The structure of smooth muscle differs from striated skeletal muscle and cardiac muscle. It consists of spindle-shaped cells with a length of 10 to 500 microns, a width of 5-10 microns, containing one nucleus. Smooth muscle cells lie in the form of parallel oriented bundles, the distance between them is filled with collagen and elastic fibers, fibroblasts, and feeding highways. The membranes of adjacent cells form nexuses, which provide electrical communication between cells and serve to transmit excitation from cell to cell. In addition, the plasma membrane of the smooth muscle cell has special invaginations - caveolae, due to which the membrane area increases by 70%. The outside of the plasma membrane is covered by the basement membrane. The complex of the basal membrane and plasma membrane is called the sarcolemma. Smooth muscle lacks sarcomeres. The basis of the contractile apparatus is made up of myosin and actin protofibrils. There are much more actin protofibrils in SMCs than in striated muscle fibers. Actin/myosin ratio = 5:1.

Thick and thin myofilaments are scattered throughout the sarcoplasm of the smooth myocyte and do not have such a harmonious organization as in striated skeletal muscle. In this case, thin filaments are attached to dense bodies. Some of these bodies are located on the inner surface of the sarcolemma, but most of them are found in the sarcoplasm. Dense bodies are composed of alpha-actinin, a protein found in the structure of the Z-membrane of striated muscle fibers. Some of the dense bodies located on inner surface the membranes are in contact with the dense bodies of the adjacent cell. Thus, the force created by one cell can be transmitted to the next. Thick smooth muscle myofilaments contain myosin, and thin ones contain actin and tropomyosin. At the same time, troponin was not found in thin myofilaments.

Smooth muscle is found in the walls of blood vessels, skin and internal organs.

Smooth muscle plays important role in regulation

lumen of the airways,

tone of blood vessels,

motor activity of the gastrointestinal tract,

uterus, etc.

Classification of smooth muscles:

Multiunitary, they are part of the ciliary muscle, the muscles of the iris, and the levator pili muscle.

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

Multiunitary smooth muscle.

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

has a high innervation density;

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

each muscle cell can contract separately and its activity is regulated by nerve impulses;

Unitary smooth muscle (visceral).

is a layer or bundle, and the sarcolemmas of individual myocytes have multiple points of contact. This allows excitation to spread from one cell to another

membranes of adjacent cells form multiple tight junctions(gap junctions), through which ions are able to move freely from one cell to another

action potentials generated at the smooth muscle cell membrane and ionic currents can propagate throughout the muscle fiber, allowing large numbers of individual cells to contract simultaneously. This type of interaction is known as functional syncytium

An important feature of smooth muscle cells is their ability to self-excitation (automation), that is, they are able to generate an action potential without the influence of an external stimulus.

There is no constant resting membrane potential in smooth muscles; it constantly drifts and averages -50 mV. The drift occurs spontaneously, without any influence, and when the resting membrane potential reaches a critical level, an action potential occurs, which causes muscle contraction. The duration of the action potential reaches several seconds, so the contraction can also last several seconds. The resulting excitation then spreads through the nexus to neighboring areas, causing them to contract.

Spontaneous (independent) activity is associated with stretching of smooth muscle cells and when they stretch, an action potential occurs. The frequency of action potentials depends on the degree of fiber stretch. For example, peristaltic contractions of the intestine are enhanced when its walls are stretched by chyme.

Unitary muscles mainly contract under the influence of nerve impulses, but spontaneous contractions are sometimes possible. A single nerve impulse is not capable of causing a response. For it to occur, several pulses must be summed.

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

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

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

slow wave oscillations of membrane potential;

depolarization caused by stretching of smooth muscle.

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

Contraction of smooth muscle.

Smooth muscles, like striated muscles, contain cross-bridged myosin, which hydrolyzes ATP and interacts with actin to cause contraction. In contrast to striated muscle, smooth muscle thin filaments contain only actin and tropomyosin and no troponin; regulation of contractile activity in smooth muscles occurs due to the binding of Ca ++ to calmodulin, which activates myosin kinase, which phosphorylates the myosin regulatory chain. This leads to ATP hydrolysis and starts the cycle of cross-bridge formation. In smooth muscle, the movement of actomyosin bridges is a slower process. The breakdown of ATP molecules and the release of energy necessary to ensure the movement of actomyosin bridges does not occur as quickly as in striated muscle tissue.

