmit a contractile impulse to contractile cardiomyocytes.

ENDOCRINE CARDIOMYOCYTES are located only in the atria, have a more process-shaped shape, poorly developed myofibrils, intercalated discs, and T-channels. They have well-developed granular ER, Golgi complex and mitochondria, and their cytoplasm contains secretion granules.

FUNCTION OF endocrine cardiomyocytes- secretion of atrial natriuretic factor (ANF), which regulates the contractility of the heart muscle, the volume of circulating fluid, blood pressure, and diuresis.

REGENERATION of cardiac muscle tissue is only physiological, intracellular. When cardiac muscle fibers are damaged, they are not restored, but are replaced by connective tissue (histotypic regeneration).

Tissue - location in the body, types, functions, structure

Tissues are a system of cells and intercellular substance that have the same structure, origin and functions.

Intercellular substance is a product of cell activity. It provides communication between cells and creates a favorable environment for them. It can be liquid, such as blood plasma; amorphous - cartilage; structured - muscle fibers; hard - bone(in the form of salt).

Tissue cells have different shapes, which determine their function. Fabrics are divided into four types:

• epithelial - border tissues: skin, mucous membrane;
• connective - the internal environment of our body;
• muscle;
• nerve tissue.

Epithelial tissue

Epithelial (border) tissues - line the surface of the body, the mucous membranes of all internal organs and cavities of the body, serous membranes, and also form the glands of external and internal secretion. The epithelium lining the mucous membrane is located on the basement membrane, and its inner surface directly faces the external environment. Its nutrition is accomplished by the diffusion of substances and oxygen from blood vessels through the basement membrane.

Features: there are many cells, there is little intercellular substance and it is represented by a basement membrane.

Epithelial tissues perform the following functions:

• protective;
• excretory;
• suction

Classification of epithelia. Based on the number of layers, a distinction is made between single-layer and multi-layer. They are classified according to shape: flat, cubic, cylindrical.

If all epithelial cells reach the basement membrane, it is a single-layer epithelium, and if only cells of one row are connected to the basement membrane, while others are free, it is multilayered. Single-layer epithelium can be single-row or multi-row, which depends on the level of location of the nuclei. Sometimes mononuclear or multinuclear epithelium has ciliated cilia facing the external environment.

Stratified epithelium Epithelial (integumentary) tissue, or epithelium, is a boundary layer of cells that lines the integument of the body, the mucous membranes of all internal organs and cavities, and also forms the basis of many glands.

Glandular epithelium The epithelium separates the organism (internal environment) from the external environment, but at the same time serves as an intermediary in the interaction of the organism with the environment. Epithelial cells are tightly connected to each other and form a mechanical barrier that prevents the penetration of microorganisms and foreign substances into the body. Epithelial tissue cells live for a short time and are quickly replaced by new ones (this process is called regeneration).

Epithelial tissue is also involved in many other functions: secretion (exocrine and endocrine glands), absorption (intestinal epithelium), gas exchange (lung epithelium).

The main feature of the epithelium is that it consists of a continuous layer of tightly adjacent cells. The epithelium can be in the form of a layer of cells lining all surfaces of the body, and in the form of large accumulations of cells - glands: liver, pancreas, thyroid, salivary glands, etc. In the first case, it lies on the basement membrane, which separates the epithelium from the underlying connective tissue . However, there are exceptions: epithelial cells in the lymphatic tissue alternate with connective tissue elements; such epithelium is called atypical.

Epithelial cells, arranged in a layer, can lie in many layers (stratified epithelium) or in one layer (single-layer epithelium). Based on the height of the cells, epithelia are divided into flat, cubic, prismatic, and cylindrical.

Single-layer squamous epithelium - lines the surface of the serous membranes: pleura, lungs, peritoneum, pericardium of the heart.

Single-layer cubic epithelium - forms the walls of the kidney tubules and the excretory ducts of the glands.

Single-layer columnar epithelium - forms the gastric mucosa.

Bordered epithelium - a single-layer cylindrical epithelium, on the outer surface of the cells of which there is a border formed by microvilli that ensure the absorption of nutrients - lines the mucous membrane of the small intestine.

