Histology LC6 Muscle Tissue PDF

The variation in diameter of muscle fibers COURSE OUTLINE depends on factors such as the specific muscle, age, gender, nutritional status, and...

The variation in diameter of muscle fibers COURSE OUTLINE depends on factors such as the specific muscle, age, gender, nutritional status, and physical training of the individual. I. MUSCLE TISSUE A. Types of Muscle Tissues 1. Skeletal Muscle A. THREE TYPES OF MUSCLE TISSUES 2. Cardiac Muscle 3. Smooth Muscle B. Muscle Cells Common Feature Skeletal Muscle II. SKELETAL MUSCLE Bundles of very long, multinucleated cells with cross A. Embryonic Development Process striations. Contraction is quick and forceful III. ORGANIZATION OF SKELETAL MUSCLE and, usually under voluntary control A. Connective Tissue Layers 1. Epimysium 2. Perimysium 3. Endomysium B. Myotendinous Junctions IV. ORGANIZATION WITHIN MUSCLE FIBER V. THICK FILAMENTS VI. THIN FILAMENTS VII. SARCOPLASMIC RETICULUM AND T-TUBULE SYSTEM A. Triad Figure 1. Skeletal Muscle. VIII. MECHANISM OF CONTRACTION IX. INNERVATION Cardiac Muscle X. SKELETAL MUSCLE FIBER TYPES Also has cross-striations XI. SMOOTH MUSCLE Composed of elongated, often XII. CARDIAC MUSCLE irregular branched cells bound to XIII. REGENERATION OF SMOOTH MUSCLE one another at structures called intercalated discs Contraction is involuntary, vigorous, I. MUSCLE TISSUE and rhythmic. Considered as a connective tissue because they give form and maintains the shape of the body Composed of cells that optimize the universal cell property of contractility Mesodermal origin Differentiate by a gradual process of cell lengthening with abundant synthesis of the myofibrillar proteins actin and myosin Three types of muscle tissue can be distinguished on the basis of morphologic Figure 2. Cardiac Muscle. and functional characteristics with the structure of each adapted to its physiologic Smooth Muscle role. BATCH TANNAWAG  Collections of grouped fusiform cells that lack striations A. EMBRYONIC DEVELOPMENT PROCESS Slow, weak involuntary contractions The density of intercellular packing During embryonic muscle development, seen reflects the small amount of mesenchymal myoblasts (L. myo, muscle) extracellular connective tissue fuse, forming myotubes with many nuclei. present Myotubes then further differentiate to form striated muscle fibers. Elongated nuclei are found peripherally just under the sarcolemma, a characteristic nuclear location unique to skeletal muscle fibers/cells. A small population of reserve progenitor cells called muscle satellite cells remains adjacent to most fibers of differentiated skeletal muscle project into the overlying aqueous layer Figure 3. Smooth Muscle. In all these types of muscle, contraction is caused by the sliding interaction of thick Figure 4. Development of Skeletal Muscle. myosin filaments along thin actin filaments. III. ORGANIZATION OF SKELETAL MUSCLE B. MUSCLE CELLS COMMON FEATURE Thin layers of connective tissue surround and organize the contractile fibers in all three Muscle specialists refer to certain muscle types of muscle. cell organelles with special names. Seen particularly well in skeletal muscle Sarcoplasm (Gr. sarkos, flesh + plasma, Resembles that in large peripheral nerves thing formed) cytoplasm Sarcoplasmic reticulum - smooth endoplasmic reticulum Sarcolemma (sarkos + Gr. lemma, husk) CONNECTIVE TISSUE LAYERS Smooth muscle membrane 1. Epimysium II. SKELETAL MUSCLE External sheath of thick layer of dense connective tissue “Striated muscle” Surrounds the entire muscle Diameters range from 10 to 100 μm. BATCH TANNAWAG  Septa of this tissue extend inward, carrying IV. ORGANIZATION WITHIN MUSCLE the larger nerves, blood vessels, and FIBER lymphatics of the muscle Longitudinally sectioned skeletal muscle Continuous with fascia and the tendon fibers show cross- striations of alternating binding muscle to bone. light and dark bands. 