Histology Lecture 7: Muscle Tissues (University of Northern Philippines, 2022)

UNIVERSITY OF NORTHERN PHILIPPINES HISTOLOGY LECTURE 7 Muscle Tissue COLLEGE OF MEDICINE, BATCH 2026 Transcri...

UNIVERSITY OF NORTHERN PHILIPPINES HISTOLOGY LECTURE 7 Muscle Tissue COLLEGE OF MEDICINE, BATCH 2026 Transcribers: Gadut, Gorospe, Kwaw, Lucas Dr. N. Lacuesta Jr. | Oct. 24, 2022 Editors: Gadut, Gorospe, Kwaw, Lucas MUSCLE TISSUE I. MUSCLE TISSUE A. Types of Muscle Tissues 1. Skeletal Muscle 2. Cardiac Muscle 3. Smooth Muscle B. Muscle Cells Common Features II. SKELETAL MUSCLE A. Embryonic Development Process III. ORGANIZATION OF SKELETAL MUSCLE 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 & T-TUBULE SYSTEM A. Triad VIII. MECHANISM OF CONTRACTION Figure 1. Skeletal muscle IX. INNERVATION X. SKELETAL MUSCLE FIBER TYPES A. Type I 2. Cardiac Muscle B. Type IIa Also has cross-striations C. Type IIb Composed of elongated, often irregular branched cells XI. SMOOTH MUSCLE bound to one another at structures called intercalated discs XII. REGENERATION OF SMOOTH MUSCLE Contraction is involuntary, vigorous, and rhythmic. XIII. REFERENCES XIV. APPENDIX I. MUSCLE TISSUE - 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 and functional characteristics with the structure of each adapted to its physiologic role. - The variation in diameter of muscle fibers depends on factors such as the specific muscle, age, gender, nutritional status, and physical training of the individual. A. THREE TYPES OF MUSCLE TISSUES 1. Skeletal Muscle Bundles of very long, multinucleated cells with cross- striations. Figure 2. Cardiac muscle Contraction is quick and forceful and, usually under voluntary control 1 [HISTOLOGY] 1.07 MUSCLE TISSUE – Dr. Nicanor B. Lacuesta Jr. 3. Smooth Muscle Collections of grouped fusiform cells that lack striations Slow, weak involuntary contractions The density of intercellular packing seen reflects the small amount of extracellular connective tissue present. Figure 4. Development of Skeletal Muscle III. ORGANIZATION OF SKELETAL MUSCLE - Thin layers of connective tissue surround and organize the contractile fibers in all three types of muscle - Seen particularly well in skeletal muscle - Resembles that in large peripheral nerves A. CONNECTIVE TISSUE LAYERS 1. Epimysium External sheath of thick layer of dense connective tissue Figure 3. Smooth muscle Surrounds the entire muscle Septa of this tissue extend inward, carrying the larger nerves, - In all these types of muscle, contraction is caused by the sliding blood vessels, and lymphatics of the muscle interaction of thick myosin filaments along thin actin filaments. Continuous with fascia and the tendon binding muscle to bone B. MUSCLE CELLS COMMON FEATURES 2. Perimysium - Muscle specialists refer to certain muscle cell organelles with Thin connective tissue layer that immediately surrounds each special names. bundle of muscle fibers termed as fascicle. Sarcoplasm (Gr. sarkos, flesh + plasma, thing formed) Fascicle of muscle fibers: functional unit in which the fibers cytoplasm work together. Sarcoplasmic reticulum - smooth endoplasmic reticulum Nerves, blood vessels, and lymphatics penetrate the Sarcolemma (sarkos + Gr. lemma, husk) - Smooth muscle perimysium to supply each fascicle. membrane 3. Endomysium II. SKELETAL MUSCLE A very thin, delicate layer of connectives tissue (reticular fibers and scattered fibroblasts) within a fascicle - “Striated muscle” Surrounds the external lamina of individual muscle fibers. - Diameters range from 10 to 100 μm. In addition to nerve fibers, capillaries form a rich network in the endomysium bringing O2 to the muscle fibers. A. EMBRYONIC DEVELOPMENT PROCESS 1. During embryonic muscle development, mesenchymal - Collagen in these connective tissue layers of muscle serve to myoblasts (L. myo, muscle) fuse, forming myotubes with transmit the mechanical forces generated by the contracting many nuclei. muscle cells/fibers. 2. Myotubes then further differentiate to form striated muscle - Individual muscle fibers seldom extend from one end of a muscle fibers. to the other. 