Tissue is a collection of cells of similar structure that are united by common 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 able to contract, but this process occurs individually in different species.

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. A prerequisite for muscle functioning, in addition to the presence of contractile proteins, is also the presence of calcium ions in the 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, which especially applies to the abdominal muscles, which prevent 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 the smooth muscles of the intestine push feces to the place where they are excreted 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 the correct position of 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:

Smooth muscle	Striated muscles	Cardiac muscle tissue
Shrinks automatically	Can be controlled consciously	Shrinks automatically
Cells are elongated, spindle-shaped	Cells are long, filamentous	Elongated cells
Fibers are not bundled	Fibers are combined into bundles	Fibers are combined into bundles
One nucleus per cell	Several nuclei in a cell	Several nuclei in a cell
Relatively small number of mitochondria	Large number of mitochondria
No myofibrils	Myofibrils present	There are myofibrils
Cells are capable of dividing	Fibers cannot divide	Cells cannot divide
Contracts slowly, weakly, rhythmically	Contracts quickly and strongly	Contracts quickly, strongly, rhythmically
Line internal organs (intestines, uterus, bladder), form sphincters	Attached to the skeleton	Shape the heart

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.

These muscles form the muscular layers of the walls of the stomach, intestines, ureters, bronchi, blood vessels and other internal organs. They are built from spindle-shaped mononuclear muscle cells. Smooth muscles are divided into two main groups: multiunitary and unitary. Multiunitary muscles function independently of each other, and each fiber can be innervated by a separate nerve ending. Such fibers are found in the ciliary muscle of the eye, the nictitating membrane and the muscular layers of some large vessels, these include the muscles that lift the hair. U unitary muscles the fibers are so closely intertwined that their membranes can fuse to form electrical contacts (nexuses). When one fiber is stimulated due to these contacts, PD quickly spreads to neighboring fibers. Therefore, despite the fact that the motor nerve endings are located on a small number of muscle fibers, the entire muscle is involved in the reaction. Such muscles are present in most organs: the digestive tract, uterus, and ureters.

A feature of smooth muscles is their ability to perform slow and prolonged tonic contractions. Slow, rhythmic contractions of the smooth muscles of the stomach, intestines, ureters and other organs ensure the movement of the contents of these organs. Long-term tonic contractions of smooth muscles ensure the functioning of the sphincters of the hollow organs, which prevent the release of their contents.

The smooth muscles of the walls of blood vessels, especially arteries and arterioles, are also in a state of constant tonic contraction. Changes in muscle tone in the walls of arterial vessels affect the size of their lumen and, consequently, the level of blood pressure and blood supply to organs. An important property of smooth muscles is their plasticity, i.e. the ability to maintain the length given to them when stretched. Normal skeletal muscle has almost no plasticity. When the tensile load is removed, the skeletal muscle quickly shortens, but the smooth muscle remains stretched. The high plasticity of smooth muscles is of great importance for the normal functioning of hollow organs. For example, the plasticity of the bladder muscles as it fills prevents excessive pressure build-up.

A strong and sharp stretch of smooth muscles causes their contraction, which is due to the depolarization of cells that increases with stretching, which ensures the automaticity of the smooth muscle. This contraction plays an important role in the autoregulation of blood vessel tone, and also contributes to the involuntary emptying of a full bladder in cases where neural regulation is absent as a result of spinal cord damage.

In smooth muscle, tetanic contraction occurs at low stimulation frequencies. Unlike skeletal muscles, smooth muscles are capable of developing spontaneous thetan-like contractions under conditions of denervation and even after blockade of the intramural ganglia. Such contractions occur due to the activity of cells with automaticity (pacemaker cells), which differ in electrophysiological properties from other muscle cells. Pacemaker potentials appear in them, depolarizing the membrane to a critical level, which causes the occurrence of an action potential.

A feature of smooth muscles is their high sensitivity to mediators, which have a modulating effect on the spontaneous activity of pacemakers. When acetylcholine is applied to the colon muscle preparation, the frequency of PD increases. The contractions they cause merge, forming an almost smooth tetanus. The higher the AP frequency, the stronger the contraction. Norepinephrine, on the contrary, hyperpolarizes the membrane, reducing the frequency of AP and the magnitude of tetanus.

Excitation of smooth muscle cells causes an increase in calcium concentration in the sarcoplasm, which activates contractile structures. Like cardiac and skeletal muscle, smooth muscle relaxes when the concentration of calcium ions decreases. Smooth muscle relaxation occurs more slowly because the removal of calcium ions is slower.

