In this article we will look at the types of muscle tissue. This is a very important topic in biology, because everyone should know how our muscles function. They represent a complex system, which we hope you will find interesting to study. And the pictures that you will find in this article will help you better imagine the types of muscle tissue. First of all, we will give a definition that is necessary when studying this topic.
This is a special group of animals, the main function of which is its contraction, causing the movement of the organism or its constituent parts in space. This function corresponds to the structure of the basic elements that make up various types of muscle tissue. These elements have a longitudinal and elongated orientation of myofibrils, which include myosin and actin. Muscle tissue, like epithelial tissue, is a composite tissue group, since its main elements develop from embryonic rudiments.
Contraction of muscle tissue
Its cells, like nerve cells, can be excited when exposed to electrical and chemical impulses. Their ability to contract (shorten) in response to a particular stimulus is associated with the presence of myofibrils, special protein structures, each of which consists of microfilaments, short protein fibers. In turn, they are divided into myosin (thicker) and actin (thin) fibers. In response to nervous stimulation, various types of muscle tissue contract. Contraction to the muscle is transmitted along the nerve process through the neurotransmitter, which is acetylcholine. Muscle cells in the body perform energy-saving functions, since the energy consumed during the contraction of various muscles is then released in the form of heat. That is why, when the body is exposed to cooling, trembling occurs. This is nothing more than frequent muscle contractions.
The following types of muscle tissue can be distinguished, depending on the structure of the contractile apparatus: smooth and striated. They consist of histogenetic types that differ in structure.
Muscle tissue is striated
Myotome cells, which are formed from the dorsal mesoderm, are the source of its development. This fabric consists of elongated cylinders, the ends of which are pointed. These formations reach 12 cm in length and 80 microns in diameter. Symplasts (multinuclear formations) are contained in the center of muscle fibers. Adjacent to them are cells called “myosatellites”. The sarcolemma is limited by the fibers. It is formed by the plasmolemma simplast and the basement membrane. Myosatelliotocytes are located under the basement membrane of the fiber - so that the plasmalemma simplast touches their plasmalemma. These cells are the cambial reserve of muscle skeletal tissue, and it is due to it that fiber regeneration occurs. Myosymplasts, in addition to the plasmalemma, also include sarcoplasm (cytoplasm) and numerous nuclei located along the periphery.
The importance of striated muscle tissue
When describing the types of muscle tissue, it should be noted that striated muscle tissue is the executive apparatus of the entire motor system. It forms In addition, this type of tissue is included in the structure of internal organs, such as the pharynx, tongue, heart, upper esophagus, etc. Its total mass in an adult is up to 40% of body weight, and in elderly people, as well as newborns , its share is 20-30%.
Features of striated muscle tissue
Contraction of this type of muscle tissue, as a rule, can be done with the participation of consciousness. It is slightly faster than the smooth one. As you can see, the types of muscle tissue differ (we will talk about smooth tissue very soon and note some other differences between them). In striated muscles, nerve endings perceive information about the current state of muscle tissue, and then transmit it along afferent fibers to the nerve centers responsible for the regulation of motor systems. Control signals come from regulators in the form of nerve impulses along motor or autonomic efferent nerve fibers.
Smooth muscle tissue
Continuing to describe the types of human muscle tissue, we move on to smooth tissue. It is formed by spindle-shaped cells, the length of which ranges from 15 to 500 microns, and the diameter ranges from 2 to 10 microns. Unlike striated muscle fibers, these cells have one nucleus. In addition, they do not have transverse striations.
The importance of smooth muscle tissue
The functioning of all body systems depends on the contractile function of this type of muscle tissue, since it is part of the structure of each of them. For example, smooth muscle tissue is involved in controlling the diameter of the respiratory tract, blood vessels, contraction of the uterus, bladder, and in the implementation of the motor functions of our digestive tract. It controls the diameter of the pupil of the eyes, and is also involved in many other functions of various body systems.
Muscle layers
This type of tissue forms muscle layers in the walls of lymphatic and blood vessels, as well as all hollow organs. Usually this is two or three layers. The thick circular one is the outer layer, the middle one is not necessarily present, the thin longitudinal one is the inner one. The blood vessels supplying the muscle tissue, as well as the nerves, run parallel to the axis of the muscle cells between their bundles. Smooth muscle cells can be divided into 2 types: unitary (united, grouped) and autonomous myocytes.
Autonomous myocytes
Autonomous cells function quite independently of each other, since each such cell is innervated by a nerve ending. They were found in the muscle layers of large blood vessels, as well as in the ciliary muscle of the eye. Also of this type are the cells that make up the muscles that lift the hair.
Unitary myocytes
Unitary muscle cells, on the contrary, are closely intertwined with each other, so that their membranes can not only adhere tightly to each other, forming desmosomes, but also merge, forming nexuses (gap junctions). Bundles are formed as a result of this combination. Their diameter is about 100 microns, and their length reaches several mm. They form a network and are woven into its cells. Fibers of autonomic neurons are innervated by bundles, and they become functional units of smooth muscle tissue. Depolarization upon excitation of one cell of the beam spreads very quickly to neighboring ones, since the resistance of the gap junctions is low. Tissues consisting of unitary cells are found in most organs. These include the ureters, uterus, and digestive tract.
Myocyte contraction
The contraction of myocytes is caused in smooth tissue, as in striated tissue, by the interaction of myosin and actin filaments. This is similar to the different types of muscle tissue in humans. These threads are distributed less orderly within the myoplasm than in the striated muscle. This is due to the lack of transverse striations in smooth muscle tissue. Intracellular calcium is the final executive link that controls the interaction of myosin and actin filaments (that is, the contraction of myocytes). The same applies to the striated muscle. However, the details of the control mechanism differ significantly from the latter.
The vegetative axons passing through the very thickness of the smooth muscle tissue do not form synapses, which is typical for striated tissue, but numerous thickenings along the entire length, which play the role of synapses. The thickenings release a transmitter that diffuses to nearby myocytes. Receptor molecules are located on the surface of these myocytes. The mediator interacts with them. It causes depolarization of the outer membrane of the myocyte.
Features of smooth muscle tissue
The nervous system, its autonomic department, is controlled without the participation of consciousness by the work of smooth muscles. The bladder muscles are the only exception. Control signals are either directly implemented or indirectly through hormonal (chemical, humoral) influences.