Ciliated epithelium (ciliated epithelium) is a pseudostratified epithelium consisting of cylindrical cells, the inner edge of which, i.e. facing the cavity or canal, is equipped with constantly oscillating hair-like formations (cilia) - the cilia ensure the movement of the egg in the tubes; removes germs and dust from the respiratory tract.

Stratified epithelium is located at the border between the body and the external environment. If keratinization processes occur in the epithelium, i.e., the upper layers of cells turn into horny scales, then such a multilayered epithelium is called keratinization (skin surface). Multilayer epithelium lines the mucous membrane of the mouth, food cavity, and cornea of ​​the eye.

Transitional epithelium lines the walls of the bladder, renal pelvis, and ureter. When these organs are filled, the transitional epithelium stretches, and cells can move from one row to another.

Glandular epithelium - forms glands and performs a secretory function (releases substances - secretions that are either released into the external environment or enter the blood and lymph (hormones)). The ability of cells to produce and secrete substances necessary for the functioning of the body is called secretion. In this regard, such an epithelium was also called secretory epithelium.

Connective tissue

Connective tissue Consists of cells, intercellular substance and connective tissue fibers. It consists of bones, cartilage, tendons, ligaments, blood, fat, it is present in all organs (loose connective tissue) in the form of the so-called stroma (framework) of organs.

In contrast to epithelial tissue, in all types of connective tissue (except adipose tissue), the intercellular substance predominates over the cells in volume, i.e., the intercellular substance is very well expressed. The chemical composition and physical properties of the intercellular substance are very diverse in various types connective tissue. For example, blood - the cells in it “float” and move freely, since the intercellular substance is well developed.

In general, connective tissue makes up what is called the internal environment of the body. It is very diverse and represented various types- from dense and loose forms to blood and lymph, the cells of which are in liquid. The fundamental differences in the types of connective tissue are determined by the ratios of cellular components and the nature of the intercellular substance.

Dense fibrous connective tissue (muscle tendons, joint ligaments) is dominated by fibrous structures and experiences significant mechanical stress.

Loose fibrous connective tissue is extremely common in the body. It is very rich, on the contrary, in cellular forms of different types. Some of them are involved in the formation of tissue fibers (fibroblasts), others, which is especially important, provide primarily protective and regulatory processes, including through immune mechanisms (macrophages, lymphocytes, tissue basophils, plasma cells).

Bone

Bone tissue Bone tissue, which forms the bones of the skeleton, is very strong. It maintains body shape (constitution) and protects organs located in the skull, chest and pelvic cavities, and participates in mineral metabolism. The tissue consists of cells (osteocytes) and intercellular substance in which nutrient channels with blood vessels are located. The intercellular substance contains up to 70% mineral salts (calcium, phosphorus and magnesium).

In its development, bone tissue passes through fibrous and lamellar stages. In various parts of the bone it is organized in the form of compact or spongy bone substance.

Cartilage tissue

Cartilage tissue consists of cells (chondrocytes) and intercellular substance (cartilage matrix), characterized by increased elasticity. It performs a supporting function, as it forms the bulk of cartilage.

There are three types of cartilage tissue: hyaline, which is part of the cartilage of the trachea, bronchi, ends of the ribs, and articular surfaces of bones; elastic, forming the auricle and epiglottis; fibrous, located in the intervertebral discs and joints of the pubic bones.

Adipose tissue

Adipose tissue is similar to loose connective tissue. The cells are large and filled with fat. Adipose tissue performs nutritional, shape-forming and thermoregulatory functions. Adipose tissue is divided into two types: white and brown. In humans, white adipose tissue predominates, part of it surrounds the organs, maintaining their position in the human body and other functions. The amount of brown adipose tissue in humans is small (it is found mainly in newborns). The main function of brown adipose tissue is heat production. Brown adipose tissue maintains the body temperature of animals during hibernation and the temperature of newborns.

Muscle

Muscle cells are called muscle fibers, because they are constantly elongated in one direction.

