Lecture 8 General med_2nd semester

1. Lecture 8 General med_2nd semester Muscle tissue General characteristics of muscle tissue and its classification Skeletal muscle tissue. Ultrastructure of the myofibrils. Mechanism of the muscle contraction Cardiac muscle tissue Smooth muscle tissue

2. function of muscle tissue is to move both the body as whole and itsparts with respect to one another morphological and functional characteristics common to all types of muscle cells are as follows: • muscle cells retain and develop contractility to the highest extent of any cells of the body • muscle cells are elongated with well-developed nuclei • the cytoplasm (termed as sarcoplasm) stains red with eosin and contains contractile proteins - actin and myosin- arranged in the form of thin fibers, called myofibrilsthat run parallel to the long axis of the cell; the plasma membrane is termed as sarcolemma 3 types of muscle tissue are to distinguish on both structural and functional basis: • smooth muscle(involuntary or visceral) - occurs chiefly in wall of hollow organs • skeletal muscle(orstriated voluntary)- is attached to bones and fasciae • cardiac muscle(or striated involuntary)- forms the wall of theheart

3. Skeletal muscle giant multinucleated cylindrical cells referred as = muscle fibers orrhabdomyocytes (gr. rhabdos = rod) the length - mm to cm, thickness varies from 10 to 100 mm

4. numerous nuclei are in each fiber (about 35 per 1 mm of itslength) they are ovoid, located peripherally at the sarcolemmawhose surface is coated withthe basal lamina thesarcoplasm contains numerous myofibrilsthat are prominently cross striated with light and dark bands cell organelles are constitutedwith mitochondria (termed as sarcosomes), sarcoplasmic reticulum (smooth ER), and small Golgi apparatuses in juxtanuclearpositions fibers contain alsocell inclusions, mainly glycogen granules and lipid droplets

5. an organelle of great importance is the sarcoplasmic reticulum, a special type of sER it consists of longitudinal sarcotubulesthat are confluent with transversally oriented channels called terminal cisternae pairs of parallel terminal cisternae run transversally across the myofibrils in close apposition to the T tubule T- tubule is an invagination of the sarcolemma andis not part of the sarcoplasmic reticulum two terminal cisternae and T tubule constitute structure called as triad of skeletal muscle triads usually occur in sites where the dark and light bands meet sarkotubules terminal cisternae

8. myofibrils -contractile fibrils resolvable with the LMthat are mostly randomly dispersed within the sarcoplasm their diameter is about 1 -2 mm (in cross sections, the myofibrils appear as dots, in longitudinal sections, they show a conspicuous cross banding) the dark band is called the anisotropic, or A band, because it is doubly refractive to polarized light and appears dark in the fresh state the light isotropic, or I band,is single refractive to polarized light and is pale in the fresh each I band is intersected by a thin dark line - the Z line, likewise, the dark A bandis bisected by a thin, light-staining band, called H band, within it M-line is seen the segment of myofibril extending between two Z lines is a sarcomere, its length varies from 3.5 to 4 mm sarcomere . . . .

10. Ultrastructure of myofibrils by electron microscopy, the myofibrils are composed of more slender filamentous unitscalled myofilaments • 2 setsoffilaments: • thick filaments contain myosin and are 10 nm in diameter and 1.5 mm in length • thin filamentscontain protein actin and are 5 nm in diameter and 2 mm in length

11. Myosin filamentsare confined to the A band and show slight thickening at their centers • these thickenings give rise to the transverse density at the midpoint of A band recognized as M line • in cross sections, they are seen to be arranged in a regular hexagonal pattern • Actin filamentsextend in either direction from the Z line and thus constitute the I band • they are not limited to this band, but extend some distance into the adjacent A bands • whereendsoffilamentsoverlapeachother, thecross bridges are developed between them • the part of the A band thatis free of actin filaments is called the H zone

12. Thick myofilament- 300 to 400 molecules of myosin (asymetric protein consisting of a double heads and tail) Molecularcomponentsoffilaments

13. Thin myofilament- from F-actin+ protein complex composed of troponin and tropomyosin tropomyosin runs around F-actin in 40 nm distances troponin: of 3 proteins troponin I, C and T

14. Mechanismofcontraction • by the sliding filament theory of Huxley • the theorysays: • as a muscle fiber is stimulated to contract, the actin and myosin filaments react by sliding past each other but with no changes in length of either myofilaments • the thick myosin filaments in the A band are relatively stationary, whereas the thin actin filaments, attached to the Z disks, extend further into the A band and may eventually obliterate the H zone • because the thin filaments are attached to Z disks, the Zdisk are drawn toward each other, the sarcomeres are compressed, the myofibril is shortened, and contraction of the muscle occurs • Note:contraction is not due to a shortening of the actin and myosin filaments but to an increase in the overlap between thosefilaments • energy for the contraction process is produced by the breakdown of ATP by ATPase localized in the cross bridges that interconnect the actin and myosin filaments

16. Muscles - structure

18. cardiac muscle cells, termed acardiomyocytes cardiac muscle occurs in the myocardium (muscle layer of the heart) and in the walls oflarge vessels joining the heart Cardiac muscle • ň