3. Elongated nuclei are found peripherally just under the sarcolemma, a characteristic nuclear location unique to B. MYOTENDINOUS JUNCTIONS skeletal muscle fibers/cells. - Epimysium is continuous with the dense regular connective tissue 4. A small population of reserve progenitor cells called muscle of a tendon at myotendinous junctions satellite cells remains adjacent to most fibers of Collagen fibers from the tendon insert among muscle differentiated skeletal muscle. fibers and associate directly with infoldings of sarcolemma 2 [HISTOLOGY] 1.07 MUSCLE TISSUE – Dr. Nicanor B. Lacuesta Jr. - All three layers are continuous with the tough connective tissue - The A and I banding pattern - due mainly to the regular of a tendon at myotendinous junctions arrangement of thick and thin myofilaments organized within each myofibril in a symmetric pattern containing thousands of each filament type. Figure 6. Parts of three muscle fibers are separated by very small amounts of endomysium. One fibroblast nucleus (F) is shown. Muscle nuclei (N) are found against the sarcolemma. Along each fiber thousands of dark-staining A bands alternate with lighter I bands. X200. H&E. Figure 5. Organization of Skeletal Muscle IV. ORGANIZATION WITHIN THE MUSCLE FIBER - Longitudinally sectioned skeletal muscle fibers show cross- striations of alternating light and dark bands. A bands - the dark bands (anisotropic or birefringent in polarized light microscopy) Figure 7. Each fiber can be seen to have three or four myofibrils, I bands here with their striations slightly out of alignment with one - the light bands (isotropic, do not alter polarized light) another. Myofibrils are cylindrical bundles of thick and thin - bisected by a dark transverse line myofilaments that fill most of each muscle fiber. The middle of Z disc each I band can be seen to have a darker Z line (or disc). X500. - a dark line (Ger. zwischen, between) that bisects each I band Giemsa. - The sarcoplasm has little RER and contains primarily long cylindrical filament bundles, called myofibrils, running parallel to the long axis of the fiber. - Underneath the light microscopy, the light band is bisected in the middle by a dark bank called the Z line. Z line to Z line makes up one sarcomere. Peripheral nuclei are interspersed underneath the sarcolemma. - The repetitive functional subunit of the contractile apparatus, the sarcomere, extends from Z disc to Z disc and is about 2.5 μm long in resting muscle. - Mitochondria and sarcoplasmic reticulum are found between the myofibrils (1 to 2 μm). Figure 8. TEM showing the more electron-dense A bands bisected - Myofibrils consist of an end-to end repetitive arrangement of by a narrow, less electron-dense region called the H zone and in sarcomeres; the lateral registration of sarcomeres in adjacent the I bands the presence of sarcoplasm with mitochondria (M), myofibrils causes the entire muscle fiber to exhibit a characteristic glycogen granules, and small cisternae of SER around the Z line. pattern of transverse striations. X24,000. 3 [HISTOLOGY] 1.07 MUSCLE TISSUE – Dr. Nicanor B. Lacuesta Jr. V. THICK FILAMENTS I bands - Bisected by a Z disc - 1.6 μm long; 15 nm wide - Consist of the portions of the thin filaments that do not - Occupy the A band at the middle region of the sarcomere overlap the thick filaments (reason why I bands stain more - Composed of Myosin - a large complex (~500 kDa) with two lightly) identical heavy chains and two pairs of light chains. A bands Two Heavy Chains - Contain both thick filaments and the overlapping portions of ▪ Thin, rod-like motor proteins thin filaments. ▪ 150 nm long; 2-3 nm thick H zone ▪ Twisted together as myosin tails - a lighter zone at the center of the A band Four Light Chains - corresponding to a region with only the rod-like portions of the ▪ Form a head at one each end of each heavy myosin molecule and no thin filaments chain. M line - The myosin heads bind both actin, forming transient cross bridges - (Ger. Mitte, middle) between the thick and thin filaments. - Bisects the H zone - ATP catalyzing energy release (actomyosin ATPase activity). - Containing a myosin-binding protein myomesin that holds the - Several hundred myosin molecules are arranged within each thick thick filaments in place filament with overlapping rod-like portions and the globular heads - Myomesin catalyzes transfer of phosphate groups from directed toward either end. phosphocreatine, a storage form of high-energy phosphate groups, to ADP, helping to supply ATP for muscle contraction. Other Proteins found within the Sarcomere: 1. Titin ▪ 3700 kDa ▪ Accessory protein in the I band ▪ The largest protein in the body ▪ With scaffolding and elastic properties ▪ Supports the thick myofilaments and connects them to the Z disc 2. Nebulin ▪ 600-900 kDa ▪ binds each thin myofilament laterally ▪ Helps anchor them to α-actinin ▪ Specifies the length of the actin polymers during myogenesis. Figure 9. A thick myofilament contains 200-500 molecules of myosin. VI. THIN FILAMENTS - 1.0 μm long; 8 nm wide - Helical in shape - Run between the thick filaments - Composed of F-actin - Anchored perpendicularly on the Z disc by actin-binding protein. Figure 10. A thin filament contains F-actin, tropomyosin, and α-actinin. troponin. - Exhibit opposite polarity on each side of the Z disc. - Thin filaments are also tightly associated with two regulatory VII. SARCOPLASMIC RETICULUM & TRANSVERSE TUBULAR SYSTEM proteins: 1. Tropomyosin - Specialized for Ca2+ sequestration. ▪ 40-nm-long - Depolarization of the sarcoplasmic reticulum membrane, which ▪ Coil of two polypeptide chains located in the causes release of calcium, is initiated at specialized motor nerve groove between the two twisted actin strands. synapses on the sarcolemma. 