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Lecture notes, cheat sheets
Histology. Muscle tissue. Muscle tissue. Skeletal muscle tissue
Directory / Lecture notes, cheat sheets Table of contents (expand) Topic 15. MUSCLE TISSUES. SKELETAL MUSCLE TISSUE Almost all types of cells have the property of contractility due to the presence in their cytoplasm of the contractile apparatus, represented by a network of thin microfilaments (5-7 nm), consisting of contractile proteins actin, myosin, tropomyosin. Due to the interaction of these microfilament proteins, contractile processes are carried out and the movement of hyaloplasm, organelles, vacuoles in the cytoplasm, the formation of pseudopodia and plasmolemma invaginations, as well as the processes of phago- and pinocytosis, exocytosis, cell division and movement are ensured. The content of contractile elements (and, consequently, contractile processes) is not equally expressed in different types of cells. Contractile structures are most pronounced in cells whose main function is contraction. Such cells or their derivatives form muscle tissues that provide contractile processes in hollow internal organs and vessels, movement of body parts relative to each other, maintaining posture and moving the body in space. In addition to movement, during contraction, a large amount of heat is released, and therefore, muscle tissues are involved in the thermoregulation of the body. Muscle tissues are not the same in structure, sources of origin and innervation, and functional features. Any kind of muscle tissue, in addition to contractile elements (muscle cells and muscle fibers), includes cellular elements and fibers of loose fibrous connective tissue and vessels that provide trophism and transfer the forces of contraction of muscle elements. Muscle tissue is divided according to its structure into smooth (non-striated) and striated (striated). Each of the two groups, in turn, is divided into species according to sources of origin, structure and functional features. Smooth muscle tissue, which is part of the internal organs and blood vessels, develops from the mesenchyme. Special muscle tissues of neural origin include smooth muscle cells of the iris, epidermal origin - myoepithelial cells of the salivary, lacrimal, sweat and mammary glands. Striated muscle tissue is divided into skeletal and cardiac. Both of these varieties develop from the mesoderm, but from its different parts: skeletal - from somite myotomes, cardiac - from visceral sheets of splanchiotomes. Striated skeletal muscle tissue As already noted, the structural and functional unit of this tissue is the muscle fiber. It is an elongated cylindrical formation with pointed ends from 1 to 40 mm long (and according to some sources - up to 120 mm), with a diameter of 0,1 mm. The muscle fiber is surrounded by a sheath of sarcolemma, in which two sheets are clearly distinguished under an electron microscope: the inner sheet is a typical plasmalemma, and the outer one is a thin connective tissue plate (basal plate). The main structural component of the muscle fiber is the myosymplast. Thus, the muscle fiber is a complex formation and consists of the following main structural components: 1) myosymplast; 2) myosatellite cells; 3) basal plate. The basal plate is formed by thin collagen and reticular fibers, belongs to the supporting apparatus and performs an auxiliary function of transferring contraction forces to the connective tissue elements of the muscle. Myosatellite cells are growth elements of muscle fibers that play an important role in the processes of physiological and reparative regeneration. The myosymplast is the main structural component of the muscle fiber, both in terms of volume and functions. It is formed by the fusion of independent undifferentiated muscle cells - myoblasts. Myosymplast can be considered as an elongated giant multinucleated cell, consisting of a large number of nuclei, cytoplasm (sarcoplasm), plasmolemma, inclusions, general and specialized organelles. In the myosymplast, there are up to 10 thousand longitudinally elongated light nuclei located on the periphery under the plasmalemma. Fragments of a weakly expressed granular endoplasmic reticulum, a lamellar Golgi complex, and a small number of mitochondria are localized near the nuclei. There are no centrioles in the symplast. The sarcoplasm contains inclusions of glycogen and myoglobin. A distinctive feature of the myosymplast is also the presence in it: 1) myofibrils; 2) sarcoplasmic reticulum; 3) tubules of the T-system. Myofibrils - the contractile elements of the myosymplast are localized in the central part of the sarcoplasm of the myosymplast. They are combined into bundles, between which there are layers of sarcoplasm. A