Muscular tissues
Muscular tissues (textus muscularis) name a tissue, various on a structure and a parentage, but similar on ability to the expressed reductions. They provide movings to organism space in whole, its parts and movement of organs in an organism (heart, tongue, an intestine, etc.).
Property of change of the form cells of many tissues possess, but in muscular tissues this ability becomes the main function.
The basic morphological signs of elements of muscular tissues - the extended form, presence it is longitudinal the located myofibrils and myofilaments - the special organellas providing contractility, a locating of mitochondrions near to contractile elements, presence of includings of a glycogen, lipids and a myoglobin.
Special contractile organellas - myofilaments or myofibrils provide reduction which arises at interaction in them of two basic fibrillar fibers - an actin and a myosin at obligatory participation of ions of calcium. Mitochondrions provide these processes by energy. The stock of energy sources is formed by a glycogen and lipids. The myoglobin - the fiber providing linkage of oxygen and creation of its stock at the moment of reduction of a muscle when blood vessels (oxygen entering thus sharply are squeezed falls).
Classification. Two principles are put in a basis of classification of muscular tissues - a morpho-functional and a histo-genetic. According to a morpho-a functional principle, depending on structure of organellas of reduction, muscular tissues section on two subgroups.
The first subgroup - transversely striated (striated) muscular tissues (textus muscularis striatus). In a cytoplasma of their elements myosine filaments are constantly polymerized, form with actinide threads constantly existing myofibrils. The last are organised in characteristic complexes - sarcomeres. In the next myofibrils structural sub units of sarcomeres are located at identical level and frame cross-section striation. Striated muscular tissues are reduced faster, than smooth.
The second subgroup - smooth (not striated) muscular tissues (textus muscularis nonstriatus). These tissues are characterised by that out of reduction myosine filaments are depolymerized. In the presence of ions of calcium they polymerise and enter interaction with actin filaments. Myofibrils formed thus have no cross-section striation: at special colourings they are presented (smooth) threads in regular intervals painted on all length.
According to a histo-a genetic principle depending on development sources (embryonal germs) muscular tissues are sectioned into 5 types: mesenchimes (from desmal a germ in structure mesenchimes), epidermal (from a dermal ectoderm and from a prechordal plate), neural (from a nervous tube), coelomic (from a myoepicardial plate of a visceral leaf of a somite) and somatic (myotomes).
First three types concern a subgroup of smooth muscular tissues, the fourth and the fifth - to a subgroup transversely striated.
Transversely striated muscular tissues
There are two basic versions of transversely striated (striated) tissues - sceletal and warm.
Sceletal muscular tissue
Histogenesis
A source of development of elements of a sceletal (somatic) transversely striated muscular tissue (textus muscularis striatus sceletalis) are cells of myotomes - myoblasts. One of them are differentiated on a place and participate in formation of so-called autochtonous muscles. Other cells migrate from myotomes in a mesenchyma. They are already determined, though outwardly do not differ from other cells of a mesenchyma. Their differentiation proceeds in places of a bookmark of other muscles of a body. During a differentiation there are two cellular lines. Cells of one of lines merge, forming the extended symplasts - myotubes (myotubes). In them there is a differentiation of special organellas - myofibrils. At this time in myotubes well developed granulous cytoplasmic reticulum becomes perceptible. Myofibrils at first settle down under a plasmolemma, and then fill the most part myotubes. Kernels, on the contrary, from the central departments are displaced to periphery. The cellular centres and microtubules thus completely disappear. The granulous cytoplasmic reticulum is reduced substantially. Such structures name myosimplasts.
Cells of other line remain independent and are differentiated in myosatellitocites (myosatellites). These cells settle down on a surface of myosymplasts.
Structure
Basic structural unit of a sceletal muscular tissue is the muscular fiber consisting of a myosymplast and myosatellitecites, covered with the general basal membrane.
The length of all fiber can be measured by centimetres at a thickness 50 - 100 microns. A complex consisting of a plasmolemma of a myosymplast and a basal membrane, name a sarcolemma.
Myosymplast structure
The myosymplast has set of the oblong kernels located immediately under a sarcolemma. Their quantity in one symplast can reach several tens thousand. At poles of kernels general meaning organellas - apparatus Goldzhi and small fragments of a granulous cytoplasmic reticulum settle down. Myofibrils fill the basic part of a myosymplast and are located is longitudinal.