The energetic and mechanical properties of this type of muscle tissue ensure the maintenance of (controlled) tone of the walls of hollow organs and blood vessels. This is due to the fact that smooth tissue functions efficiently and does not require large amounts of ATP. It has a lower speed of action than striated muscle tissue, but it is capable of contracting for a longer time, in addition, it can develop significant tension and change its length over a wide range.
So, we looked at the types of muscle tissue and the features of their structural organization. Of course, this is just basic information. You can describe the types of muscle tissue for a long time. The pictures will help you visualize them.
Muscle tissues are specialized tissues whose primary function is contraction. Thanks to them, all motor processes in the body are ensured (hemocirculation in the vessels, rhythmic activity of the myocardium, peristalsis of the digestive tract and others, as well as movement of the body in space). Contraction of the structural elements of muscle tissue is carried out with the help of special organelles - myofibrils - and is the result of the interaction of contractile protein molecules.
There are two classifications muscle tissue – morphofunctional and genetic. According to the first classification, muscle tissue is divided into two groups: 1) smooth(non-striated) muscle tissue, which is characterized by containing myofibrils that do not have cross-striations; 2) striated(striated) muscle tissue, the myofibrils of which form transverse striations. In turn, it is divided into skeletal And cardiac. According to genetic classification (by origin), muscle tissue is divided into 5 types: 1) mesenchymal(develop from mesenchyme and are found in internal organs and blood vessels); 2) epidermal(develop from the skin ectoderm, include non-muscle contracting cells - myoepithelial cells of the sweat, mammary, salivary and lacrimal glands); 3) neural(develop from the neural tube, smooth myocytes of the muscles of the iris belong to them); 4) somatic(develop from mesoderm myotomes and form skeletal muscle tissue); 5) coelomic(develop from the visceral layer of the splanchnotome and form cardiac muscle tissue). The first three types are smooth muscle tissues, the rest are striated. General structural features characteristic of muscle tissue include the presence of: 1) special organelles - myofibrils, due to the interaction of their contractile proteins, contraction occurs; 2) a developed trophic apparatus that ensures the contractile function - mitochondria, smooth endoplasmic reticulum, inclusions of glycogen and myoglobin; 3) a developed supporting apparatus in the form of a two-layer shell with a surrounding network of connective tissue fibers.
Smooth muscle tissue
Smooth muscle tissue of mesenchymal origin is located in the wall of internal organs and blood vessels. Its structural unit is smooth myocyte. This is a spindle-shaped, sometimes process-shaped cell (uterus, endocardium, aorta), 20-500 µm long, with a centrally located nucleus (Fig. 7-1). The cytolemma of the smooth myocyte forms numerous invaginations - caveolae(small bubbles). The outside of the cytolemma is covered by a thin basement membrane. In the basement membrane of each myocyte there are openings where cells contact each other using nexuses that carry out metabolic connections.
Organelles of general importance - the Golgi complex, mitochondria, free ribosomes, sarcoplasmic reticulum - are localized mainly near the poles of the nucleus. The most developed and numerous of them are mitochondria. The sarcoplasmic reticulum is involved in the synthesis of glycosaminoglycans and protein molecules, from which the components of the basement membrane, fibers, and amorphous substance surrounding the cells are assembled. The synthetic capacity of definitive myocytes decreases. Long narrow tubes of the smooth sarcoplasmic reticulum are adjacent to the caveolae and, together with them, serve to deposit calcium ions.
Special organelles are visible in the form of filaments, oriented predominantly along the long axis of the cell and without transverse striations. In the cytoplasm of myocytes, only thin filaments are consistently detected - myofilaments, consisting of the protein actin. They attach to the inner side of the cytolemma, forming dense bodies consisting of the protein actinin. When the cell membrane potential changes, calcium ions coming from the depot activate the assembly of myosin (thicker) filaments and their interaction with actin filaments. As actin-myosin bridges form, the actin myofilaments shift towards each other, the thrust is transmitted to the cytolemma, and the cell shortens. When calcium levels decrease, myosin loses its affinity for actin. As a result, the myocyte relaxes and the myosin filaments disassemble. The contraction is slow, tonic.
Rice. 7-1. Smooth muscle cell.
1. Mitochondria.
2. Basement membrane.
3. Dense bodies.
4. Zone of gap contacts.
5. Actin myofilaments.
6. Core.
7. Caveolae.
(After Lentz T. L. 1971).
Innervation smooth muscle tissue is carried out by the autonomic nervous system - sympathetic and parasympathetic nerve fibers, the terminals of which form varicose veins on smooth muscle cells. Smooth myocytes do not function in isolation, but in cellular complexes. Cells contact each other using nexuses. The latter contribute to the conduction of excitation from cell to cell, immediately covering a group of myocytes. The complexes also contain pacemaker myocytes, which themselves generate an action potential and transmit it to neighboring cells.
A mesh is formed around each smooth myocyte from reticular, elastic and collagen fibers - endomysium. Groups of 10-12 cells are combined into muscle layers, surrounded by connective tissue with blood vessels and nerves, called perimysium. In organs, bundles of muscle cells form layers of muscle tissue. The combination of bundles forms a muscle, which is surrounded by a thicker layer of connective tissue - epimysium. With increased functional load, smooth myocytes hypertrophy, as, for example, in the uterus during pregnancy, exhibiting a high ability for physiological regeneration. During reparative regeneration, restoration is possible due to the division of poorly differentiated myocytes, which are part of muscle complexes, as well as from adventitial cells and myofibroblasts.
In vertebrates and humans there are three different muscle groups:
- striated muscles of the skeleton;
- striated muscle of the heart;
- smooth muscles of internal organs, blood vessels and skin.
Rice. 1. Types of human muscles
Smooth muscle
Of the two types of muscle tissue (striated and smooth), smooth muscle tissue is at a lower stage of development and is characteristic of lower animals.
They form the muscular layer of the walls of the stomach, intestines, ureters, bronchi, blood vessels and other hollow organs. They consist of spindle-shaped muscle fibers and do not have transverse striations, since the myofibrils in them are located less orderly. In smooth muscles, individual cells are connected to each other by special sections of outer membranes - nexuses. Due to these contacts, action potentials propagate from one muscle fiber to another. Therefore, the entire muscle is quickly involved in the excitation reaction.
Smooth muscles carry out movements of internal organs, blood and lymphatic vessels. In the walls of internal organs, they are usually located in the form of two layers: the inner annular and the outer longitudinal. They form spiral-shaped structures in the walls of the artery.