2. Perimysium Thin connective tissue layer that immediately A. A BANDS surrounds each bundle of muscle fibers the dark bands (anisotropic or termed as fascicle. birefringent in polarized light microscopy) Fascicle of muscle fibers: functional unit in B. I BANDS which the fibers work together. Nerves, blood vessels, and lymphatics the light bands (isotropic, do not alter penetrate the perimysium to supply each polarized light) fascicle. bisected by a dark transverse line 3. Endomysium A very thin, delicate layer of connectives C. Z DISC tissue (reticular fibers and scattered a dark line (Ger. zwischen, between) that fibroblasts) within a fascicle bisects each I band Surrounds the external lamina of individual The sarcoplasm has little RER and contains muscle fibers. primarily long cylindrical filament bundles, In addition to nerve fibers, capillaries form a called myofibrils, running parallel to the long rich network in the endomysium bringing O2 axis of the fiber. to the muscle fibers. Underneath the light microscopy, the light Collagen in these connective tissue layers of band is bisected in the middle by a dark muscle serve to transmit the mechanical bank called the Z line. Z line to Z line makes forces generated by the contracting muscle up one sarcomere. Peripheral nuclei are cells/fibers. interspersed underneath the sarcolemma. Individual muscle fibers seldom extend from The repetitive functional subunit of the one end of a muscle to the other. contractile apparatus, the sarcomere, extends from Z disc to Z disc and is about B. MYOTENDINOUS JUNCTIONS 2.5 μm long in resting muscle. Epimysium is continuous with the dense Mitochondria and sarcoplasmic reticulum are regular connective tissue of a tendon at found between the myofibrils (1 to 2 μm). myotendinous junctions Myofibrils consist of an end-to end repetitive arrangement of sarcomeres; the lateral All three layers are continuous with registration of sarcomeres in adjacent the tough connective tissue of a tendon at myofibrils causes the entire muscle fiber to myotendinous junctions exhibit a characteristic pattern of transverse striations The A and I banding pattern - due mainly to the regular arrangement of thick and thin myofilaments organized within each myofibril in a symmetric pattern containing thousands of each filament type. Figure 5. Organization of Skeletal Muscle BATCH TANNAWAG  V. THICK FILAMENTS 1.6 μm long; 15 nm wide Occupy the A band at the middle region of the sarcomere Composed of Myosin - a large complex (~500 kDa) with two identical heavy chains and two pairs of light chains. Two Heavy Chains Thin, rod-like motor proteins Figure 6. Parts of three muscle fibers are separated by very 150 nm long; 2-3 nm thick small amounts of endomysium. One fibroblast nucleus (F) is Twisted together as myosin shown. Muscle nuclei (N) are found against the sarcolemma. tails Along each fiber thousands of dark-staining A bands alternate with lighter I bands. X200. H&E. Four Light Chains Form a head at one each end of each heavy chain. The myosin heads bind both actin, forming transient cross bridges between the thick and thin filaments. ATP catalyzing energy release (actomyosin ATPase activity). Several hundred myosin molecules are arranged within each thick filament with overlapping rod-like portions and the globular heads directed toward either end. Figure 7. Each fiber can be seen to have three or four myofibrils, here with their striations slightly out of alignment with one another. Myofibrils are cylindrical bundles of thick and thin myofilaments that fill most of each muscle fiber. The middle of each I band can be seen to have a darker Z line (or disc). X500. Giemsa. Figure 9. A thick myofilament contains 200-500 molecules of myosin. VI. THIN FILAMENTS 1.0 μm long; 8 nm wide Helical in shape Figure 8. TEM showing the more electron-dense A bands Run between the thick filaments bisected by a narrow, less electron-dense region called the H Composed of F-actin zone and in the I bands the presence of sarcoplasm with Anchored perpendicularly on the Z disc by mitochondria (M), glycogen granules, and small cisternae of actin-binding protein. α-actinin. SER around the Z line. X24,000. Exhibit opposite polarity on each side of the Z disc. BATCH TANNAWAG  Thin filaments are also tightly associated binds each thin myofilament with two regulatory proteins: laterally Tropomyosin Helps anchor them to α-actinin 40-nm-long Specifies the length of the actin Coil of