Classification of muscle tissue is carried out on the basis of the structure of the tissue (histologically): by the presence or absence of transverse striations, and on the basis of the mechanism of contraction - voluntary (as in skeletal muscle) or involuntary (smooth or cardiac muscle).

Muscle tissue has excitability and the ability to actively contract under the influence of the nervous system and certain substances. Microscopic differences allow us to distinguish two types of this tissue - smooth (unstriated) and striated (striated).

Smooth muscle tissue has a cellular structure. It forms the muscular membranes of the walls of internal organs (intestines, uterus, bladder, etc.), blood and lymphatic vessels; its contraction occurs involuntarily.

Striated muscle tissue consists of muscle fibers, each of which is represented by many thousands of cells, fused, in addition to their nuclei, into one structure. It forms skeletal muscles. We can shorten them at will.

A type of striated muscle tissue is cardiac muscle, which has unique abilities. During life (about 70 years), the heart muscle contracts more than 2.5 million times. No other fabric has such strength potential. Cardiac muscle tissue has transverse striations. However, unlike skeletal muscle, there are special areas where the muscle fibers meet. Thanks to this structure, the contraction of one fiber is quickly transmitted to neighboring ones. This ensures simultaneous contraction of large areas of the heart muscle.

Also, the structural features of muscle tissue are that its cells contain bundles of myofibrils formed by two proteins - actin and myosin.

Nervous tissue

Nervous tissue consists of two types of cells: nerve (neurons) and glial. Glial cells are closely adjacent to the neuron, performing supporting, nutritional, secretory and protective functions.

Neuron is the basic structural and functional unit of nervous tissue. Its main feature is the ability to generate nerve impulses and transmit excitation to other neurons or muscle and glandular cells of working organs. Neurons can consist of a body and processes. Nerve cells are designed to conduct nerve impulses. Having received information on one part of the surface, the neuron very quickly transmits it to another part of its surface. Since the processes of a neuron are very long, information is transmitted over long distances. Most neurons have two types of processes: short, thick, branching near the body - dendrites, and long (up to 1.5 m), thin and branching only at the very end - axons. Axons form nerve fibers.

A nerve impulse is an electrical wave traveling at high speed along a nerve fiber.

Depending on the functions performed and structural features, all nerve cells are divided into three types: sensory, motor (executive) and intercalary. Motor fibers, running as part of the nerves, transmit signals to the muscles and glands; sensory fibers transmit information about the state of the organs to the central nervous system.

Now we can combine all the information received into a table.

Types of fabrics (table)

These tissues belong to excitable tissues, i.e. They are capable of responding to irritation with excitement and conducting it at a distance.

Muscle tissue

In origin and structure, muscle tissue differs significantly from each other, but they are united by the ability to contract, which ensures the motor function of organs and the body as a whole. The muscle elements are elongated and connected either with other muscle elements or with supporting structures.

There are smooth, striated muscle tissue and cardiac muscle tissue (Fig. 5).

Smooth muscle tissue.

This tissue is formed from mesenchyme. The structural unit of this tissue is the smooth muscle cell. It has an elongated spindle-shaped shape and is covered with a cell membrane. These cells adhere tightly to each other, forming layers and groups separated from each other by loose, unformed connective tissue.

The cell nucleus has an elongated shape and is located in the center. Myofibrils are located in the cytoplasm; they run along the periphery of the cell along its axis. They consist of thin threads and are the contractile element of the muscle.

The cells are located in the walls of blood vessels and most internal hollow organs (stomach, intestines, uterus, bladder). The activity of smooth muscles is regulated by the autonomic nervous system. Muscle contractions do not obey the will of a person and therefore smooth muscle tissue is called involuntary muscles.

Striated muscle tissue.

This tissue is formed from myotomes, derivatives of the mesoderm. The structural unit of this tissue is striated muscle fiber. This cylindrical body is a symplast. It is covered with a membrane - sarcolemma, and the cytoplasm is called sarcoplasm, which contains numerous nuclei and myofibrils. Myofibrils form a bundle of continuous fibers running from one end of the fiber to the other parallel to its axis. Each myofibril consists of discs that have a different chemical composition and appear dark and light under a microscope. The homogeneous disks of all myofibrils coincide, and therefore the muscle fiber appears striated. Myofibrils are the contractile apparatus of muscle fiber.

