Skeletal Muscle Physiology.pdf

Skeletal Muscle Cellular Physiology Inefta M. Reid, PhD [email protected] 631-638-3696 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 • Exte...

Skeletal Muscle Cellular Physiology Inefta M. Reid, PhD [email protected] 631-638-3696 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 1 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 Skeletal Muscle 2 Structure of the Contractile Apparatus Organization of a Skeletal Muscle Fiber Structure of a Skeletal Muscle Fiber Mitochondria Sarcoplasmic reticulum Nucleus Thick filament Thin filament T-tubules Myofibril Sarcolemma 3 Electromicroscopy Longitudinal: alternating light/dark bands Transverse: hexagonal arrangement of filaments A-band I-band Z-line M-line 4 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. Sarcomere A band H zone I band Z disk Z disk M line 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 Ultrastructure Cont’d 5 Myosin Structure Cartoon representation: EM of myosin monomers: Thick filament assembly: “Bare zone” 6 Actin Structure • G-actin monomer: Globular protein • F-actin Filament: polymerization of G-actin subunits • forms a double helix analogous to double string of pearls • associated w/ regulatory proteins: troponin & tropomyosin Cartoon representation: Actual High-res. structure: Troponin G- Sarcomere structure Rest and contraction The sarcomere shortens during contraction. As contraction takes place, actin and myosin do not change length but instead slide past one another. A band I 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 7 Figure 12.7 Summary map of muscle contraction Filaments form “ratchet” -- like a bumper jack process repeats at next binding site... 1 2 3 4 5 actin filament myosin filament 1 2 3 4 5 myosin head group binds to actin site 2 3 4 5 6 2 3 4 5 6 conformational change; filaments slide by one another head group releases; change back to original conformation 8 Contractile Cycle No ATP: No release! (rigor mortis) Tight Binding in the Rigor State G-actin molecule Myosin binding sites Myosin filament NAVIGATOR ATP binds to myosin. Myosin releases actin. ATP binds. ADP releases. Myosin releases ADP at the end of the power stroke. Contractionrelaxation Myosin hydrolyzes ATP. Energy from ATP rotates the myosin head to the cocked position. Myosin binds weakly to actin. The Power Stroke Actin filament moves toward M line. Sliding filament Head swivels. Myosin releases Pi. Ca2+ signal Power stroke begins when tropomyosin (not shown) moves off the binding site. ADP Pi ADP and Pi remain bound. 9 Calcium regulates contraction • Troponin has 3 subunits: – TnC: Two calcium binding sites – TnT: rod-like portion bound to tropomyosin – TnI: links the three subunits together • Tropomyosin normally covers myosin dinging sites on actin – prevents myosin-actin attachment at rest – Note: myosin ready to go; has previously hydrolyzed ATP and is phosphorylated • Sarcoplasmic Ca++ concentration: – Resting:  100 nM (none bound to troponin) – Contracting: 1000 nM (TnC sites saturated with Ca++) Calcium Initiates Muscle Contraction Troponin-tropomyosin conformational change 10 Where does sarcoplasmic Ca++ come from? • Not from outside! • Proof: bathe in Ca++-free media, fiber still contracts! • Reason: diffusion time too long (> 1 sec) • See for yourself: remember, x2 = 2Dt • Comes from sarcoplasmic reticulum (SR) • major Ca++ reservoir • Process called excitation-contraction (EC) coupling • Excitation: AP on sarcolemma (surface membrane) • Contraction: thick/thin filament interactions Recall… ultrastructure I-band A-band SR Transverse (t) tubules filled with extracellular fluid! Z-disk 11 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 ACh - - ++ + + + + T-tubule + + + + + - Motor end plate RyR + + + - Z disk Somatic motor neuron releases ACh at neuromuscular junction. Na+ Net entry of Na+ through ACh receptor-channel initiates a muscle action potential. Sarcoplasmic reticulum Ca2+ DHP Troponin Actin Tropomyosin M line Myosin head Myosin thick filament Excitation-Contraction Coupling and Relaxation KEY DHP = dihydropyridine L-type calcium channel RyR = ryanodine receptor-channel Action potential in t-tubule alters conformation of DHP receptor. Excitation-Contraction Coupling + + + + - DHP receptor opens RyR Ca2+ release channels in sarcoplasmic reticulum, and Ca2+ enters cytoplasm.  + - + - - + + Ca2+ released. Ca2+ binds to troponin, allowing actin-myosin binding. Myosin thick filament Myosin heads execute power stroke. Distance actin moves Note… ryanodine receptor in #4 associated with SR Ca++ channel Actin filament slides toward center of sarcomere. 12 Excitation-Contraction Coupling and Relaxation Configuration of skeletal-muscle DHP receptor: resting conditions Resting Vm T-tubule lumen + Ryanodine receptor (a Ca++ channel) DHP receptor (Vm sensitive) SR fenestrations (holes) SR membrane T-tubule membrane 13 Configuration of skeletal-muscle DHP receptor: during action potential Depolarized V m T-tubule lumen + Ryanodine receptor (a Ca++ channel) Ca++ efflux (into sarcoplasm) DHP receptor (Vm sensitive) SR fenestrations (holes) SR membrane T-tubule membrane Finally…what terminates contraction? • Answer: reduction of Ca++ from 1000 nM to 100 nM (resting level) • Mediated by. . . • Repolarization of t-tubule membrane • DHP receptor “blocks” ryanodine-receptor channel • Ca++ pumped back into SR via Ca++-ATPase (pump) • SR Ca++ sequesteration aided by Ca++-binding “buffer” protein called calsequestrin • calsequestrin located within SR 14 Figure 12.11 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 +30 Muscle fiber Action potential from CNS Neuron membrane potential in mV 70 Time Motor end plate Axon terminal Recording electrodes Muscle Fiber Action Potential +20 Muscle action potential Muscle fiber membrane potential in mV 80 2 msec Time NAVIGATOR Neuromuscular junction (NMJ) Development of Tension during One Muscle Twitch Latent period FIGURE QUESTIONS Muscle twitch Movement of what ion(s) in what direction(s) creates (a) the neuronal action potential? (b) the muscle action potential? Contraction Relaxation phase phase Tension E-C coupling 10–100 msec Time 15 Tension (percent of maximum) Length-tension relationship 100 80 60 40 20 0 1.3 m 2.0 m 2.3 m Decreased length Optimal resting length 3.7 m Adapted from A.M. Gordon et al., J Physiol 184: 170–192, 1966. Increased length 16 Muscle Mechanics Isotonic (“same tension”) contraction Muscle shortens while lifting load Isometric (“same length”) contraction Muscle generates force, doesn’t shorten! 17 Skeletal Muscle Contractions Skeletal Muscle – Fiber Types Slow-twitch fibers vs. Fast-twitch fibers 18 Strenuous vs. endurance exercise energy expenditure energy from O2 metabolism A1 Rate of energy expenditure A2 called “O2 debt”; restores energy (A1) expended during strenuous exercise A2 exercise 0 recovery 2 4 6 energy expenditure 8 10 minutes During endurance, less O2 debt since metabolism keeps up energy from O2 metabolism exercise 0 10 20 recovery 30 40 50 minutes Muscle fiber types 19 Muscle Disorders • Inherited disorders – Duchenne muscular dystrophy • Dystrophin – McArdle’s disease (aka Glycogen storage disease type V (GSD-V) • Myophosphorylase deficiency • Glycogen not converted to glucose 6-phosphate 20