ch 20 muscle - Animal Physiology.txt

  1. Ch 20
  2. skeletal muscle composed of…
    • Bundles of muscle FIBERS
    • Each muscle fiber contains hundreds of parallel MYOFIBRILS
  3. Skeletal muscle nucleation and membrane
    • Muscle (fibers) cells are multinucleated as they are developed from the CELL FUSION of myoblasts
    • Sarcolemma: muscle fiber cell membrane
  4. MyoFIBRIL contains…
    • myoson: thick filaments
    • actin: thin filaments
    • sarcomeres that contain both filaments
    • Also contains intermediate filaments
  5. Each sarcomere contains:
    • M line (with myosin attached to it)
    • H zone: contains just the myosin
    • A band: contains H zone + myosin overlap with actin
    • Z disc (z shaped with actin attached)
    • I band: contains just the Actin
  6. Cross-bridges
    Myosin heads: interact with actin to generate muscle CONTRACTION
  7. Titin and nebulin
    Structural PROTEINS that aid in alignment of actin and myosin
  8. Myosin and actin movement during contraction
    during the sliding, heads of MYOSIN bind to sites on actin and draw the ACTIN TOWARDS CENTER of each sarcomere
  9. Steps in muscle contraction
    • 1. ATP BINDING to myosin DISSOCIATES it from actin (G-actin monomer)
    • 2. Myosin ATPase HYDROLYZES ATP to ADP + Phosphate (both remain bound)
    • 3. Myosin MOVES to cocked position and binds to G-actin molecule
    • 4. the binding triggers RELEASE of PHOSPHATE and the POWER STROKE occurs (moves 10nm towards sarcomere center)
    • 5. ADP dissociates from myosin and it remains bound to actin (rigor - a transient state)
  10. Key regulation of muscle contraction and the proteins involved
    • Calcium is the key regulator, acting through..
    • Troponin: Contains a binding site for Ca, so when Ca binds, it causes..
    • Tropomyosin: Ca binding to troponin causes this protein to shift from its location where it blocked the myosin binding sites, allowing actin to bind and muscle contraction to occur
  11. Excitation-contraction coupling
    • Neural excitation role in contraction: depolarization (excitation) leads to activation of contraction muscle machinery
    • Coupling occurs b/w two membrane systems
    • 1. T-tubules: travel perpendicular to muscle fibers, forming invaginations in the sarcolemma. T-tubule continuous with EXTRACELLULAR space
    • 2. Sarcoplasmic reticulum (SR): branch of tubules within the muscle fibers, come into close association with t-tubules.
  12. How action potential and depolarization of the t-tubule membrane cause Ca release from sarcoplasmic reticulum?
    • Voltage gated Ca channels DHPR and RyRs: DHPRs are on the t-tubules and face the RyRs in the SR when action potential is not occurring
    • AFTER Depolarization: T-tubules alter conformation of DHPR, changing conformation of RyRs, allowing it to OPEN and RELEASE Ca into the Cytoplasm, where Ca travels to bind to the Troponin, allowing MUSCLE CONTRACTION to occur
  13. What happens after action potential and contraction has occurred (after excitation-contraction coupling)
    • RyR channels close and SR uses ATPase PUMPS to pump 2 Ca back into the SR for every ATP
    • As Ca conc decreases, they unbind from Troponin, thus blocking MYOSIN Binding sites
  14. ATPases in SR
    For each molecule of ATP hydrolyzed, 2 Ca molecules are moved INTO the SR (from cytoplasm)
  15. Muscle antagonist pairs
    When one muscle shortens, its antagonist muscle lengthens
  16. Isometric contraction
    SAME LENGTH contraction: tightening of a muscle without muscle length changing (ie flexing) although the Sarcomeres shorten slightly
  17. CONCentric contraction
    • Isotonic contraction
    • MUSCLE shortening contraction: whole muscle shortens, such as when doing curls
  18. Eccentric contractions
    • isotonic contraction
    • LENGTHENING contraction: when muscles are lengthening but contraction is occurring (ie hiking down an incline)
  19. Tension
    • Measures the Force exerted on a LOAD by a unit of cross-sectional AREA of muscle
    • Isometric contractions measure tension in muscle
  20. Force-velocity relationship
    The velocity of Shortening DECREASES with INCREASING force (increasing load)
  21. Length-Tension relationship
    • The Amount of TENSION developed by a muscle depends on the LENGTH of the muscle
