MP 10-08-14 - Mammalian Physiology(21:120:340 Dr Tracy S Tran([email protected] Muscle Tissues Nearly half of body's mass Transforms chemical

MP 10-08-14 - Mammalian Physiology(21:120:340 Dr Tracy S...

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Unformatted text preview: Mammalian Physiology (21:120:340) Dr. Tracy S. Tran ([email protected]) 10-­‐08-­‐14 Muscle Tissues •  Nearly half of body's mass •  Transforms chemical energy (ATP) to directed mechanical energy exerts force •  Three types Skeletal Cardiac Smooth •  Cardiac muscle Smooth muscle -­‐  Only in heart; bulk of heart walls -­‐ In walls of hollow organs, e.g., -­‐  Striated (striped) stomach, urinary bladder, and airways -­‐ Not striated -­‐ Can contract without nervous system sRmulaRon -­‐ Involuntary •  Skeletal muscles -­‐ Organs aSached to bones and skin -­‐ Elongated cells called muscle fibers -­‐  Striated -­‐  Voluntary (i.e., conscious control) -­‐ Contract rapidly; Rre easily; powerful -­‐ Require nervous system sRmulaRon Skeletal Muscles •  Four important func8ons -­‐ Movement of bones or fluids (e.g., blood) -­‐ Maintaining posture and body posiRon -­‐ Stabilizing joints -­‐ Heat generaRon (especially skeletal muscle) •  Addi8onal func8ons -­‐ Protects organs, forms valves, controls pupil size, causes "goosebumps” Proper8es: •  Each muscle served by one artery, one nerve, and one or more veins Enter/exit near central part and branch through connecRve Rssue sheaths Every skeletal muscle fiber supplied by nerve ending that controls its acRvity Huge nutrient and oxygen need; generates large amount of waste •  Connec8ve 8ssue sheaths of skeletal muscle Support cells; reinforce whole muscle External to internal Epimysium: dense irregular connecRve Rssue surrounding enRre muscle; may blend with fascia Perimysium: fibrous connecRve Rssue surrounding fascicles (groups of muscle fibers) Endomysium: fine areolar connecRve Rssue surrounding each muscle fiber Skeletal Muscle Architecture 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 OrganizaRonal Levels of Skeletal Muscle Microscopic Anatomy of a Skeletal Muscle Fiber •  Long, cylindrical cell 10 to 100 µm in diameter; up to 30 cm long •  Mul8ple peripheral nuclei •  Sarcolemma = plasma membrane •  Sarcoplasm = cytoplasm -­‐  Glycosomes for glycogen storage, -­‐  myoglobin for O2 storage •  Modified structures: myofibrils, sarcoplasmic re8culum, and T tubules •  Myofibril: Densely packed, rodlike elements, ~80% of cell volume -­‐ Contain sarcomeres -­‐ contracRle units; Sarcomeres contain myofilaments Exhibit striaRons -­‐ perfectly aligned repeaRng series of dark A bands and light I bands Sarcolemma Diagram of part of a muscle fiber showing the myofibrils. One myofibril extends from the cut end of the fiber. Mitochondrion Dark A band Myofibril Light Nucleus I band StriaRons on a Myofibril •  H zone: lighter region in midsecRon 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 enRre length of an A band •  Thin filaments: run length of I band and partway into A band •  Sarcomere: region between two successive Z discs Thin (actin) filament Z disc H zone Z disc Each sarcomere extends from one Z disc to the next. Thick (myosin) filament I band I band A band Sarcomere M line •  Sarcomere: Smallest contracRle unit (funcRonal unit) of muscle fiber -­‐ RepeRRve segments along the myofibril -­‐ Contains A band with ½ I band at each end -­‐ Composed of thick and thin myofilaments made of contracRle proteins The Sarcomere •  Orderly arrangement of acRn and myosin myofilaments within sarcomere Ac8n 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 Z disc Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments. Sarcomere M line Z disc Thin (actin) filament Elastic (titin) filaments Thick (myosin) filament Myosin protein: composed of 2 heavy and four light polypep8de chains •  tails contain 2 interwoven, heavy polypep8de chains •  heads contain 2 smaller, light polypep8de chains that act as Longitudinal section of filaments within one sarcomere of a myofibril Ac8n protein: Twisted double strand of fibrous protein -­‐ F ac8n F ac8n consists of G (globular) ac8n subunits Tropomyosin and troponin -­‐ regulatory proteins bound to ac8n Thick filament cross bridges during contrac8on •  Binding sites for ac8n of thin filaments •  Binding sites for ATP 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 oppositeends of the filament. 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 thick filament Myosin head Portion of a thin filament Tropomyosin Troponin Actin Actin-binding sites Heads ATPbinding site Flexible hinge region Myosin molecule Tail Active sites for myosin attachment Actin subunits Actin subunits Sarcoplasmic ReRculum (SR) and T Tubules SR: Network of smooth endoplasmic reRculum surrounding each myofibril -­‐ Most run longitudinally -­‐ Pairs of terminal cisterns form perpendicular cross channels -­‐ FuncRons in regulaRon of intracellular Ca2+ levels: stores and releases Ca2+ Part of a skeletal muscle fiber (cell) I band A band I band Z disc H zone Z disc M line Myofibril Sarcolemma Triad: Sarcolemma • T tubule • Terminal cisterns of the SR (2) Tubules of the SR Mitochondria T Tubules: ConRnuaRons of sarcolemma -­‐ Increase muscle fiber's surface area by penetraRng the cell's interior at each A band–I band juncRon Associate with paired terminal cisterns to form a triad that encircle