About This Chapter. Skeletal muscle Mechanics of body movement Smooth muscle Cardiac muscle Pearson Education, Inc.

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1 About This Chapter Skeletal muscle Mechanics of body movement Smooth muscle Cardiac muscle

2 Skeletal Muscle Usually attached to bones by tendons Origin: closest to the trunk or to more stationary bone Insertion: more distal or more mobile attachment Flexor: brings bones together Extensor: moves bones away Antagonistic muscle groups: flexor-extensor pairs

3 Figure 12.1 The three types of muscles Skeletal muscle fibers are large, multinucleate cells that appear striped or striated under the microscope. Nucleus Muscle fiber (cell) Striations Cardiac muscle fibers are also striated but they are smaller, branched, and uninucleate. Cells are joined in series by junctions called intercalated disks. Nucleus Muscle fiber Intercalated disk Striations Smooth muscle fibers are small and lack striations. Nucleus Muscle fiber

4 Figure 12.2 Antagonistic muscles Flexion moves bones closer together. Extension moves bones away from each other. Triceps muscle relaxes. Biceps muscle contracts (flexor). Triceps muscle contracts (extensor). Biceps muscle relaxes.

5 Anatomy of Skeletal Muscle Muscle cells are called muscle fibers Fused cells with many nuclei Satellite cells differentiate into muscle Cells bundled into fascicles surrounded by connective tissue

6 Anatomy of Skeletal Muscle Fiber internal structures Sarcolemma: cell membrane Sarcoplasm: cytoplasm Sarcoplasmic reticulum: endoplasmic reticulum Network of transverse tubules: T-tubules connected with the sarcolemma Myofibrils are the contractile structures Actin: thin filament Myosin: thick filament Accessory proteins: troponin and tropomyosin

7 Figure 12.3a Skeletal Muscles

8 Figure 12.3b Skeletal Muscles Structure of a Skeletal Muscle Fiber Mitochondria Sarcoplasmic reticulum Nucleus Thick filament Thin filament T-tubules Myofibril Sarcolemma

9 Figure 12.3e Skeletal Muscles Thick Filaments M line Myosin heads Myosin tail Hinge region Myosin Molecule

10 Figure 12.3f Skeletal Muscles Thin Filaments Titin Troponin Nebulin Tropomyosin G-actin molecule Actin Chain

11 Sarcomere: Contractile Unit Light and dark bands seen under light microscopy One sarcomere between by two Z disks I band is made of thin filaments only (actin) A band is made of darker regions where both light and heavy filaments overlap H zone is a clear band in the middle of the A band, heavy filaments only M line represents the proteins to which heavy filaments attach

12 Z disk M line Z disk Figure 12.5a-c The Sarcomere Organization of a Sarcomere The Z disk (not shown in part (c)) has accessory proteins that link the thin filaments together, similar to the accessory proteins shown for the M line. Myosin heads are omitted for simplicity. I band Sarcomere A band H zone I band KEY Actin Myosin I band Actin only H zone Myosin only M line Myosin linked with accessory proteins A band (outer edge) Actin and myosin overlap

13 Figure 12.5de The Sarcomere The sarcomere shortens during contraction. As contraction takes place, actin and myosin do not change length but instead slide past one another. I band A band Z Muscle Relaxed Myosin Actin Sarcomere shortens with contraction. Half of I band H zone Half of I band Z line Muscle Contracted H zone and I band both shorten, while A band remains constant. I H I

14 Muscle Contraction Muscle tension: force created by muscle Load: weight or force opposing contraction Contraction: creation of tension in muscle Events at the neuromuscular junction Excitation-contraction coupling Sliding filaments Relaxation: release of tension

15 Figure 12.7 Summary map of muscle contraction

16 Skeletal Muscle Tissue Interactive Physiology Animation: Anatomy Review: Skeletal Muscle Tissue

17 A&P Flix: Events at the Neuromuscular Junction

18 Sliding Filament Theory Actin and myosin slide past each other during contraction Power stroke cycle: myosin crossbridges move actin filament Calcium release from T-tubules Calcium binds troponin Troponin pulls tropomyosin from myosin-binding sites on actin Myosin binds tightly to and moves actin Rigor: myosin stays tightly bound to actin until ATP binds to myosin which is released from actin

19 Sliding Filament Theory Myosin breaks down ATP The energy rotates the myosin head that binds weakly to actin down the molecule Head of myosin is cocked ready for the next power stroke ATP must bind to myosin to release myosin from actin Rigor mortis: muscles freeze if no ATP is available to release myosin Relaxation: Calcium ions pumped back to sarcoplasmic reticulum

20 Figure 12.8 Troponin and tropomyosin Relaxed state. Myosin head cocked. Tropomyosin partially blocks binding site on actin. Myosin is weakly bound to actin. Initation of contraction. A calcium signal initiates contraction. Troponin G-actin Cytosolic Ca 2+ Tropomyosin shifts, exposing binding site on actin. Ca2+ levels increase in cytosol. Ca 2+ binds to troponin (TN). Tropomyosin TN P i ADP Myosin head Power stroke TN ADP Actin moves Troponin-Ca 2+ complex pulls tropomyosin away from actin s myosin-binding site. P i Myosin binds strongly to actin and completes power stroke. Actin filament moves.

