Muscle tissue: structural features and functions. Structure and basic properties of muscle tissue. Functions of smooth muscles

Muscle tissues are tissues that differ both in their structure and origin. However, what they have in common is that they are capable of pronounced contractions. Muscle tissue is based on elongated cells, which receive impulses from the central nervous system, and the reaction to this is their contraction. Thanks to muscle tissue, the body and internal organs and systems (heart, lungs, intestines, etc.) of which it consists are able to move, changing their position in space. Cells of other tissues also have the ability to change shape and contract. However, in muscle tissue this function is basic.

Features of the structure of muscle tissue

The most important features of the main components of muscle tissue are their elongated shape, the presence of elongated and appropriately arranged myofilaments and myofibrils (which ensure muscle contractility), as well as the presence of mitochondria, lipids, glycogen and myoglobin. Inside the contractile organelles, myosin and actin interact (with the simultaneous participation of Ca ions in the reaction), resulting in muscle contraction. The source of energy for contractile processes is mitochondria, lipids and glycogen. Oxygen is bound and stored through a protein called myoglobin, which occurs when muscle contraction and simultaneous compression of blood vessels.

Classification of muscle fibers

Taking into account the nature of the contraction, tonic and phasic muscle fibers are distinguished. In particular, the first type of fibers is designed to provide tone (or static muscle tension), which is especially important for maintaining a particular body position relative to spatial coordinates. Phasic fibers are designed to ensure the ability to perform rapid contractions, but are not able to maintain the shortening of the muscle fiber at a certain level for a long time. Taking into account biochemical features, as well as colors, distinguish between white and red fibers. The color of muscle tissue is determined by the concentration of myoglobin in it (the so-called degree of vascularization). One of the features of red muscle fiber is the presence in its composition of chains of mitochondria surrounded by myofibrils. Slightly lower number of mitochondria in white muscle fiber. They are usually evenly distributed in the sarcoplasm.

Depending on the characteristics of oxidative metabolism, muscle fibers can be glycolytic, oxidative and intermediate. Fibers are distinguished based on information about the degree of activity of the SDH enzyme, which is a marker for the so-called Krebs cycle and mitochondria. The intensity of energy metabolism can be determined by the degree of activity of this enzyme. Glycolytic fibers (or A-type fibers) are characterized by low activity of the above enzyme, while oxidative (or C-type fibers), on the contrary, have increased succinate dehydrogenase activity. B-type fibers are fibers that occupy an intermediate position. The process of transition from type A fibers to type C fibers is a transition to oxygen-dependent metabolism from anaerobic glycolysis. An example would be a situation where sports training in combination with nutrition, they are aimed at the rapid development and formation of glycolytic muscle fibers, which contain glycogen in large quantities, and energy production is carried out anaerobically. This type of training is usually reserved for bodybuilders or sprinters. At the same time, for those sports that require endurance, it is necessary to develop oxidative muscle fibers, which have more blood vessels and mitochondria that provide aerobic glycolysis.

Muscle tissue can be of several types, if we consider their sources of development. That is, depending on the type of embryonic rudiments, they can be mesenchymal (desmal rudiment), epidermal (prechordal plate or cutaneous ectoderm), coelomic (myoepicardial plate of the so-called visceral section of the splanchnotome), neural (neural tube) or somatic/myotome.

Types of muscle tissue

There are smooth and striated (skeletal and cardiac) muscle tissue. Included smooth fabric predominantly myocytes (mononuclear cells) are present, having the shape of a spindle. The cytoplasm of such myocytes is homogeneous and does not have transverse stripes. Smooth muscle tissue has special properties. First of all, it relaxes and contracts extremely slowly. In addition, she is uncontrollable by humans and usually all her reactions are involuntary. The walls of the vessels of the lymphatic and circulatory systems, urinary tract, stomach and intestines are composed of smooth muscle tissue. Striated skeletal tissue contains very long multinucleated (one hundred or more nuclei) myocytes. If you examine the cytoplasm under a microscope, it will look like alternating light and dark stripes. Striated skeletal muscle tissue is characterized by a fairly high rate of contraction and relaxation. The activity of this type of tissue can be controlled by a person, and it itself is present in the composition skeletal muscles, in the upper esophagus, in the tongue, as well as in the muscles responsible for the movements of the eyeball.