large number of mitochondria (sacrosomes) are localized between the myofibrils. Each myofibril extends longitudinally throughout the entire myosymplast and, with its free ends, is attached to its plasmolemma at the conical ends. The diameter of the myofibril is 0,2 - 0,5 microns. According to their structure, myofibrils are heterogeneous in length, divided into dark (anisotropic), or A-disks, and light (isotropic), or I-disks. Dark and light discs of all myofibrils are located at the same level and cause the transverse striation of the entire muscle fiber. The disks, in turn, consist of thinner fibers - protofibrils, or myofilaments. Dark discs are made up of myosin, light discs are made up of actin. In the middle of the I-disk across the actin microfilaments, there is a dark strip - a telophragm (or Z-line), in the middle of the A-disk there is a less pronounced mesophragm (or M-line). Actin myofilaments in the middle of the I-disk are held together by proteins that make up the Z-line, and with their free ends partially enter the A-disk between thick myofilaments. In this case, six actin filaments are located around one myosin filament. With a partial contraction of the myofibril, the actin filaments seem to be drawn into the A-disk, and a light zone (or H-strip) is formed in it, bounded by the free ends of the microfilaments. The width of the H-band depends on the degree of contraction of the myofibril. The section of the myofibril located between the two Z-bands is called the sarcomere and is the structural and functional unit of the myofibril. The sarcomere includes the A-disk and two halves of the I-disk located on either side of it. Therefore, each myofibril is a collection of sarcomeres. It is in the sarcomere that contraction processes take place. It should be noted that the terminal sarcomeres of each myofibril are attached to the myosymplast plasmolemma by actin myofilaments. Structural elements of a sarcomere in a relaxed state can be expressed by the formula: Z + 1/2I = 1/2A + b + 1/2A + 1/2I + Z. The contraction process is carried out by the interaction of actin and myosin filaments with the formation of actomyosin "bridges" between them, through which the actin filaments are drawn into the A-disk and the sarcomere is shortened. Three conditions are necessary for the development of this process: 1) the presence of energy in the form of ATP; 2) the presence of calcium ions; 3) presence of biopotential. ATP is produced in sarcosomes (mitochondria), located in large quantities between myofibrils. The fulfillment of the second and third conditions is carried out with the help of special organelles of muscle tissue - the sarcoplasmic reticulum (an analogue of the endoplasmic reticulum of ordinary cells) and the T-tubules system. The sarcoplasmic reticulum is a modified smooth endoplasmic reticulum and consists of dilated cavities and anastomosing tubules surrounding the myofibrils. In this case, the sarcoplasmic reticulum is subdivided into fragments surrounding individual sarcomeres. Each fragment consists of two terminal cisterns connected by hollow anastomosing tubules - L-tubules. In this case, the terminal tanks cover the sarcomere in the region of the I-disk, and the tubules - in the region of the A-disk. The terminal cisterns and tubules contain calcium ions, which, upon receipt of a nerve impulse and reaching a wave of depolarization of the membranes of the sarcoplasmic reticulum, leave the cisterns and tubules and are distributed between actin and myosin microfilaments, initiating their interaction. After the wave of depolarization ceases, calcium ions rush back to the terminal cisterns and tubules. Thus, the sarcoplasmic reticulum is not only a reservoir for calcium ions, but also plays the role of a calcium pump. The wave of depolarization is transmitted to the sarcoplasmic reticulum from the nerve ending, first through the plasmalemma, and then through the T-tubules, which are not independent structural elements. They are tubular invaginations of the plasmalemma into the sarcoplasm. Penetrating deep, T-tubules branch and cover each myofibril within one bundle strictly at a certain level, usually at the level of the Z-band or somewhat more medially - in the area of \uXNUMXb\uXNUMXbjunction of actin and myosin filaments. Therefore, each sarcomere is approached and surrounded by two T-tubules. On the sides of each T-tubule are two terminal cisterns of the sarcoplasmic reticulum of neighboring sarcomeres, which, together with the T-tubules, form a triad. Between the wall of the T-tubule and the walls of the terminal cisterns there are contacts through which the depolarization wave is transmitted to the membranes of the cisterns and causes the release of calcium ions from them and the onset of