two polypeptide chains polymers during myogenesis. located in the groove between the two twisted actin strands. Troponin A complex of three subunits: TnT -which attaches to tropomyosin TnC -which binds Ca2+ Tnl-which Figure 10. A thin filament contains F-actin, tropomyosin and regulates the actin-myosin troponin. interaction (I for Inhibition) I BANDS VII. SARCOPLASMIC RETICULUM AND Bisected by a Z disc T-TUBULE SYSTEM Consist of the portions of the thin filaments that do not overlap the thick Specialized for Ca2+ sequestration. filaments (reason why I bands stain Depolarization of the sarcoplasmic reticulum more lightly) membrane, which causes release of calcium, A BANDS is initiated at specialized motor nerve Contain both thick filaments and the synapses on the sarcolemma. overlapping portions of thin filaments. Sarcolemma is folded into a system of H ZONE transverse or T tubules - penetrate deeply a lighter zone at the center of the A into the sarcoplasm and encircle every band myofibril near the aligned A - and I - band corresponding to a region with only the boundaries of sarcomeres to trigger Ca2+ rod-like portions of the myosin molecule release from sarcoplasmic reticulum and no thin filaments throughout the fiber simultaneously and M LINE cause uniform contraction of all myofibrils (Ger. Mitte, middle) Bisects the H zone TRIAD Containing a myosin-binding protein myomesin that holds the thick filaments Complex of a T tubule with two closely in place associated small cisterns of sarcoplasmic Myomesin catalyzes transfer of reticulum on each phosphate groups from After depolarization, calcium ions phosphocreatine, a storage form of concentrated within these cisternae are high-energy phosphate groups, to ADP, released through Ca2+ channels in the helping to supply ATP for muscle membrane into cytoplasm. contraction. Ca2+ binds troponin and allows bridging OTHER PROTEINS FOUND IN between actin and myosin molecules. SARCOMERE: When depolarization ends, the Titin sarcoplasmic reticulum pumps Ca2+ 3700 kDa back into the cisternae, ending Accessory protein in the I band contractile activity. The largest protein in the body Together, the triad components make up With scaffolding and a signaling apparatus for converting elastic properties repeated cell membrane depolarizations Supports the thick myofilaments into spikes of free cytoplasmic Ca2+ that and connects them to the Z disc trigger the contraction. Nebulin 600-900 kDa BATCH TANNAWAG  transverse tubule (T) and two adjacent terminal cisternae (TC) extending from the sarcoplasmic reticulum. Centrally located is the Z disc. Besides elements of the triad, sarcoplasm surrounding the myofibril also contains dense glycogen granules (G). VIII. MECHANISM OF CONTRACTION Filaments do not change their length. Results as the overlapping thin and thick filaments of each sarcomere slide past one another. Induced when an action potential arrives at a Figure 11.. Skeletal muscle fibers are composed mainly of synapse, the neuromuscular junction (NMJ), myofibrils. Each myofibril extends the length of the fiber and and is transmitted along the T tubules to the is surrounded by parts of the sarcoplasmic reticulum. The sarcoplasmic reticulum to trigger Ca2+ sarcolemma has deep invaginations called T-tubules, each release. of which becomes associated with two terminal cisternae of Stimuli neurotransmitter secretion → the sarcoplasmic reticulum. A T-tubule and its two Excitation of T-System → Release of calcium associated terminal cisterna comprise a “triad” of small → Formation of cross-bridges → Sliding of spaces along the surface of the myofibrils. actin filaments → H band diminishes 1. A nerve impulse triggers release of ACh from the synaptic knob into the synaptic cleft. ACh binds to Ach receptors in the motor end plate of the neuromuscular junction, initiating a muscle impulse in the sarcolemma of the muscle fiber. 2. As the muscle impulse spreads quickly from the sarcolemma along T tubules, calcium ions are released from terminal cisternae into the sarcoplasm. Figure 12. Muscle shows portions of two fibers and the endomysium (E) between them. Several transverse or T-tubules (T) are shown, perpendicular to the fiber surface, penetrating between myofibrils (M). Figure 14. Schematic representation of number 1 and number 2. 