A characteristic feature of smooth muscles is their ability to spontaneous automatic activity (muscles of the stomach, intestines, gallbladder, ureters). This property is regulated by nerve endings. Smooth muscles are plastic, i.e. are able to maintain the length given by stretching without changing the tension. Skeletal muscle, on the contrary, has low plasticity and this difference can be easily established in the following experiment: if you stretch both smooth and striated muscles with the help of weights and remove the load, then the skeletal muscle immediately shortens to its original length, and the smooth muscle takes a long time may be in a stretched state.
This property of smooth muscles is of great importance for the functioning of internal organs. It is the plasticity of smooth muscles that ensures only a slight change in pressure inside the bladder when it is filled.
Rice. 2. 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.
Smooth muscle has the same basic properties as striated skeletal muscle, but also some special properties:
- automation, i.e. the ability to contract and relax without external irritation, but due to excitations that arise within themselves;
- high sensitivity to chemical irritants;
- pronounced plasticity;
- contraction in response to rapid stretch.
The contraction and relaxation of smooth muscles occurs slowly. This contributes to the onset of peristaltic and pendulum-like movements of the digestive tract organs, which leads to the movement of the food bolus. Prolonged contraction of smooth muscles is necessary in the sphincters of hollow organs and prevents the release of contents: bile in the gallbladder, urine in the bladder. The contraction of smooth muscle fibers occurs regardless of our desire, under the influence of internal reasons not subordinate to consciousness.
Striated muscles
Striated muscles are located on the bones of the skeleton and contraction sets individual joints and the entire body in motion. They form a body, or soma, which is why they are also called somatic, and the system that innervates them is the somatic nervous system.
Thanks to the activity of skeletal muscles, the body moves in space, the varied work of the limbs, the expansion of the chest during breathing, the movement of the head and spine, chewing, and facial expressions. There are more than 400 muscles. The total muscle mass makes up 40% of the weight. Typically, the middle part of the muscle consists of muscle tissue and forms the belly. The ends of the muscles - tendons - are built from dense connective tissue; they are connected to the bones using the periosteum, but can also attach to other muscles and to the connective layer of the skin. In a muscle, muscle and tendon fibers are combined into bundles using loose connective tissue. Nerves and blood vessels are located between the bundles. proportional to the number of fibers making up the muscle belly.
Rice. 3. Functions of muscle tissue
Some muscles pass through only one joint and, when contracted, cause it to move—single-joint muscles. Other muscles pass through two or more joints - multi-joint muscles, they produce movement in several joints.
As the ends of the muscles attached to the bones move closer to each other, the size of the muscle (length) decreases. Bones connected by joints act as levers.
By changing the position of the bone levers, the muscles act on the joints. In this case, each muscle affects the joint in only one direction. A uniaxial joint (cylindrical, trochlear) has two muscles or groups of muscles acting on it, which are antagonists: one muscle is a flexor, the other is an extensor. At the same time, each joint is acted in one direction, as a rule, by two or more muscles, which are synergists (synergism is a joint action).
In a biaxial joint (ellipsoidal, condyle, saddle-shaped) the muscles are grouped according to its two axes around which movements are performed. To a ball-and-socket joint, which has three axes of movement (multi-axial joint), muscles are adjacent on all sides. For example, in the shoulder joint there are flexor and extensor muscles (movements around the frontal axis), abductors and adductors (sagittal axis) and rotators around the longitudinal axis, inward and outward. There are three types of muscle work: overcoming, yielding and holding.
If, due to muscle contraction, the position of a body part changes, then the resistance force is overcome, i.e. overcoming work is performed. Work in which the muscle force yields to the action of gravity and the load being held is called yielding. In this case, the muscle functions, but it does not shorten, but lengthens, for example, when it is impossible to lift or support a body that has a large mass. With great muscle effort, you have to lower this body onto some surface.
Holding work is performed due to muscle contraction; the body or load is held in a certain position without moving in space, for example, a person holds a load without moving. In this case, the muscles contract without changing length. The force of muscle contraction balances the weight of the body and the load.
When a muscle, contracting, moves the body or its parts in space, they perform overcoming or yielding work, which is dynamic. Statistical work is holding work, in which there is no movement of the whole body or part of it. The mode in which the muscle can freely shorten is called isotonic(there is no change in muscle tension and only its length changes). The condition in which the muscle cannot shorten is called isometric- only the tension of the muscle fibers changes.
Rice. 4. Human muscles
The structure of striated muscles
Skeletal muscles consist of a large number of muscle fibers, which are combined into muscle bundles.
One bundle contains 20-60 fibers. Muscle fibers are cylindrical cells 10-12 cm long and 10-100 microns in diameter.
Each muscle fiber has a membrane (sarcolemma) and cytoplasm (sarcoplasm). Sarcoplasm contains all the components of an animal cell and thin filaments are located along the axis of the muscle fiber - myofibrils, Each myofibril consists of protofibrils, which include threads of the proteins myosin and actin, which are the contractile apparatus of muscle fiber. Myofibrils are separated from each other by partitions called Z-membranes into sections - sarcomeres. At both ends of the sarcomeres, thin actin filaments are attached to the Z-membrane, and thick myosin filaments are located in the middle. The ends of the actin filaments partially fit between the myosin filaments. In a light microscope, myosin filaments appear as a light stripe in a dark disk. Under electron microscopy, skeletal muscles appear striated (cross-striped).
Rice. 5. Cross bridges: Ak - actin; Mz - myosin; Gl - head; Ш - neck
On the sides of the myosin filament there are projections called cross bridges(Fig. 5), which are located at an angle of 120° relative to the axis of the myosin filament. Actin filaments appear as a double filament twisted into a double helix. In the longitudinal grooves of the actin helix there are filaments of the protein tropomyosin, to which the protein troponin is attached. In the resting state, tropomyosin protein molecules are arranged in such a way as to prevent the attachment of myosin cross bridges to actin filaments.
Rice. 6. A - organization of cylindrical fibers in skeletal muscle attached to bones by tendons. B - structural organization of filaments in a skeletal muscle fiber, creating a pattern of transverse stripes.
Rice. 7. Structure of actin and myosin
In many places, the surface membrane deepens in the form of microtubes inside the fiber, perpendicular to its longitudinal axis, forming a system transverse tubules(T-system). Parallel to the myofibrils and perpendicular to the transverse tubules between the myofibrils there is a system longitudinal tubules(sarcoplasmic reticulum). The terminal extensions of these tubes are terminal tanks - come very close to the transverse tubules, forming together with them so-called triads. The bulk of intracellular calcium is concentrated in the cisterns.
