Muscles and Muscle Tissue

1. Muscles and Muscle Tissue Form and Function of Movement

2. Muscle Tissue Anatomy and Histology

3. Functional Characteristics of Muscle Tissue • Excitability, or irritability – the ability to receive and respond to stimuli • Contractility – the ability to shorten forcibly • Extensibility – the ability to be stretched or extended • Elasticity – the ability to recoil and resume the original resting length

4. Muscle Overview • The three types of muscle tissue are skeletal, cardiac, and smooth • These types differ in structure, location, function, and means of activation

5. Cardiac muscle cells branch, are striated, are uninucleate (B) and have intercalated discs (A). • Locations: heart • Function: involuntary, rhythmic contraction

6. Skeletal Muscle • Skeletal muscle cells run the full length of a muscle. Line A show the width of one cell (fiber). Note the striations characteristics of this muscle type. These cells are multicellular, B marks one nucleus. • Location: muscles associated with the skeleton • Function: voluntary movement • Muscles are connected to bones by tendons. Bones are connected to other bones at their joints by ligaments.

7. Smooth Muscle • Smooth muscle cells are spindle shaped and uninucleate. (B). • Locations: walls of hollow organs, i.e. stomach, intestine, uterus, ureter • Functions: involuntary movement - i.e. churning of food, movement of urine from the kidney to the bladder, partuition

8. Skeletal Muscle Figure 9.2 (a)

9. Myofibrils Figure 9.3 (b)

10. Sarcomeres Figure 9.3 (c)

11. Myofilaments: Banding Pattern

12. Ultrastructure of Myofilaments: Thick Filaments Figure 9.4 (a)(b)

13. Ultrastructure of Myofilaments: Thick Filaments • Thick filaments are composed of the protein myosin • Each myosin molecule has a rodlike tail and two globular heads • Tails – two interwoven, heavy polypeptide chains • Heads – two smaller, light polypeptide chains called cross bridges

14. Ultrastructure of Myofilaments: Thin Filaments Figure 9.4 (c)

15. Ultrastructure of Myofilaments: Thin Filaments • Thin filaments are chiefly composed of the protein actin • Tropomyosin and troponin are regulatory subunits bound to actin

16. Arrangement of the Filaments in a Sarcomere

17. Sarcoplasmic Reticulum (SR) Figure 9.5

18. Sarcoplasmic Reticulum (SR) • SR smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril • Paired terminal cisternae form perpendicular cross channels • Functions in the regulation of intracellular calcium levels

19. T Tubules • T tubules are continuous with the sarcolemma • They conduct impulses to the deepest regions of the muscle • These impulses signal for the release of Ca2+ from adjacent terminal cisternae

20. Muscle Cell Contraction Sliding Filament Theory

21. Excitation of a Muscle Fiber

22. Excitation (steps 1 & 2) • Nerve signal stimulates voltage-gated calcium channels that result in exocytosis of synaptic vesicles containing ACh = ACh release

23. Excitation (steps 3 & 4) • Binding of ACh to the surface of muscle cells opens Na+ and K+ channels resulting in an end-plate potential (EPP)

24. Excitation (step 5) • Voltage change in end-plate region (EPP) opens nearby voltage-gated channels in plasma membrane producing an action potential

25. Excitation-Contraction Coupling

26. Excitation-Contraction Coupling(steps 6&7) • Action potential spreading over sarcolemma reaches and enters the T tubules -- voltage-gated channels open in T tubules causing calcium gates to open in SR

27. Excitation-Contraction Coupling(steps 8&9) • Calcium released by SR binds to troponin • Troponin-tropomyosin complex changes shape and exposes active sites on actin

28. Contraction (steps 10 & 11) • Myosin ATPase in myosin head hydrolyzes an ATP molecule, activating the head and “cocking” it in an extended position • It binds to an active site on actin

29. Contraction (steps 12 & 13) • Power stroke = shows myosin head releasing the ADP & phosphate as it flexes pulling the thin filament along • With the binding of more ATP, the myosin head releases the thin filament and extends to attach to a new active site further down the thin filament • at any given moment, half of the heads are bound to a thin filament, preventing slippage • thin and thick filaments do not become shorter, just slide past each other (sliding filament theory) 12. Power Stroke; sliding of thin filament over thick

30. Relaxation (steps 14 & 15) • Nerve stimulation ceases and acetylcholinesterase removes ACh from receptors so stimulation of the muscle cell ceases

