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PowerPoint® Lecture Slides prepared by Barbara Heard, Atlantic Cape Community College CHAPTER 9 Muscles and Muscle Tissue: Part A © Annie Leibovitz/Contact Press Images © 2013 Pearson Education, Inc. Muscle Tissue • Nearly half of body's mass • Transforms chemical energy (ATP) to directed mechanical energy exerts force • Three types – Skeletal – Cardiac – Smooth • Myo, mys, and sarco - prefixes for muscle © 2013 Pearson Education, Inc. Types of Muscle Tissue • Skeletal muscles – Organs attached to bones and skin – Elongated cells called muscle fibers – Striated (striped) – Voluntary (i.e., conscious control) – Contract rapidly; tire easily; powerful – Require nervous system stimulation © 2013 Pearson Education, Inc. Types of Muscle Tissue • Cardiac muscle – Only in heart; bulk of heart walls – Striated – Can contract without nervous system stimulation – Involuntary © 2013 Pearson Education, Inc. Types of Muscle Tissue • Smooth muscle – In walls of hollow organs, e.g., stomach, urinary bladder, and airways – Not striated – Can contract without nervous system stimulation – Involuntary © 2013 Pearson Education, Inc. Table 9.3 Comparison of Skeletal, Cardiac, and Smooth Muscle (1 of 4) © 2013 Pearson Education, Inc. Special Characteristics of Muscle Tissue • Excitability (responsiveness): ability to receive and respond to stimuli • Contractility: ability to shorten forcibly when stimulated • Extensibility: ability to be stretched • Elasticity: ability to recoil to resting length © 2013 Pearson Education, Inc. Muscle Functions • Four important functions – Movement of bones or fluids (e.g., blood) – Maintaining posture and body position – Stabilizing joints – Heat generation (especially skeletal muscle) • Additional functions – Protects organs, forms valves, controls pupil size, causes "goosebumps" © 2013 Pearson Education, Inc. Skeletal Muscle • Connective tissue sheaths of skeletal muscle – Support cells; reinforce whole muscle – External to internal • Epimysium: dense irregular connective tissue surrounding entire muscle; may blend with fascia • Perimysium: fibrous connective tissue surrounding fascicles (groups of muscle fibers) • Endomysium: fine areolar connective tissue surrounding each muscle fiber © 2013 Pearson Education, Inc. Figure 9.1 Connective tissue sheaths of skeletal muscle: epimysium, perimysium, and endomysium. Bone Epimysium Epimysium Perimysium Tendon Endomysium Muscle fiber in middle of a fascicle Blood vessel Perimysium wrapping a fascicle Endomysium (between individual muscle fibers) Muscle fiber Fascicle Perimysium © 2013 Pearson Education, Inc. Table 9.1 Structure and Organizational Levels of Skeletal Muscle (1 of 3) © 2013 Pearson Education, Inc. Table 9.1 Structure and Organizational Levels of Skeletal Muscle (2 of 3) © 2013 Pearson Education, Inc. Table 9.1 Structure and Organizational Levels of Skeletal Muscle (3 of 3) © 2013 Pearson Education, Inc. Microscopic Anatomy of a Skeletal Muscle Fiber • Long, cylindrical cell – 10 to 100 µm in diameter; up to 30 cm long • Multiple peripheral nuclei • Sarcolemma = plasma membrane • Sarcoplasm = cytoplasm – Glycosomes for glycogen storage, myoglobin for O2 storage • Modified structures: myofibrils, sarcoplasmic reticulum, and T tubules © 2013 Pearson Education, Inc. Myofibrils • Densely packed, rodlike elements • ~80% of cell volume • Contain sarcomeres - contractile units – Sarcomeres contain myofilaments • Exhibit striations - perfectly aligned repeating series of dark A bands and light I bands © 2013 Pearson Education, Inc. Figure 9.2b Microscopic anatomy of a skeletal muscle fiber. Diagram of part of a muscle fiber showing the myofibrils. One myofibril extends from the cut end of the fiber. Sarcolemma Mitochondrion Myofibril Dark A band © 2013 Pearson Education, Inc. Light Nucleus I band Striations • H zone: lighter region in midsection of dark A band where filaments do not overlap • M line: line of protein myomesin bisects H zone • Z disc (line): coin-shaped sheet of proteins on midline of light I band that anchors thin filaments and connects myofibrils to one another • Thick filaments: run entire length of an A band • Thin filaments: run length of I band and partway into A band • Sarcomere: region between two successive Z discs © 2013 Pearson Education, Inc. Sarcomere • Smallest contractile unit (functional unit) of muscle fiber • Align along myofibril like boxcars of train • Contains A band with ½ I band at each end • Composed of thick and thin myofilaments made of contractile proteins © 2013 Pearson Education, Inc. Figure 9.2c Microscopic anatomy of a skeletal muscle fiber. Thin (actin) filament Small part of one myofibril enlarged to show the myofilaments responsible for the banding pattern. Thick Each sarcomere extends from one Z (myosin) filament disc to the next. © 2013 Pearson Education, Inc. Z disc I band H zone Z disc I band A band Sarcomere M line Figure 9.2d Microscopic anatomy of a skeletal muscle fiber. Z disc Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments. © 2013 Pearson