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

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

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 with an 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 - 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 striated filaments, 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 enough large number important internal organs in the animal body. So, she was educated:

• intestines;
• genitals;
• blood vessels of all types;
• glands;
• organs of the excretory system;
• respiratory tract;
• 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 myocytes smooth fabric if 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.

Electrical activity. Visceral smooth muscles are characterized by unstable membrane potential. Fluctuations in membrane potential, regardless of neural influences, cause irregular contractions that maintain the muscle in a state of constant partial contraction - 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.

In some smooth muscles, such as the ureter, stomach, and lymphatic vessels, APs have a prolonged plateau during repolarization. Plateau-shaped PDs ensure the entry into the cytoplasm of myocytes of a significant 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 AP is necessary to maintain tone and develop contraction; therefore, blocking the 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 for correction of motility of the digestive tract and vascular tone in the treatment of patients with hypertension.

Automation. The action potentials of smooth muscle cells are autorhythmic (pacemaker) in nature, similar to the potentials of the conduction system of the heart. Pacemaker potentials are recorded in various areas of smooth muscle. This indicates that any visceral smooth muscle cells are capable of spontaneous automatic activity. Automaticity of smooth muscles, i.e. the ability for automatic (spontaneous) activity is inherent in many internal organs and vessels.

Tensile response. 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 filled, its wall stretches. 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. Dr. etc., stretching of the uterine muscles by the growing fetus is one of the reasons for the onset of labor.

Plastic. If visceral smooth muscle is stretched, its tension will increase, but if the muscle is held in the state of elongation caused by stretching, then the tension will gradually decrease, sometimes not only to the level that existed before the stretch, but also below this level. The plasticity of smooth muscles contributes to the normal functioning of internal hollow organs.

Relationship between excitation and contraction. Under conditions of relative rest, a single AP can be recorded. The contraction of smooth muscle, as in skeletal muscle, is based on the sliding of actin in relation to myosin, where the Ca2+ ion performs a trigger function.

The mechanism of contraction of smooth muscle has a feature that distinguishes it from the mechanism of contraction of skeletal muscle. This feature is that before smooth muscle myosin can exhibit its ATPase activity, it must be phosphorylated. The mechanism of phosphorylation of smooth muscle myosin is as follows: the Ca2+ ion combines with calmodulin (calmodulin is a receptive protein for the Ca2+ ion). The resulting complex activates the enzyme myosin light chain kinase, which in turn catalyzes the process of myosin phosphorylation. Actin then slides against myosin, which forms the basis of contraction. That. The trigger for smooth muscle contraction is the addition of Ca2+ ion to calmodulin, while in skeletal and cardiac muscle the trigger is the addition of Ca2+ to troponin.

Chemical sensitivity. Smooth muscles are highly sensitive to various physiologically active substances: adrenaline, norepinephrine, ACh, histamine, etc. This is due to the presence of specific receptors on the smooth muscle cell membrane.

Norepinephrine acts on α- and β-adrenergic receptors on the smooth muscle cell membrane. The interaction of norepinephrine with β-receptors reduces muscle tone as a result of activation of adenylate cyclase and the formation of cyclic AMP and a subsequent increase in the binding of intracellular Ca2+. The effect of norepinephrine on α-receptors inhibits contraction by increasing the release of Ca2+ ions from muscle cells.

ACh has an effect on membrane potential and contraction of intestinal smooth muscle that is opposite to the effect of norepinephrine. The addition of ACh to an intestinal smooth muscle preparation reduces the membrane potential and increases the frequency of spontaneous APs. As a result, the tone increases and the frequency of rhythmic contractions increases, i.e., the same effect is observed as when the parasympathetic nerves are excited. ACh depolarizes the membrane and increases its permeability to Na+ and Ca++.

Related information.