Ministry of Education and Science of the Russian Federation

Federal State Budgetary Educational Institution of Higher Professional Education

«***»

ABSTRACT

at the rate

"fundamentals of anatomy and physiology"

on the topic

"Smooth muscles. Structure, functions, contraction mechanism"

Leading teacher:

Art. teacher **

Work completed:

Group student**

Score based on the results of the abstract defense:

_____________________

"___"__________20__

Moscow 2013

1. Introduction……………………………………………………………………………….2
2. The structure of smooth muscles……………………………………………………………...3
3. Functions of smooth muscles………………………………………………………………...5
4. Reduction mechanism………………………………………………………………..8
5. Excitatory and inhibitory mediators secreted at the neuromuscular junctions of smooth muscles………………………………………………………………………………...10
6. Conclusion…………………………………………………………………………………...11
7. List of references……………………………………………………………….12

Introduction

Muscles or muscles (from lat. musculus muscle) organs bodies of animals and humans, consisting of elastic, elastic muscle tissue , capable of contracting under the influencenerve impulses. Designed to perform various actions: body movements, vocal contractions ligaments, breathing . Muscles allow you to move parts of the body and express thoughts and feelings in actions. A person performs any movements, from such simple ones as blinking or smile , to thin and energetic, such as we see in jewelers or athletes due to the ability of muscle tissue to contract.

Smooth muscles are an integral part of some internal organs and are involved in providing the functions performed by these organs. In particular, they regulate the patency of the bronchi for air, blood flow in various organs and tissues, the movement of fluids and chyme (in the stomach, intestines, ureters, urinary and gall bladders), expel the fetus from the uterus, dilate or constrict the pupils (by reducing the radial or circular muscles of the iris), change the position of hair and skin relief.

The structure of smooth muscles

There are three groups of smooth (non-striated) muscle tissues: mesenchymal, epidermal and neural.
Muscle tissue of mesenchymal origin.
Stem cells and progenitor cells in smooth muscle tissue at the stages of embryonic development have not yet been precisely identified. Apparently, they are related to mechanocytes of tissues of the internal environment. Probably, in the mesenchyme they migrate to the sites of organ formation, being already determined. Differentiating, they synthesize the components of the matrix and collagen of the basement membrane, as well as elastin. In definitive cells (myocytes), the synthetic ability is reduced, but does not disappear completely. Smooth myocyte spindle-shaped cell 20 500 µm long, 5 8 µm wide. The nucleus is rod-shaped and is located in its central part. When a myocyte contracts, its nucleus bends and even twists. Organelles of general importance, including many mitochondria, are concentrated near the poles of the nucleus (in the endoplasm). The Golgi apparatus and granular endoplasmic reticulum are poorly developed, which indicates low activity of synthetic functions. Ribosomes are mostly located
free. Myocytes are united into bundles, between which there are thin layers of connective tissue. Reticular and elastic fibers surrounding the myocytes are woven into these layers. Blood vessels and nerve fibers pass through the layers. The terminals of the latter end not directly on the myocytes, but between them. Therefore, after the arrival of a nerve impulse, the transmitter spreads diffusely, exciting many cells at once.
Smooth muscle tissue of mesenchymal origin is presented mainly in the walls of blood vessels and many tubular internal organs, and also forms individual small muscles (ciliary).

Muscle tissue of epidermal origin.Myoepithelial cells develop from the epidermal primordium. They are found in the sweat, mammary, salivary and lacrimal glands and have common precursors with

their secretory cells. Myoepithelial cells are directly adjacent to the epithelial cells proper and have a common basement membrane with them. During regeneration, both cells are also restored from common poorly differentiated precursors. Most myoepithelial cells are stellate in shape. These cells are often called basket cells: their processes cover the terminal sections and small ducts of the glands.
In the cell body there is a nucleus and organelles of general importance, and in the processes there is a contractile apparatus, organized as in cells of mesenchymal muscle tissue.

Muscle tissue of neural origin.
Myocytes of this tissue develop from the cells of the neural primordium as part of the inner wall of the optic cup. The bodies of these cells are located in the epithelium of the posterior surface of the iris. Each of them has a process that is directed into the thickness of the iris and lies parallel to its surface. The process contains a contractile apparatus, organized in the same way as in all smooth myocytes. Depending on the direction of the processes (perpendicular or parallel to the edge of the pupil), myocytes form two muscles: the constrictor and the dilator of the pupil.

It should be remembered that the composition of smooth muscle tissue, regardless of its origin, also includes specific constituent elements that are directly related to the contraction mechanism itself, these are myofibrils. It contains “contractile” proteins called actin and myosin.

Myosin - protein of muscle contractile fibers. Its content in muscles is about 40% of the mass of all proteins (for example, in other tissues it is only 1-2%). The myosin molecule is a long thread-like rod, as if two ropes were woven together, forming two pear-shaped heads at one end.

Actin also a protein of contractile muscle fibers, much smaller in size than myosin, and occupying only 15-20% of the total mass of all proteins. It consists of two threads woven into a rod, with grooves.

Functions of smooth muscles

Smooth muscles, like skeletal muscles, have excitability, conductivity and contractility. Unlike skeletal muscles, which have elasticity, smooth muscles are plastic (able to maintain the length given to them by stretching for a long time without increasing tension). This property is important for the function of depositing food in the stomach or liquids in the gall or bladder.

