Animal tissues 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 precisely the muscular one, since its importance in the formation of the outer and inner 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, properties.
Varieties of these fabrics
There are few types of muscles in the animal body:
- cross-striped;
- smooth muscle tissue.
Both of them have their own characteristic features of the structure, functions performed and properties exhibited. In addition, they are easy to distinguish from each other. After all, both have their own unique pattern, which is formed due to the protein components that make up the cells.
Striated is also divided into two main types:
- skeletal;
- hearty.
Samothe name reflects the main areas of location in the body. Its functions are extremely important, because it is this muscle that provides the contraction of the heart, the movement of the limbs and all other moving parts of the body. However, smooth muscles are no less significant. What are its features, we will consider further.
In general, it can be seen 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 more or less significant of them.
Smooth muscle tissue: structural features
The main unusual features of the structure under consideration are 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. General features of the structure can be identified in several points.
- Each cell is surrounded by a dense plexus of connective tissue fibers that looks like a capsule.
- Each structural unit tightly adjoins the other, intercellular spaces are practically absent. This allows the entire fabric to be tightly packed, structured and strong.
- Unlike the striated colleague, this structure may include cells of different shapes.
This, of course, is not the whole characteristic that smooth muscle tissue has. Structural features, as already mentioned, lie precisely in the myocytes themselves, their functioning and composition. Therefore, this issue will be considered in more detail below.
Smooth muscle myocytes
Myocytes have different shapes. Depending on the localization in a particular organ, they can be:
- oval;
- fusiform elongated;
- round;
- 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.
Of course, and the cytoplasm with the usual inclusions is also present. An interesting fact is that smooth muscle myocytes are covered on the outside not only with a plasma membrane, but also with a membrane (basal). This provides them with an additional opportunity to contact each other.
These points of contact are features of smooth muscle tissue. The places of contact are called nexuses. It is through them, as well as through the pores that are in these places in the membrane, that impulses are transmitted between cells, the exchange of information, water molecules and other compounds.
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. That's 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.
- Secretory. Their main function is toproduction and accumulation of glycogen granules, preservation of many mitochondria, polysomes and ribosomal units. These structures got their name because of the proteins contained in them. These are actin filaments and contractile fibrin filaments. These cells are most often localized along the periphery of the tissue.
- Smooth muscle fibers. They look like spindle-shaped elongated structures containing an oval nucleus, displaced to the middle of the cell. Another name for leiomyocytes. They differ in that they are larger. 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 well-coordinated work of the indicated types of myocytes, as well as their organization, provide 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 origins. This explains the differences that the structure of smooth muscle tissue has.
- Mesenchymal origin. most of the smooth fibers have this. It is from the mesenchyme that almost all the tissues that line the inside of the hollow organs are formed.
- Epidermal origin. Samothe name speaks of the places of localization - these are all skin glands and their ducts. It is they that are formed by smooth fibers that have this variant of appearance. Sweat, salivary, milk, lacrimal - all these glands secrete their secret due to irritation of the cells of myoepitheliocytes - the structural particles of the organ in question.
- Neural origin. Such fibers are localized in one specific place - this is the iris, one of the membranes of the eye. The contraction or expansion of the pupil is innervated and controlled by these smooth muscle cells.
Despite their different origins, the internal composition and performance properties of all cell types in the tissue under consideration 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 is:
- conductivity;
- excitability;
- lability;
- contractility.
At the same time, there is one rather specific feature. If the striated skeletal muscles are able to contract rapidly (this is a good illustration of the trembling in the human body), then the smooth one can be held in a compressed state for a long time. In addition, its activities are not subject to the will and mind of man. Since it is innervated by the autonomic nervous system.
A very important property is the ability to long-term slow stretching (contraction) and the same relaxation. So, this is the basis of the work of the bladder. Under the action of a biological fluid (its filling), it is able tostretch 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 precisely 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, bundles or strands called myofibrils form in some organs in animals.
The location in the tissue of these beams 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 rapid and correct cutting of the entire tissue.
The contraction itself occurs as follows: the internal environment of the cell contains pinocytic 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 involvement of the plasmalemma and as a result, the myocyte is reduced.
Smooth muscle tissue drawing
If we talk about striated fabric, then it is easy to recognize bystriation. But with regard to the structure we are considering, this does not happen. Why does smooth muscle tissue have a completely different pattern than its close neighbor? This is due to the presence and location of protein components in myocytes. In the composition of smooth muscles, the threads of myofibrils of different nature are localized randomly, without a definite ordered state.
That's why the pattern of the fabric is simply missing. In the striated filament, actin is successively replaced by transverse myosin. The result is a pattern - striation, thanks to which the fabric got its name.
Under the microscope, the smooth tissue looks very even and ordered, thanks to the longitudinally arranged elongated myocytes that are tightly adjacent to each other.
Regions of spatial arrangement in the body
Smooth muscle tissue forms a sufficiently large number of important internal organs in the animal body. So, her educated:
- intestines;
- sexual organs;
- blood vessels of all types;
- glands;
- organs of the excretory system;
- respiratory tract;
- parts of the visual analyzer;
- organs of the digestive system.
Obviously, the localization of the tissue in question is 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 tissueforms sufficiently important structures to have the ability to regenerate. Therefore, it is characterized by two main ways of recovery from damage of various kinds.
- Mitotic division of myocytes until the required amount of tissue is formed. The most common simple and fastest way to regenerate. This is how the internal part of any organ formed by smooth muscles is restored.
- Myofibroblasts are able to transform into myocytes of smooth tissue if necessary. This is a more complex and rare way to regenerate this tissue.
Smooth muscle innervation
Smooth muscle tissue performs its functions regardless of the desire or unwillingness of a living being. This is due to the fact that its innervation is carried out by the autonomic nervous system, as well as the processes of the nerves of the ganglia (spinal).
An example of this 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 meaning of this structure? Why is smooth muscle tissue important? Its functions are as follows:
- prolonged contraction of organ walls;
- working out secrets;
- ability to respond to stimuli and excitability.