Mechanism of skeletal muscle contraction
Muscle consists of cells called muscle fibers. Outside, the fiber is surrounded by a sheath - the sarcolemma. Inside the sarcolemma is the cytoplasm (sarcoplasm), which contains nuclei and mitochondria. It contains a huge number of contractile elements called myofibrils. Myofibrils run from one end of a muscle fiber to the other. They exist for a relatively short period of time - about 30 days, after which they are completely replaced. Intense protein synthesis occurs in the muscles, which is necessary for the formation of new myofibrils.
Muscle fiber contains a large number of nuclei, which are located directly under the sarcolemma, since the main part of the muscle fiber is occupied by myofibrils. It is the presence of a large number of nuclei that ensures the synthesis of new myofibrils. Such a rapid change of myofibrils ensures high reliability of the physiological functions of muscle tissue.
Rice. 7. A - diagram of the organization of the sarcoplasmic reticulum, transverse tubules and myofibrils. B - diagram of the anatomical structure of the transverse tubules and sarcoplasmic reticulum in an individual skeletal muscle fiber. B - the role of the sarcoplasmic reticulum in the mechanism of skeletal muscle contraction
Each myofibril consists of regularly alternating light and dark areas. These areas, having different optical properties, create transverse striations in the muscle tissue.
In skeletal muscle, contraction is caused by the arrival of an impulse along a nerve. The transmission of a nerve impulse from a nerve to a muscle occurs through the neuromuscular synapse (contact).
A single nerve impulse, or single irritation, leads to an elementary contractile act - a single contraction. The onset of contraction does not coincide with the moment of application of irritation, since there is a hidden, or latent, period (the interval between the application of irritation and the beginning of muscle contraction). During this period, the development of the action potential, the activation of enzymatic processes and the breakdown of ATP occur. After this the contraction begins. The breakdown of ATP in muscle leads to the conversion of chemical energy into mechanical energy. Energy processes are always accompanied by the release of heat and thermal energy is usually intermediate between chemical and mechanical energies. In muscle, chemical energy is converted directly into mechanical energy. But heat in the muscle is formed both due to the shortening of the muscle and during its relaxation. The heat generated in the muscles plays a large role in maintaining body temperature.
Unlike the heart muscle, which has the property of automation, i.e. it is capable of contracting under the influence of impulses arising within itself, and unlike smooth muscles, which are also capable of contracting without receiving signals from the outside, skeletal muscle contracts only when signals from outside are received by it. Signals to muscle fibers are directly transmitted through the axons of motor cells located in the anterior horns of the gray matter of the spinal cord (motoneurons).
Reflex nature of muscle activity and coordination of muscle contractions
Skeletal muscles, unlike smooth muscles, are capable of performing voluntary rapid contractions and thereby producing significant work. The working element of a muscle is muscle fiber. A typical muscle fiber is a structure with several nuclei, pushed to the periphery by a mass of contractile myofibrils.
Muscle fibers have three main properties:
- excitability - the ability to respond to the actions of a stimulus by generating an action potential;
- conductivity - the ability to conduct an excitation wave along the entire fiber in both directions from the point of irritation;
- contractility - the ability to contract or change tension when excited.
In physiology, there is the concept of a motor unit, which means one motor neuron and all the muscle fibers that this neuron innervates. Motor units vary in size: from 10 muscle fibers per unit for muscles that perform precise movements, to 1000 or more fibers per motor unit for “power-oriented” muscles. The nature of the work of skeletal muscles can be different: static work (maintaining a posture, holding a load) and dynamic work (moving the body or load in space). Muscles are also involved in the movement of blood and lymph in the body, the production of heat, the acts of inhalation and exhalation, they are a kind of depot for water and salts, and they protect internal organs, for example, the muscles of the abdominal wall.
Skeletal muscle is characterized by two main modes of contraction - isometric and isotonic.
The isometric mode manifests itself in the fact that tension increases in the muscle during its activity (force is generated), but due to the fact that both ends of the muscle are fixed (for example, when trying to lift a very large load), it does not shorten.
The isotonic regime is manifested in the fact that the muscle initially develops tension (force) capable of lifting a given load, and then the muscle shortens - changes its length, maintaining tension equal to the weight of the load being held. It is practically impossible to observe a purely isometric or isotonic contraction, but there are techniques of so-called isometric gymnastics, when the athlete tenses the muscles without changing the length. These exercises develop muscle strength to a greater extent than exercises with isotonic elements.
The contractile apparatus of skeletal muscle is represented by myofibrils. Each myofibril with a diameter of 1 micron consists of several thousand protofibrils - thin, elongated polymerized molecules of the proteins myosin and actin. Myosin filaments are twice as thin as actin filaments, and in the resting state of the muscle fiber, actin filaments fit in loose rings between the myosin filaments.
In the transmission of excitation, calcium ions play an important role, which enter the interfibrillar space and trigger the contraction mechanism: mutual retraction of actin and myosin filaments relative to each other. Retraction of the threads occurs with the obligatory participation of ATP. In active centers located at one end of the myosin filaments, ATP is broken down. The energy released during the breakdown of ATP is converted into movement. In skeletal muscles, the ATP reserve is small - only enough for 10 single contractions. Therefore, constant re-synthesis of ATP is necessary, which occurs in three ways: first, through creatine phosphate reserves, which are limited; the second is the glycolytic pathway during the anaerobic breakdown of glucose, when two molecules of ATP are formed for one molecule of glucose, but at the same time lactic acid is formed, which inhibits the activity of glycolytic enzymes, and finally the third is the aerobic oxidation of glucose and fatty acids in the Krebs cycle, which occurs in mitochondria and forms 38 ATP molecules per 1 glucose molecule. The last process is the most economical, but very slow. Constant training activates the third oxidation pathway, resulting in increased muscle endurance for long-term exercise.
Muscle tissues are classified into smooth and striated or striated. Striated is divided into skeletal and cardiac. Depending on their origin, muscle tissue is divided into 5 types:
mesenchymal (smooth muscle tissue);
epidermal (smooth muscle tissue);
neural (smooth muscle tissue);
coelomic (cardiac);
somatic or myotome (skeletal striated).