3. Calcium ions bind to troponin. Troponin changes shape, moving tropomyosin on the actin to expose active sites on actin molecules of thin filaments. Myosin heads of thick filaments attach to exposed active sites to form cross-bridges. Figure 13. Skeletal muscle in the longitudinal section shows four membranous triads (Tr) cut transversely near the A-band–I-band junctions. Each triad consists of a central BATCH TANNAWAG  IX. INNERVATION Innervation is to supply nerves to something, but it can also mean to energize. Myelinated motor nerves branch out within the perimysium connective tissue which gives rise to several unmyelinated terminal twigs that pass-through endomysium and form synapses with individual muscle fibers. Schwann cells enclose the small axon branches and cover their points of contact with the muscle cells; the external lamina of Figure 15. Schematic representation of number 3. the Schwann cell fuses with that of the sarcolemma of the muscle cell it is attached 4. Myosin heads pivot, moving thin filaments to. toward the sarcomere center. ATP binds myosin Motor end plate (MEP), or NMJ is a dilated heads and is broken down into ADP and P. Myosin termination of each axonal branch. heads detach from thin filaments and return to their pre-pivot position. The repeating cycle of attach pivot-detach-return slides thick and thin filaments past one another. The sarcomere shortens and the muscle contracts. The cycle continues as long as calcium ions remain bound to troponin to keep active sites exposed. Figure 18. Neuromuscular Junction which is carried to the deeper myofibrils by the T-tubules. Within the axon terminal are mitochondria and numerous synaptic acetylcholine. vesicles containing Figure 16. Schematic representation of number 4. Between the axon and the muscle is a 5. When the impulse stops, calcium ions are actively space, the synaptic cleft. transported into the sarcoplasmic reticulum, Adjacent to the synaptic cleft, the tropomyosin re-covers active sites, and filaments sarcolemma is thrown into numerous deep passively slide back to their relaxed state. junctional folds to provide greater postsynaptic surface area and more transmembrane acetylcholine receptors. Figure 17. Schematic representation of number 5. BATCH TANNAWAG  Figure 19. Diagram of enclosed portion of the SEM indicating key features of a typical MEP: synaptic vesicles of acetylcholine (ACh), a synaptic cleft, and a postsynaptic membrane. This membrane, the sarcolemma, is highly folded to increase the number of ACh receptors at the MEP. Receptor binding initiates muscle fiber depolarization, which is carried to the deeper myofibrils by the T-tubules. X. SKELETAL MUSCLE FIBER TYPES A. Type I - Slow Oxidative Adapted for slow contractions over long periods without fatigue, having many mitochondria, many surrounding capillaries, and much myoglobin, all features that make fresh tissue rich in these fibers dark or red in color Use aerobic respiration (oxygen and glucose) to produce ATP B. Type IIa - Fast, Oxidative - Glycolytic Fibers Have physiological and histological features intermediate between those of the other two types. Primarily use aerobic respiration but because they may switch to anaerobic respiration (glycolysis), fatigue more than SO fibers C. Type IIb - Fast, Glycolytic Fibers Figure 20. Cross section of skeletal muscle demonstrating the distribution of slow (S) type I Specialized for rapid, short-term contraction, fibers, intermediate (I) type IIa fibers, and fast (F) having few mitochondria or capillaries and type IIb fibers. X40. depending largely on anaerobic metabolism of glucose derived from the stored glycogen, features that make such fibers appear white. XI. CARDIAC MUSCLE Rapid contractions lead to rapid fatigue as lactic acid produced by glycolysis accumulates. During embryonic development The FG fibers fatigue more mesenchymal cells around the primitive quickly than the others. heart tube align into chainlike arrays. Cardiac muscle cells form complex junctions between interdigitating processes. Cells within