contraction. Thus, the functional role of T-tubules is to transfer excitation from the plasma membrane to the sarcoplasmic reticulum. For the interaction of actin and myosin filaments and subsequent contraction, in addition to calcium ions, energy is also needed in the form of ATP, which is produced in sarcosomes, which are located in large numbers between myofibrils. Under the influence of calcium ions, the ATP-ase activity of myosin is stimulated, which leads to the breakdown of ATP with the formation of ADP and the release of energy. Thanks to the released energy, "bridges" are established between the heads of the myosin protein and certain points on the actin protein, and due to the shortening of these "bridges", actin filaments are pulled between myosin filaments. Then these bonds break down, using the energy of ATP and the myosin head, new contacts are formed with other points on the actin filament, but located distal to the previous ones. This is how the gradual retraction of actin filaments between myosin filaments and shortening of the sarcomere occurs. The degree of this contraction depends on the concentration of free calcium ions near the myofilaments and on the content of ATP. When the sarcomere is fully contracted, the actin filaments reach the M-band of the sarcomere. In this case, the H-band and I-disks disappear, and the sarcomere formula can be expressed as follows: Z + 1/2IA + M + 1/2AI + Z. With a partial reduction, the sarcomere formula will look like this: Z + 1/nI + 1/nIA + 1/2H + M + 1/2H + 1/nAI + 1/nI + Z. Simultaneous and friendly contraction of all sarcomeres of each myofibril leads to contraction of the entire muscle fiber. The extreme sarcomeres of each myofibril are attached by actin myofilaments to the plasmolemma of the myosymplast, which has a folded character at the ends of the muscle fiber. At the same time, at the ends of the muscle fiber, the basal plate does not enter the folds of the plasmalemma. It is perforated by thin collagen and reticular fibers, penetrate deep into the folds of the plasmolemma and attach in those places where the actin filaments of the distal sarcomeres are attached from the inside. This creates a strong connection between the myosymplast and the fibrous structures of the endomysium. Collagen and reticular fibers of the end sections of muscle fibers, together with the fibrous structures of endomysium and perimysium, together form muscle tendons that attach to certain points of the skeleton or are woven into the reticular layer of the dermis of the skin in the facial area. Due to muscle contraction, parts or the whole body move, as well as a change in the relief of the face. Not all muscle fibers are the same in their structure. There are two main types of muscle fibers, between which there are intermediate ones that differ primarily in the features of metabolic processes and functional properties and, to a lesser extent, in structural features. Type I fibers - red muscle fibers, are characterized primarily by a high content of myoglobin in the sarcoplasm (which gives them a red color), a large number of sarcosomes, high activity of the succinate dehydrogenase enzyme in them, and high activity of slow-acting ATPase. These fibers have the ability of slow but prolonged tonic contraction and low fatigue. Type II fibers - white muscle fibers, characterized by a low content of myoglobin, but a high content of glycogen, high activity of phosphorylase and fast-type ATPase. Functionally, fibers of this type are characterized by the ability of a faster, stronger, but shorter contraction. Between the two extreme types of muscle fibers are intermediate, characterized by a different combination of these inclusions and different activities of the listed enzymes. Any muscle contains all types of muscle fibers in their various quantitative ratios. In the muscles that maintain the posture, red muscle fibers predominate, in the muscles that provide movement of the fingers and hands, red and transitional fibers predominate. The nature of the muscle fiber can change depending on the functional load and training. It has been established that the biochemical, structural and functional features of the muscle fiber depend on the innervation. Cross transplantation of efferent nerve fibers and their endings from red fiber to white (and vice versa) leads to a change in metabolism, as well as structural and functional features in these fibers to the opposite type. The structure and physiology of the muscle A muscle as an organ consists of muscle fibers, fibrous connective tissue, blood vessels, and nerves. A muscle is an anatomical formation, the main and functionally leading structural component of which is muscle tissue. Fibrous connective tissue forms layers in the muscle: endomysium, perimysium, epimysium, and tendons. Endomysium surrounds each muscle fiber, consists of loose fibrous connective tissue and contains blood and lymphatic vessels, mainly capillaries, through which the trophic fiber is provided. The perimysium surrounds several muscle fibers collected in bundles. Epimysium (or fascia) surrounds the entire muscle, contributes to the functioning of the muscle as an organ. Histogenesis of skeletal striated muscle tissue From the myotomes of the mesoderm, poorly differentiated cells - myoblasts - are evicted to certain areas of the mesenchyme. In the area of contacts of myoblasts, the cytolemma disappears, and a symplastic formation is formed - a myotube, in which nuclei in the form of a chain are located in the middle, and along the periphery, myofibrils begin to differentiate from myofilaments. Nerve fibers grow to the myotube, forming motor nerve endings. Under the influence of efferent nerve innervation, the restructuring of the muscle tube into a muscle fiber begins: the nuclei move to the periphery of the symplast to the plasmolemma, and the myofibrils occupy the central part. From the folds of the endoplasmic reticulum, the sarcoplasmic reticulum develops, surrounding each myofibril throughout its entire length. The plasmalemma of the myosymplast forms deep tubular protrusions - T-tubules. Due to the activity of the granular endoplasmic reticulum, first of the myoblasts, and then of the muscular tubes, proteins and polysaccharides are synthesized and secreted using the lamellar complex, from which the basal plate of the muscle fiber is formed. During the formation of the myotube, and then the differentiation of the muscle fiber, part of the myoblasts is not part of the symplast, but is adjacent to it, located under the basal plate. These cells are called myosatellites and play an important role in the process of physiological and reparative regeneration. It has been established that the laying of striated skeletal muscles occurs only in the embryonic period. In the postnatal period, their further differentiation and hypertrophy are carried out, but the number of muscle fibers does not increase even under conditions of intensive training. Regeneration of skeletal muscle tissue In muscle, as in other tissues, two types of regeneration are distinguished: physiological and reparative. Physiological regeneration is manifested in the form of hypertrophy of muscle fibers. This is expressed in an increase in their thickness and length, an increase in the number of organelles, mainly myofibrils, the number of nuclei, which is manifested by an increase in the functional ability of the muscle fiber. It has been established by radioisotope methods that an increase in the content of nuclei in muscle fibers is achieved by the division of myosatellite cells and the subsequent entry into the myosymplast of daughter cells. The increase in the number of myofibrils is carried out using the synthesis of actin and myosin proteins by free ribosomes and the subsequent assembly of these proteins into actin and myosin myofilaments in parallel with the corresponding sarcomere filaments. As a result of this, myofibrils thicken first, and then their splitting and the formation of daughter ones. It is possible that new actin and myosin myofilaments are formed not in parallel, but end to end with existing ones, which results in their elongation. The sarcoplasmic reticulum and T-tubules in a hypertrophied muscle fiber are formed due to the growth of the previous elements. With certain types of muscle training, a predominantly red type of muscle fibers (for stayers in athletics) or a white type can be formed. Age-related hypertrophy of muscle fibers is intensely manifested with the onset of motor activity of the body (1-2 years), which is primarily due to increased nervous stimulation. In old age, as well as under conditions of slight muscle load, atrophy of special and general organelles, thinning of muscle fibers and a decrease in their performance occurs. Reparative regeneration develops after damage to muscle fibers. With this method, regeneration depends on the size of the defect. With significant damage along the muscle fiber, myosatellites in the area of damage and in adjacent areas are disinhibited, proliferate intensively, and then migrate to the area of the defect in the muscle fiber, where they are embedded in chains, forming a microtubule. The subsequent differentiation of the microtubule leads to the replacement of the defect and restoration of the integrity of the muscle fiber. Under conditions of a small defect in the muscle fiber at its ends, due to the regeneration of intracellular organelles, muscle buds are formed that grow towards each other and then merge, leading to the