Features of excitability smooth muscle fibers are to some extent associated with their low transmembrane potential (E 0 = 30-70 mV). Many of these fibers are automatic. The duration of their action potential can reach tens of milliseconds. This happens because the action potential in these fibers develops mainly due to the entry of calcium into the sarcoplasm from the intercellular fluid through the so-called slow Ca 2+ channels.

Visceral smooth muscles are characterized by unstable membrane potential. Fluctuations in membrane potential, independent of neural influences, cause irregular contractions that maintain the muscle in a state of constant partial contraction and tone. The tone of smooth muscles is clearly expressed in the sphincters of hollow organs: the gall bladder, bladder, at the junction of the stomach into the duodenum and the small intestine into the large intestine, as well as in the smooth muscles of small arteries and arterioles. The membrane potential of smooth muscle cells does not reflect the true value of the resting potential. When the membrane potential decreases, the muscle contracts; when it increases, it relaxes. During periods of relative rest, the membrane potential is on average 50 mV. In visceral smooth muscle cells, slow wave-like fluctuations of the membrane potential of several millivolts are observed, as well as the action potential (AP). The value of PD can vary widely. In smooth muscles, AP duration is 50 x 250 ms; PDs of various shapes are found. In some smooth muscles, such as the ureter, stomach, and lymphatic vessels, APs have a prolonged plateau during depolarization, reminiscent of the potential plateau in myocardial cells. Plateau-shaped PDs ensure the entry into the cytoplasm of myocytes of significant

the amount of extracellular calcium, which subsequently participates in the activation of contractile proteins of smooth muscle cells. The ionic nature of smooth muscle PD is determined by the characteristics of the smooth muscle cell membrane channels. The main role in the mechanism of occurrence of PD is played by Ca2+ ions. Calcium channels in the membrane of smooth muscle cells allow not only Ca2+ ions to pass through, but also other doubly charged ions (Ba2+, Mg2+), as well as Na+. The entry of Ca2+ into the cell during PD is necessary to maintain tone and develop contraction, therefore blocking calcium channels of the smooth muscle membrane, leading to a limitation of the entry of Ca2+ ion into the cytoplasm of myocytes of internal organs and blood vessels, is widely used in practical medicine to correct motility of the digestive tract and vascular tone in the treatment of patients with hypertension.

Speed carrying out the initiationin smooth muscle cells small 2-10 cm/s. Unlike skeletal muscles, excitation in smooth muscle can be transmitted from one fiber to another nearby. This conduction occurs due to the presence of nexuses between smooth muscle fibers (contact areas between twocell membranes, where the channels for exchange are located ions and micromolecules) , which have low resistance to electric current and ensure the exchange of Ca between cells 2+ and other molecules. As a result of this, smooth muscle has the properties of functional syncytium (represents several cells fused with each other and containing several nuclei).

Contractility smooth muscle fibers are distinguished by a long latent period (the time between the onset of the stimulus and the onset of the response) (0.25-1.00 s) and a long duration (up to 1 min) of a single contraction. Smooth muscles have a low contractile force, but are able to remain in tonic contraction for a long time without developing fatigue. This is due to the fact that smooth muscle spends 100-500 times less energy to maintain a tonic contraction (long-term contraction) than skeletal muscle. Therefore, the ATP reserves consumed by smooth muscle have time to be restored even during contraction, and the smooth muscles of some body structures are in a state of tonic contraction throughout their lives (are actually a type of tetanic contraction,

representing a long-term shortening of muscles and causing mainly muscle tone - constant slight muscle tension that occurs in muscle tissue at rest. This constant tension of muscle tissue occurs even in a state of sleep).

Relationship between excitation and contraction. It is more difficult to study the relationship between electrical and mechanical manifestations in visceral smooth muscle than in skeletal or cardiac muscle, since visceral smooth muscle is in a state of continuous activity. Under conditions of relative rest, a single AP can be recorded. The contraction of both skeletal and smooth muscle is based on the sliding of actin in relation to myosin, where the Ca2+ ion performs a trigger function (the ability to remain in one state for a long time).

A unique feature of visceral smooth muscle is its response to stretch. In response to stretch, smooth muscle contracts. This is because stretching reduces the cell membrane potential, increases AP frequency and ultimately smooth muscle tone. In the human body, this property of smooth muscles serves as one of the ways to regulate the motor activity of internal organs. For example, when the stomach is full, it stretches walls . An increase in the tone of the stomach wall in response to its stretching helps maintain the volume of the organ and better contact of its walls with incoming food. In blood vessels, distension created by fluctuations in blood pressure is a major factor in the myogenic self-regulation of vascular tone. Finally, stretching of the uterine muscles by the growing fetus is one of the reasons for the onset of labor.