one fiber often branch and join with cells in adjacent fibers. Consists of tightly knit bundles of cells, interwoven in spiraling layers that provide for a characteristic wave of BATCH TANNAWAG  contraction that resembles wringing out of the heart ventricles. Mature cardiac muscle cells are 15-30 μm in diameter and 85-120 μm long, with a striated banding pattern Each cardiac muscle cell usually has only one nucleus and is centrally located. Surrounding the muscle cells is a Figure 21. (a) TEM of cardiac muscle shows abundant delicate sheath of endomysium with a mitochondria (M) and rather sparse sarcoplasmic reticulum rich capillary network. A thicker (SR) in the areas between myofibrils. T-tubules are less perimysium separates bundles and well-organized and are usually associated with one layers of muscle fibers and in specific expanded terminal cistern of SR, forming dyads (D) rather areas forms larger masses of fibrous than the triads of skeletal muscle. Functionally, these connective tissue comprising the structures are similar in these two muscle types. (X30,000) “cardiac skeleton.” (b) Muscle cells from the heart atrium show the presence of Presence of transverse lines that cross the membrane-bound granules (G), mainly aggregated at the fibers at irregular intervals where the nuclear poles. These granules are most abundant in muscle myocardial cells join. These intercalated cells of the right atrium (~600 per cell), but smaller quantities discs represent the interfaces between are also found in the left atrium and the ventricles. The atrial adjacent cells and consist of many junctional granules contain the precursor of a polypeptide hormone, complexes. Transverse regions of these atrial natriuretic factor (ANF). ANF targets cells of the irregular, steplike discs are composed of kidneys to bring about sodium and water loss (natriuresis many desmosomes and fascia adherens and diuresis). This hormone thus opposes the actions of aldosterone and antidiuretic hormone, whose effects on junctions, which together provide strong kidneys result in sodium and water conservation. (X10,000) intercellular adhesion during the cells’ constant contractile activity. The less abundant, longitudinally oriented XII. SMOOTH MUSCLE regions of each intercalated disc run parallel to the myofibrils and are filled with gap Also called visceral muscle junctions that provide ionic continuity Major component of blood vessels and of the between the cells. These regions serve as digestive, respiratory, urinary, and “electrical synapses,” promoting rapid reproductive tracts and their associated impulse conduction through many cardiac organ muscle cells simultaneously and contraction Smooth muscle is specialized for slow, of many adjacent cells as a unit. steady contraction under the influence of Muscle of the heart ventricles is much thicker autonomic nerves and various hormones, than that of the atria. T-tubules in ventricular and is controlled by a variety of involuntary muscle fibers are well-developed, with large mechanisms. lumens and penetrate the sarcoplasm in the This type of muscle is a major component of vicinity of the myofibrils’ Z discs. In atrial blood vessels and of the digestive, muscle T-tubules are much smaller or respiratory, urinary, and reproductive tracts entirely absent. The junctions between its and their associated organs. terminal cisterns and T-tubules typically Fibers are elongated, tapering, and involve only one structure of each type, non-striated cells, each of which is enclosed forming profiles called dyads rather than by a thin basal lamina and a fine network of triads in TEM reticular fibers, the endomysium. BATCH TANNAWAG  Concentrated near the nucleus are mitochondria, polyribosomes, RER, and the Golgi apparatus. The short membrane invaginations, called caveolae, are often frequent at the smooth muscle cell surface. Smooth muscle cells also have an elaborate array of 10-nm intermediate filaments, usually composed of desmin. Figure 22. (a) In a cross section of smooth muscle in the wall of the small intestine, cells of the inner circular (IC) layer are cut lengthwise and cells of the outer longitudinal layer (OL) are cut