Muscle Structure and Function: Zooming in on a Sarcomere

1. Unit 5 Notes

2. To Review… • Muscle surrounded by epimysium • Fascicle surrounded by perimysium • Individual muscle fibers (cells) surrounded by endomysium

4. Let’s zoom in on an individual muscle fiber… Note: The sarcolemma (cell membrane) is surrounded by endomysium (connective tissue).

6. Let’s zoom in on a myofibril…

7. Definitions Sarcolemma: the specific name for the plasma membrane of a muscle cell; surrounded by endomysium.

8. Definitions Myofibril: contractile organelles found in the cytoplasm of muscle cells.

9. Definitions Sarcomere: tiny contractile units that make up the myofibril; aligned end-to-end.

10. Definitions Myofilaments: filaments composing the myofibrils; the two types are actin (thin) and myosin (thick).

11. Sarcoplasmic reticulum: specialized smooth ER that surrounds myofibrils; stores calcium and releases on demand

12. There are also multiple nuclei and mitochondria scattered throughout the muscle fiber, surrounding the myofibrils.

13. So, in summary, as you work your way smaller in a muscle…. • Muscle – surrounded by epimysium • Fascicle – surrounded by perimysium • Muscle fiber (cell) – surrounded by endomysium/sarcolemma • Myofibril – sections called sarcomeres, surrounded by sarcoplasmic reticulum, mitochondria, and nuclei • Myofilaments – 2 types are actin & myosin

14. The light band is also called the I band, and has a dark midline interruption called the Z disc. The Z disc marks the end of the sarcomere, and is formed by a disc-like membrane.

15. The dark band is also called the A band, and has a lighter central area called the H zone (which contains the M line). TheHzone lacks thin filament, so it looks a bit lighter than the rest of the A band.

16. Notice that the I and A bands are responsible for the striations of skeletal muscle tissue! So, striations are formed by the locations of actin and myosin filaments.

17. Zooming in on a sarcomere… The bands of myofilaments are actually formed by many myofilaments packed together.

19. THICK FILAMENTS… • Contain myosin protein • ATPase help split ATP to generate power for muscle contraction • Form the “A” band • Have projections that are myosin “heads” – called cross bridges • Cross bridges link thick and thin filaments together in contraction of muscle

21. THIN FILAMENTS… • Contain actin protein • Also contain tropomyosin protein filaments (which block specific binding sites) and troponin protein • Thin filaments anchored to the Z disc (a disc-like membrane separating the sarcomeres) • The zone where thin filaments do not exist in the sarcomere is called the H zone

24. What functional properties allow a muscle to perform its duties? • Irritability • Ability to receive and respond to a stimulus • Contractility • Ability to shorten when adequate stimulus is received

25. What functional properties allow a muscle to perform its duties? • Conductivity • Ability for impulse to travel along plasma membrane of muscle cell • Elasticity • Ability to recoil and resume original length

26. What role does the nervous system play in muscle movement? • Motor Unit – one neuron and all the skeletal muscle cells it stimulates

27. Within a motor unit… • Muscle Fibers • Axons • Axon Terminals (neuromuscular junctions)

30. Nerve endings and muscle fibers don’t physically touch… • Neuromuscular juction – where axon terminals match up with muscle fibers • Snyaptic cleft – space between nerve endings and muscle fibers; chemical impulses travel here between nerve endings and muscle

31. What steps occur to stimulate muscle movement? • 1. Nerve impulse reaches axon terminals • 2. Chemical Neurotransmitter (ACh – acetylcholine) released • 3. ACh diffuses across synaptic cleft and attaches to receptors

32. What steps occur to stimulate muscle movement? • 4. ACh causes the sarcolemma to become temporarily permeable to Na+ • 5. Na+ rush into the muscle cell • 6. Excess of positive ions creates electric current (action potential) • 7. Muscle contracts (another whole set of steps!)

33. So, we know how muscle contraction is stimulated… but now we need to know the steps that help muscle contraction to happen! • Called the Sliding Filament Theory

34. The Sliding Filament Theory • Muscle fibers activated by nervous system due to action potential • Calcium ions (Ca+2) happen to be released • Do you remember which structure releases those calcium ions???

35. The Sliding Filament Theory • Muscle fibers activated by nervous system due to action potential • Calcium ions (Ca+2) happen to be released • Do you remember which structure releases those calcium ions??? • THE SARCOPLASMIC RETICULUM

37. The Sliding Filament Theory • Release of Ca+2 allows troposin & tropomyosin to stop blocking binding sites on the actin… which allows the cross-bridges on the thick filaments to attach to the binding sites on the thin filaments • Let the sliding begin!

38. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__action_potentials_and_muscle_contraction.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__myofilament_contraction.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__action_potentials_and_muscle_contraction.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__myofilament_contraction.html

39. The Sliding Filament Theory • Energized by energy from ATP, cross-bridges attach and detach from thin filaments • Works like an oar to keep moving thin filaments closer and closer together (Attach, pull, detach!)

41. http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__sarcomere_contraction.htmlhttp://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter10/animation__sarcomere_contraction.html

42. Neurotransmitter released diffuses across the synaptic cleft and attaches to ACh receptors on the sarcolemma. Axon terminal Synaptic cleft Synaptic vesicle Sarcolemma T tubule 1 Net entry of Na+ Initiates an action potential which is propagated along the sarcolemma and down the T tubules. ACh ACh ACh Ca2+ Ca2+ SR tubules (cut) SR Ca2+ Ca2+ 2 Action potential in T tubule activates voltage-sensitive receptors, which in turn trigger Ca2+ release from terminal cisternae of SR into cytosol. ADP Pi Ca2+ Ca2+ Ca2+ Ca2+ 6 Tropomyosin blockage restored, blocking myosin binding sites on actin; contraction ends and muscle fiber relaxes. 3 Calcium ions bind to troponin; troponin changes shape, removing the blocking action of tropomyosin; actin active sites exposed. Ca2+ 5 Removal of Ca2+ by active transport into the SR after the action potential ends. Ca2+ 4 Contraction; myosin heads alternately attach to actin and detach, pulling the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers the cycling process.

43. The Sliding Filament Theory • As this process is happening in every sarcomere throughout the muscle, the muscle itself is contracting! • The whole series of events (beginning with the nervous system signal) takes just a few thousandths of a second!!!

44. The Sliding Filament Theory • Notice in the contracted muscle, the H zone has disappeared • The I band has shortened significantly (all that’s left is the Z disc) • The A band (the dark striations!) have stayed the same thickness

45. Where’s all this energy coming from? • As a reminder, energy comes from ATP because of breaking a phosphate bond • Breaking a bond releases energy • When this energy is used by your body, it releases heat • Because ATP is the only energy source that can be used to move the cross-bridges back and forth (which contract the muscle), ATP must be regenerated continuously

46. ATP Regeneration – 3 Sources • Direct phosphorylation of ADP by creatine phosphate • When ATP used, changes to ADP • Creatine phosphate adds that missing phosphorous back on! • PROBLEM: only makes 1 ATP at a time… so not very much. And, only supplies energy for 15-20 seconds of activity! • Your body will always do this, but it’s not very effective. Therefore, we have to have other ways of supplying energy…..

47. ATP Regeneration – 3 Sources • Aerobic respiration • Occurs in the mitochondria • Glucose broken down to pyruvic acid (releasing 2 ATP), and then into carbon dioxide and water (releasing 34 ATP) • 36 ATP made for 1 glucose! A lot of energy! And, can supply energy for hours at a time! • PROBLEM: NEEDS OXYGEN • But what if you’re out of oxygen??? Then your muscles will begin……..

48. ATP Regeneration – 3 Sources • Anaerobic glycolysis and lactic acid formation • Glucose broken down to pyruvic acid, releasing 2 ATP • If oxygen present, process continues to the rest of aerobic respiration… • BUT… if oxygen is inadequate, or muscle activity is intense, pyruvic acid is instead changed to lactic acid • PROBLEM: Buildup of lactic acid is not good… promotes muscle fatigue and soreness. And, only supplies energy for 30 seconds of activity!

49. ATP Regeneration • 95% of ATP produced through aerobic respiration • C6H12O6 (aq) + 6O2 (g) → 6CO2 (g) + 6H2O + ATP • If you don’t have proper blood circulation or breathing, muscles can’t get oxygen needed for aerobic respiration • If they can’t get oxygen, they can’t produce enough ATP, which means muscles can’t contract!

50. Muscle Fatigue • A muscle is fatigued when it is unable to contract even though it is being stimulated – means you don’t have ATP to move the cross-bridges! • Lack of oxygen can cause… • Lactic acid buildup (anaerobic glycolysis) • ATP supply low (production can’t keep up with usage) • Muscle will contract less and less effectively, eventually stopping contraction completely