Sarcomere - structural unit of a myofibril. Each myofibril has the cross-section dark and light disks having unequal refraction (anisotropic A-disks and isotropic I-disks). Each myofibril is surrounded is longitudinal the loops of an agranular cytoplasmic reticulum located and anastomosing among themselves - a sarcoplasmic network. The next sarcomeres have the general boundary structure - Z - line.
It is constructed in the form of a network of albuminous fibrillar molecules among which an essential role plays a - an actinine. The extremities of actinide filaments are bound to this network. From next Z - lines actinide filaments are referred to the sarcomere centre, but do not reach its middle. Actin filaments are united with Z - line and myosin threads fibrillar not extensible molecules nebuline. In the middle of a dark disk of a sarcomere the network constructed from myomesine settles down. It forms a line in section of M-. In knots of this line M - are fixed the extremities of myosine filaments. Their other extremities are referred aside Z - lines and settle down between actin filaments, but to Z - lines too do not reach. At the same time these extremities are fixed in relation to Z - to lines by extensible huge albuminous molecules ticine.
Myosin molecules two heads have a long tail and on one of its extremities. At rising of concentration of ions of calcium in the field of joining of heads (a hinged site) the molecule changes the configuration. Thus (as between myosine filaments are located actinide) myosin heads contact an actin (with the assistance of auxiliary fibers - a tropomyosine and a troponin). Then the myosin head bends and pulls behind itself an actinide molecule towards line M-. Z - lines approach, the sarcomere is shortened.
Network alpha-actinium Z - lines of the next myofibrils are bound with each other by intermediate filaments. They approach to an internal surface of a plasmolemma and are fixed in a cortical layer of a cytoplasma so sarcomeres of all myofibrils settle down at one level. It also makes at observation in a microscope impression of cross-section striation of all fiber.
Types of muscular fibers
Different muscles (as organs) function in unequal biomechanical conditions. Therefore and muscular fibers as a part of different muscles possess different force, rate and duration of reduction, and also fatigability. Enzymes in them possess different activity and are presented in various isomeric forms. Difference in them maintenances of respiratory enzymes - glycolytic and oxidising is appreciable.
On a parity of myofibrils, mitochondrions and a myoglobin distinguish white, red and intermediate fibers. On functional features muscular fibers section on fast, slow and intermediate. Most considerably muscular fibers differ with features of the molecular organisation of a myosin. Among its various isoforms there are two cores - "fast" and "slow". At statement of histochemical reactions them distinguish on ATPhased activity. With these properties correlates also activity of respiratory enzymes. Usually in fast fibers glycolytic processes prevail, they are richer with a glycogen, in them there is less than myoglobin, therefore them name also white. In slow fibers, on the contrary, above activity of oxidising enzymes, they more richly a myoglobin, look more red.
If on activity ATPhase muscular fibers differ sharply enough the degree of activity of respiratory enzymes varies rather considerably, therefore along with the white and red there are also intermediate fibers. In a muscular tissue different fibers are often located mosaically.
Warm muscular tissue
Histogenesis and kinds of cells. Sources of development of a warm transversely striated muscular tissue (textus muscularis striatus cardiacus) - symmetric sites of a visceral leaf of a splanchnotome in a cervical part of a germ - myoepicardial plates. From them cells mesothelium an epicardium are differentiated also. During a histogenesis there are 5 kinds of cardiomyocytes - working (contractile), sinus, transitive, spending, and also secretory.
Working (contractile) cardiomyocytes form the chains. They, being shortened, provide force of reduction of all cardiac muscle. Working cardiomyocytes are capable to transfer controlling signals each other. Sinus cardiomyocytes are capable to replace automatically in a certain rhythm a reduction condition with a relaxation condition. They perceive controlling signals from nervous fibers, in the answer, on what change a rhythm of contractile activity. Sinus cardiomyocytes transfer controlling signals to transitive cardiomyocytes, and the last - spending. Spending cardiomyocytes form chains of the cells bridged by the extremities. The first cell in a chain perceives controlling signals from sinus cardiomyocytes and transfers them further - to other spending cardiomyocytes. The cells closing a chain, transfer a signal through transitive cardiomyocytes to workers. Secretory cardiomyocytes carry out special function. They develop sodium - uretic the factor (hormone) participating in processes of regulation of an uropoiesis and in some other processes. All cardiomyocytes are covered by a basal membrane.
Smooth muscular tissues
Distinguish three groups of smooth (not striated) muscular tissues (textus muscularis nonstriatus) - mesenchymal, epidermal and neural.