Reduction mechanism

Conditions for smooth muscle contraction.

The most important feature of smooth muscle fibers is that they are excited under the influence of numerous stimuli. Skeletal muscle contraction is normally initiated only by a nerve impulse passing to the neuromuscular junction. Contraction of smooth muscle can be caused by both a nerve impulse and biologically active substances (hormones, many neurotransmitters, some metabolites), as well as by the influence of physical factors, such as stretching. In addition, contraction of smooth muscle can occur spontaneously due to automaticity.

The very high reactivity of smooth muscles and their ability to respond to contractions under the influence of various factors create significant difficulties for correcting disorders of the tone of these muscles in medical practice. This can be seen in the example of bronchial asthma, arterial hypertension and other diseases that require correction of the contractile activity of smooth muscles.

The molecular mechanism of smooth muscle contraction also has a number of differences from skeletal muscle contraction. The filaments of actin and myosin in smooth muscle fibers are located less orderly than in skeletal fibers, and therefore smooth muscle does not have cross-striations. In smooth muscle actin filaments there is no protein troponin and the molecular centers of actin are always open to interaction with myosin heads. For such an interaction to occur, the ATP molecule must be broken down and phosphate transferred to the myosin heads. This is followed by the rotation of the myosin heads, during which the actin filaments are pulled between the myosin filaments and contraction occurs.

Phospholation of myosin heads occurs with the help of the enzyme myosin light chain kinase, and dephospholation occurs with the help of myosin light chain phosphatase. If the activity of myosin phosphatase prevails over the kinase, then the myosin heads are dephosphorylated, the actin-myosin bond is broken and the muscles relax.

Therefore, for smooth muscle contraction to occur, an increase in the activity of myosin light chain kinase is necessary. Its activity is regulated by Ca levels 2+ in the sarcoplasm. When the smooth muscle fiber is excited, the calcium content in its sarcoplasm increases. This increase is due to the intake of Ca 2+ from two sources: 1) intercellular space; 2) sarcoplasmic reticulum. Next, calcium ions form a complex with the protein calmodulin, which converts myosin kinase into an active state.

Sequence of processes leading to the development of smooth muscle contraction: Ca entry 2+ into the sarcoplasm activation of calmodulin activation of myosin light chain kinase phosphorylation of myosin heads binding of myosin heads to actin and rotation of the heads, in which actin filaments are pulled between myosin filaments.

Conditions necessary for smooth muscle relaxation.

1. Reduction (up to 10 -7 M/l or less) Ca content 2+ in sarcoplasm;
2. Disintegration of the 4 Ca complex 2+ - calmodulin, leading to a decrease in the activity of myosin light chain kinase, dephosphorylation of myosin heads, leading to the rupture of bonds between actin and myosin filaments

After this, elastic forces cause a relatively slow restoration of the original length of the smooth muscle fiber and its relaxation.

Excitatory and inhibitory mediators secreted at smooth muscle neuromuscular junctions.

The most important transmitters secreted by the autonomic nerves innervating smooth muscles are acetylcholine and norepinephrine, but they are never secreted by the same nerve fibers. Acetylcholine is an excitatory mediator for the smooth muscles of some organs, and acts as an inhibitory agent for the smooth muscles of other organs. While acetylcholine excites a muscle fiber, norepinephrine usually inhibits it. Conversely, if acetylcholine inhibits a fiber, norepinephrine tends to excite it. But why do such different reactions occur? The answer is that acetylcholine and norepinephrine excite or inhibit smooth muscle by first binding to a receptor protein on the surface of the muscle cell membrane. Some of these receptor proteins are excitatory receptors, while others are inhibitory receptors. Consequently, the type of receptor determines how the smooth muscle will react with inhibition or excitation, as well as which of the two mediators (acetylcholine or norepinephrine) will exhibit an excitatory or inhibitory effect.

Conclusion

There are many smooth muscles in the skin, they are located at the base of the hair follicle. By contracting, these muscles lift the hair and squeeze out fat from the sebaceous gland. In the eye around the pupil there are smooth circular and radial muscles. They work all the time: in bright light the circular muscles constrict the pupil, and in the dark the radial muscles contract and the pupil dilates. In the walls of all tubular organs - the respiratory tract, blood vessels, digestive tract, urethra, etc. - there is a layer of smooth muscle. Under the influence of nerve impulses it contracts. Due to the contraction and relaxation of smooth cells in the walls of blood vessels, their lumen either narrows or expands, which contributes to the distribution of blood in the body. The smooth muscles of the esophagus contract and push a bolus of food or a sip of water into the stomach. Complex plexuses of smooth muscle cells are formed in organs with a wide cavity - in the stomach, bladder, uterus. The contraction of these cells causes compression and narrowing of the organ lumen. The force of each cell contraction is negligible, because they are very small. However, the addition of the forces of entire bundles can create a contraction of enormous force. Powerful contractions create a sensation of intense pain. Excitation in smooth muscles spreads relatively slowly, which causes a slow, long-term contraction of the muscle and an equally long period of relaxation. Muscles are also capable of spontaneous rhythmic contractions. Stretching the smooth muscles of a hollow organ when filling it with contents immediately leads to its contraction - this ensures that the contents are pushed further.