All skeletal muscles are built from striated muscle tissue. Musculature is voluntary, because its contraction can occur under the influence of neurons in the motor zone of the cerebral cortex.

Muscle tissue of the heart.

Myocardium - the middle layer of the heart - is built from striated muscle cells(cardiomyocytes). There are two types of cells: typical contractile cells and atypical cardiac myocytes that make up the conduction system of the heart.

Typical muscle cells perform a contractile function; They rectangular shape, in the center there are 1-2 nuclei, myofibrils are located along the periphery. There are intercalary discs between adjacent myocytes. With their help, myocytes are collected into muscle fibers, separated from each other by fine fibrous connective tissue. Connective fibers pass between adjacent muscle fibers, which ensure contraction of the myocardium as a whole.

The conduction system of the heart is formed by muscle fibers consisting of atypical muscle cells. They are larger than contractile ones, richer in sarcoplasm, but poorer in myofibrils, which often intersect. The nuclei are larger and are not always in the center. The fibers of the conduction system are surrounded by a dense plexus of nerve fibers.

Nervous tissue.

Nervous tissue consists of nerve cells that have a specific function and neuroglia that perform protective, trophic and support functions. Originates from the ectoderm.

A nerve cell, or neuron, is characterized by the ability to perceive stimuli, enter a state of excitation and transmit it to other cells of the body. Thanks to this, the interconnection of organs and tissues, the regulation of all functions of the body and its adaptation to the environment are realized.

Nerve cells have different shapes and sizes and consist of a body and processes (Fig. 6).

Nerve cell processes are divided into two types:

• · Neurites, or axons, along which excitation (impulse) is transmitted from the cell body to the periphery. The axon always leaves the cell alone and ends with a terminal apparatus in the working organ or on another neuron.
• · Dendrites- processes along which impulses are transmitted from the periphery to the cell body. There are many of them and they branch.

Based on the number of processes, nerve cells are divided into three types (Fig. 7):

• · Unipolar - cells with one process. Not found in humans.
• · Bipolar- have one neurite in the central nervous system and one dendrite going to the periphery. They are located in the spinal nerve ganglia.
• · Multipolar- have one neurite and many dendrites. A person has the most of them.

The nucleus of a nerve cell is round in shape and located in the center.

In the cytoplasm of neurons there are neurofibrils, which are thin threads. In the body of the nerve cell they form a dense network. In the processes, neurofibrils are located parallel to each other.

Neuroglia represented by cells of various shapes with a large number of processes. There are more of these cells than nerve cells.

Nerve fibers. The processes of nerve cells with membranes are called nerve fibers. There are myelin (pulp) and non-myelin (pulpless). The processes are located in the center of the nerve fiber and are called the axial cylinder, which is covered with a sheath formed by neuroglial cells (lemmocytes).

Unmyelinated the fibers are an axial cylinder covered only by a membrane of lemmocytes.

myelin- much thicker. They also consist of an axial cylinder, but have two layers of the membrane: the inner, thicker one - myelin, and the outer, thin one, consisting of lemmocytes. On the outside, the myelin fiber is covered with a thin connective tissue sheath - neurilemma.

Nerve endings. All nerve fibers end in nerve endings. There are three groups:

• · Efferent. There can be two types: motor and secretory. Motor endings are the terminal devices of the axons of the somatic and autonomic nervous system.
• · Sensitive(receptors) are the terminal devices of the dendrites of sensory neurons. They are divided into free, consisting of a branch of the axial cylinder, and non-free, containing all the components of the nerve fiber, covered with a capsule.
• · terminal processes, forming interneuron synapses that communicate between neurons.

The body of all animals, including humans, consists of four types of tissue: epithelial, nervous, connective and muscle. The latter will be discussed in this article.