Muscular tissue of a mesenchymal parentage
Histogenesis
Stem cells and cells-predecessors in a smooth muscular tissue at stages of embryonal development while precisely are not identified. Apparently, they are related mechanocites tissues of internal medium. Possibly, in a mesenchyma they migrate to places of a bookmark of organs, being already determined. Being differentiated, they synthesise components of a matrix and collagen of a basal membrane, and also elastin. At attributive cells (myocytes) synthetic ability is lowered, but does not disappear completely.
Structure of cells
A smooth myocyte - a spindle cell in length 20 - 500 microns, width 5 - 8 microns.
The kernel rhabdoid, is in its central part. When the myocyte is reduced, its kernel is bent and even twists. General meaning organellas among which it is a lot of mitochondrions, are concentrated about kernel poles (in an endosarc). Apparatus Goldzhi and a granulous cytoplasmic reticulum are developed weakly, that testifies to small activity of synthetic functions. Ribosomes in the majority are located freely.
Muscular tissue of mesenchymal type as a part of organs
Myocytes are united in fascicles between which thin layers of a connecting tissue settle down. In these layers the reticular and elastic fibers surrounding myocytes are intertwined. In layers there pass blood vessels and nervous fibers. Terminals of the last terminate not immediately on myocytes, and between them. Therefore after entering of nervous impulse the mediator extends diffusively, raising at once many cells. The smooth muscular tissue of a mesenchymal parentage is presented mainly in walls of blood vessels and many tubular internal organs, and also forms separate fine muscles (ciliary).
The smooth muscular tissue as a part of concrete organs has unequal functional properties. It is caused by that on a surface of organs there are different receptors to concrete biologically active substances. Therefore and on many medicinal preparations their reaction is not identical. Probably, different functional properties of tissues are bound and to the concrete molecular organisation of actinide filaments.
Muscular tissue of an epidermal parentage
Myoepithelial cells develop from an epidermal germ.
They meet in sudoral, milk, salivary and plaintive glands and have the general precursors with their secretory cells. Myoepithelial cells immediately adjacent to actually epithelial also have the general with them a basal membrane. At those neogeneses and other cells too are restored from the general few - the differentiated precursors. The majority of myoepithelial cells have the stellar form. These cells quite often name basket-like: their processes cover trailer departments and fine ducts of glands. In a cell body settle down a kernel and general meaning organellas, and in processes - the contractile apparatus organised, as well as in cells of a muscular tissue of mesenchymal type.
Muscular tissue of a neural parentage
Myocytes of this tissue develop from cells of a neural germ as a part of an internal wall of an eyecup. Bodies of these cells settle down in an epithelium of a back surface of an iris. Each of them has a process which is referred to a depth of an iris and its surfaces lay down in parallel. In a process there is the contractile apparatus organised the same as and in all smooth myocytes. Depending on a direction of processes (perpendicularly or in parallel pupil edge) myocytes are formed by two muscles - narrowing and dilating a pupil.
Reduction of muscles
The theory of sliding of strands
H.E. Huxley and A.F. Huxley it is independent from each other in 1954 have offered for an explanation of the mechanism of a muscular contraction the theory of sliding of strands. According to the given theory, the sarcomere shorting, and, hence, and a muscular fiber at the moment of reduction descends thanks to active sliding of thin (actinide) strands concerning thick (myosine) strands. The shorting comes to an end, when actinide filaments are deeply involved on a direction to the centre of a disk which defines borders of sarcomeres. At a release phenomenon or a distention of a muscle the range of mutual overlap of thin and thick filaments is narrowed.
Slipping locomotion of myosine and actinide filaments is from each other caused by the forces generated at interaction of cross-section bridges with actinide filaments.
Cross-section ponticuluses should be attached consistently to an actinide filament, educe force, depart and again be attached in other place. To sustain active reduction, cross-section ponticuluses should work asynchronously, i.e. at any moment the part from them is attached to an actin whereas others are disconnected. After a detachment the cross-section ponticulus should be attached again to an actinide filament, but already further, aside Z - blades, leading thereby the contribution to active sliding along the specified direction.
Researches of mechanical properties of the reduced muscle, spent Huxley and Simmons, have confirmed such point of view on function of cross-section bridges. Authors have shown, that the basic part of an elastic component of the muscle, included consistently with a contractile element, is in cross-section ponticuluses, presumably in the bridged hinge. They have stated thought, that the elastic distention of the hinge serves as an important point in the course of a reserving of mechanical energy at rotation of a head of a miosin round an actinide filament. According to the given hypothesis rotation is generated by the several centres of a myosine head which serially co-operate with the centres on an actinide filament.