31. Relaxation (step 16) • Active transport pumps calcium from sarcoplasm back into SR where it binds to calsequestrin • ATP is needed for muscle relaxation as well as muscle contraction

32. Relaxation (steps 17 & 18)

33. Muscle Tone • Muscle tone: • Is the constant, slightly contracted state of all muscles, which does not produce active movements • Keeps the muscles firm, healthy, and ready to respond to stimulus • Spinal reflexes account for muscle tone by: • Activating one motor unit and then another • Responding to activation of stretch receptors in muscles and tendons

34. Isotonic Contractions • In isotonic contractions, the muscle changes in length (decreasing the angle of the joint) and moves the load • The two types of isotonic contractions are concentric and eccentric • Concentric contractions – the muscle shortens and does work • Eccentric contractions – the muscle contracts as it lengthens

35. Isotonic Contractions Figure 9.17 (a)

36. Isometric Contractions • Tension increases to the muscle’s capacity, but the muscle neither shortens nor lengthens • Occurs if the load is greater than the tension the muscle is able to develop

37. Isometric Contractions Figure 9.17 (b)

38. Muscle Metabolism: Energy for Contraction • ATP is the only source used directly for contractile activity • As soon as available stores of ATP are hydrolyzed (4-6 seconds), they are regenerated by: • The interaction of ADP with creatine phosphate (CP) • Anaerobic glycolysis • Aerobic respiration

39. Muscle Metabolism: Energy for Contraction Figure 9.18

40. Muscle Metabolism: Anaerobic Glycolysis • When muscle contractile activity reaches 70% of maximum: • Bulging muscles compress blood vessels • Oxygen delivery is impaired • Pyruvic acid is converted into lactic acid

41. Muscle Metabolism: Anaerobic Glycolysis • The lactic acid: • Diffuses into the bloodstream • Is picked up and used as fuel by the liver, kidneys, and heart • Is converted back into pyruvic acid by the liver

42. Muscle Fatigue • Muscle fatigue – the muscle is in a state of physiological inability to contract • Muscle fatigue occurs when: • ATP production fails to keep pace with ATP use • There is a relative deficit of ATP, causing contractures • Lactic acid accumulates in the muscle • Ionic imbalances are present

43. Muscle Fatigue • Intense exercise produces rapid muscle fatigue (with rapid recovery) • Na+-K+ pumps cannot restore ionic balances quickly enough • Low-intensity exercise produces slow-developing fatigue • SR is damaged and Ca2+ regulation is disrupted

44. Oxygen Debt • Vigorous exercise causes dramatic changes in muscle chemistry • For a muscle to return to a resting state: • Oxygen reserves must be replenished • Lactic acid must be converted to pyruvic acid • Glycogen stores must be replaced • ATP and CP reserves must be resynthesized • Oxygen debt – the extra amount of O2 needed for the above restorative processes

45. Heat Production During Muscle Activity • Only 40% of the energy released in muscle activity is useful as work • The remaining 60% is given off as heat • Dangerous heat levels are prevented by radiation of heat from the skin and sweating

46. Force of Muscle Contraction • The force of contraction is affected by: • The number of muscle fibers contracting – the more motor fibers in a muscle, the stronger the contraction • The relative size of the muscle – the bulkier the muscle, the greater its strength • Degree of muscle stretch – muscles contract strongest when muscle fibers are 80-120% of their normal resting length

47. Force of Muscle Contraction Figure 9.20 (a)

48. Muscle Fiber Type: Functional Characteristics • Speed of contraction – determined by speed in which ATPases split ATP • The two types of fibers are slow and fast • ATP-forming pathways • Oxidative fibers – use aerobic pathways • Glycolytic fibers – use anaerobic glycolysis • These two criteria define three categories – slow oxidative fibers, fast oxidative fibers, and fast glycolytic fibers

49. Muscle Fiber Type: Speed of Contraction • Slow oxidative fibers contract slowly, have slow acting myosin ATPases, and are fatigue resistant • Fast oxidative fibers contract quickly, have fast myosin ATPases, and have moderate resistance to fatigue • Fast glycolytic fibers contract quickly, have fast myosin ATPases, and are easily fatigued

50. Smooth Muscle • Composed of spindle-shaped fibers with a diameter of 2-10 m and lengths of several hundred m • Lack the coarse connective tissue sheaths of skeletal muscle, but have fine endomysium • Organized into two layers (longitudinal and circular) of closely apposed fibers • Found in walls of hollow organs (except the heart) • Have essentially the same contractile mechanisms as skeletal muscle