This list of examples of smooth muscles in the human body can be continued indefinitely, thereby showing the enormous importance of smooth muscles.

List of used literature

1. Histology. Yu.I. Afanasyev, N.A. Yurina, E.F. Kotovsky, 2002
2. Atlas of histology and embryology.I.V. Almazov, L.S. Sutulov, 1978
3. Human anatomy. M.F. Ivanitsky, 2008
4. Anatomy. I.V. Gaivoropsky, G.I. Nichiporuk, 2006
5. Human physiology. A.A. Semenovich, 2009

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Smooth muscles are part of the internal organs. Thanks to contraction, they provide the motor (motor) function of their organs (digestive canal, genitourinary system, blood vessels, etc.). Unlike skeletal muscles, smooth muscles are involuntary.
Morpho-functional structure of smooth (non-striated) muscles. The main structural unit of smooth muscle is the muscle cell, which has a spindle-shaped shape and is covered on the outside with a plasma membrane. Under an electron microscope, numerous depressions can be seen in the membrane - caveolae, which significantly increase the total surface of the muscle cell. The sarcolemma of a muscle cell includes a plasma membrane along with the basement membrane, which covers it from the outside, and adjacent collagen fibers. Main intracellular elements:
nucleus, mitochondria, lysosomes, microtubules, sarcoplasmic reticulum and contractile proteins.
Muscle cells form muscle bundles and muscle layers. The intercellular space (100 nm or more) is filled with elastic and collagen fibers, capillaries, fibroblasts, etc. In some areas, the membranes of neighboring cells lie very tightly (the gap between cells is 2-3 nm). It is assumed that these areas (nexus) serve for intercellular communication and transmission of excitation. It has been proven that some smooth muscles contain a large number of nexus (pupillary sphincter, circular muscles of the small intestine, etc.), while others have little or no nexus (vas deferens, longitudinal muscles of the intestines). There is also an intermediate, or desmopodibny, connection between non-skinned muscle cells (through thickening of the membrane and with the help of cell processes). Obviously, these connections are important for the mechanical connection of cells and the transmission of mechanical force by cells.
Due to the chaotic distribution of myosin and actin protofibrils, smooth muscle cells are not striated, like skeletal and cardiac cells. Unlike skeletal muscles, smooth muscles do not have a T-system, and the sarcoplasmic reticulum makes up only 2-7% of the myoplasm volume and has no connections with the external environment of the cell.
Physiological properties of smooth muscles. Smooth muscle cells, like striated ones, contract due to the sliding of actin protofibrils between myosin protofibrils, but the speed of sliding and hydrolysis of ATP, and therefore the speed of contraction, is 100-1000 times less than in striated muscles. Thanks to this, smooth muscles are well adapted for long-term gliding with little energy expenditure and without fatigue.
Smooth muscles, taking into account the ability to generate AP in response to threshold or supra-horn stimulation, are conventionally divided into phasic and tonic. Phasic muscles generate a full-fledged potential action, while tonic muscles generate only a local one, although they also have a mechanism for generating full-fledged potentials. The inability of tonic muscles to perform AP is explained by the high potassium permeability of the membrane, which prevents the development of regenerative depolarization.
The value of the membrane potential of smooth muscle cells of non-skinned muscles varies from -50 to -60 mV. As in other muscles, including nerve cells, mainly +, Na +, Cl- take part in its formation. In the smooth muscle cells of the digestive canal, uterus, and some vessels, the membrane potential is unstable; spontaneous fluctuations are observed in the form of slow waves of depolarization, at the top of which AP discharges may appear. The duration of smooth muscle action potential ranges from 20-25 ms to 1 s or more (for example, in the muscles of the bladder), i.e. she
longer than the duration of skeletal muscle AP. In the mechanism of action of smooth muscles, next to Na +, Ca2 + plays an important role.
Spontaneous myogenic activity. Unlike skeletal muscles, smooth muscles of the stomach, intestines, uterus, and ureters have spontaneous myogenic activity, i.e. develop spontaneous tetanohyodine contractions. They are stored under conditions of isolation of these muscles and with pharmacological switching off of the intrafusal nerve plexuses. So, AP occurs in the smooth muscles themselves, and is not caused by the transmission of nerve impulses to the muscles.
This spontaneous activity is of myogenic origin and occurs in muscle cells that function as a pacemaker. In these cells, the local potential reaches a critical level and passes into AP. But after membrane repolarization, a new local potential spontaneously arises, which causes another AP, etc. The AP, spreading through the nexus to neighboring muscle cells at a speed of 0.05-0.1 m/s, covers the entire muscle, causing its contraction. For example, peristaltic contractions of the stomach occur with a frequency of 3 times per 1 minute, segmental and pendulum-like movements of the colon - 20 times per 1 minute in the upper sections and 5-10 per 1 minute in the lower sections. Thus, the smooth muscle fibers of these internal organs have automaticity, which is manifested by their ability to contract rhythmically in the absence of external stimuli.