SMOOTH MUSCLE TISSUE DEVELOPING FROM SPLANCHNOTOMIC MESENCHYME
localized in the walls of hollow organs (stomach, blood vessels, respiratory tract, etc.) and non-hollow organs (in the muscle of the ciliary body of the eye of mammals). Smooth muscle cells develop from mesenchymocytes that lose their processes. They develop the Golgi complex, mitochondria, granular ER and myofilaments. At this time, type V collagen is actively synthesized on the granular EPS, due to which a basement membrane is formed around the cell. With further differentiation, organelles of general importance atrophy, the synthesis of collagen molecules in the cell decreases, but the synthesis of contractile myofilament proteins increases.
STRUCTURE OF SMOOTH MUSCLE TISSUE. It consists of smooth myocytes, spindle-shaped, with a length of 20 to 500 microns. with a diameter of 6-8 microns. Externally, myocytes are covered with plasmalemma and basement membrane.
Myocytes are closely adjacent to each other. There are contacts between them - nexuses. In the place where there are nexuses, there are holes in the basement membrane of the myocyte membrane. At this point, the plasmalemma of one myocyte approaches the plasmalemma of another myocyte at a distance of 2-3 nm. Through the nexuses, ions are exchanged, water molecules are transported, and the contractile impulse is transmitted.
On the outside, myocytes are covered with type V collagen, which forms the exocytoskeleton of the cell. The cytoplasm of myocytes is stained oxyphilic. It contains poorly developed organelles of general importance: granular ER, Golgi complex, smooth ER, cell center, lysosomes. These organelles are located at the poles of the nucleus. Well-developed organelles are mitochondria. Cores have a rod-shaped form.
Myocytes have well-developed myofilaments, which are the contractile apparatus of the cells. Among the myofilaments there are
thin, actin, consisting of actin protein;
thick myosin, consisting of the contractile protein myosin, which appear only after an impulse arrives to the cell;
intermediate filaments consisting of connectin and nebulin.
There is no striation in myocytes because all of the above filaments are arranged in a disorderly manner.
ACTIN Filaments connect to each other and to the plasmalemma using dense bodies. In those places where they connect to each other, the bodies contain alpha-actinin; in those places where the filaments connect to the plasmalemma, the bodies contain vinculin. The arrangement of actin filaments is predominantly longitudinal, but they can be located at an angle relative to the longitudinal axis. Myosin filaments are also located predominantly longitudinally. The filaments are arranged so that the ends of the actin filaments are located between the ends of the myosin filaments.
FUNCTION OF FILAMENTS- contractile. The contraction process is carried out as follows: after the arrival of the contractile impulse, pinocytosis vesicles containing calcium ions approach the filaments; Calcium ions trigger the contractile process, which involves the ends of actin filaments moving deeper between the ends of myosin filaments. The traction force is applied to the plasmalemma, to which actin filaments are connected using dense bodies, as a result of which the myocyte contracts.
FUNCTIONS OF MYOCYTES: 1) contractile (ability for long-term contraction); 2) secretory (they secrete type V collagen, elastin, proteoglycans, since they have granular EPS).
REGENERATION smooth muscle tissue is carried out in 2 ways: 1) mitotic division of myocytes; 2) transformation of myofibroblasts into smooth myocytes.
STRUCTURE OF SMOOTH MUSCLE TISSUE AS AN ORGAN. In the wall of hollow organs, smooth myocytes form bundles. These bundles are surrounded by layers of loose connective tissue called perimysium. The layer of connective tissue around the entire layer of muscle tissue is called epimysium. The perimysium and epimysium contain blood and lymphatic vessels and nerve fibers.
INNERVATION OF SMOOTH MUSCLE TISSUE carried out by the autonomic nervous system, therefore contractions of smooth muscles do not obey the will of the person (involuntary). Sensory (afferent) and motor (efferent) nerve fibers approach smooth muscle tissue. Efferent nerve fibers end in motor nerve endings in the connective tissue layer. When an impulse arrives, mediators are released from the endings, which, spreading diffusely, reach the myocytes, causing them to contract.
SMOOTH MUSCLE TISSUE OF EPIDERMAL ORIGIN located in the terminal sections and small ducts of the glands that develop from the skin ectoderm (salivary, sweat, mammary and lacrimal glands). Smooth myocytes (myoepitheliocytes) are located between the basal surface of glandular cells and the basement membrane, covering the basal part of the glandulocytes with their processes. When these processes contract, the basal part of the glandulocytes is compressed, causing secretion to be released from the glandular cells.
SMOOTH MUSCLE TISSUE OF NEURAL ORIGIN develops from optic cups growing from the neural tube. This muscle tissue forms only 2 muscles located in the iris of the eye: the constrictor pupillary muscle and the dilator pupillary muscle. It is believed that the muscles of the iris develop from neuroglia.
STRIPED SKELETAL MUSCLE TISSUE develops from the myotomes of mesodermal somites, and is therefore called somatic. Myotome cells differentiate in two directions: 1) from some, myosatellite cells are formed; 2) myosymplasts are formed from others.
FORMATION OF MYOSYMPLASTS. Myotome cells differentiate into myoblasts, which fuse together to form myotubes. During the process of maturation, myotubes transform into myosymplasts. In this case, the nuclei are shifted to the periphery, and the myofibrils - to the center.
STRUCTURE OF MUSCLE FIBER. Muscle fiber (miofibra) consists of 2 components: 1) myosatellite cells and 2) myosymplast. The muscle fiber is approximately the same length as the muscle itself, with a diameter of 20-50 microns. The fiber is covered on the outside with a sheath - sarcolemma, consisting of 2 membranes. The outer membrane is called the basement membrane, and the inner membrane is called the plasmalemma. Between these two membranes are myosatellite cells.
MUSCLE FIBER NUCLEI are located under the plasmalemma, their number can reach several tens of thousands. They have an elongated shape and do not have the ability for further mitotic division. The CYTOPLASM of a muscle fiber is called SARCOPLASMA. The sarcoplasm contains a large amount of myoglobin, glycogen inclusions and lipids; There are organelles of general importance, some of which are well developed, others less well developed. Organelles such as the Golgi complex, granular ER, and lysosomes are poorly developed and are located at the poles of the nuclei. Mitochondria and smooth ER are well developed.