The composition of striated cardiac muscle tissue includes cardiomyocytes with one or two nuclei, as well as cytoplasm, striated along the periphery of the cytolemma with transverse stripes. Cardiomyocytes are quite highly branched and form intercalated discs with cytoplasm integrated into them at the junctions. Cells also contact through cytolemmas, resulting in the formation of anastomoses. Striated cardiac muscle tissue is found in the myocardium. Key Feature of a given tissue is the ability, in the case of cellular excitation, to rhythmic contractions and subsequent relaxations. Striated cardiac muscle tissue belongs to involuntary tissues (so-called atypical cardiomycytes). There is also a third type of cardiomycytes - these are secretory cardiomycytes, which lack fibrils.

The most important functions of muscle tissue

The main functional features of muscle tissue include its abilities such as conductivity, excitability, and contractility. Muscle tissue provides the functions of heat exchange, movement and protection. In addition to the above, one more functional feature of muscle tissue can be identified - facial (or, as it is also called, social). In particular, facial muscles a person is controlled by his facial expressions, thereby transmitting a certain information message to other people around him.

Blood supply to muscle tissue

Blood enters muscle tissue due to its work. This provides the muscle with the necessary amount of oxygen. If a muscle is at rest, then it, as a rule, requires much less oxygen (usually this figure is five hundred times less than the figure reflecting the oxygen requirement of an actively working muscle). Thus, during active muscle contractions, the volume of blood entering the muscle increases many times over. This is approximately 300 to 500 capillaries per cubic millimeter, or approximately twenty times more than the amount of blood required by a muscle at rest.

Based on morphological characteristics, three muscle groups are distinguished:

1) striated muscles (skeletal muscles);

2) smooth muscles;

3) heart muscle (or myocardium).

Functions of striated muscles:

1) motor (dynamic and static);

2) ensuring breathing;

3) mimic;

4) receptor;

5) depositing;

6) thermoregulatory.

Functions of smooth muscles:

1) maintaining pressure in hollow organs;

2) regulation of pressure in blood vessels;

3) emptying of hollow organs and advancement of their contents.

Cardiac muscle function– pumping room, ensuring the movement of blood through the vessels.

1) excitability (lower than in the nerve fiber, which is explained by the low membrane potential);

2) low conductivity, about 10–13 m/s;

3) refractoriness (occupies a longer period of time than that of the nerve fiber);

4) lability;

5) contractility (the ability to shorten or develop tension).

There are two types of abbreviations:

a) isotonic contraction (length changes, tone does not change);

b) isometric contraction (tone changes without changing fiber length). There are single and titanic contractions. Single contractions occur under the action of a single irritation, and titanic contractions occur in response to a series of nerve impulses;

6) elasticity (the ability to develop tension when stretched).

Physiological characteristics of smooth muscles.

Smooth muscles have the same physiological properties as skeletal muscles, but also have their own characteristics:

1) unstable membrane potential, which maintains muscles in a state of constant partial contraction - tone;

2) spontaneous automatic activity;

3) contraction in response to stretching;

4) plasticity (decreasing elongation with increasing elongation);

5) high sensitivity to chemicals.

Physiological feature of the heart muscle is hers automatism . Excitation occurs periodically under the influence of processes occurring in the muscle itself. Certain atypical muscle areas of the myocardium, poor in myofibrils and rich in sarcoplasm, have the ability to automate.

2. Mechanisms of muscle contraction

Electrochemical stage of muscle contraction.

1. Generation of action potential. The transfer of excitation to the muscle fiber occurs with the help of acetylcholine. The interaction of acetylcholine (ACh) with cholinergic receptors leads to their activation and the appearance of an action potential, which is the first stage of muscle contraction.

2. Action potential propagation. The action potential propagates into the muscle fiber through the transverse tubule system, which is the connecting link between the surface membrane and the contractile apparatus of the muscle fiber.

3. Electrical stimulation of the contact site leads to activation of the enzyme and the formation of inosyl triphosphate, which activates membrane calcium channels, which leads to the release of Ca ions and an increase in their intracellular concentration.

Chemomechanical stage of muscle contraction.

The theory of the chemomechanical stage of muscle contraction was developed by O. Huxley in 1954 and supplemented in 1963 by M. Davis. The main provisions of this theory:

1) Ca ions trigger the mechanism of muscle contraction;

2) due to Ca ions, thin actin filaments slide relative to myosin filaments.

At rest, when there are few Ca ions, sliding does not occur, because this is prevented by troponin molecules and the negative charges of ATP, ATPase and ADP. The increased concentration of Ca ions occurs due to its entry from the interfibrillar space. In this case, a number of reactions occur with the participation of Ca ions:

1) Ca2+ reacts with tryponine;

2) Ca2+ activates ATPase;

3) Ca2+ removes charges from ADP, ATP, ATPase.