What is the reason for the appearance of potential in pacemaker smooth muscle cells? Obviously, it occurs due to a decrease in potassium and an increase in sodium and (or) calcium permeability of the membrane. As for the regular occurrence of slow waves of depolarization, most pronounced in the muscles of the gastrointestinal tract, there is no reliable data on their ionic origin. Perhaps a certain role is played by a decrease in the initial inactivating component of the potassium current during depolarization of muscle cells due to inactivation of the corresponding potassium ion channels. Thanks to this, the occurrence of repeated G1D becomes possible.
Elasticity and extensibility of smooth muscles. Unlike skeletal muscles, smooth muscles act as plastic, elastic structures when stretched. Thanks to plasticity, smooth muscle can be completely relaxed in both contracted and stretched states. For example, the plasticity of the smooth muscles of the wall of the stomach or bladder as these organs fill prevents an increase in intracavitary pressure. Excessive stretching often leads to stimulation of contraction, which is caused by the depolarization of pacemaker cells that occurs when the muscle is stretched, and is accompanied by an increase in the frequency of action potential, and as a result, an increase in contraction. Contraction, which activates the stretching process, plays a large role in the self-regulation of the basal tone of blood vessels.
The mechanism of smooth muscle contraction. A prerequisite for the occurrence is a contraction of smooth muscles, as well as skeletal muscles, and an increase in the concentration of Ca2 + in the myoplasm (up to 10-5 M). It is believed that the contraction process is activated primarily by extracellular Ca2+, which enters muscle cells through voltage-gated Ca2+ channels.
The peculiarity of neuromuscular transmission in smooth muscles is that innervation is carried out by the autonomic nervous system and it can have both an excitatory and an inhibitory effect. By type, there are cholinergic (mediator acetylcholine) and adrenergic (mediator norepinephrine) mediators. The former are usually found in the muscles of the digestive system, the latter in the muscles of the blood vessels.
The same transmitter in some synapses can be excitatory, and in others - inhibitory (depending on the properties of the cytoreceptors). Adrenergic receptors are divided into a- and b-. Norepinephrine, acting on α-adrenergic receptors, constricts blood vessels and inhibits the motility of the digestive tract, and acting on B-adrenergic receptors, stimulates the activity of the heart and dilates the blood vessels of some organs, relaxes the muscles of the bronchi. Described neuromuscular-. transmission in smooth muscles for the help of other mediators.
In response to the action of an excitatory transmitter, depolarization of smooth muscle cells occurs, which manifests itself in the form of an excitatory synaptic potential (ESP). When it reaches a critical level, PD occurs. This happens when several impulses approach the nerve ending one after another. The occurrence of PGI is a consequence of an increase in the permeability of the postsynaptic membrane for Na +, Ca2 + and SI."
The inhibitory transmitter causes hyperpolarization of the postsynaptic membrane, which is manifested in the inhibitory synaptic potential (ISP). Hyperpolarization is based on an increase in membrane permeability, mainly for K +. The role of inhibitory mediator in smooth muscles excited by acetylcholine (for example, muscles of the intestine, bronchi) is played by norepinephrine, and in smooth muscles for which norepinephrine is an excitatory mediator (for example, muscles of the bladder), acetylcholine plays the role.
Clinical and physiological aspect. In some diseases, when the innervation of skeletal muscles is disrupted, their passive stretching or displacement is accompanied by a reflex increase in their tone, i.e. resistance to stretching (spasticity or rigidity).
If blood circulation is impaired, as well as under the influence of certain metabolic products (lactic and phosphoric acids), toxic substances, alcohol, fatigue, or a decrease in muscle temperature (for example, during prolonged swimming in cold water), contracture may occur after prolonged active muscle contraction. The more the muscle function is impaired, the more pronounced the contracture aftereffect is (for example, contracture of the masticatory muscles in pathology of the maxillofacial region). What is the origin of contracture? It is believed that the contracture arose due to a decrease in the concentration of ATP in the muscle, which led to the formation of a permanent connection between the cross bridges and actin protofibrils. In this case, the muscle loses flexibility and becomes hard. The contracture goes away and the muscle relaxes when the ATP concentration reaches normal levels.
In diseases such as myotonia, muscle cell membranes are excited so easily that even a slight irritation (for example, the introduction of a needle electrode during electromyography) causes the discharge of muscle impulses. Spontaneous APs (fibrillation potentials) are also recorded at the first stage after denervation of the muscle (until inaction leads to its atrophy).
Tonic contractions of some smooth muscles, especially the muscles of the vascular walls (basal or myogenic tone) are activated predominantly by extracellular Ca 2 +. Physiologically active substances and mediators can cause a decrease in smooth muscle tone by closing chemosensitive Ca2 + channels (through activation of chemoreceptors) or hyperpolarization, which causes suppression of spontaneous APs and closing voltage-dependent Ca2 + channels.