2. Troponin - Sarcolemma is folded into a system of transverse or T tubules - ▪ A complex of three subunits: penetrate deeply into the sarcoplasm and encircle every myofibril 1. TnT – which attaches to tropomyosin near the aligned A - and I - band boundaries of sarcomeres to 2. TnC – which binds Ca2+ trigger Ca2+ release from sarcoplasmic reticulum throughout the 3. Tnl – which regulates the actin-myosin fiber simultaneously and cause uniform contraction of all interaction myofibrils 4 [HISTOLOGY] 1.07 MUSCLE TISSUE – Dr. Nicanor B. Lacuesta Jr. A. TRIAD Complex of a T tubule with two closely associated small cisterns of sarcoplasmic reticulum on each side. ▪ After depolarization, calcium ions concentrated within these cisternae are released through Ca2+ channels in the membrane into cytoplasm. ▪ Ca2+ binds troponin and allows bridging between actin and myosin molecules. ▪ When depolarization ends, the sarcoplasmic reticulum pumps Ca2+ back into the cisternae, ending contractile activity. ▪ Together, the triad components make up a signaling apparatus for converting repeated cell membrane depolarizations into spikes of free cytoplasmic Ca2+ that Figure 13. Skeletal muscle in longitudinal section shows four trigger the contraction. membranous triads (Tr) cut transversely near the A-band–I-band junctions. Each triad consists of a central transverse tubule (T) and two adjacent terminal cisterns (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 synapse, the neuromuscular junction (NMJ), and is transmitted along the T tubules to the sarcoplasmic reticulum to trigger Ca2+ release. Figure 11. Skeletal muscle fibers are composed mainly of myofibrils. Each Stimuli neurotransmitter secretion Excitation of T-System myofibril extends the length of the fiber and is surrounded by parts of the Release of calcium Formation of cross-bridges Sliding of sarcoplasmic reticulum. The sarcolemma has deep invaginations called T- actin filaments H band diminishes tubules, each of which becomes associated with two terminal cisternae of the sarcoplasmic reticulum. A T-tubule and its two associated terminal 1. A nerve impulse triggers release of ACh from the synaptic cisterna comprise a “triad” of small spaces along the surface of the knob into the synaptic cleft. ACh binds to Ach receptors in the myofibrils. 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, Figure 14. Schematic representation of number 1 and number 2. perpendicular to the fiber surface, penetrating between myofibrils (M). 5 [HISTOLOGY] 1.07 MUSCLE TISSUE – Dr. Nicanor B. Lacuesta Jr. 3. Calcium ions bind to troponin. Troponin changes shape, IX. INNERVATION moving tropomyosin on the actin to expose active sites on actin molecules of thin filaments. Myosin heads of thick - Innervation is to supply nerves to something, but it can also mean filaments attach to exposed active sites to form cross- to energize. bridges. - 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 the Schwann cell fuses with that of the sarcolemma of the muscle cell it is attached to. - Motor end plate (MEP), or NMJ is a dilated termination of each axonal branch. Figure 15. Schematic representation of number 3. 4. Myosin heads pivot, moving thin filaments toward the sarcomere center. ATP binds myosin heads and is broken down into ADP and P. Myosin 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. Neurumuscular Junction - Within the axon terminal are mitochondria and numerous synaptic vesicles containing acetylcholine. - Between the axon and the muscle is a space, the synaptic cleft. - Adjacent to the synaptic cleft, the sarcolemma, is thrown into numerous deep junctional folds to provide greater postsynaptic surface area and more transmembrane acetylcholine receptors. Figure 16. Schematic representation of number 4. 5. When the impulse stops, calcium ions are actively transported into the sarcoplasmic reticulum, tropomyosin re- covers active sites, and filaments passively slide back to their relaxed state. 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, Figure 17. Schematic representation of number 5. which is carried to the deeper myofibrils by the T-tubules. 6 [HISTOLOGY] 1.07 MUSCLE TISSUE – Dr. Nicanor B. Lacuesta Jr. 