    • Optimal overlap of thick and thin filaments causes HIGHEST Tension
  22. Work muscle does depends on..
    • Force produced by muscle and the DISTANCE the muscle shortens
    • ie: if isometric force, NO WORK done b/c it does not move any length
  23. Velocity of shortening
    The GREATER LENGTH of muscle fiber, the INCREASED Velocity of shortening
  24. How far does a Ca ion travel from SR to TN-binding site? (CH 20 SUMMARY #1)
    The distance Ca must travel to get to the site will vary depending on if the TN-binding site is near the t-tubule, and therefore is close to the RyR channel where the Ca is released from the SR. Since the distance of a myosin power stroke is 10nm, I estimate that it can travel at a minimum 100nm, but can go much further if the distance is greater to the RyR channel
  25. Why must ATP must be present and Ca must NOT be in order to distinguish b/w thick and thin filaments in experiments? (Ch 20 SUMMARY #2)
    • Ca: Ca in solution will allow for the attachment of myosin to myosin binding sites of actin due to Ca binding to Troponin, causing tropomyosin shift to expose the binding sites
    • ATP: ATP needs to be present because when MYOSIN is bound to ATP, it triggers the release of it from G-actin
  26. Explain events and structures taking place b/w excitation of skeletal muscle by action potential and cross-bridge action (CH 20 SUMMARY #3)
    • 1. Excitation in motor neuron triggers RELEASE of ACh, and binding of it to LIGAND-Gated channels
    • 2. These channels allow Na to flow INTO Muscle, DEPOLARIZING the T-TUBULES
    • 3. This Depolarization causes DHPR to change conformation to allow RyR to alter conformation and allow Ca to release from the SR and into the cytoplasm
    • 4. Ca release causes binding to the troponin on actin and then tropomyosin shifts, allowing myosin heads to bind and contraction to occur
  27. Why is advantagous for Oxidative muscle fibers to have SMALLER diameters vs glycolytic muscle fibers (CH 20 SUMMARY #4)
    • SO fibers (slow oxidative) have smaller diameters because they generate LESS of the tension of the muscle, as they are used for isometric functions and SMALL, slow movements.
    • FG fibers (fast glycolytic) have LARGE diameters because they generate rapid contractions and LARGE increments of TENSION and generate LARGER FORCE movements, although they are bursts that fatigue quicker
  28. Difference btw SIngle cross-bridge power stroke and single-twitch of skeletal muscle (CH 20 SUMMARY #5)
    • Twitches generated from Twitch fibers: fibers generate Action Potential, which give rise to muscle twitch
    • Cross-bridge power stroke: amount of tension developed PER cross bridge power stroke the same, but the number of cycles Per unit of time differs.
    • ie: FG fibers split 600 ATP molecules per second, so the rate of ATP hydrolysis determines the RATE of cross-bridge cycling, therefore higher ATP hydrolysis activity means.. Faster contraction
  29. Why is amount of tension produced by TRAIN OF ACTION POTENTIALS greater than amount of tension from SINGLE Twitch? (CH 20 SUMMARY #6)
    • Since action potentials only last 1-2ms, and a muscle twitch lasts many ms, there can be more than one action potential during a muscle twitch, therefore ..
    • If you INCREASE the FREQUENCY of stimulation causes a SUMMATION of twitches up to a MAXIMUM contractile response called fused TETANUS. This maximum response has the GREATER tension, whereas a single twitch
  30. How do arthropod muscle fibers that do NOT generate action potentials still activate their contractile elements in coordinated and rapid fashion? (CH 20 SUMMARY #7)
    • Arthropod fibers contain overlapping motor units, termed POLYNEURONAL INNERVATION. Each fiber is innervated with multiple neurons. Each neuron also makes multiple contacts per fiber
    • Instead of action potentials…neurons generate POST-synaptic potentials to provide graded ELECTRIC signals to trigger contractions (greater depolarization, greater Ca released to SR and greater tension)
    • Also in arthropods, the velocity of contraction is ASSOCIATED with the Sarcomere length
  31. What's a Motor unit and how does recruitment of motor unit influence tension produced by whole muscle? (CH20 SUMMARY #8)
    • Motor unit: Composed of a single MOTOR NEURON and all of the muscle fibers it innervates
    • *one motor neuron may innervate many fibers, but each fiber is only innervated by ONE neuron
    • The amount of tension generated by a muscle depends on the amount of DEPOLARIZATION produced by EPSPs.
    • Motor neurons producing action potentials generate EPSPs (excitatory post-synaptic potential) and this spreads to fibers via excitation-contraction coupling
  32. Two muscles with same diameter, one is twice as long, which produces more work? (CH 20 SUMMARY #9)
    • The one with longer length produces more work, as work is related to the distance and the force generated.
    • Note: Length of muscle fiber does NOT contribute to how much force it generates, but does contribute to how much Work the muscle can do
  33. 3 means of ATP production
    • Transfer of high energy phosphate from creatine phosphate to ADP
    • Glycolysis
    • Oxidative phosphorylation: lowest rate of ATP synthesis
  34. What roles does ATP play in muscle contraction
    • 1. ATP binding of cross-bridge aids in DETACHMENT of myosin from actin
    • 2. Hydrolysis of ATP cocks myosin in preparation of binding to actin and power stroke
    • 3. Energy from hydrolysis of ATP (via ATPases) drives Calcium pump that transports Ca into SR
Card Set
ch 20 muscle - Animal Physiology.txt