The Muscular System

1. The Muscular System Chapter 8

2. Types of muscle & function • Skeletal- 40-50% of total body weight- voluntary • mostly movement of bone & body parts • Stabilizing body positions • Cardiac- only in heart- involuntary • Heart only • Develops pressure for arterial blood flow • Smooth- grouped in walls of hollow organs • Sphincters regulate flow in tubes • Maintain diameter of tubes • Move material in GI tract and reproductive organs

3. Muscle Functions • Produce body movements • Stabilize body positions • Regulate organ volume • Moving substances internally • Producing heat

4. Skeletal Muscle Tissue • Muscle includes: muscle fibers, connective tissue, nerves & blood vessels • Wrapped in Epimysium • Perimysium surrounds fiber bundles called fascicles • Endomysium surrounds each individual fiber

5. Skeletal Muscle Tissue • Well-supplied with blood vessels and nerves • Terminal of a neuron on each muscle fiber

6. Figure 8.1

7. Muscle Histology • elongated cylindrical cells = muscle fibers • plasma membrane = sarcolemma • Transverse (T- tubules) tunnel from surface to center of each fiber • Multiple nuclei lie near surface • Cytoplasm = sarcoplasm

8. Figure 8.2a

9. Muscle histology (cont.) • Throughout sarcoplasm is sarcoplasmic reticulum • Stores Calcium ions • Sarcoplasm contains myoglobin • Red pigmented protein related to Hemoglobin that carries oxygen • Along entire length are myofibrils • Myofibrils made of protein filaments • Come in thick and thin filaments

10. Figure 8.2b

11. Sarcomere • Filaments overlap in repeating patterns • Unit structure is called sarcomere • Separated by Z-discs • Darker area = A-band associated with thick filaments • H-zone has no thin filaments • I-band has thin filaments no thick filaments

12. Figure 8.2c

13. Figure 8.3a

14. Figure 8.3b

15. Functional Structure • Thick filament (myosin) has moveable heads • Thin filaments (actin) are anchored to Z-discs • Contain myosin binding sites for myosin head • Also contain tropomyosin & troponin • Tropomyosin blocks myosin binding site at rest

16. Sliding Filament Mechanism • During contraction myosin heads bind actin sites • Pull and slide actin molecules (and Z-discs) toward H-zone • I-bands and H-zones narrow • Sliding generates force and shortens sarcomeres and thus fibers.

17. Figure 8.4

18. Neuromuscular Interaction • Nerve signal triggers muscle action potential • Delivered by motor neuron • One neuron can trigger 1 or more fibers at the same time • Neuron plus triggered fibers = motor unit

19. Neuromuscular Junction • neuronal ending to muscle fiber = Neuromuscular junction • Synaptic end bulbs (at neuron terminal) • Release neurotransmitter • Muscular area = Motor end plate • Between is synaptic cleft

20. Figure 8.5

21. Action at NMJ • Release of acetylcholine (ACh) • Diffuses across cleft 2.Activation of ACh receptors 3. Generation of Muscle Action Potential Repeats with each neuronal action potential 4. Breakdown of ACh

22. Contraction Trigger • Muscle action potential=> Ca2+ release from Sacroplasmic Reticulum (SR) • Ca2+ binds to troponin => • Moves tropomyosin off actin sites => • Myosin binds & starts cycle

23. Contraction Cycle • Myosin binds to actin & releases phosphate group (Forming crossbridges) • Crossbridge swivels releasing ADP & shortening sarcomere (Power stroke) • ATP binds to Myosin => release of myosin from actin • ATP broken down to ADP & Pi => activates myosin head to bind and start again • Repeats as long as Ca2+ concentration is high

24. Figure 8.6

25. Relaxation • Breakdown of Ach to stop muscle Action potentials • Ca2+ ions transported back into SR lowering concentration=> • This takes ATP • tropomyosin covers actin binding sites

26. Figure 8.7

27. Muscle Tone • Even at rest some motor neuron activity occurs = Muscle Tone • If nerves are cut fiber becomes flaccid (very limp)

28. Metabolism • Rapid changes from very low ATP consumption to high levels of consumption • Creatine phosphate (high energy store) • Fast & good for ~ 15 sec

29. Figure 8.8a

30. Glycolysis • Break down glucose to 2 pyruvates getting 2 ATPs • If insufficient mitochondria or oxygen pyruvate => lactic acid • Get about 30-40 seconds more at max.

31. Figure 8.8b

32. Aerobic Cellular Respiration • Production of ATP in mitochondria • Requires oxygen and carbon substrate • Produces CO2 and H2O and heat.

33. Fatigue • Inability to contract forcefully after prolonged activity • Limiting factors can include: • Ca2+ • Creatine Phosphate • Oxygen • Build up of acid • Neuronal failure

34. Oxygen Use After Exercise • Convert lactic acid back to glucose in liver • Resynthesize Creatine Phosphate and ATP • Replace oxygen removed from myoglobin

35. Control of Muscle Contraction • Single Action Potential(AP) =>twitch • Smaller than maximum muscle force • Total tension of fiber depends on frequency of APs (number/second) • Require wave summation • Maximum = tetanus • Total tension of muscle depends on number of fibers contracting in unison • Increasing numbers = Motor unit recruitment

36. Figure 8.9

37. Figure 8.10

38. Fiber types • Slow oxidative (SO)- small diameter & red • large amounts of myoglobin and mitochondria • ATP production primarily oxidative • Fatigue resistant- • Fast oxidative- glycolytic (FOG) • Large diameter = many myofibrils • Many mitochondria and high glycolytic capacity • Fast glycolytic fibers (FG) • white, fast & powerful and fast fatiguing • For strong, short term use

39. Recruitment • Muscle contractions only use the fibers required for the work • Recruited in order: SO=>FOG=>FG • if force is constant and the muscle shortens = Isotonic Contraction • If length is constant and the force varies = Isometric Contraction • The latter is often a postural muscle activity

40. Effects of Exercise • SO/FG fiber ratio genetically determined • High FG => sprinters • High SO=> marathoners • Endurance exercise gives FG=> FOG • Increased diameter and numbers of mitochondria • Strength exercise increases size & strength of FG fibers

41. Cardiac Muscle • Striated, short fibers and branched • Single central nucleus; Cells joined by gap junctions & desmosomes • Thickened joint area called intercalated discs • Some cardiac muscles generate own AP- autorhythmicity • Involuntary

42. Cardiac muscle • No nerve- internal pacemaker • Ca2+- from S.R. and extracellular space • separate cells with gap junctions -> electrical connections

43. Figure 15.2b

44. Smooth muscle • Involuntary • In internal organs • Filaments not regular so not striated • Visceral (single unit) type or • Form sheets and are autorhythmic • Contract as a unit • Multi-unit type- • each has own nerve and can contract independently

45. Smooth Muscle • Graded contractions and slow responses • Often sustain long term tone • Often triggered by autonomic nerves • modulated chemically, nerves, by mechanical events (stretching)

46. Figure 8.11

47. Aging • Like bone there is a slow progressive loss of skeletal muscle mass • Relative number of SO fibers tends to increase

48. Movement • Move one bone relative to another • Origin => most stationary end • Location where the tendon attaches • Insertion => the most mobile end • Location where tendon inserts • Action => the motion or function of the muscle

49. Figure 8.12

50. Movement (cont.) • Generally arranged in opposing pairs • Flexors- extensors; abductors- adductors • The major actor = Prime mover or agonist • The one with opposite effect = antagonist • Synergists- help prime mover • Fixators- stabilize origin of prime mover • Role of muscle varies with motion