each sarcomere Sliding filament model of contracRon 1 Fully relaxed sarcomere of a muscle fiber 1) In relaxed state, thin and thick filaments overlap only at ends of A band Sliding filament model of contrac8on 2) During contracRon, thin filaments slide past thick filaments acRn and myosin overlap more -­‐ Occurs when myosin heads bind to acRn cross bridges 3) Myosin heads bind to acRn; sliding begins -­‐ Cross bridges form and break several Rmes, pulling 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 but length stays same! H A Z I Z I 2 Fully contracted sarcomere of a muscle fiber Z Z I A I Generates Force! Actin The Cross Bridge Cycle Ca2+ Thin filament Myosin cross bridge Thick filament Myosin 1) Cross bridge formaRon. Energized myosin head aVaches to an ac8n 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,” posi8on. * *This cycle will con8nue as long as ATP is available and Ca2+ is bound to troponin. 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 ac8n filament toward the M line. In the absence of ATP, myosin heads will not detach, causing rigor mor8s. 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”). Physiology of Skeletal Muscle Fibers For skeletal muscle to contract: Phase 1 Motor neuron stimulates muscle fiber. ACh effects quickly terminated by enzyme acetylcholinesterase in synapRc clem -­‐ Breaks down ACh to aceRc acid and choline -­‐ Prevents conRnued muscle fiber contracRon in absence of addiRonal sRmulaRon Action potential (AP) arrives at axon terminal and neuromuscular junction ACh released; binds to receptors on sarcolemma Ion permeability of sarcolemma changes Local change in membrane voltage (depolarization) occurs Local depolarization (end plate potential) ignites AP in sarcolemma AP travels across the entire sarcolemma AP travels along T tubules Phase 2: Excitation-contraction coupling occurs. SR releases Ca2+; Ca2+ binds to troponin; myosin-binding sites (active sites) on actin exposed Myosin heads bind to actin; contraction begins ExcitaRon-­‐ContracRon (E-­‐C) Coupling Steps in E-C Coupling: Setting the stage The events at the neuromuscular junction (NMJ) set the stage for E-C coupling by providing excitation. Released acetylcholine binds to receptor proteins on the sarcolemma and triggers an action potential in a muscle fiber. Synaptic cleft Voltage-sensitive tubule protein Sarcolemma T tubule Ca2+ release channel 1 The action potential (AP) propagates along the sarcolemma and down the T tubules. 2Calcium ions are released. Transmission of the AP along the T tubules of the triads causes the voltage-sensitive tubule proteins to change shape. This shape change opens the Ca2+ release channels in the terminal cisterns of the sarcoplasmic reticulum (SR), allowing Ca2+ to flow into the cytosol. Terminal cistern of SR Axon terminal of motor neuron at NMJ Action potential is generated ACh Actin Sarcolemma Troponin T tubule Terminal cistern of SR Muscle fiber Tropomyosin blocking active sites Myosin Triad Active sites exposed and ready for myosin binding One sarcomere One myofibril Myosin cross bridge 3Calcium binds to troponin and removes the blocking action of tropomyosin. When Ca2+ binds, troponin changes shape, exposing binding sites for myosin (active sites) on the thin filaments. 4Contraction begins: Myosin binding to actin forms cross bridges and contraction (cross bridge cycling) begins. At this point, E-C coupling is over. The aftermath When the muscle AP ceases, the voltage-sensitive tubule proteins return to their original shape, closing the Ca2+ release channels of the SR. Ca2+ levels in the sarcoplasm fall as Ca2+ is continually pumped back into the SR by active transport. Without Ca2+, the blocking action of tropomyosin is restored, myosin-actin interaction is inhibited, and relaxation occurs. Each time an AP arrives at the neuromuscular junction, the sequence of E-C coupling is repeated. The sequence of events by which transmission of an acRon potenRal along the sarcolemma leads to the sliding of myofilaments. Steps in E-­‐C Coupling: Sarcolemma Voltage-sensitive tubule protein 1) The acRon potenRal (AP) propagates along the sarcolemma and down the T tubules. T tubule Ca2+ release channel 2) Calcium ions are released. Transmission of the AP along the T tubules of the triads causes the voltage-­‐sensiRve Ca2+ channels, in the terminal cisterns of the sarcoplasmic reRculum (SR), to open allowing Ca2+ to flow into the cytosol. Terminal cistern of SR Actin Troponin Tropomyosin blocking active sites Myosin 3) Calcium binds to troponin and removes the blocking acRon of tropomyosin. When Ca2+ binds, troponin changes shape, exposing binding sites for myosin (ac8ve sites) on the thin filaments. Active sites exposed and ready for myosin binding Myosin cross bridge 4) ContracRon begins: Myosin binding to ac8n forms cross bridges and contrac8on (cross bridge cycling) begins. At this point, E-­‐C coupling is over. •  When the muscle AP ceases, the voltage-­‐ sensi8ve Ca2+ channels of the SR close. •  Ca2+ levels in the sarcoplasm fall as Ca2+ is con8nually pumped back into the SR by ac8ve transport. •  Without Ca2+, the blocking ac8on of tropomyosin is restored, myosin-­‐ac8n interac8on is inhibited, and relaxa8on occurs. •  Each 8me an AP arrives at the neuromuscular junc8on, the sequence of E-­‐C coupling is repeated. ...
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