PHYSIOLOGY OF SMOOTH MUSCLES

Smooth muscles are built from muscle fibers that have a diameter of 2 to 5 microns and a length of only 20 to 500 microns, which is significantly smaller than in skeletal muscles, the fibers of which have a diameter 20 times greater and a length thousands of times. They do not have transverse striations. The mechanism of contraction of smooth muscle fibers is fundamentally the same as in lumbar muscles. It is built on the interaction between the contractile proteins actin and myosin, although there are some differences - they are not characterized by an ordered arrangement of filaments. The analogue of Z-lines in smooth muscles is dense bodies, which are contained in the myoplasm and are connected to the cell membrane and actin filaments. Contraction of various smooth muscles lasts from 0.2 s to 30 s. Their absolute strength is 4-6 kg/cm2, in skeletal muscles - 3-17 kg/cm2.

Types of smooth muscles: smooth muscles are divided into visceral, or unitary, polyelement, or multiunitary, And vascular smooth muscle, possessing properties of both previous types.

Visceral or unitary muscles are contained in the walls of hollow organs - the digestive canal, uterus, ureters, gall bladder and bladder. Their peculiarity is that they transmit excitation from cell to cell through low-resistance gap junctions, which allows the muscles to respond as a functional syncytium, that is, as one cell, hence the term unitary muscles. They are spontaneously active, have pacemakers (pacemakers), which are modulated under the influence of hormones or neurotransmitters. The resting potential is not typical for these muscle fibers, since in the active state of the cell it is low, during its inhibition it is high, and at rest it is about -55 mV. They are characterized by so-called sinusoidal slow waves of depolarization, on which peak APs are superimposed, lasting from 10 to 50 ms (Fig. 2.34).

The mechanism of generation of smooth muscle action potential and their contraction is largely initiated by Ca2 ions. Contraction occurs 100-200 ms after excitation, and the maximum develops only 500 ms after the onset of the peak. Therefore, smooth muscle contraction is a slow process. However visceral muscles have a high degree of electrical coupling between cells, ensuring high coordination of their contraction.

Polyelement, or multiunit smooth muscles are composed of individual units without connecting bridges, and the response of the whole muscle to stimulation consists of the response of individual muscle fibers. Each muscle fiber is innervated by one nerve ending, as in skeletal muscles. These include the muscles of the iris, the ciliary muscle of the eye, and the pilorector muscle of the skin. They do not have voluntary regulation; they contract due to nerve impulses that are transmitted through neuromuscular synapses by the autonomic nervous system, whose neurotransmitters can cause both excitation and inhibition.

Mechanisms of contraction and relaxation of smooth muscles

The mechanism of coupling excitation and contraction differs from a similar process occurring in skeletal muscles, since smooth muscles do not contain troponin.

The sequence of processes in smooth muscles that leads to contraction and relaxation has the following steps:

1. When the cell membrane is depolarized, voltage-gated calcium channels and ions open

RICE. 2.34.

Ca 2+ enters the cell with an electrochemical gradient, the concentration of Ca 2+ ions in the cell increases.

2. The entry of Ca 2+ ions through the cell membrane can cause additional exit of Ca 2+ ions from the sarcoplasmic reticulum (SRR) through the Ca 2+ dependent gate of calcium channels. Hormones and neurotransmitters also stimulate the release of Ca 2+ ions from the SPR through inositol triphosphatide (I-S-P)-dependent calcium channel gates.

3. intracellular concentration of Ca 2+ ions increases.

4. Ca 2+ ions bind to calmodulin, a regulatory protein that has 4 Ca 2+ bindings and plays an important role in enzyme activation. Calcium calmodulin complex activates enzyme kinase the myosin light chain, which leads to phosphorylation of the myosin head molecules. Myosin hydrolyzes ATP, energy is generated and the cycle of formation of transverse actin-myosin bridges and actin sliding along myosin chains begins. Phosphorylated myosin bridges repeat their cycle until they are dephosphorylated myosin phosphatase.

5. Dephosphorylation of myosin leads to a relaxation of the muscle fiber, or a state of residual tension due to the cross-bridges formed, until the final dissociation of the calcium-calmodulin complex occurs.

AGE CHANGES IN EXCITING STRUCTURES

During the process of ontogenesis, the properties of excitable structures change in connection with the development of the musculoskeletal system and its regulation.

Muscle mass increases - from 23.3% of body weight in a newborn to 44.2% at the age of 17-18 years. Muscle tissue grows due to lengthening and thickening of muscle fibers, and not an increase in their number.