Education, Inc. Sarcomere M line Z disc Thin (actin) filament Elastic (titin) filaments Thick (myosin) filament Myofibril Banding Pattern • Orderly arrangement of actin and myosin myofilaments within sarcomere – Actin myofilaments = thin filaments • Extend across I band and partway in A band • Anchored to Z discs – Myosin myofilaments = thick filaments • Extend length of A band • Connected at M line © 2013 Pearson Education, Inc. Ultrastructure of Thick Filament • Composed of protein myosin • Each composed of 2 heavy and four light polypeptide chains – Myosin tails contain 2 interwoven, heavy polypeptide chains – Myosin heads contain 2 smaller, light polypeptide chains that act as cross bridges during contraction • Binding sites for actin of thin filaments • Binding sites for ATP • ATPase enzymes © 2013 Pearson Education, Inc. Ultrastructure of Thin Filament • Twisted double strand of fibrous protein F actin • F actin consists of G (globular) actin subunits • G actin bears active sites for myosin head attachment during contraction • Tropomyosin and troponin - regulatory proteins bound to actin © 2013 Pearson Education, Inc. Figure 9.3 Composition of thick and thin filaments. Longitudinal section of filaments within one sarcomere of a myofibril Thick filament Thin filament In the center of the sarcomere, the thick filaments lack myosin heads. Myosin heads are present only in areas of myosin-actin overlap. Thick filament. Thin filament Each thick filament consists of many myosin molecules whose heads protrude at opposite ends of the filament. Portion of a thick filament Myosin head A thin filament consists of two strands of actin subunits twisted into a helix plus two types of regulatory proteins (troponin and tropomyosin). Portion of a thin filament Tropomyosin Troponin Actin Actin-binding sites Heads ATPbinding site Flexible hinge region Myosin molecule © 2013 Pearson Education, Inc. Tail Active sites for myosin attachment Actin subunits Actin subunits Structure of Myofibril • Elastic filament – Composed of protein titin – Holds thick filaments in place; helps recoil after stretch; resists excessive stretching • Dystrophin – Links thin filaments to proteins of sarcolemma • Nebulin, myomesin, C proteins bind filaments or sarcomeres together; maintain alignment © 2013 Pearson Education, Inc. Sarcoplasmic Reticulum (SR) • Network of smooth endoplasmic reticulum surrounding each myofibril – Most run longitudinally • Pairs of terminal cisterns form perpendicular cross channels • Functions in regulation of intracellular Ca2+ levels – Stores and releases Ca2+ © 2013 Pearson Education, Inc. T Tubules • • • • Continuations of sarcolemma Lumen continuous with extracellular space Increase muscle fiber's surface area Penetrate cell's interior at each A band–I band junction • Associate with paired terminal cisterns to form triads that encircle each sarcomere © 2013 Pearson Education, Inc. Figure 9.5 Relationship of the sarcoplasmic reticulum and T tubules to myofibrils of skeletal muscle. Part of a skeletal muscle fiber (cell) I band Z disc Myofibril Sarcolemma A band H zone M line I band Z disc Sarcolemma Triad: • T tubule • Terminal cisterns of the SR (2) Tubules of the SR Myofibrils Mitochondria © 2013 Pearson Education, Inc. Triad Relationships • T tubules conduct impulses deep into muscle fiber; every sarcomere • Integral proteins protrude into intermembrane space from T tubule and SR cistern membranes and connect with each other • T tubule integral proteins act as voltage sensors and change shape in response to voltage changes • SR integral proteins are channels that release Ca2+ from SR cisterns when voltage sensors change shape © 2013 Pearson Education, Inc. Sliding Filament Model of Contraction • Generation of force • Does not necessarily cause shortening of fiber • Shortening occurs when tension generated by cross bridges on thin filaments exceeds forces opposing shortening © 2013 Pearson Education, Inc. Sliding Filament Model of Contraction • In relaxed state, thin and thick filaments overlap only at ends of A band • Sliding filament model of contraction – During contraction, thin filaments slide past thick filaments actin and myosin overlap more – Occurs when myosin heads bind to actin cross bridges © 2013 Pearson Education, Inc. Sliding Filament Model of Contraction • Myosin heads bind to actin; sliding begins • Cross bridges form and break several times, ratcheting thin filaments toward center of sarcomere – Causes shortening of muscle fiber – Pulls Z discs toward M line • I bands shorten; Z discs closer; H zones disappear; A bands move closer (length stays same) © 2013 Pearson Education, Inc. Figure 9.6 Sliding filament model of contraction. Slide 2 1 Fully relaxed sarcomere of a muscle fiber Z I © 2013 Pearson Education, Inc. H A Z I Figure 9.6 Sliding filament model of contraction. Slide 3 2 Fully contracted sarcomere of a muscle fiber Z © 2013 Pearson Education, Inc. I Z A I The Nerve Stimulus and Events at the Neuromuscular Junction • Skeletal muscles stimulated by somatic motor neurons • Axons of motor neurons