Smooth muscles are present in hollow organs, blood vessels and skin. Smooth muscle fibers do not have transverse striations. Cells shorten as a result of relative sliding of the filaments. The speed of sliding and the rate of breakdown of adenosine triphosphate are 100-1000 times less than in . Thanks to this, smooth muscles are well adapted for long-term, sustained contraction without fatigue, with less energy expenditure.

Smooth muscle are an integral part of the walls of a number of hollow internal organs and are involved in providing the functions performed by these organs. In particular, they regulate blood flow in various organs and tissues, bronchial patency for air, movement of fluids and chyme (in the stomach, intestines, ureters, urinary and gall bladder), uterine contraction during childbirth, pupil size, and skin texture.

Smooth muscle cells are spindle-shaped, 50-400 µm long, 2-10 µm thick (Fig. 5.6).

Smooth muscles are involuntary muscles, i.e. their reduction does not depend on the will of the macroorganism. The characteristics of the motor activity of the stomach, intestines, blood vessels and skin to a certain extent determine the physiological characteristics of the smooth muscles of these organs.

Characteristics of smooth muscles

• Has automaticity (the influence of the intramural nervous system is corrective in nature)
• Plasticity - the ability to maintain length for a long time without changing tone
• Functional syncytium - individual fibers are separated, but there are special areas of contact - nexuses
• The resting potential value is 30-50 mV, the amplitude of the action potential is less than that of skeletal muscle cells
• Minimum “critical zone” (excitation occurs if a certain minimum number of muscle elements are excited)
• The interaction between actin and myosin requires the Ca 2+ ion, which comes from outside
• The duration of a single contraction is long

Features of smooth muscles- their ability to exhibit slow rhythmic and prolonged tonic contractions. Slow rhythmic contractions of the smooth muscles of the stomach, intestines, ureters and other hollow organs help move their contents. Long-term tonic contractions of the smooth muscles of the sphincters of the hollow organs prevent the voluntary release of their contents. The smooth muscles of the walls of blood vessels are also in a state of constant tonic contraction and affect the level of blood pressure and blood supply to the body.

An important property of smooth muscles is their mysticism, those. the ability to retain its shape caused by stretching or deformation. The high plasticity of smooth muscles is of great importance for the normal functioning of organs. For example, the plasticity of the bladder allows, when it is filled with urine, to prevent an increase in pressure in it without disrupting the process of urine formation.

Excessive stretching of smooth muscles causes them to contract. This occurs as a result of depolarization of cell membranes caused by their stretching, i.e. smooth muscles have automaticity.

Contraction caused by stretching plays an important role in the autoregulation of blood vessel tone, movement of gastrointestinal contents and other processes.

Rice. 1. 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.

Automaticity in smooth muscles is due to the presence of special pacemaker (rhythm-setting) cells in them. Their structure is identical to other smooth muscle cells, but they have special electrophysiological properties. Pacemaker potentials arise in these cells, depolarizing the membrane to a critical level.

Excitation of smooth muscle cells causes an increase in the entry of calcium ions into the cell and the release of these ions from the sarcoplasmic reticulum. As a result of an increase in the concentration of calcium ions in the sarcoplasm, contractile structures are activated, but the mechanism of activation in smooth fiber differs from the mechanism of activation in striated muscles. In smooth cells, calcium interacts with the protein calmodulin, which activates myosin light chains. They connect to the active centers of actin in protofibrils and perform a “stroke”. Smooth muscles relax passively.

Smooth muscles are involuntary, and do not depend on the will of the animal.

Physiological properties and features of smooth muscles

Smooth muscles, like skeletal muscles, have excitability, conductivity and contractility. Unlike skeletal muscles, which have elasticity, smooth muscles have plasticity - the ability to maintain the length given to them when stretched for a long time without increasing tension. This property is important for the function of depositing food in the stomach or liquids in the gall and bladder.

The characteristics of the excitability of smooth muscle cells are to a certain extent associated with the low potential difference on the membrane at rest (E 0 = (-30) - (-70) mV). Smooth myocytes can be automatic and spontaneously generate action potentials. Such cells, the pacemakers of smooth muscle contraction, are found in the walls of the intestine, venous and lymphatic vessels.

Rice. 2. Structure of a smooth muscle cell (A. Guyton, J. Hall, 2006)

The duration of AP in smooth myocytes can reach tens of milliseconds, since AP in them develops primarily due to the entry of Ca 2+ ions into the sarcoplasm from the intercellular fluid through slow calcium channels.