In a newborn child, the activity of sodium-potassium pumps located in the membranes of myocytes is still low and therefore the concentration of K + ions in the cell is almost half that of an adult, and only begins to increase at 3 months. APs are already generated after birth, but have a smaller amplitude and longer duration. The generation of muscle fiber action potential in newborns is not blocked by tetrodotoxin.

After birth, the length and diameter of the axial cylinders in the nerve fibers increases from 1-3 microns to 7 microns at 4 years, and their formation is completed at 5-9 years. By the age of 9, myelination of nerve fibers ends. The speed of excitation after birth does not exceed 50% of the speed in adults and increases over 5 years. The increase in conduction velocity is due to: an increase in the diameter of nerve fibers, their myelination, the formation of ion channels and an increase in the amplitude of action potentials. Reducing the duration of the AP and, accordingly, the absolute refractory phase leads to an increase in the number of APs that the nerve fiber can generate.

The muscle receptor apparatus develops faster than motor nerve endings are formed. The duration of neuromuscular transmission after birth is 4.5 ms, in an adult it is 0.5 ms. During ontogenesis, the synthesis of acetylcholine, acetylcholinesterase, and the density of cholinergic receptors in the lamina terminalis increases.

During the aging process, the duration of AP in excitable structures increases, and the number of AP that generate muscle fibers per unit time (lability) decreases. Muscle mass decreases due to a decrease in metabolic rate.

Smooth muscles that form the walls (muscle layers) of internal organs are divided into two types - visceral(i.e. internal) smooth muscles lining the walls gastrointestinal tract and urinary tract, and unitary – smooth muscles located in the walls of blood vessels, in the pupil and lens of the eye and at the roots of the hair of the skin (muscles that ruffle the hair of animals). These muscles are built from spindle-shaped mononuclear cells that do not have transverse striations, which is due to the chaotic arrangement of contractile proteins in their fibers. The muscle fibers are relatively short (from 50 to 200 µm), they have branches at both ends and fit tightly together, forming long and thin cylindrical bundles with a diameter of 0.05-0.01 mm, which branch and connect with other bundles. Their network forms either layers (layers) or even thicker bundles in the internal organs.

Neighboring cells in smooth muscles are functionally connected to each other by low-resistance electrical contacts - nexuses. Due to these contacts, action potentials and slow waves of depolarization propagate unhindered from one muscle fiber to another. 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 contractile reaction. Consequently, smooth muscles represent not only a morphological, but also a functional syncytium.

As in skeletal muscle, smooth muscle contractile proteins are activated by increased concentrations of calcium ions in the sarcoplasm. However, calcium does not come from the cisterns of the sarcoplasmic reticulum, as in skeletal muscles, but from the extracellular environment, along a concentration gradient, through the plasma membrane of the cell, through slow voltage-sensitive calcium channels, which are activated as a result of depolarization of the membrane when it is excited. This significantly affects the development of the action potential of smooth muscle cells, which is clearly reflected by the AP curve (Fig. 12. 1).

Fig. 12. Action potential (1) and curve

contractions (2) of smooth muscle cells.

A – depolarization phase (Na + - input);

B – “calcium plateau” (Ca 2+ - input);

B – repolarization phase (K + - output);

(the dotted line indicates the PP of the skeletal muscle)

A slow, but quite significant incoming calcium current forms a characteristic “calcium plateau” on the AP curve, which does not allow rapid depolarization of the membrane, which leads to a significant increase in the duration of the refractory period. Calcium is removed from the cell even more slowly, through Ca 2+ - ATPases of the plasma membrane. All this significantly affects both the characteristics of excitability and the contractility of smooth muscles. Smooth muscles are much less excitable than striated muscles and excitation spreads through them at a very low speed - 2-15 cm/s; in addition, they contract and relax very slowly, and the time of a single contraction can last several seconds.

Due to the long refractory period, the duration of the action potential of the smooth muscle fiber practically coincides with the time of entry and removal of calcium ions from the cell, that is, the time of development of AP and the duration of contraction practically coincide (Fig. 12. 2) As a result, smooth muscles are practically not capable of forming a classical tetanus. Due to the very slow relaxation, fusion of single contractions (“smooth muscle tetanus”) occurs even at low frequencies of stimulation and is, to a large extent, the result of a slow wave-like involvement of cells adjacent to the stimulated cell in a long contraction.