Elasticity of the bridged hinge promotes rotation of a head without appreciable intermittent fluctuations of educed force. Having stretched, the bridged hinge will transfer the effort to a thick filament softly, promoting activation of sliding of filaments. One of the main arguments is that, according to Huxley and Simmons, consistently coherent elastic component of a muscular fiber is proportional to size of mutual overlap of thin and thick filaments and consequently, it is proportional to number of the attached cross-section bridges. Authors also have established, that subitaneously arising small shorting is accompanied by very fast ascending of educed effort; they explain it only turn of heads of the cross-section bridges co-operating with an actin, in more stable position.
The short description of processes of reduction and release phenomenon
Let's result the list of the processes supervising reduction of a skeletal muscle.
- The superficial membrane of a muscular fiber will depolarize under the influence of an action potential or (in some muscles) under the influence of synaptic potentials.
- The action potential arrives in depth of a muscular fiber on T-tubules.
- In reply to depolarisation T - tubules the signal which, possibly, is mediated by moleculas Inositol trisphosphate, extends from these tubules to trailer tanks of a sarcoplasmic reticulum.
- This chemical messenger causes discovering of calcium channels in a sarcoplasmic reticulum and liberation sequestrated calcium ions.
- Concentration free Ca2+ in a myoplasm increases from value of 10-7 M and more low (in dormancy) to approximately 10-6 M and more (in an active state). Calcium is bridged to a troponin, causing in a molecula of this protein conformational changes.
- Conformational changes of a molecula of a tropomyosine eliminate a regional interrupting for apposition of cross-section bridges to actinide filaments.
- Myosine cross-section ponticuluses are attached to actinide filaments and enter consecutive interaction with their centres that causes rotation of a myosine head concerning actinide filaments and a tension of the bridged hinge.
- The tension of the bridged hinge leads to active occurrence of actinide filaments in A - disk. The sarcomere is slightly shortened.
- Before there will be a following motion cycle of a myosine cross-section ponticulus, ATP (bound with ATPhased the centre on a myosine head) is hydrolyzed and the energy released thus is reserved in the form of a conformational change in a miosin molecula. The myosine head departs and then is again ready to join the following centre located on length of an actinide filament, and to cycle a loop, described in subitem 7 and 8. During the solitary reduction each cross-section ponticulus in process of the progression to Z - a blade along an actinide filament is attached, tightened and disconnected set of times.
- At last, as a result of active work of a sarcoplasmic reticulum level Ca2+ in a sarcoplasm again goes down, and the tropomyosine starts to interfere with apposition of cross-section bridges. The muscle remains relaxed until will descend following membrane depolarisation.
Between structure sarko - tubular system and muscle function there is an interesting communication. Those muscles which are reduced and relaxed very quickly, have advanced a sarcoplasmic reticulum and an extensive network T - tubules. And those muscles, reduction and which release phenomenon descends slowly, accordingly have less educed a sarcoplasmic reticulum. Various rates of reduction and a release phenomenon, apparently, correlate with efficacyy of a sarcoplasmic reticulum in regulation of changes of concentration of calcium which in turn start and intercept the contractile mechanism.
As it has already been noted, muscular tissues are a bunch of tissues of an organism of the various parentage, united on the basis of contractility: transversely striated (sceletal and warm), smooth, and also specialised retractile tissues - is epithelial-muscular and nejro - glial, a part irises of an eye.
The transversely striated sceletal muscular tissue arises from the myotomes which are a part of elements of the segmented mesoderm - somites.
The smooth muscular tissue of the human and vertebrates educes as a part of mesenchyma derivatives, as well as internal environment tissues. However for all muscular tissues similar isolation as a part of an embryonal germ in the form of cells of the spindle-shaped form - a muscle - of formative cells, or myoblasts is characteristic.
Reduction of a muscular fiber consists in a shorting of myofibrils within each sarcomere. Thick (myosine) and thin (actinide) strands, in the relaxed state bound only trailer departments, at the moment of reduction carry out slipping locomotions towards each other. Abjection of energy necessary for reduction descends as a result of metamorphosis ATP in ADP under the influence of a miosin. Fermental activity of a miosin shows under condition of optimum content Ca2+ which collect in sarcoplasmic reticulum.
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