21 Figure The contraction cycle Tight Binding in the Rigor State G-actin molecule Myosin binding sites Myosin filament ATP binds to myosin. Myosin releases actin. ADP releases. NAVIGATOR ATP binds. Myosin releases ADP at the end of the power stroke. The Power Stroke Actin filament moves toward M line. Contractionrelaxation Sliding filament Myosin hydrolyzes ATP. Energy from ATP rotates the myosin head to the cocked position. Myosin binds weakly to actin. Head swivels. Myosin releases Pi. Power stroke begins when tropomyosin (not shown) moves off the binding site. Ca 2+ signal ADP and P i remain bound. ADP P i

22 Sliding Filament Theory Interactive Physiology Animation: Muscular System: Sliding Filament Theory

23 A&P Flix: The Cross Bridge Cycle

24 Excitation-Contraction Coupling Acetylcholine (ACh) is released from the somatic motor neuron. ACh binds to receptors on the sarcolemma Na + and K + ions flow in the cell Depolarization of the muscle fiber membrane creates an end-plate potential (EPP)

25 Excitation-Contraction Coupling Calcium release from the sarcoplasmic reticulum Mediated by two membrane proteins L-type calcium channel dihydropyridine (DHP) receptor and ryanodine receptors (RyR) A muscle twitch is a single contraction-relaxation cycle A latent period is the short delay between the muscle action potential and beginning of muscle tension development. Time required for calcium release and binding to troponin

26 Figure 12.10a-0 Excitation-Contraction Coupling and Relaxation Initiation of Muscle Action Potential Axon terminal of somatic motor neuron KEY DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel Muscle fiber T-tubule Z disk Actin Na RyR DHP Ca 2+ Motor end plate Troponin Tropomyosin ACh Sarcoplasmic reticulum M line Somatic motor neuron releases ACh at neuromuscular junction. Net entry of Na + through ACh receptor-channel initiates a muscle action potential. Myosin head Myosin thick filament

27 Figure 12.10b-0 Excitation-Contraction Coupling and Relaxation KEY DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel Excitation-Contraction Coupling Ca 2+ released. Action potential in t-tubule alters conformation of DHP receptor. DHP receptor opens RyR Ca 2+ release channels in sarcoplasmic reticulum, and Ca 2+ enters cytoplasm. Ca 2+ binds to troponin, allowing actin-myosin binding. Distance actin moves Myosin thick filament Myosin heads execute power stroke. Actin filament slides toward center of sarcomere.

28 A&P Flix: Excitation-Contraction Coupling

29 A&P Flix: Events at the Neuromuscular Junction

30 Events at the Neuromuscular Junction Interactive Physiology Animation: Muscular System: Events at the Neuromuscular Junction

31 Tension Figure Timing of E-C coupling Action potentials in the axon terminal (top graph) and in the muscle fiber (middle graph) are followed by a muscle twitch (bottom graph). Motor Neuron Action Potential Muscle fiber Action potential from CNS +30 Neuron membrane potential in mv 70 Time Motor end plate Axon terminal Muscle action potential Recording electrodes Muscle Fiber Action Potential +20 Muscle fiber membrane potential in mv NAVIGATOR 80 2 msec Time Neuromuscular junction (NMJ) E-C coupling Development of Tension during One Muscle Twitch Latent period Contraction phase Relaxation phase Muscle twitch FIGURE QUESTIONS Movement of what ion(s) in what direction(s) creates (a) the neuronal action potential? (b) the muscle action potential? Time msec

32 Three Metabolic Pathways to Obtain Energy Phosphocreatine breakdown: short burst of energy Anaerobic glycolysis produces lactate and acid: quick, no oxygen required, small amount of energy released Aerobic respiration (citric acid cycle and electron transport chain): slow, requires oxygen, large amount of energy released

33 Causes of Muscle Fatigue during Exercise Central fatigue due to CNS Peripheral fatigue due to neuron or muscle Extended submaximal exercise Depletion of glycogen stores Short-duration maximal exertion Increased levels of inorganic phosphate May slow P i release from myosin

34 Causes of Muscle Fatigue during Exercise Decrease calcium release Maximal exercise K + leaves muscle fiber, leading to increased extracellular concentration, altering the membrane potential Changes Na +_ K +_ ATPase activity

35 Figure Muscle fatigue

36 Fast-Twitch and Slow-Twitch Muscle Fibers Slow-twitch fibers Rely primarily on oxidative phosphorylation Fast-twitch fibers Develop tension faster Split ATP more rapidly

37 Fast-Twitch and Slow-Twitch Muscle Fibers Pump Ca 2+ into sarcoplasmic reticulum more rapidly Fast-twitch glycolytic fibers Rely primarily on anaerobic glycolysis Fast-twitch oxidative-glycolytic fiber Use oxidative and glycolytic metabolism

38 Figure Fast-twitch and slow-twitch muscles Slow-Twitch Oxidative Muscle Fibers. Note smaller diameter, darker color due to myoglobin. Fatigue-resistant. Capillaries Mitochondria Cross section of slow-twitch muscle fibers (LM 170) Fast-Twitch Glycolytic Muscle Fibers. Larger diameter, pale color. Easily fatigued. Cross section of fast-twitch muscle fibers (LM 170)

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