travel from central nervous system via nerves to skeletal muscle • Each axon forms several branches as it enters muscle • Each axon ending forms neuromuscular junction with single muscle fiber – Usually only one per muscle fiber © 2013 Pearson Education, Inc. Events of Excitation-Contraction (E-C) Coupling • AP propagated along sarcomere to T tubules • Voltage-sensitive proteins stimulate Ca2+ release from SR – Ca2+ necessary for contraction © 2013 Pearson Education, Inc. Role of Calcium (Ca2+) in Contraction • At low intracellular Ca2+ concentration – Tropomyosin blocks active sites on actin – Myosin heads cannot attach to actin – Muscle fiber relaxed © 2013 Pearson Education, Inc. Role of Calcium (Ca2+) in Contraction • At higher intracellular Ca2+ concentrations – Ca2+ binds to troponin • Troponin changes shape and moves tropomyosin away from myosin-binding sites • Myosin heads bind to actin, causing sarcomere shortening and muscle contraction – When nervous stimulation ceases, Ca2+ pumped back into SR and contraction ends © 2013 Pearson Education, Inc. Cross Bridge Cycle • Continues as long as Ca2+ signal and adequate ATP present • Cross bridge formation—high-energy myosin head attaches to thin filament • Working (power) stroke—myosin head pivots and pulls thin filament toward M line © 2013 Pearson Education, Inc. Cross Bridge Cycle • Cross bridge detachment—ATP attaches to myosin head and cross bridge detaches • "Cocking" of myosin head—energy from hydrolysis of ATP cocks myosin head into high-energy state © 2013 Pearson Education, Inc. Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments toward the center of the sarcomere. Actin Ca2+ Thin filament Myosin cross bridge Thick filament Myosin 1 Cross bridge formation. Energized myosin head attaches to an actin myofilament, forming a cross bridge. ATP hydrolysis 4 Cocking of the myosin head. As ATP is hydrolyzed to ADP and Pi, the myosin head returns to its prestroke high-energy, or “cocked,” position. * *This cycle will continue as long as ATP is available and Ca2+ is bound to troponin. © 2013 Pearson Education, Inc. 2 The power (working) stroke. ADP and Pi are released and the myosin head pivots and bends, changing to its bent low-energy state. As a result it pulls the actin filament toward the M line. In the absence of ATP, myosin heads will not detach, causing rigor mortis. 3 Cross bridge detachment. After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches (the cross bridge “breaks”). Slide 1 Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments toward the center of the sarcomere. Actin Myosin cross bridge Slide 2 Thin filament Thick filament Myosin 1 Cross bridge formation. Energized myosin head attaches to an actin myofilament, forming a cross bridge. © 2013 Pearson Education, Inc. Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments toward the center of the sarcomere. Slide 3 2 The power (working) stroke. ADP and Pi are released and the myosin head pivots and bends, changing to its bent low-energy state. As a result it pulls the actin filament toward the M line. © 2013 Pearson Education, Inc. Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments toward the center of the sarcomere. Slide 4 3 Cross bridge detachment. After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches (the cross bridge “breaks”). © 2013 Pearson Education, Inc. Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments toward the center of the sarcomere. Slide 5 ATP hydrolysis 4 Cocking of the myosin head. *This cycle will continue as long as ATP is available and Ca2+ is bound to troponin. © 2013 Pearson Education, Inc. As ATP is hydrolyzed to ADP and Pi, the myosin head returns to its prestroke high-energy, or “cocked,” position. * Figure 9.12 The cross bridge cycle is the series of events during which myosin heads pull thin filaments toward the center of the sarcomere. Actin Ca2+ Thin filament Myosin cross bridge PLAY A&P Flix™: The Cross Bridge Cycle Slide 6 Thick filament Myosin 1 Cross bridge formation. Energized myosin head attaches to an actin myofilament, forming a cross bridge. ATP hydrolysis 4 Cocking of the myosin head. As ATP is hydrolyzed to ADP and Pi, the myosin head returns to its prestroke high-energy, or “cocked,” position. * *This cycle will continue as long as ATP is available and Ca2+ is bound to troponin. © 2013 Pearson Education, Inc. 2 The power (working) stroke. ADP and Pi are released and the myosin head pivots and bends, changing to its bent low-energy state. As a result it pulls the actin filament toward the M line. In the absence of ATP, myosin heads will not detach, causing rigor mortis. 3 Cross bridge detachment. After ATP attaches to myosin, the link between myosin and actin weakens, and the myosin head detaches (the cross bridge “breaks”). Homeostatic Imbalance • Rigor mortis – Cross bridge detachment requires ATP – 3–4 hours after death muscles begin to stiffen with weak rigidity at 12 hours post mortem • Dying cells take in calcium cross bridge formation • No ATP generated to break cross bridges © 2013 Pearson Education, Inc.