[HISTO]LEC_007_MUSCLE TISSUE.pdf

(006) MUSCULAR TISSUES DR. LACUESTA | 10/12/2020 OUTLINE FOUND IN: Any muscle that connects to and controls the motions...

(006) MUSCULAR TISSUES DR. LACUESTA | 10/12/2020 OUTLINE FOUND IN: Any muscle that connects to and controls the motions of the bones I. MUSCLE TISSUE A. Definition of Terms 2. CARDIAC MUSCLE B. Overview of Types of Muscle Tissue - Also has cross-striations II. SKELETAL MUSCLE A. Organization of the Muscle - Composed of elongated, often branched cells bound to one B. Organization of the Skeletal Muscle another at structures called intercalated discs C. Sarcoplasmic Reticulum and Transverse Tubular - Contraction is involuntary, vigorous, and rhythmic System D. Mechanism of Contraction FOUND IN: the heart E. Types of Skeletal Muscles F. Innervation 3. SMOOTH MUSCLE III. SMOOTH MUSCLES - Collections of fusiform cells that lack striations - Slow, involuntary contractions IV. REGENERATION OF MUSCLE TISSUE FOUND IN: The walls of hollow organs, such as the intestines, uterus and stomach. You can also find smooth I. MUSCLE TISSUE muscle in the walls of passageways, including arteries and veins - Connective tissues of the cardiovascular system. This type of involuntary non-striated - Have optimized function of contractility muscle is also found in the tracts of the urinary, respiratory and - Mesodermal origin reproductive systems. In addition to that, you can find smooth - Differentiate by a gradual process of a cell lengthening with muscle in the eyes, where it acts to change the size of the iris and abundant synthesis of the myofibrillar proteins actin and the shape of the lens. The skin also contains smooth muscle myosin which allows hair to raise in response to cold temperatures or fear. DEFINITION OF TERMS II. SKELETAL MUSCLE 1. Sarcoplasm (Gr. sarkos – flesh, plasma – thing formed) - Cytoplasm - Also referred to as striated muscle 2. Sarcoplasmic reticulum - Consists of muscle fibers that are long, cylindrical, - Smooth endoplasmic reticulum multinucleated cells with diameters of 10 to 100 m 3. Sarcolemma (Gr. lemma – husk) - During embryonic muscle development, mesenchymal - Smooth muscle membrane myoblasts (L. myo – muscle) fuse, forming myotubes with many nuclei TYPES OF MUSCLE TISSUE - 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 populations of reserve progenitor cells called muscle satellite cells remain adjacent to most fibers of differentiated skeletal muscle 1. SKELETAL MUSCLE Figure 1 Development of Skeletal Muscle - Bundles of very long, multinucleated cells with cross- striations - Contraction is quick, forceful - Under voluntary control PREPARED AND EDITED BY: GROUP 8  (006) MUSCULAR TISSUES DR. LACUESTA | 10/12/2020 ORGANIZATION OF MUSCLES A. ORGANIZATION OF SKELETAL MUSCLE - Longitudinally sectioned skeletal muscle fibers show cross striations of alternating light and dark bands Thin layers of connective tissue surround and organize the o Dark bands – called A bands, anisotropic or contractile fibers in all three types of muscle birefringent in polarized light microscopy Seen particularly well in skeletal muscle o Light bands – called I bands, isotropic or do not Resembles that in large peripheral nerves alter polarized light, bisected by a dark transverse line called Z disc (Ger. zwischen – between) Connective Tissue Coverings - The repetitive functional subunit of the contractile ✓ Collagen in these layers serve to transmit the mechanical apparatus, the sarcomere, extends from Z disc to Z disc forces generated by the contracting muscle cells/fibers and is about 2. 5 m long in resting muscle ✓ Individual muscle fibers seldom extend from one end of a - The sarcoplasm has little RER and contains primarily long muscle to the other cylindrical filament bundles, called myofibrils, running ✓ All three layers plus the dense irregular connective tissue of parallel to the long axis of the fiber the deep fascia are continuous with the dense regular - Mitochondria and sarcoplasmic reticulum are found connective tissue of a tendon at myotendinous junctions o Collagen fibers from the tendon insert among between the myofibrils (1 to 2 m) muscle fibers and associate directly with - Myofibrils consist of an end-to-end repetitive arrangement infoldings of sarcolemma of sarcomeres; the lateral registration of sarcomeres in adjacent myofibrils causes the entire muscle fiber to exhibit 1. Epimysium a characteristic pattern of transverse striations - External sheath of dense connective tissue - The A and I banding pattern is due mainly to the regular - Surrounds the entire muscle arrangement of thick and thin myofilaments