19. cardiomyocytes: contractingandnon contracting Contracting (working) cardiomyocytes cells are of cylindrical form, 20 mm in diameter and 50 to 200 mm in length at specialized junctional zones called intercalated discs, cardiac muscle cells are joined end to end and form a linear unit - cardiac muscle fiber intercalated discs- sites of intercellular contacts

21. the cardiomyocyte is enveloped by a thin sarcolemma and contains one elongated nucleus situated centrally between diverging myofibrils, in its vicinity a small Golgi apparatus, lipid droplets and deposits of lipofuscin pigment are found myofibrils are oriented parallel and run from one end to another and show the cross banding, the sarcomeres are composed of the same parts as in skeletal muscle (A, I bands, Z, M lines, H zone) numerous mitochondria arranged in rows and sarcoplasmic reticulum are seen among the myofibrils

22. The intercalated discs are areas of extensive cell contact at sites of intercalated discs, special adhering and communicating junctions arefound:fascia adherens (corresponds to the zonulaadherens), macula adherens (spot desmosome) and nexus (gap junction) Non-contractingcardiomyocytes in addition to the cardiomyocytes whose primary function is contraction, there are specialized cells whose function is to generate the stimuli for the heart beat cellsare located just beneath the endocardium and in a whole represent impuls conducting system of the heart (sinoatrial node - node of Keith-Flack, atrio- ventricular node - node of Tawara, atrioventricular bundle - bundle of Hiss) by light microscopy, cells of theconducting system are larger, thicker and more palely stained, with abundant central sarcoplasm and relatively few myofibrils, located usually in a peripheral position the cells contain also large quantities of glycogen noncontractingcells form similar to contractile cardiac muscle cells fibers called Purkinje fibers

23. smooth muscle tissue is structurally the simplest of the muscle types, it is called smooth because it has no visible crossstriation, involuntary because it is not under conscious control, and visceral because it is predominantly found in organs is composed of spindle shaped cells with one nucleus 3–10 µm thick and 20 - 50 µm long - leiomyocytes (gr. leios = smooth) Smoothmuscle

24. cell nuclei have a cigar shaped form in dilated cells, but are of corkscrew appearance in cells which are contracted cells vary greatly in length, from 20 mm (in the walls of small ducts and blood vessels) to 500 mm (in the pregnant human uterus) the cytoplasm - called sarcoplasm - appears rather homogeneous and is filled with fine contractile filaments (myofilaments) that are hardly seen Remember: the spindle-shaped form is imperceptible only in the longitudinal sections of smooth muscle in the transverse section the smooth muscle cells appear as homogenous discs of various diameters - the largest of the discs are cut through the middle of the cells and usually contain the nucleus

25. Fine structural parameters of smooth muscle cells: • - nucleus is elongated or cigar- shaped and • smoothly contoured • - juxtanuclear sarcoplasm contains long slender mitochondria, a few tubules of smooth endoplasmic reticulum, free ribosomes and small Golgi apparatus • - the plasma membrane is characteristically studded with small vesicular inpocketings or caveoli (micropinocytosis) • - the surface of each smooth muscle cell is invested by a thick extracellular coating that corresponds to the basal lamina of epithelial cells • in certain limited areas of the surface of smooth muscle cells, the basal lamina is lacking and the membranes of neighboring cells come into very close association; such sites are termed as nexuses or gap junctions • they constitute low resistance pathways, permitting a spread of excitation from one cellular unit to another

26. - the bulk of the cytoplasm is occupied by two sets of myofilaments: actin filaments are dominant myosin filaments occuronly in a small amount myofilaments do not exhibit the paracrystalline organization that is seen in the skeletal or cardiac muscle Physiology: smooth mucle cells contract slowly and have low force of contraction they are innervated with adrenergic and cholinergic nerve fibers that act antagonistically nucleus thick filament dense body thin filaments intermediate filament

28. Regeneration of muscle tissue • cardiac musclecells have no regenerative capacity (defects or damage of heart muscle are generally replaced by proliferation of connective tissue forming myocardial scars) • skeletal muscle fibers (rhabdomyocytes) undergoe regeneration in a limited extent as a source for regeneration serveso calledsatellite cells, mononucleated spindle cells located beneth the lasal lamina surrounding mature muscle fiber (satellite cells are inactive myoblasts that persist after muscle differentiation, cells are activated following injury, they proliferate and fuse to form new skeletal muscle fibers) • smooth muscle cells are capable of a modest regenerative response (following injury, viable mononucleated smooth muscle cells undergo mitosis and provide for the replacement of the damaged tissue)

29. Histogenesisofmuscletissue in vertebrates, muscles develop from mesoderm, except some intrinsic muscles of the eye and skinthat are ectodermal in origin • smooth muscle, the most primitive of the three forms of muscle, derives from theembryonic connective tissue - mesenchyme • mostskeletal muscles develop from paired mesodermalcondensationscalledsomites the middle portion of eachsomite, the myotome(muscle plate) thickens and differentiates into myoblaststhat give rise to multinucleated cylindrical cells, called muscle fibers • cardiac muscle cells develop from splanchnic mesoderm that surrounds the primitive heart tubes