In muscle fibers, myofibrils are well developed, which are the contractile apparatus of the fiber. Myofibrils have striations because the myofilaments in them are arranged in a strictly defined order (unlike smooth muscle). There are 2 types of myofilaments in myofibrils: 1) thin actin, consisting of actin protein, troponin and tropomyosin; 2) thick myosin consists of the protein myosin. Actin filaments are arranged longitudinally, their ends are at the same level and extend somewhat between the ends of the myosin filaments. Around each myosin filament there are 6 actin filament ends. The muscle fiber has a cytoskeleton, including intermediate filaments, telophragm, mesophragm, and sarcolemma. Thanks to the cytoskeleton, identical myofibril structures (actin, myosin filaments, etc.) are arranged in an orderly manner.
That part of the myofibril in which only actin filaments are located is called disk I (isotropic or light disk). A Z-stripe, or telophragm, about 100 nm thick and consisting of alpha-actinin, passes through the center of disk I. Actin filaments are attached to the telophragm (the zone of attachment of thin filaments).
Myosin filaments are also arranged in a strictly defined order. Their ends are also at the same level. Myosin filaments, together with the ends of actin filaments extending between them, form disk A (an anisotropic disk with birefringence). Disc A is also divided by the mesophragm, which is similar to the telophragm and consists of M protein (myomysin).
In the middle part of disk A there is an H-stripe, bounded by the ends of actin filaments that extend between the ends of the myosin filaments. Therefore, the closer the ends of the actin filaments are located to each other, the narrower the H-band.
SARCOMER is a structural and functional unit of myofibrils, which is a section located between two telophragms. Sarcomere formula: 1.5 disks I + disk A + 1.5 disks I. Myofibrils are surrounded by well-developed mitochondria and well-developed smooth ER.
SMOOTH EPS forms a system of L-tubules that form complex structures in each disc. These structures consist of L-tubules located along the myofibrils and connecting to transversely directed L-tubules (lateral cisterns). FUNCTIONS of smooth ER (L-tubule system): 1) transport; 2) synthesis of lipids and glycogen; 3) deposition of calcium ions.
T-CHANNELS- these are invaginations of the plasmalemma. At the border of the disks from the plasma membrane deep into the fiber, an invagination occurs in the form of a tube located between two lateral cisterns.
TRIAD includes: 1) T-canal and 2) 2 lateral cisterns of smooth EPS. THE FUNCTION OF TRIADS is that in the relaxed state of myofibrils, calcium ions accumulate in the lateral cisterns; at the moment when an impulse (action potential) moves along the plasmalemma, it passes to the T-channels. When an impulse moves along the T-channel, calcium ions come out of the lateral cisterns. Without calcium ions, contraction of myofibrils is impossible, because in actin filaments the centers of interaction with myosin filaments are blocked by tropomyosin. Calcium ions unblock these centers, after which the interaction of actin filaments with myosin filaments begins and contraction begins.
MECHANISM OF MYOFIBRILL CONTRACTION. When actin filaments interact with myosin filaments, Ca ions unblock the adhesion centers of actin filaments with the heads of myosin molecules, after which these outgrowths attach to the adhesion centers on the actin filaments and, like a paddle, carry out the movement of actin filaments between the ends of the myosin filaments. At this time, the telophragm approaches the ends of the myosin filaments, since the ends of the actin filaments also approach the mesophragm and each other, and the H-stripe narrows. Thus, during myofibril contraction, disc I and the H-stripe narrow. After the termination of the action potential, calcium ions return to the L-tubules of the smooth ER, and tropomyosin again blocks the centers of interaction with myosin filaments in actin filaments. This leads to the cessation of contraction of myofibrils, their relaxation occurs, i.e. actin filaments return to their original position, the width of disk I and the H-band is restored.
MYOSATELLITOCYTES muscle fibers are located between the basement membrane and the plasmalemma of the sarcolemma. These cells are oval in shape, their oval nucleus is surrounded by a thin layer of organelle-poor and weakly stained cytoplasm. FUNCTION of myosatellite cells- these are cambial cells involved in the regeneration of muscle fibers when they are damaged.
STRUCTURE OF MUSCLE AS AN ORGAN . Each muscle of the human body is a unique organ with its own structure. Each muscle is made up of muscle fibers. Each fiber is surrounded by a thin layer of loose connective tissue - endomysium. Blood and lymphatic vessels and nerve fibers pass through the endomysium. The muscle fiber together with blood vessels and nerve fibers is called "myon". Several muscle fibers form a bundle surrounded by a layer of loose connective tissue called perimysium. The entire muscle is surrounded by a layer of connective tissue called the epimysium.
CONNECTION OF MUSCLE FIBERS WITH COLLAGEN FIBERS OF TENDON.
At the ends of the muscle fibers there are invaginations of the sarcolemma. These invaginations include collagen and reticular fibers of the tendons. Reticular fibers pierce the basement membrane and, using molecular linkages, connect to the plasmalemma. Then these fibers return to the lumen of the invagination and braid the collagen fibers of the tendon, as if tying them to the muscle fiber. Collagen fibers form tendons that attach to the bone skeleton.
TYPES OF MUSCLE FIBERS. There are 2 main types of muscle fibers:
Type I (red fibers) and type II (white fibers). They differ mainly in the speed of contraction, the content of myoglobin, glycogen and enzyme activity.
TYPE 1 (red fibers) are characterized by a high myoglobin content (that's why they are red), high succinate dehydrogenase activity, slow type ATPase, not so rich in glycogen content, duration of contraction and low fatigue.
TYPE 2 (white fibers) are characterized by low myoglobin content, low succinate dehydrogenase activity, fast-type ATPase, rich glycogen content, rapid contraction and high fatigue.
The slow (red) and fast (white) types of muscle fibers are innervated by different types of motor neurons: slow and fast. In addition to the 1st and 2nd types of muscle fibers, there are intermediate ones that have the properties of both.
Each muscle contains all types of muscle fibers. Their number may vary and depends on physical activity.
REGENERATION OF STRIPED SKELETAL MUSCLE TISSUE . When muscle fibers are damaged (ruptured), their ends at the site of injury undergo necrosis. After rupture, macrophages arrive at the fragments of fibers, which phagocytose the necrotic areas, clearing them of dead tissue. After this, the regeneration process is carried out in 2 ways: 1) due to increased reactivity in muscle fibers and the formation of muscle buds at the sites of rupture; 2) due to myosatellite cells.