closure of the defect. Reparative regeneration and restoration of the integrity of muscle fibers can be carried out only under certain conditions: if motor innervation of muscle fibers is preserved and if elements of connective tissue (fibroblasts) do not get into the area of damage. Otherwise, a connective tissue scar is formed at the site of the defect. At present, the possibility of autotransplantation of muscle tissue, including whole muscles, has been proven under the following conditions: 1) mechanical grinding of the transplant muscle tissue in order to disinhibit satellite cells for their subsequent proliferation; 2) placing the crushed tissue in the fascial bed; 3) suturing the motor nerve fiber to the crushed graft; 4) the presence of contractile movements of antagonist and synergist muscles. Skeletal muscle innervation Skeletal muscles receive motor, sensory and trophic (vegetative) innervation. The motor (efferent) innervation of the skeletal muscles of the trunk and limbs is received from the motor neurons of the anterior horns of the spinal cord, and the muscles of the face and head - from the motor neurons of certain cranial nerves. In this case, either the axon of the motor neuron itself, or its branch, approaches each muscle fiber. In muscles that provide coordinated movements (muscles of the hands, forearm, neck), each muscle fiber is innervated by one motor neuron, which ensures greater accuracy of movements. In the muscles that predominantly maintain the posture, tens and even hundreds of muscle fibers receive motor innervation from one motor neuron through the branching of its axon. The motor nerve fiber, approaching the muscle fiber, penetrates under the endomysium and basal plate and breaks up into terminals, which, together with the adjacent specific area of the myosymplast, form an axonomuscular synapse (or motor plaque). Under the influence of a nerve impulse, the depolarization wave propagates further along the T-tubules and, in the region of the triads, is transmitted to the terminal cisterns of the sarcoplasmic reticulum, causing the release of calcium ions and the beginning of the process of contraction of the muscle fiber. Sensitive innervation of skeletal muscles is carried out by pseudounipolar neurons of the spinal ganglia through various receptor endings in the dendrites of these cells. The receptor endings of skeletal muscles can be divided into two groups: 1) specific receptor devices that are characteristic only for skeletal muscles - muscle spindles and the Golgi tendon complex; 2) non-specific receptor endings of a bushy or tree-like form, distributed in the loose connective tissue of the endo-, peri- and epineurium. Muscle spindles are complex encapsulated formations. Each muscle contains several to hundreds of muscle spindles. Each muscle spindle contains not only nerve elements, but also 10-12 specific muscle fibers - intrafusal, surrounded by a capsule. These fibers are located parallel to the contractile muscle fibers (extrafusally) and receive not only sensitive, but also special motor innervation. Muscle spindles perceive irritation both when the given muscle is stretched, caused by contraction of the antagonist muscles, and when it contracts, and thereby regulate the degree of contraction and relaxation. Tendon organs are specialized encapsulated receptors, which include in their structure several tendon fibers surrounded by a capsule, among which the terminal branches of the pseudounipolar neuron dendrite are distributed. When the muscle contracts, the tendon fibers come together and compress the nerve endings. Tendon organs perceive only the degree of contraction of a given muscle. Through muscle spindles and tendon organs, with the participation of spinal centers, automatic movement is ensured, for example, when walking. Trophic innervation of skeletal muscles is carried out by the autonomic nervous system - its autonomic part and is mainly carried out indirectly through the innervation of blood vessels. Blood supply Skeletal muscles are richly supplied with blood. Loose connective tissue (perimysium) contains a large number of arteries and veins, arterioles, venules and arterio-venular anastomoses. In the endomysium there are capillaries, mostly narrow (4,5 - 7 microns), which provide the trophism of the nerve fiber. The muscle fiber, together with the surrounding capillaries and motor endings, make up the mion. The muscles contain a large number of arteriovenular anastomoses that provide adequate blood supply during various muscle activity. Authors: Selezneva T.D., Mishin A.S., Barsukov V.Yu. << Back: Connective tissues. Skeletal connective tissues >> Forward: Muscle tissue. Cardiac and smooth muscle tissue
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