01:377:370 Lecture 2: Chapter 18

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Exercise Science and Sport Studies
Professor Pellegrino

Chapter 18- Skeletal Muscle Structure and Function Gross Structure of Skeletal Muscle • Fiber- a muscle cell o Long, slender, and multinucleated • Connective tissue layers o Endomysium- wraps each fiber and separates it from neighboring fibers o Perimysium- surrounds several fibers and forms bundles (Fasciculi) o Epimysium- surrounds all the bundles to form the entire muscle • Tendons- connective tissue connecting muscle to periosteum of bone o Origin (location where tendon joins skeletal part) = more stable bone o Insertion (point of attachment to moving bone) = moving bone • Sarcolemma- muscle cell membrane that encloses fiber’s cellular contents • Satellite cells- myogenic stem cells located within the sarcolemma o Help regenerative cell growth and repair of muscle cell o Role in hypertrophy Microstructures of Skeletal Muscle • Mitochondria- derive energy o Network or matrix all around the cell o Gradient across membrane- use proteins to keep equilibrium o Make up 1-20% of cell’s volume o Increased surface area = increased energy able to be produced o *Important for endurance activities o Mitochondrial biogenesis- making new mitochondria ▪ Apoptosis- send out signals for cell death • Sarcoplasmic Reticulum (SR) o ER in the muscle o Extensive lattice-like network of tubules and vesicles o Provides structural integrity o Stores, releases, and reuptakes calcium from the cytosol o *Important for power production and speed activities • Myofibrils o Make up more than 80% of muscle’s weight o *Important for force production and strength o Contains hexagonal packaging of ▪ Thick filaments- myosin • 2/3 myosin by weight ▪ Thin filaments- actin • 2:1 filament ratio o Includes several other structural and functional proteins (Titin/Nebulin) Strength = increased myofibrils / decreased mitochondria Endurance = decreased myofibrils / increased mitochondria Blood Supply • Skeletal muscle has rich vascular network to supply oxygen and nutrient demands during exercise o Blood vessels run through perimysium • Rhythmic flow o Vessels compressed during contraction phase o Vessels open during relaxation phase • Sustained contractions o Over 60% force-generating capacity o Elevated intramuscular pressure occludes local blood flow ▪ Primary reason anaerobic processes supply ATP (no oxygen supply) • Training enhances capillarization (especially endurance) o Expedites removal of metabolic by-products and heat o Increases oxygen delivery o Gets rid of carbon dioxide Skeletal Muscle Ultra-Structure • Fibrils- smaller functional units of fibers that contain myofilaments actin and myosin o Troponin and tropomyosin o Several other structural proteins • Sarcomere- functional unit of muscle fiber o Runs from Z disk to Z disk ▪ Contraction- A band stays same (myosin not moving) and I band and H zone shorten o Z disk- made of connective tissue and proteins o M band- helps hold sarcomere in place with proteins o I band- thin filaments only o H zone- thick filaments only o A band- where thick and thin filaments overlap Proteins • Actin o Lie in a hexagonal pattern around myosin o Consists of 2 twisted chains of monomers bound by tropomyosin polypeptide chains • Myosin o Consists of bundles of molecules with polypeptide chains and globular heads o Cross-bridges spiral around myosin where actin and myosin overlap o Myosin light chain- determines how fast myosin ATP degradation occurs • Tropomyosin o Lies along actin in the groove formed by the double helix o Covers cross-bridge binding site so myosin cannot bind to actin • Troponin o Embedded at regular intervals along actin o Interacts with calcium o Moves tropomyosin- reveals cross-bridge binding sites for myosin to bind to actin • Titin- protein with contractile properties and is believed to control the number of myosin molecules in the thick filament • Nebulin- protein with contractile properties and is believed to control the number of actin joined to each other in a thin filament • Myomyosin- anchoring protein • Tropomodulin- protects and anchors thin and thick filaments Chemical and Mechanical Events During Contraction/Relaxation • Sliding filament model o Contraction occurs as myosin and actin slide past one another o Energy is provided by ATP h
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