The 1st PATH is characterized by the fact that at the ends of broken fibers the granular ER is hypertrophied, on the surface of which the proteins of myofibrils, membrane structures inside the fiber and sarcolemma are synthesized. As a result, the ends of the muscle fibers thicken and transform into muscle buds. These buds, as they grow, move closer to each other from one torn end to the other, and finally the buds connect and grow together. Meanwhile, due to the endomysium cells, new formation of connective tissue occurs between the muscle buds growing towards each other. Therefore, by the time the muscle buds join, a connective tissue layer is formed, which will become part of the muscle fiber. Consequently, a connective tissue scar is formed.
The 2nd WAY of regeneration is that myosatellite cells leave their habitats and undergo differentiation, as a result of which they turn into myoblasts. Some myoblasts join the muscle buds, some join into myotubes, which differentiate into new muscle fibers.
Thus, during reparative muscle regeneration, old muscle fibers are restored and new ones are formed.
INNERVATION OF SKELETAL MUSCLE TISSUE carried out by motor and sensory nerve fibers ending in nerve endings. MOTOR (motor) nerve endings are the terminal devices of the axons of motor nerve cells of the anterior horns of the spinal cord. The end of the axon, approaching the muscle fiber, is divided into several branches (terminals). The terminals pierce the basement membrane of the sarcolemma and then plunge deep into the muscle fiber, dragging the plasmalemma with them. As a result, a neuromuscular ending (motor plaque) is formed.
STRUCTURE OF THE NEUROMUSCULAR endings The neuromuscular ending has two parts (poles): nervous and muscular. There is a synaptic gap between the nerve and muscle parts. The nerve part (axon terminals of the motor neuron) contains mitochondria and synaptic vesicles filled with the neurotransmitter acetylcholine. In the muscular part of the neuromuscular ending there are mitochondria, an accumulation of nuclei, and there are no myofibrils. The synaptic cleft, 50 nm wide, is bounded by a presynaptic membrane (axon plasmalemma) and a postsynaptic membrane (muscle fiber plasmalemma). The postsynaptic membrane forms folds (secondary synaptic clefts), it contains receptors for acetylcholine and the enzyme acetylcholinesterase.
FUNCTION of neuromuscular endings. The impulse moves along the axon plasmalemma (presynaptic membrane). At this time, synaptic vesicles with acetylcholine approach the plasmalemma, from the vesicles acetylcholine flows into the synaptic cleft and is captured by receptors of the postsynaptic membrane. This increases the permeability of this membrane (muscle fiber plasma membrane), as a result of which sodium ions move from the outer surface of the plasma membrane to the inner surface, and potassium ions move to the outer surface - this is a depolarization wave or a nerve impulse (action potential). After the occurrence of an action potential, acetylcholinesterase of the postsynaptic membrane destroys acetylcholine and the transmission of the impulse through the synaptic cleft stops.
SENSITIVE NERVE ENDINGS(neuromuscular spindles - fusi neuro-muscularis) end the dendrites of the sensory neurons of the spinal ganglia. Neuromuscular spindles are covered with a connective tissue capsule, inside which there are 2 types of intrafusal (intraspindle) muscle fibers: 1) with a nuclear bursa (in the center of the fiber there is a thickening in which there is an accumulation of nuclei), they are longer and thicker; 2) with a nuclear chain (the nuclei in the form of a chain are located in the center of the fiber), they are thinner and shorter.
Thick nerve fibers penetrate into the endings, which entwine both types of intrafusal muscle fibers in a ring and thin nerve fibers ending in grape-shaped endings on muscle fibers with a nuclear chain. At the ends of the intrafusal fibers there are myofibrils and motor nerve endings approach them. Contractions of intrafusal fibers do not have great strength and do not add up to the rest (extrafusal) muscle fibers.
FUNCTION of neuromuscular spindles consists in the perception of the speed and force of muscle stretching. If the tensile force is such that it threatens to rupture the muscle, then the contracting antagonist muscles from these endings reflexively receive inhibitory impulses.
CARDIAC MUSCLE TISSUE develops from the anterior section of the visceral layers of the splanchnotome. From these sheets, 2 myoepicardial plates stand out: right and left. The cells of the myoepicardial plates differentiate in two directions: from some the mesothelium covering the epicardium develops, from others - cardiomyocytes of five varieties;
contractile
pacemaker
conductive
intermediate
secretory or endocrine
STRUCTURE OF CARDIOMYOCYTES . Cardiomyocytes have a cylindrical shape, 50-120 µm long, 10-20 µm in diameter. Cardiomyocytes connect their ends to each other and form functional cardiac muscle fibers. The junction of cardiomyocytes is called intercalated discs (discus intercalatus). The discs contain interdigitations, desmosomes, attachment sites for actin filaments, and nexuses. Metabolism between cardiomyocytes occurs through nexuses.
On the outside, cardiomyocytes are covered with a sarcolemma, consisting of an outer (basal) membrane and a plasmalemma. Processes extend from the lateral surfaces of the cardiomyocytes and intertwine into the lateral surfaces of the cardiomyocytes of the adjacent fiber. These are muscle anastomoses.
CORE cardiomyocytes (one or two), oval in shape, usually polyploid, located in the center of the cell. MYOFIBRILLS are localized along the periphery. ORGANELLES - some are poorly developed (granular ER, Golgi complex, lysosomes), others are well developed (mitochondria, smooth ER, myofibrils). The oxyphilic CYTOPLASMA contains inclusions of myoglobin, glycogen and lipids.
STRUCTURE OF MYOFIBRILLS the same as in skeletal muscle tissue. Actin filaments form a light disk (I), separated by a telophragm; due to myosin filaments and actin ends, disk A (anisotropic) is formed, separated by a mesophragm. In the middle part of disk A there is an H-stripe bounded by the ends of actin filaments.
Cardiac muscle fibers differ from skeletal muscle fibers in that they consist of individual cells - cardiomyocytes, the presence of muscle anastomoses, the central location of the nuclei (in the skeletal muscle fiber - under the sarcolemma), the increased thickness of the diameter of T-channels, since they include plasmalemma and basement membrane (in skeletal muscle fibers - only plasmalemma).
REDUCTION PROCESS in the fibers of the heart muscle is carried out according to the same principle as in the fibers of skeletal muscle tissue.
CONDUCTING CARDIOMYOCYTES characterized by a thicker diameter (up to 50 μm), lighter cytoplasm, central or eccentric arrangement of nuclei, low content of myofibrils, and a simpler arrangement of intercalary discs. The discs have fewer desmosomes, interdigitations, nexuses, and actin filament attachment sites.