organized - Extend inward, carrying the larger nerves, blood vessels, with each myofibril in a symmetric pattern containing and lymphatics of the muscle thousands of each filament type - allows a muscle to contract and move powerfully while maintaining its structural integrity - 2. Perimysium - Thin connective tissue layer that immediately surrounds each bundle of muscle fibers termed a fascicle - Fascicle of muscle fibers – functional unit in which the fibers work together - Nerves, blood vessels, and lymphatics penetrate the perimysium to supply each fascicle 3. Endomysium - Within fascicles a very thin, delicate layer of reticular fibers and scattered fibroblasts - Surrounds the external lamina of individual muscle fibers - The endomysium surrounds the extracellular matrix of the cells and plays a role in transferring force produced by the muscle fibers to the tendons. Figure 2 Thick and Thin Filaments - In addition to nerve fibers, capillaries form a rich network in Thick filaments the endomysium bringing O2 to the muscle fibers - 1.6 m long and 15 nm wide - They occupy the A band at the middle region of the sarcomere - Composed of myosin o Myosin – a large complex (~500 kDa) with two identical heavy chains and two pairs of light chains o Two heavy chains – thin rodlike motor proteins (150 nm long and 2-3 nm thick) twisted together as myosin tails o Four myosin light chains – form a head at one end of each heavy chain - The myosin heads bind both actin, forming transient cross- bridges between the thick and thin filaments, and ATP, catalysing energy release (actomyosin ATPase activity). PREPARED AND EDITED BY: GROUP 8  (006) MUSCULAR TISSUES DR. LACUESTA | 10/12/2020 - Several hundred myosin molecules are arranged within each thick filament with overlapping rodlike portions and the globular heads directed toward either end Thin Filaments - Composed of F-actin - Helical - 1.0 m long and 8 nm wide - Run between the thick filaments - Anchored perpendicularly on the Z disc by actin-binding protein -actinin - Exhibit opposite polarity on each side of the Z disc - Tightly associated with two regulatory proteins o Tropomyosin – a 40-nm-long coil of 2 polypeptide chains located in the groove between the two twisted actin strands o Troponin – a complex of 3 subunits: TnT – attaches to tropomysin TnC – binds Ca2+ TnI – regulated the actin-myosin interaction - Troponin complexes attach at specific sites regularly spaced along each tropomyosin molecule A bands - The A bands contain both thick filaments and the overlapping portions of thin filaments - H zone o a lighter zone at the center of the A band o corresponding to a region with only the rodlike portions of the myosin molecule and no thin Figure 3 Myofibril filaments - M line (Ger. mitte – middle) o Bisects the H zone B. SARCOPLASMIC RETICULUM & o Containing a myosin-binding protein myomesin TRANSVERSE TUBULE SYSTEM that holds the thick filaments in place. This enzyme catalyzes transfer of phosphate groups - Specialized for Ca2+ sequestration from phosphocreatine, a storage form of high- - Depolarization of the sarcoplasmic reticulum membrane, energy phosphate groups, to ADP, helping to which causes release of calcium, is initiated at specialized supply ATP for muscle contraction motor nerve synapses on the sarcolemma - Sarcolemma is folded into a system of transverse or T I bands tubules – penetrate deeply into the sarcoplasm and encircle - Each bisected by a Z disc, consist of the portions of the thin every myofibril near the aligned A- and I-band boundaries filaments that do not overlap the thick filaments (which is of sarcomeres – to trigger Ca2+ release from sarcoplasmic why I bands stain more lightly) reticulum throughout the fiber simultaneously and cause - Accessory proteins in I bands: uniform contraction of all myofibrils o Titin (3700 kDa) The largest protein in the body Triad With scaffolding and elastic properties - Complex of a T tubule with two closely associated small Supports the thick myofilaments and cisterns of sarcoplasmic reticulum on each side connects them to the Z disc o After depolarization, calcium ions concentrated within these cisternae are released through Ca2+ o Nebulin (600-900 kDa) channels in the membrane into cytoplasm - Ca2+ Binds each thin myofilament laterally binds troponin and allows bridging between actin Helps anchor them to -actinin and myosin molecules o When depolarization ends, the sarcoplasmic Specifies the length of the actin reticulum pumps Ca2+ back into the cisternae, polymers during myogenesis ending contractile activity - The triad components make up a signalling apparatus