The interaction of Ca ions with troponin leads to a change in the location of the latter on the actin filament, and the active centers of the thin protofibril open. Due to them, cross bridges are formed between actin and myosin, which move the actin filament into the spaces between the myosin filament. When the actin filament moves relative to the myosin filament, muscle tissue contracts.

So, the main role in the mechanism of muscle contraction is played by the protein troponin, which closes the active centers of the thin protofibril and Ca ions.

Physiology of skeletal and smooth muscles

Lecture 5

In vertebrates and humans three types of muscles: striated muscles of the skeleton, striated muscle of the heart - myocardium and smooth muscles, forming the wall of the hollow internal organs and vessels.

The anatomical and functional unit of skeletal muscle is neuromotor unit - a motor neuron and the group of muscle fibers it innervates. The impulses sent by the motor neuron activate all the muscle fibers that form it.

Skeletal muscles consist of a large number of muscle fibers. The fiber of the striated muscle has an elongated shape, its diameter is from 10 to 100 microns, the length of the fiber is from several centimeters to 10-12 cm. The muscle cell is surrounded by a thin membrane - sarcolemma, contains sarcoplasm(protoplasm) and numerous kernels. The contractile part of the muscle fiber is the long muscle filaments - myofibrils, consisting mainly of actin, running inside the fiber from one end to the other, having transverse striations. Myosin in smooth muscle cells is dispersed, but contains a lot of protein that plays an important role in maintaining long-term tonic contraction.

During the period of relative rest, skeletal muscles do not relax completely and maintain a moderate degree of tension, i.e. muscle tone.

Main functions of muscle tissue:

1) motor – ensuring movement

2) static – ensuring fixation, including in a certain position

3) receptor – muscles have receptors that allow them to perceive their own movements

4) storage - water and some nutrients are stored in the muscles.

Physiological properties of skeletal muscles:

Excitability . Lower than the excitability of nervous tissue. Excitation spreads along the muscle fiber.

Conductivity . Less conductivity of nerve tissue.

Refractory period muscle tissue lasts longer than nervous tissue.

Lability muscle tissue is significantly lower than nervous tissue.

Contractility – the ability of a muscle fiber to change its length and degree of tension in response to stimulation of a threshold force.

At isotonic reduction the length of the muscle fiber changes without changing tone. At isometric reduction muscle fiber tension increases without changing its length.

Depending on the conditions of stimulation and the functional state of the muscle, a single, continuous (tetanic) contraction or contracture of the muscle may occur.

Single muscle contraction. When a muscle is irritated by a single current pulse, a single muscle contraction occurs.

The amplitude of a single muscle contraction depends on the number of myofibrils contracting at that moment. The excitability of individual groups of fibers is different, so the threshold current strength causes a contraction of only the most excitable muscle fibers. The amplitude of such a reduction is minimal. As the strength of the irritating current increases, less excitable groups of muscle fibers are also involved in the excitation process; the amplitude of contractions is summed up and grows until there are no fibers left in the muscle that are not covered by the excitation process. In this case, the maximum contraction amplitude is recorded, which does not increase, despite a further increase in the strength of the irritating current.

Tetanic contraction. IN natural conditions muscle fibers receive not single, but a series of nerve impulses, to which the muscle responds with a prolonged, tetanic contraction, or tetanus . Only skeletal muscles are capable of tetanic contraction. Smooth muscle and striated muscle of the heart are not capable of tetanic contraction due to a long refractory period.

Tetanus occurs due to the summation of single muscle contractions. For tetanus to occur, the action of repeated irritations (or nerve impulses) on the muscle is necessary even before its single contraction ends.

If the irritating impulses are close together and each of them occurs at the moment when the muscle has just begun to relax, but has not yet had time to completely relax, then a jagged type of contraction occurs ( serrated tetanus ).

If the irritating impulses are so close together that each subsequent one occurs at a time when the muscle has not yet had time to move to relaxation from the previous irritation, that is, it occurs at the height of its contraction, then a long continuous contraction occurs, called smooth tetanus .

Smooth tetanus – the normal working state of skeletal muscles is determined by the arrival of nerve impulses from the central nervous system with a frequency of 40-50 per second.

Serrated tetanus occurs at a frequency of nerve impulses up to 30 per 1 s. If a muscle receives 10-20 nerve impulses per second, then it is in a state muscular tone , i.e. moderate degree of tension.