Conducting cardiomyocytes lack T channels. Conducting cardiomyocytes can connect to each other not only with their ends, but also with their lateral surfaces. The FUNCTION of conductive cardiomyocytes is to produce and transmit a contractile impulse to contractile cardiomyocytes.
ENDOCRINE CARDIOMYOCYTES are located only in the atria, have a more process-shaped shape, poorly developed myofibrils, intercalated discs, and T-channels. They have well-developed granular ER, Golgi complex and mitochondria, and their cytoplasm contains secretion granules.
FUNCTION OF endocrine cardiomyocytes- secretion of atrial natriuretic factor (ANF), which regulates the contractility of the heart muscle, the volume of circulating fluid, blood pressure, and diuresis.
REGENERATION of cardiac muscle tissue is only physiological, intracellular. When cardiac muscle fibers are damaged, they are not restored, but are replaced by connective tissue (histotypic regeneration).
Muscle- this is a group of animal and human tissues, the main function of which is contraction, which, in turn, causes the movement of the body or its parts in space. This function corresponds to the structure of the main elements of muscle tissue, which have an elongated shape and longitudinal orientation of myofibrils, which include contractile proteins - actin and myosin. Like epithelial tissue, muscle tissue is a composite tissue group, since its main components develop from various embryonic rudiments.
Depending on the structure of its contractile apparatus, muscle tissue is divided into striated (skeletal) and smooth tissue, consisting of various histogenetic types that differ in structure. The following scheme gives a general idea of the classification of muscle tissue:
Striated muscle tissue
The source of its development are myotome cells formed from the dorsal mesoderm. Striated muscle tissue consists of elongated formations - muscle fibers, which look like cylinders with pointed ends. The fibers reach 80 microns in diameter and 12 cm in length. In the center of the muscle fibers there are multinucleated formations (symplasts), to which cells - myosatelites - are adjacent to the outside. The fibers are limited by the sarcolemma formed by the basement membrane and the plasmolemma simplast.Myosatelliotocytes are located under the basement membrane of the muscle fiber so that their plasmalemma touches the symplast plasmalemma. These cells represent the cambial reserve of skeletal muscle tissue, due to which the regeneration of its fibers is carried out.
In addition to the plasmalemma, myosimplasts include cytoplasm (sarcoplasm) and numerous nuclei located along the periphery. In the perinuclear region there is a poorly developed granular endoplasmic reticulum and Golgi complex. A muscle fiber with its sheath, nerve endings, blood and lymphatic capillaries is called a muscle unit (Mion).
A characteristic feature of skeletal muscle fibers is transverse striation, caused by the alternation of double-folding (anisotropic) A-disks and single-folding (isotropic) I-discs. The discs contain myofibrils, which form the contractile apparatus of the fibers. Myofibrils are composed of ordered filaments of the contractile proteins actin and myosin. These threads are secured by transversely located telophragms and mesophragms,
which are made up of other proteins. The segment of myofibril between adjacent telophragms is called a sarcomere. It is a morphofunctional unit of the fiber contractile apparatus. In its middle part there is a mesophragm (M-line on longitudinal sections). Thick (about 11 nm in diameter) myosin filaments extend from the mesophragm towards the telophragm, and thin (about 5 nm) actin filaments extend from the telophragm towards them.
Myosin filaments are the main component of dark discs, and actin filaments are the main component of light discs. Within the dark disk, actin and myosin filaments are located in parallel. The middle segment of the A-disc has only myosin filaments and is called the H-stripe (light zone).
For the convenience of considering the structure of the contractile apparatus of the muscle fiber, it is necessary to remember the so-called sarcomere formula, which reflects the sequential placement of its main components and looks like this: telophragm + 1/2 disk 1 + 1/2 disk A + stripe M + + 1/2 disk A + 1/2 disk I + telophragm.
The cytolemma of the symplastic part of the muscle fiber at the level of the telophragm is formed by deep protrusions - transverse or T-tubules (from the Latin Transversus - transverse). Parallel to these tubes are located expanded sections of the tubules of the agranular endoplasmic reticulum (terminal cisterns), which accompany them on both sides. Together with T-tubules they form triads.
In the terminal cisterns of the agranular endoplasmic reticulum, calcium ions accumulate in the relaxed state of the muscle fiber. Under the influence of the propagation of the action potential along the cytolemma of the fiber and T-tubules, calcium ions leave the terminal cisterns entering the myofibrils and, interacting with special reticular proteins - troponin and tropomyosin, begin to actively contract. In this case, the actin and myosin filaments, interacting with each other, move towards each other. Actin filaments come between the myosin filaments and approach the M-line, and therefore, when the muscle fiber contracts, the width of the H-strip and H-disk decreases. The width of the A-disc remains unchanged. (The structure of different functional types of muscle fibers is discussed in histology textbooks).
Smooth muscle tissue
Smooth muscle tissue of mesenchymal origin forms the muscular membranes of internal organs. Smooth myocytes are often spindle-shaped, their length ranges from 15 to 500 μm, and their thickness ranges from 5 to 8 MNM. Cell nuclei elongate in length. As cells shrink, they can take on a gimlet-like appearance. The organelles in these cells are poorly developed. The cytolemma, stretching, forms numerous pinocytotic vesicles, which transmit irritation into the cell, which, in turn, causes its contraction.The contractile apparatus of smooth myocytes (myofibrils) consists of thin myofilaments formed by actin and thick ones formed by myosin. Myocytes are bounded by a basement membrane as well as collagen (reticular) elastic fibers. These structural components of smooth muscle tissue are formed by smooth myocytes. Efferent (motor) innervation of smooth myocytes is carried out by postganglionic fibers of the autonomic nervous system. Neighboring myocytes, through holes in the basement membrane, form slit-like communications (nexus) with each other, which ensure functional cell interactions.
Smooth muscle tissue of epidermal origin is formed by myoepithelial cells, which are formed from the skin mesoderm. They have a star-shaped (bucket-shaped) shape and are part of the sweat, mammary and salivary glands. Located between the epithelial cells and the basement membrane of the secretory sections of the glands and small excretory ducts, they, by contracting, contribute to the excretion of secretions.
Smooth muscle tissue of neural origin is formed during the embryonic development of the eyeball from the cells of the wall of the optic cup. It is part of the muscles of the iris of the eyeball, which dilate or contract the pupil.