SKELETAL MUSCLE PHYSIOLOGY

1. SKELETAL MUSCLE PHYSIOLOGY Abraham D. Lee, Ph.D.,P.T. Department of Physical Therapy Office: Collier Building # 4206 Phone #: 419-383-3437 Email: abraham.lee2@utoledo.edu

2. Contents 1. Muscle structure & organization 2. Muscle fiber type 3. Muscle action 4. Muscle mechanics 5. Motor unit and its recruitment 6. Local muscle control 7. Muscle plasticity 8. Summary

3. Muscle organization • Epimysium: wraps an entire muscle • Perimysium: wraps a bundle of muscle fibers. This bundle is called fascicle or fasciculus • Endomysium: wraps an individual muscle fiber • Sarcolemma: muscle membrane • Myofibrils: contractile filaments

7. Myofibrils • Thin filament • Actin filaments • Troponin • Tropomyosin • Thick filament • Myosin: 4 light chains and 2 heavy chains • Heavy chains • Myosin head region: heavy meromyosin • Myosin tail region: light memromysin

8. Muscle pennation • Longitudinal (non-pennated) architecture: muscle fibers in parallel to the muscle force generating axis • Example: biceps brachii, sartorius muscle • In these muscles fibers are said to be fusiform or spindle shaped. • “Pennate” architecture: muscle fibers are oriented at an angle or multiple angles relative to force-generating axis. • Unipennate: soleus-25 degree; vastus medialis-5 degree • Bipennate: gastrocnemius, rectus femoris • Multipennate: deltoid

10. Effect of pennation Force loss Space saving

11. Comparison b/n non-pennated & pennated muscle • Contraction • Fiber packing • w/ given volume • Force loss due • to pennation • # fiber • Muscle force • Production • CSA Non-pennatedPennated Fast Slow Less More No Yes Less More Less Greater Less Greater

12. Muscle Fiber Type

13. Muscle fiber type • Type I, • Slow-oxidative (SO) fibers • Type IIa, • Fast-oxidative-glycolytic (FOG) fibers • Type IIb, • Fast-glycolytic (FG) fibers

14. Characteristics of different fibers #Mitochondria Resistance to fatigue Energy ATPase activity Vmax Efficiency • Type I Type IIa Type IIb • H H/M L • H H/M L • A A+AN AN • L H HH • L H HH • H M L L: low H: high M: moderate A: aerobic AN: anaerobic HH: highest

15. Muscle composition in athletes % Type I %Type IIa &IIb 70-80 20-30 25-30 70-75 45-55 45-55 47-53 47-53 Distance runners Track sprinters Weight lifters Non-athletes Will fiber type change with training?

16. Muscle Action • Excitation-contraction coupling • Type of muscle action

17. Excitation-Contraction Coupling • Nerve impulse generation and propagation • Neuromuscular junction transmission • Muscle action potential propagation • Ca2+ release from SR • Ca2+ binding to troponin • Interaction of myosin head and actin • Cross bridge moves: tension development • Ca2+ taken up to SR • Ca2+ removal from troponin • Relaxation

19. E-C coupling DHPR: dihydropyridine receptors RyR: ryanodine receptor Other possible mechanism: Inositol 1,4,5-triphosphate (InsP3) InsP3 receptor activation Ca2+ release from SR May play a role in slow twitch muscle in developmental stage (Talon et al., Am. J. Physiol 282: R1164-R1173, 2002)

20. E-C coupling

21. Sliding Filament Theory

23. Changes during shortening muscle action • Sarcomere length (distance between two adjacent Z lines): shortens • A band: no change • I band: shortens • H zone: shortens

24. Different type of muscle action (contraction) • Isometric action • Isotonic action (dynamic action) • Concentric action • Eccentric action

25. Muscle mechanics It deals with how muscle force is generated and regulated.

26. Factors that affect muscle force generation • Rate of muscle stimulation • Muscle length • Joint angle • Speed of action (speed of contraction) • Muscle fiber type • # of MU recruitment

27. Rate of Muscle Stimulation • Twitch: • Tetanus:

28. Muscle twitch

29. Muscle tetanus

30. Effect of Muscle Length

31. Force-Length Relationship • Isolated muscle • In vivo human muscles

32. Force-Length Relationship • Isolated muscle

34. Force-Length Relationship • In vivo human muscles • Two things are considered: muscle length and joint angle • In general, a group of muscles produces more force (torque) when muscles are lengthened before contraction. But some muscles do not follow this rule.

35. Shoulder muscles Shoulder flexors (anterior deltoid): causes to flex shoulder joint Shoulder extensors(posterior deltoid): causes to extend shoulder joint 180° 135° 90° 45° 40° 0°

37. Knee flexors • A person is lying on the stomach (prone position) 90° 120° 45° 0° Trunk Lower leg Thigh Knee joint Hip joint Knee flexors (hamstring muscles): causes to flex knee joint

39. Hip flexors • A person is lying on the back (supine position) 90° 120° 45° 0° Lower leg Thigh Trunk Knee joint hip joint Hip flexors (iliopsoas, sartorius): causes to flex hip joint

41. Knee extensors • A person is sitting on the bench 0° 45° 120° 90° Knee extensors (quadriceps muscles): causes to extend knee joint

43. Elbow flexors Elbow flexors (biceps brachii): causes to flex elbow joint

45. Force arm distance

46. Effect of Velocity (Speed of Action)

47. Force-velocity curve

48. Effect of Muscle Fiber

49. Effectof muscle fiber type on force

50. Muscle Power Need to consider two factors: 1. Muscle force 2. Speed of action