for converting repeated cell membrane depolarizations into spikes of free, cytoplasmic Ca2+ that trigger contraction PREPARED AND EDITED BY: GROUP 8  (006) MUSCULAR TISSUES DR. LACUESTA | 10/12/2020 C. MECHANISM OF CONTRACTION - Acetylcholine quickly dissociates from its receptors, and all free neurotransmitter is removed from the synaptic cleft by the extracellular enzyme acetylcholinesterase, preventing Filaments do not change their length prolonged contact of the transmitter with its receptors. Results as the overlapping thin and thick filaments of each - Motor unit – single axon and all the muscle fibers in sarcomere slide past one another contact with its branches Induced when an action potential arrives at a synapse, the - “all or nothing” contraction neuromuscular junction (NMJ), and is transmitted along the T - An axon from a single motor neuron can form MEPs with tubules to the sarcoplasmic reticulum to trigger Ca 2+ release one or many muscle fibers - The lesser the number of fibers innervated by one axon, the 1. A nerve impulse triggers release of ACh from the synaptic greater the precision of its control and movement knob into the synaptic cleft. ACh binds to ACh receptors in the motor end plate of the NMJ, initiating a muscle impulse in the sarcolemma of the muscle fiber D. SKELETAL MUSCLE FIBER TYPES 2. As the muscle impulse spreads quickly from the sarcolemma along T tubules, calcium ions are released 1. Type 1 – slow oxidative from terminal cisternae into the sarcoplasm - For slow contractions over long periods without fatigue 3. Calcium ions bind to troponin. Troponin changes shape, - use aerobic respiration (oxygen and glucose) to produce moving tropomyosin on the actin to expose active sites on ATP. actin molecules of thin filaments. Myosin heads of thick 2. Type IIa – fast, oxidative-glycolytic fibers filaments attach to exposed active sites to form cross - For rapid, short-term contraction bridges. - primarily use aerobic respiration, but because they may 4. Myosin heads pivot, moving thin filaments toward the switch to anaerobic respiration (glycolysis), can fatigue sarcomere center. ATP binds myosin heads and is broken more quickly than SO fibers. down into ADP and P. Myosin heads detach from thin 3. Type IIb – fast, glycolytic fibers filaments and return to their pre-pivot position. The - primarily use anaerobic glycolysis. The FG fibers fatigue repeating cycle of attach-pivot-detach-return slides thick more quickly than the others. and thin filaments past one another. The sarcomere shortens and the muscle contracts. The cycle continues as III. SMOOTH MUSCLES long as calcium ions remain bound to troponin to keep active sites exposed. - Also called as visceral muscle 5. When the impulse stops, calcium ions are actively - Specialized for slow, steady contraction an is controlled by transported into the sarcoplasmic reticulum, tropomyosin a variety of involuntary mechanisms covers active sites, and filaments passively slide back to - Fibers are elongated, tapering, and nonstriated cells, each their relaxed state. of which is enclosed by a thin basal lamina and a fine network of reticular fibers, the endomysium INNERVATION - Connective tissues serve to combine the forces generated by each smooth muscle fiber into a concerted action - Myelinated motor nerves branch out within the perimysium - May range in length from 20 m in small blood vessels to connective tissue – gives rise to several unmyelinated 500 m in the pregnant uterus terminal twigs that pass through endomysium and form - Each cell has a single long nucleus in the center of the synapses with individual muscle fibers. Schwann cells cell’s broadest part. enclose the small axon branches and cover their points of - The cells stain uniformly along their lengths contact with the muscle cells; the external lamina of the - The narrow part of one cell lies adjacent to the broad parts Schwann cell fuses with that of the sarcolemma of neighboring cells - Adjacent to the synaptic cleft, the sarcolemma is thrown - All cells are linked by numerous gap junctions into numerous deep junctional folds, to provide greater - The borders of the cell become scalloped when smooth postsynaptic surface area and more transmembrane muscle contracts and the nucleus becomes distorted acetylcholine receptors - Concentrated near the nucleus are mitochondria, - Within the axonal terminal are mitochondria and numerous polyribosomes, RER, and the Golgi apparatus synaptic vesicles containing acetylcholine - The short membrane invaginations, called caveolae, are - Between the axon and the muscle is a space, the synaptic often