Fatigue muscles . With prolonged rhythmic stimulation in the muscle, fatigue develops. Its signs are a decrease in the amplitude of contractions, an increase in their latent periods, an extension of the relaxation phase, and, finally, the absence of contractions with continued irritation.

Another type of prolonged muscle contraction is contracture. It continues even when the stimulus is removed. Muscle contracture occurs when there is a metabolic disorder or a change in the properties of contractile proteins of muscle tissue. The causes of contracture may be poisoning with certain poisons and drugs, metabolic disorders, increased body temperature and other factors leading to irreversible changes in muscle tissue proteins.

The main morphological characteristics of muscle tissue elements: elongated shape, the presence of longitudinally located myofibrils and myofilaments - special organelles that ensure contractility, the location of mitochondria next to the contractile elements, the presence of inclusions of glycogen, lipids and myoglobin.

Special contractile organelles - myofilaments or myofibrils - provide contraction, which occurs when two main fibrillar proteins interact in them - actin and myosin - with the obligatory participation of calcium ions. Mitochondria provide these processes with energy. The supply of energy sources is formed by glycogen and lipids. Myoglobin is a protein that ensures the binding of oxygen and the creation of its reserve at the time of muscle contraction, when the blood vessels are compressed (the oxygen supply drops sharply).

Properties of muscle tissue

1. Contractility

Types of muscle tissue

Smooth muscle tissue

Consists of mononuclear cells - spindle-shaped myocytes with a length of 20-500 microns. Their cytoplasm in a light microscope looks uniform, without transverse striations. This muscle tissue has special properties: it contracts and relaxes slowly, is automatic, and is involuntary (that is, its activity is not controlled by the will of a person). It is part of the walls of internal organs: blood and lymphatic vessels, urinary tract, digestive tract (contraction of the walls of the stomach and intestines).

Striated skeletal muscle tissue

Consists of myocytes that are long (up to several centimeters) and have a diameter of 50-100 microns; these cells are multinucleated, containing up to 100 or more nuclei; in a light microscope, the cytoplasm looks like alternating dark and light stripes. The properties of this muscle tissue are high speed of contraction, relaxation and volition (that is, its activity is controlled by the will of the person). This muscle tissue is part of the skeletal muscles, as well as the wall of the pharynx, the upper part of the esophagus, it forms the tongue, and the extraocular muscles. Fibers are 10 to 12 cm long.

Striated cardiac muscle tissue

Consists of 1 or 2 nuclear cardiomyocytes with transverse striations of the cytoplasm (along the periphery of the cytolemma). Cardiomyocytes are branched and form connections with each other - intercalary discs, in which their cytoplasm is united. There is also another intercellular contact - anostamosis (invagination of the cytolemma of one cell into the cytolemma of another) This type of muscle tissue forms the myocardium of the heart. Develops from the myoepicardial plate (visceral layer of the splanchnotome of the fetal neck). A special property of this tissue is automaticity - the ability to rhythmically contract and relax under the influence of excitation that occurs in the cells themselves (typical cardiomyocytes). This tissue is involuntary (atypical cardiomyocytes). There is a 3rd type of cardiomyocytes - secretory cardiomyocytes (they do not have fibrils). They synthesize the hormone troponin, which lowers blood pressure and dilates the walls of blood vessels.

Functions of muscle tissue

Motor. Protective. Heat exchange. You can also highlight one more function - facial (social). Facial muscles, controlling facial expressions, transmit information to others.

Notes

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See what “Muscle tissue” is in other dictionaries:

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

Varieties of these fabrics

There are several types of muscles in the animal body:

• transversely striped;
• smooth muscle tissue.

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

Striated is also divided into two main types:

• skeletal;
• cardiac.

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

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

Smooth structural features

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

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

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

Smooth muscle myocytes

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

• oval;
• fusiform elongated;
• rounded;
• process.

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

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

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

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

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

There are two main types of myocytes.

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

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

• glycoproteins;
• procollagen;
• elastane;
• intercellular substance;
• proteoglycans.

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

Origin of this muscle

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

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

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

Main properties of this fabric

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

• conductivity;
• excitability;
• lability;
• contractility.

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

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

Cell proteins

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

• myosin filaments;
• actin;
• nebulin;
• connectin;
• tropomyosin.

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

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

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

Smooth muscle tissue: drawing

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

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

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

Areas of spatial location in the body

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

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

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

Recovery methods

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

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

Innervation of smooth muscles

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

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

Functions of smooth muscle tissue

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

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

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. Structural unit This tissue is a 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 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.