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 Figure 21. (a) In a cross section of smooth muscle in the wall of those of the other two types. the small intestine, cells of the inner circular (IC) layer are cut - Primarily use aerobic respiration but because they may switch to lengthwise and cells of the outer longitudinal layer (OL) are cut anaerobic respiration (glycolysis), fatigue more than SO fibers transversely (b) Section of smooth muscle in bladder shows fibers in cross section (XS) and longitudinal section (LS) with the same C. Type IIb - Fast, Glycolytic Fibers fascicle. There is much collagen in the branching perimysium (P), - Specialized for rapid, short-term contraction, having few but very little evidence of endomysium is apparent. mitochondria or capillaries and depending largely on anaerobic metabolism of glucose derived from the stored glycogen, features - Connective tissues serve to combine the forces generated by each that make such fibers appear white. smooth muscle fiber into a concerted action. - May range in length from 20μm in small blood vessels to 500μm - Rapid contractions lead to rapid fatigue as lactic acid produced by in the pregnant uterus. glycolysis accumulates. - Each cell has a single long nucleus in the center of the cell’s - The FG fibers fatigue more quickly than the others. central, broadest part. - The cells stain uniformly along their lengths - The narrow part of one cell lies adjacent to the broad parts of neighboring cells. Figure 20. Cross section of skeletal muscle demonstrating the distribution of slow (S) type I fibers, intermediate (I) type IIa fibers, and fast (F) type IIb fibers. X40 XI. SMOOTH MUSCLE - Also called visceral muscle Figure 22. Micrograph showing a contracted (C) region of smooth muscle, with contraction decreasing the cell length and deforming - Smooth muscle is specialized for slow, steady contraction under the nuclei. The long nuclei of individual fibers assume a cork-screw the influence of autonomic nerves and various hormones, and is shape when the fibers contract, reflecting the reduced cell length controlled by a variety of involuntary mechanisms. at contraction. Connective tissue (CT) of the perimysium outside - This type of muscle is a major component of blood vessels and of the muscle fascicle is stained blue. X240. Mallory trichrome. the digestive, respiratory, urinary, and reproductive tracts and their associated organs. - All cells are linked by numerous gap junctions. - Fibers are elongated, tapering, and non-striated cells, each of - The borders of the cell become scalloped when smooth muscle contracts and the nucleus becomes distorted. which is enclosed by a thin basal lamina and a fine network of - Concentrated near the nucleus are mitochondria, polyribosomes, reticular fibers, the endomysium. RER, and the Golgi apparatus. 7 [HISTOLOGY] 1.07 MUSCLE TISSUE – Dr. Nicanor B. Lacuesta Jr. - The short membrane invaginations, called caveolae, are often is determined largely by the degree of autonomic innervation and frequent at the smooth muscle cell surface. the density of the gap junctions; both conditions vary considerably - Smooth muscle cells also have an elaborate array of 10-nm in different organs. intermediate filaments, usually composed of desmin. - 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. Figure 23. Contraction of Smooth Muscle Fiber XII. REGENERATION OF MUSCLE TISSUE - The intermediate filaments and F-actin filaments both insert into - Repair and regeneration can occur in skeletal muscle because of a cytoplasmic and plasmalemma- associated dense bodies. population of reserve muscle satellite cells that can proliferate, - Dense bodies contain α-actin in and are functionally similar to the fuse, and form new muscle fibers. Z discs of striated and cardiac muscle. - Cardiac muscle lacks satellite cells and has little capacity for - The attachments of thin and intermediate filaments to the dense regeneration. Defects or damage (e.g., infarcts) to heart muscle bodies helps transmit contractile force to adjacent smooth muscle are generally replaced by proliferating fibroblasts and growth of cells and their surrounding network of reticular fibers connective tissue, forming only myocardial scars - Not under voluntary control, and its fibers lack MEPs. - Regeneration is rapid in smooth muscle because the cells/fibers are small and relatively less differentiated, which allow renewed mitotic activity after injury. XIII. REFERENCES Mescher, A.L. (2016). Junqueira’s Basic Histology Text and Atlas 15th Edition. McGraw Hill Education. Dr. Lacuesta’s Powerpoint Presentation: Muscle Tissue. University of Northern Philippines – College of Medicine. School Year 2022- 2023. Figure 24. Thin filaments attachment to dense bodies located at the cell membrane and deep in the cytoplasm. - Control can involve autonomic nerves, a variety of hormones and similar substances, and local physiologic conditions such as the degree of stretch. - Whether smooth muscle fibers contract as small groups or throughout an entire muscle to produce waves of contraction, it 8 [HISTOLOGY] 1.07 MUSCLE TISSUE – Dr. Nicanor B. Lacuesta Jr. XIV. APPENDIX 9