frequent at the smooth muscle cell surface. cleft - Smooth muscle cells also have an elaborate array of 10-nm - Motor end plate (MEP) or NMJ – a dilated termination of intermediate filaments, usually composed of desmin each axonal branch - The intermediate filaments and F-actin filaments both insert - Nerve action potential reaches the MEP – acetylcholine is into cytoplasmic and plasmalemma-associated dense liberated from the axon terminal – diffuses across the cleft – bodies binds to its receptors in the folded sarcolemma - Dense bodies contain -actinin and are functionally similar - Acetylcholine receptor contains a nonselective cation to the Z discs of striated and cardiac muscle channel that opens upon neurotransmitter binding – influx of Na+ - depolarizing the sarcolemma – producing the muscle action potential PREPARED AND EDITED BY: GROUP 8  (006) MUSCULAR TISSUES DR. LACUESTA | 10/12/2020 - The attachments of thin and intermediate filaments to the dense bodies helps transmit contractile force to adjacent TEST YOUR KNOWLEDGE smooth muscle cells and their surrounding network of reticular fibers 1. A 40-nm-long coil of 2 polypeptide chains located in the - Not under voluntary control groove between the two twisted actin strands - Fibers lack MEPs a. Tropomyosin - Control can involve autonomic nerves, a variety of b. Troponin hormones and similar substances, and local physiologic c. TnC conditions such as the degree of stretch. d. TnI - Whether smooth muscle fibers contract as small groups or throughout an entire muscle to produce waves of 2. What is the role of acetylcholine in calcium release? contraction is determined largely by the degree of a. Stimulates cellular contraction by opening voltage- autonomic innervation and the density of the gap junctions; gated ion channel both conditions vary considerably in different organs b. Stimulates postsynaptic acetylcholine receptors to - Swellings of autonomic nerve axons with synaptic vesicles release intracellular calcium simply lie in close contact with the sarcolemma with little or c. Binds to sarcoplasmic reticulum, causing calcium no specialized structure to the junctions release - Smooth muscle is most often spontaneously active without d. Binds to the T-tubules, causing calcium release nervous stimuli, its nerve supply serves primarily to modify activity rather than to initiate it. 3. Which of the following correctly identifies muscle - Receives both adrenergic and cholinergic nerve endings components in order from largest to smallest? that act antagonistically, stimulating or depressing its a. Muscle fiber, fasciculus, myofibril activity b. Sarcolemma, sarcoplasm, fasciculu - Supplement fibroblast activity, synthesizing collagen, c. Fasciculus, muscle bundle, myofibril elastin, and proteoglycans, with a major influence on the d. Fasciculus, muscle fiber, myofibril extracellular matrix (ECM) in tissues - Active synthesis of ECM by the small cells/fibers of smooth 4. The space between two Z lines constitutes the: muscle may reflect less specialization for strong a. Sarcolemma contractions than in skeletal and cardiac muscle and is b. Sarcophagus similar to this synthetic function in other contractile cells, c. Sarcomere such as myofibroblasts and pericytes d. Sarcoplasm IV. REGENERATION OF MUSCLE TISSUE 5. We would expect to find smooth muscle in all of the following EXCEPT: - Repair and regeneration can occur in skeletal muscle a. Pulmonary circulatory tissue because of a population of reserve muscle satellite cells b. Esophagus that can proliferate, fuse, and form new muscle fibers c. Small intestine - Cardiac muscle lacks satellite cells and has little capacity d. Myocardial tissue for regeneration (tendons form after myocardial infarction) - Regeneration is rapid in smooth muscle because the 6. Intercalated discs are most likely to be observed in: cells/fibers are small and relatively less differentiated, which a. Longitudinal section of skeletal muscle allow renewed mitotic activity after injury b. Transverse section of skeletal muscle c. Transverse section of cardiac muscle d. Longitudinal section of cardiac muscle e. Transverse section of smooth muscle 7. When Skeletal muscle contracts an arrangement of the alternating light and dark bands traversing each skeletal muscle cell changes. Which of the following statements is not correct? a. The dark A bands will remain a constant length b. The space occupied by the H zone will not change c. The light I bands will shorten d. The z lines come closer together e. The I band consists solely of thin actin filaments ANSWERS: a, a, d, c, d, d, c PREPARED AND EDITED BY: GROUP 8