Types of muscle tissue

It comes in three types:

• striated;
• smooth;
• cardiac.

The functions of muscle tissue of different types are somewhat different. Yes, and the building too.

Where are muscle tissues located in the human body?

Muscle tissues of different types occupy different locations in the body of animals and humans. So, as the name implies, the heart is built from cardiac muscles.

Skeletal muscles are formed from striated muscle tissue.

Smooth muscles line the inside of the cavities of organs that need to contract. These are, for example, the intestines, bladder, uterus, stomach, etc.

The structure of muscle tissue varies between species. Let's talk about it in more detail later.

How is muscle tissue structured?

It consists of large cells - myocytes. They are also called fibers. Muscle tissue cells have several nuclei and a large number of mitochondria - organelles responsible for energy production.

In addition, the structure of muscle tissue in humans and animals provides for the presence of a small amount of intercellular substance containing collagen, which gives the muscles elasticity.

Let's look at the structure and functions of muscle tissue of different types separately.

Structure and role of smooth muscle tissue

This tissue is controlled by the autonomic nervous system. Therefore, a person cannot consciously contract muscles made of smooth tissue.

It is formed from mesenchyme. This is a type of embryonic connective tissue.

This tissue contracts much less actively and quickly than striated tissue.

Smooth tissue is built from spindle-shaped myocytes with pointed ends. The length of these cells can range from 100 to 500 micrometers, and the thickness is about 10 micrometers. The cells of this tissue are mononuclear. The nucleus is located in the center of the myocyte. In addition, organelles such as the agranular ER and mitochondria are well developed. Also in the cells of smooth muscle tissue there are a large number of inclusions from glycogen, which represent reserves of nutrients.

The element that ensures the contraction of this type of muscle tissue is myofilaments. They can be built from two contractile proteins: actin and myosin. The diameter of myofilaments that are composed of myosin is 17 nanometers, and those that are built of actin are 7 nanometers. There are also intermediate myofilaments, the diameter of which is 10 nanometers. The orientation of myofibrils is longitudinal.

The composition of muscle tissue of this type also includes an intercellular substance made of collagen, which provides communication between individual myocytes.

Functions of muscle tissue of this type:

• Sphincteric. It consists in the fact that smooth tissues are made of circular muscles that regulate the transition of contents from one organ to another or from one part of an organ to another.
• Tow truck. The point is that smooth muscles help the body remove unnecessary substances and also take part in the birth process.
• Creation of vascular lumen.
• Formation of the ligamentous apparatus. Thanks to it, many organs, such as the kidneys, are kept in place.

Now let's look at the next type of muscle tissue.

Cross-striped

It is regulated by the somatic nervous system. Therefore, a person can consciously regulate the work of muscles of this type. Skeletal muscles are formed from striated tissue.

This fabric consists of fibers. These are cells that have many nuclei located closer to the plasma membrane. In addition, they contain a large number of glycogen inclusions. Organelles such as mitochondria are well developed. They are located near the contractile elements of the cell. All other organelles are localized near the nuclei and are poorly developed.

Structures thanks to which striated fabric contracts are myofibrils. Their diameter ranges from one to two micrometers. Myofibrils occupy most of the cell and are located in its center. The orientation of myofibrils is longitudinal. They consist of light and dark discs that alternate, which creates the transverse “striation” of the tissue.

Functions of muscle tissue of this type:

• Provide movement of the body in space.
• Responsible for the movement of body parts relative to each other.
• Capable of maintaining body posture.
• They participate in the process of temperature regulation: the more actively the muscles contract, the higher the temperature. When frozen, striated muscles may begin to contract involuntarily. This explains the trembling in the body.
• Perform a protective function. This is especially true for the abdominal muscles, which protect many internal organs from mechanical damage.
• Act as a depot of water and salts.

Cardiac muscle tissue

This fabric looks like both cross-striped and smooth. Like smooth, it is regulated by the autonomic nervous system. However, it contracts just as actively as the striated one.

It consists of cells called cardiomyocytes.

Functions of this type of muscle tissue:

• There is only one thing: ensuring the movement of blood throughout the body.