The speed of AP conduction along the membrane of smooth myocytes is low - 2-10 cm/s. Unlike skeletal muscles, excitation can be transmitted from one smooth myocyte to others nearby. This transmission occurs due to the presence of nexuses between smooth muscle cells, which have low resistance to electric current and ensure the exchange of Ca 2+ ions and other molecules between cells. As a result, smooth muscle exhibits the properties of functional syncytium.

The contractility of smooth muscle cells is characterized by a long latent period (0.25-1.00 s) and a long duration (up to 1 min) of a single contraction. Smooth muscles develop a low contractile force, but are able to remain in tonic contraction for a long time without developing fatigue. This is due to the fact that smooth muscle spends 100-500 times less energy to maintain tonic contraction than skeletal muscle. Therefore, the ATP reserves consumed by smooth muscle have time to be restored even during contraction, and the smooth muscles of some body structures are almost constantly in a state of tonic contraction. The absolute strength of smooth muscle is about 1 kg/cm2.

Mechanism of smooth muscle contraction

The most important feature of smooth muscle cells is that they are excited under the influence of numerous stimuli. under natural conditions, it is initiated only by a nerve impulse coming to. Contraction of smooth muscle can be caused by both the influence of nerve impulses and the action of hormones, neurotransmitters, prostaglandins, some metabolites, as well as the influence of physical factors, such as stretching. In addition, excitation and contraction of smooth myocytes can occur spontaneously - due to automation.

The ability of smooth muscles to respond by contraction to the action of various factors will create significant difficulties for correcting disturbances in the tone of these muscles in medical practice. This can be seen in the examples of difficulties in treating bronchial asthma, arterial hypertension, spastic colitis and other diseases that require correction of the contractile activity of smooth muscles.

The molecular mechanism of smooth muscle contraction also has a number of differences from the mechanism of skeletal muscle contraction. The filaments of actin and myosin in smooth muscle cells are less ordered than in skeletal cells, and therefore smooth muscle does not have cross-striations. Smooth muscle actin filaments do not contain the protein troponin, and actin centers are always open to interact with myosin heads. At the same time, myosin heads are not energized at rest. In order for actin and myosin to interact, it is necessary to phosphorylate the myosin heads and give them excess energy. The interaction of actin and myosin is accompanied by rotation of the myosin heads, in which the actin filaments are retracted between the myosin filaments and contraction of the smooth myocyte occurs.

Phosphorylation of myosin heads is carried out with the participation of the enzyme myosin light chain kinase, and dephosphorylation is carried out with the help of phosphatase. If myosin phosphatase activity predominates over kinase activity, the myosin heads are dephosphorylated, the myosin-actin bond is broken, and the muscle relaxes.

Therefore, for smooth myocyte contraction to occur, the activity of myosin light chain kinase must be increased. Its activity is regulated by the level of Ca 2+ ions in the sarcoplasm. Neurotransmitters (acetylcholine, noradrsnaline) or hormones (vasopressin, oxytocin, adrenaline) stimulate their specific receptor, causing dissociation of the G-protein, the a-subunit of which further activates the enzyme phospholipase C. Phospholipase C catalyzes the formation of inositol trisphosphate (IFZ) and diacylglycerol from phospho-inositol diphosphate cell membranes. IPE diffuses to the endoplasmic reticulum and, after interacting with its receptors, causes the opening of calcium channels and the release of Ca 2+ ions from the depot into the cytoplasm. An increase in the content of Ca 2+ ions in the cytoplasm is a key event for the initiation of smooth myocyte contraction. An increase in the content of Ca 2+ ions in the sarcoplasm is also achieved due to its entry into the myocyte from the extracellular environment (Fig. 3).

Ca 2+ ions form a complex with the protein calmodulin, and the Ca 2+ -calmodulin complex increases the kinase activity of myosin light chains.

The sequence of processes leading to the development of smooth muscle contraction can be described as follows: entry of Ca 2+ ions into the sarcoplasm - activation of calmodulin (through the formation of the 4Ca 2 -calmodulin complex) - activation of myosin light chain kinase - phosphorylation of myosin heads - binding of myosin heads to actin and the rotation of the heads, in which the actin filaments are retracted between the myosin filaments - contraction.

Rice. 3. Pathways for Ca 2+ ions entering the sarcoplasm of a smooth muscle cell (a) and removing them from the sarcoplasm (b)

Conditions necessary for smooth muscle relaxation:

• decrease (up to 10-7 M/l or less) in the content of Ca 2+ ions in the sarcoplasm;
• disintegration of the 4Ca 2+ -calmodulin complex, leading to a decrease in the activity of myosin light chain kinase - dephosphorylation of myosin heads under the influence of phosphatase, leading to the rupture of bonds between actin and myosin filaments.

Under these conditions, elastic forces cause a relatively slow restoration of the original length of the smooth muscle fiber and its relaxation.