Smooth muscles are capable of performing relatively slow and prolonged tonic abbreviations. Slow, rhythmic contractions of the smooth muscles of the stomach, intestines, ureters and other organs ensure the movement of the contents of these organs. Prolonged tonic contractions of smooth muscles are especially pronounced in the sphincters of the hollow organs, which prevent the release of the contents of these organs.

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. These differences can be easily observed with slow stretching of smooth and skeletal muscle. 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. Due to its high plasticity, smooth muscle can be completely relaxed in both shortened and extended states. For example, the plasticity of the muscles of the bladder as it fills prevents an excessive increase in pressure inside it.

An adequate stimulus for smooth muscles is their rapid and strong stretching, which causes their contraction due to the increasing depolarization of cells during stretching. The frequency of action potentials (and, accordingly, the frequency of contractions) is greater, the more and faster the smooth muscle is stretched. Thanks to this mechanism, in particular, the movement of the food bolus through the digestive tract is ensured. The muscular wall of the intestine, stretched by a bolus of food, responds with contraction and thus pushes the bolus into the next section of the intestine. Stretch-induced contraction plays an important role in the autoregulation of blood vessel tone and also provides involuntary (automatic) emptying of a full bladder in cases where neural regulation is absent as a result of spinal cord injury.

Nervous regulation of smooth muscles is carried out through sympathetic and parasympathetic fibers of the autonomic nervous system.

The peculiarity of visceral smooth muscle cells is that they are capable of contracting in the absence of direct nervous influences under conditions of their isolation and denervation, and even after blockade of autonomic ganglion neurons.

In this case, contractions occur not as a result of the transmission of nerve impulses from the nerve, but as a result of the activity of one’s own cells ( pacemakers), which are identical in structure to other muscle cells, but differ in electrophysiological properties - they have automaticity. In these cells, the activity of membrane ion channels is regulated in such a way that their membrane potential is not balanced, but constantly “drifts”. As a result, the membrane regularly produces prepotentials or pacemaker potentials, with a certain frequency depolarizing the membrane to a critical level. When an action potential occurs in a pacemaker, excitation spreads from them to neighboring ones, which leads to their excitation and contraction. As a result, one section of the muscle layer after another is sequentially reduced.

It follows from this that visceral smooth muscles are controlled by the autonomic nervous system, which performs not a triggering, but a tuning, regulating function in relation to these muscles. This means that the activity of visceral smooth muscles itself occurs spontaneously, without nervous influences, but the level of this activity (strength and frequency of contractions) changes under the influence of the autonomic nervous system. In particular, by changing the rate of “drift” of the membrane potential, nerve impulses from autonomic fibers affect the frequency of contractions of visceral smooth muscle fibers.

Unitary smooth muscles can also be spontaneously active, but they contract mainly under the influence of nerve impulses from autonomic fibers. Their peculiarity is that a single nerve impulse arriving at them is not capable of causing a contraction; in response, only a temporary subthreshold depolarization of the muscle cell membrane occurs. Only when a series of impulses follows along the autonomic nerve fiber with a frequency of about 1 impulse per 1 second. and more, it is possible to develop the action potential of the muscle fiber and its contraction. That is, unitary muscle fibers “summarize” nerve impulses and respond to stimulation when the impulse frequency reaches a certain value.

In unitary smooth muscle, as in visceral smooth muscle, excited muscle cells exert influence on neighboring cells. As a result, excitation captures many cells (hence the name of these muscles - unitary, i.e., consisting of unit - “units” with a large number of muscle fibers in each of them).

Two mediators are involved in the nervous regulation of smooth muscle contraction: acetylcholine (ACh) and adrenaline (norepinephrine). The mode of action of ACh in smooth muscles is the same as in skeletal muscles: ACh increases the ionic permeability of the membrane, causing its depolarization. The mechanism of action of adrenaline is unknown. Skeletal muscle fibers respond to the action of the mediator only in the area of ​​the end plate (neuromuscular synapse), while smooth muscle fibers respond to the action of the mediator regardless of the site of its application. Therefore, smooth muscles can be influenced by mediators contained in the blood (for example, adrenaline, which has a long-term effect on smooth muscles, causes them to contract).

From all of the above, another characteristic feature of smooth muscles follows: their contraction does not require large energy expenditures.