transversely (b) Section of smooth muscle in bladder shows fibers in cross section (XS) and longitudinal section (LS) with the same fascicle. There is much collagen in the branching perimysium (P), but very little evidence of endomysium is apparent. Connective tissues serve to combine the forces generated by each smooth muscle fiber into a concerted action. May range in length from 20μm in small Figure 24. Contraction of Smooth Muscle Fiber blood vessels to 500μm in the pregnant The intermediate filaments and F-actin uterus. filaments both insert into cytoplasmic and Each cell has a single long nucleus in the plasmalemma associated dense bodies. center of the cell’s central, broadest part. Dense bodies contain α-actinin and are The cells stain uniformly along their lengths functionally similar to the Z discs of striated The narrow part of one cell lies adjacent to and cardiac muscle. the broad parts of neighboring cells. The endomysium and other connective tissue layers help combine the force generated by the smooth muscle fibers into a concerted action, for example peristalsis in the intestine. The attachments of thin and intermediate filaments to the dense bodies helps transmit contractile force to adjacent smooth muscle cells and their surrounding network of reticular fibers. Not under voluntary control, and its fibers lack MEPs. Figure 23. Micrograph showing a contracted (C) region of smooth muscle, with contraction decreasing the cell length and deforming the nuclei. The long nuclei of individual fibers assume a cork-screw shape when the fibers contract, reflecting the reduced cell length at contraction. Connective tissue (CT) of the perimysium outside the muscle fascicle is stained blue. X240. Mallory trichrome. All cells are linked by numerous gap junctions. The borders of the cell become scalloped Figure 25. Thin filament attachment to dense bodies located when smooth muscle contracts and the at the cell membrane and deep in the cytoplasm. nucleus becomes distorted. BATCH TANNAWAG  Control can involve autonomic nerves, a References: variety of hormones and similar substances, Dr. Lacuesta’s Powerpoint Presentation: Muscle Tissue. University and local physiologic conditions such as the of Northern Philippines – College of Medicine. School Year degree of stretch. 2023-2024. Whether smooth muscle fibers contract as Mescher, A.L. (2016). Junqueira’s Basic Histology Text and Atlas 15th Edition. McGraw Hill Education. small groups or throughout an entire muscle to produce waves of contraction, it is determined largely by the degree of autonomic innervation and the density of the gap junctions; both conditions vary considerably in different organs. Swellings of autonomic nerve axons with synaptic vesicles simply lie in close contact with the sarcolemma with little or no specialized structure to the junctions. Smooth muscle is most often spontaneously active without nervous stimuli, its nerve supply serves primarily to modify activity rather than to initiate it. Receives both adrenergic and cholinergic nerve endings that act antagonistically, stimulating or depressing its activity. Supplement fibroblast activity, synthesizing collagen, elastin, and proteoglycans, with a major influence on the extracellular matrix (ECM) in tissues. Active synthesis of ECM by the small cells/fibers of smooth muscle may reflect less specialization for strong contractions than in skeletal and cardiac muscle and is similar to this synthetic function in other contractile cells, such as myofibroblasts and pericytes. XIII. REGENERATION OF SMOOTH MUSCLE Repair and regeneration can occur in skeletal muscle because of a population of reserve muscle satellite cells that can proliferate, fuse, and form new muscle fibers. Cardiac muscle lacks satellite cells and has little capacity for regeneration. Defects or damage (e.g., infarcts) to heart muscle are generally replaced by proliferating fibroblasts and growth of connective tissue, forming only myocardial scars Regeneration is rapid in smooth muscle because the cells/fibers are small and relatively less differentiated, which allow renewed mitotic activity after injury. BATCH TANNAWAG  XIV. APPENDIX BATCH TANNAWAG