Animal tissues perform a very important function in the organisms of living things - they form and line all organs and their systems. Of particular importance among them is precisely muscle, 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, the features of its structure, and properties.
Varieties of these fabrics
In the composition of the animal body there are few types of muscles:
- striated;
- smooth muscle tissue.
Both of them have their own characteristic features of the structure, the functions performed and the properties shown. In addition, they are easy to distinguish among themselves. After all, both of them 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:
The name itself 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 muscle is no less significant. What are its features, we will consider further.
In general, you can see that only well-coordinated work that smooth and striated muscle tissue performs 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 this structure 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 structural features can be identified in several paragraphs.
- Each cell is surrounded by a dense plexus of connective tissue fibers that looks like a capsule.
- Each structural unit is tightly adjacent to the other, intercellular spaces are practically absent. This allows the entire fabric to be tightly packed, structured and durable.
- Unlike the striated counterpart, this structure may include cells that are not uniform in shape.
This, of course, is not the whole characteristic that smooth muscle tissue has. Structural features, as already mentioned, are precisely in the myocytes themselves, their functioning and composition. Therefore, this question will be considered in more detail below.
Smooth muscle myocytes
Myocytes have a different shape. Depending on the location in a particular organ, they can be:
- oval;
- fusiform elongated;
- rounded
- process.
However, in any case, their overall composition is similar. They contain organelles such as:
- well-defined and functioning mitochondria;
- Golgi complex;
- core, often elongated in shape;
- endoplasmic reticulum;
- lysosomes.
Naturally, a cytoplasm with ordinary inclusions is also present. An interesting fact is that smooth muscle myocytes on the outside are covered not only with a plasmolemma, but also with a membrane (basal). This provides them with an additional opportunity for contact with each other.
These places of contact make up the features of smooth muscle tissue. Contact points are called nexus. 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. Therefore, the nexus is so important. So that no cell is left without innervation, and the impulse could be transmitted through the neighboring structure through the tissue.
There are two main types of myocytes.
- Secretory. Their main function is the production 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 at the periphery of the tissue.
- Smooth muscle fibers. They look like spindle-shaped elongated structures containing an oval core displaced to the middle of the cell. Another name is leiomyocytes. Differ in that they have larger sizes. Some uterine organ particles 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;
- elastanes;
- intercellular substance;
- proteoglycans.
Joint interaction and coordinated work of the indicated types of myocytes, as well as their organization, provide the structure of smooth muscle tissue.
The origin of this muscle
The source of the formation of this type of muscle in the body is not one. There are three main variants of origin. This explains the differences that the structure of smooth muscle tissue has.
- Mesenchymal origin. this has most of the smooth fibers. It is from mesenchyme that almost all the tissues lining the inner part of the hollow organs are formed.
- Epidermal origin. The name itself speaks of places of localization - these are all skin glands and their ducts. They are formed by smooth fibers having such a variant of appearance. Sweat, salivary, mammary, lacrimal - all these glands secrete their secret due to irritation of the cells of myoepithelial cells - structural particles of the organ in question.
- Neural origin. Such fibers are localized in one particular place - this is the iris, one of the membranes of the eye. The contraction or expansion of the pupil is innervated and controlled precisely by these smooth muscle cells.
Despite the different origin, the internal composition and the performed properties of all types of cells in the tissue under consideration remain approximately the same.
The main properties of this fabric
The properties of smooth muscle tissue correspond to those for striated. In this they are one. It:
- conductivity;
- excitability;
- lability;
- contractility.
At the same time, there is one rather specific feature. If the striated skeletal muscles are able to contract quickly (this illustrates well the trembling in the human body), then the smooth muscle can be kept in a compressed state for a long time. In addition, her activities are not subject to the will and mind of man. Since its autonomic nervous system innervates .
A very important property is the ability to long slow stretching (contraction) and the same relaxation. So, this is the basis of the work of the bladder. Under the influence of a biological fluid (its filling), it is able to stretch and then contract. Its walls are lined with smooth muscles.
Cell proteins
Myocytes of the tissue in question contain many different compounds. However, the most important of them, providing the fulfillment of the functions of contraction and relaxation, are precisely protein molecules. Of these, it contains:
- myosin filaments;
- actin;
- nebulin;
- Connectin
- tropomyosin.
These components are usually located in the cytoplasm of cells in isolation from each other, without forming clusters. However, bundles or cords called myofibrils form in some organs in animals.
The location in the tissue of these bundles 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 quick and proper contraction of the entire tissue.
The reduction itself occurs as follows: in the composition of the internal environment of the cell there are pinocytotic vesicles, which necessarily contain calcium ions. When a nerve impulse arrives, indicating the need for contraction, this vesicle approaches the fibril. As a result, the calcium ion irritates actin and it moves deeper between the myosin strands. This leads to the involvement of the plasmalemma and as a result, the myocyte contracts.
Smooth muscle tissue: pattern
If we talk about striated fabric, then it is easy to recognize by striation. But as for the structure under consideration, this does not happen. Why does smooth muscle tissue have a completely different pattern than a neighbor close to it? This is due to the presence and location of protein components in myocytes. As part of the smooth muscles, the filaments of myofibrils of various nature are localized randomly, without a certain ordered state.
That is why the fabric pattern is simply missing. In a striated actin filament, 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 orderly, thanks to the longitudinally located elongated elongated myocytes.
Spatial Areas in the Body
Smooth muscle tissue forms a fairly large number of important internal organs in the animal body. So, she is educated:
- intestines;
- genitals
- blood vessels of all types;
- glands;
- organs of the excretory system;
- Airways;
- parts of the visual analyzer;
- digestive organs.
Obviously, the localization sites of the tissue in question are extremely diverse and important. In addition, it should be noted that such muscles mainly form those organs that are subject to automatic control.
Recovery Methods
Smooth muscle tissue forms quite important structures that have the ability to regenerate. Therefore, it is characterized by two main ways of recovery from injuries of various kinds.
- Mitotic division of myocytes to the formation of the desired amount of tissue. The most common simple and fast way to regenerate. Thus, the restoration of the internal part of any organ formed by smooth muscles occurs.
- Myofibroblasts are able to transform into smooth tissue myocytes if necessary. This is a more complex and rarely seen way of regenerating this tissue.
Smooth muscle innervation
Smooth muscle tissue performs its functions regardless of the desire or unwillingness of a living creature. This is due to the fact that its innervation is carried out by the autonomic nervous system, as well as processes of the nerves of the ganglia (spinal).
An example of this and proof is the reduction or increase in the size of the stomach, liver, spleen, distension and contraction of the bladder.
Smooth muscle function
What is the significance of this structure? Why is smooth muscle tissue needed ? Its functions are as follows:
- prolonged contraction of the walls of organs;
- making secrets;
- ability to respond to irritations and irritability.