Muscles and Muscle Tissue

1. Muscles and Muscle Tissue LAB 6

2. Muscle Overview • Muscle tissue makes up nearly half the body mass. • The most distinguishing functional characteristic of muscles is their ability to transform chemical energy ATP into directed mechanical energy • The three types of muscle tissue are: skeletal, cardiac, and smooth • These types differ in structure, location, function, and means of activation

3. MARTINI PG 133

4. Muscle Similarities • Skeletal and smooth muscle cells are elongated and are called muscle fibers • Muscle contraction depends on two kinds of myofilaments – actin and myosin • Muscle terminology is similar • Sarcolemma – muscle plasma membrane • Sarcoplasm – cytoplasm of a muscle cell • Prefixes – myo, mys, and sarco all refer to muscle

5. 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

6. Muscle Function • Skeletal muscles are responsible for all locomotion • Cardiac muscle is responsible for coursing the blood through the body • Smooth muscle helps maintain blood pressure, and squeezes or propels substances (i.e., food, feces) through organs • Muscles also maintain posture, stabilize joints, and generate heat

7. Muscle Classification: Functional Groups • Prime movers – provide the major force for producing a specific movement • Antagonists – oppose or reverse a particular movement • Synergists • Add force to a movement • Reduce undesirable or unnecessary movement • Fixators – synergists that immobilize a bone or muscle’s origin

8. Naming Skeletal Muscles • Location of muscle – bone or body region associated with the muscle • Shape of muscle – e.g., the deltoid muscle (deltoid = triangle) • Relative size – e.g., maximus (largest), minimus (smallest), longus (long) • Direction of fibers – e.g., rectus (fibers run straight), transversus, and oblique (fibers run at angles to an imaginary defined axis)

9. Naming Skeletal Muscles • Number of origins – e.g., biceps (two origins) and triceps (three origins) • Location of attachments – named according to point of origin or insertion • Action – e.g., flexor or extensor, as in the names of muscles that flex or extend, respectively

10. Bone-Muscle Relationships: Lever Systems • Lever – a rigid bar that moves on a fulcrum, or fixed point • Effort – force applied to a lever • Load – resistance moved by the effort

11. Bone-Muscle Relationships: Lever Systems Figure 10.2a

12. Bone-Muscle Relationships: Lever Systems Figure 10.2b

13. Lever Systems: Classes • First class – the fulcrum is between the load and the effort • Second class – the load is between the fulcrum and the effort • Third class – the effort is applied between the fulcrum and the load

14. Lever Systems: First Class Figure 10.3a

15. Lever Systems: Second Class Figure 10.3b

16. Lever Systems: Third Class Figure 10.3c

17. Major Skeletal Muscles: Anterior View The 40 superficial muscles here are divided into 10 regional areas of the body: • 1.- Facial • 2.- Neck • 3.-Thorax • 4.- Shoulder • 5.- Arm • 6.- Forearm • 7.- Abdomen • 8.- Pelvis • 9.- Thigh • 10.- Leg Figure 10.4b

19. Major Skeletal Muscles: Posterior View The 27 superficial muscles here are divided into seven regional areas of the body: 1.- Neck 2.- Shoulder 3.-Arm 4.- Forearm 5.- Hip 6.-Thigh 7.- Leg Figure 10.5b

21. Muscles of the Face • 11 muscles are involved in lifting the eyebrows, flaring the nostrils, opening and closing the eyes and mouth, and smiling • All are innervated by cranial nerve VII (facial nerve) • Usually insert in skin (rather than bone), and adjacent muscles often fuse

22. Muscles of the Face Figure 10.6

23. Muscles of Mastication • There are four pairs of muscles involved in mastication • Prime movers – temporalis and masseter • Grinding movements – pterygoids and buccinators • All are innervated by cranial nerve V (trigeminal nerve)

24. Muscles of Mastication Figure 10.7a

25. Muscles of Mastication Figure 10.7b

26. Extrinsic Tongue Muscles • Three major muscles that anchor and move the tongue • All are innervated by cranial nerve XII (hypoglossal nerve)

27. Extrinsic Tongue Muscles Figure 10.7c

28. Homeostatic Imbalance • Many toxins, drugs and diseases interfere with events at the neuromuscular junction Ex: Myastenia gravis: Characterize by: 1.- Drooping of the upper eyelids 2.- Difficulty of swallowing and talking 3.- Muscle weakness 4.- Serum antibodies against acetilcholine (Ach) receptor

29. Developmental Aspects: Male and Female • There is a biological basis for greater strength in men than in women • Women’s skeletal muscle makes up 36% of their body mass • Men’s skeletal muscle makes up 42% of their body mass

30. Action Potential: Electrical Conditions of a Polarized Sarcolemma • The outside (extracellular) face is positive, while the inside face is negative • This difference in charge is the resting membrane potential Figure 9.8 (a)

31. Action Potential: Electrical Conditions of a Polarized Sarcolemma • The predominant extracellular ion is Na+ • The predominant intracellular ion is K+ • The sarcolemma is relatively impermeable to both ions Figure 9.8 (a)

32. Action Potential: Depolarization and Generation of the Action Potential • An axonal terminal of a motor neuron releases ACh and causes a patch of the sarcolemma to become permeable to Na+ (sodium channels open) Figure 9.8 (b)

33. Action Potential: Depolarization and Generation of the Action Potential • Na+ enters the cell, and the resting potential is decreased (depolarization occurs) • If the stimulus is strong enough, an action potential is initiated Figure 9.8 (b)

34. Action Potential: Propagation of the Action Potential • Polarity reversal of the initial patch of sarcolemma changes the permeability of the adjacent patch • Voltage-regulated Na+ channels now open in the adjacent patch causing it to depolarize Figure 9.8 (c)

35. Action Potential: Propagation of the Action Potential • Thus, the action potential travels rapidly along the sarcolemma • Once initiated, the action potential is unstoppable, and ultimately results in the contraction of a muscle Figure 9.8 (c)

36. Action Potential: Repolarization • Immediately after the depolarization wave passes, the sarcolemma permeability changes • Na+ channels close and K+ channels open • K+ diffuses from the cell, restoring the electrical polarity of the sarcolemma Figure 9.8 (d)

37. Action Potential: Repolarization • Repolarization occurs in the same direction as depolarization, and must occur before the muscle can be stimulated again (refractory period) • The ionic concentration of the resting state is restored by the Na+-K+ pump Figure 9.8 (d)

38. Excitation-Contraction Coupling • Once generated, the action potential: • Is propagated along the sarcolemma • Travels down the T tubules • Triggers Ca2+ release from terminal cisternae • Ca2+ binds to troponin and causes: • The blocking action of tropomyosin to cease • Actin active binding sites to be exposed

39. Excitation-Contraction Coupling • Myosin cross bridges alternately attach and detach • Thin filaments move toward the center of the sarcomere • Hydrolysis of ATP powers this cycling process • Ca2+ is removed into the SR, tropomyosin blockage is restored, and the muscle fiber relaxes

40. Excitation-Contraction Coupling Figure 9.9

41. Role of Ionic Calcium (Ca2+) in the Contraction Mechanism • At low intracellular Ca2+ concentration: • Tropomyosin blocks the binding sites on actin • Myosin cross bridges cannot attach to binding sites on actin • The relaxed state of the muscle is enforced Figure 9.10 (a)

42. Role of Ionic Calcium (Ca2+) in the Contraction Mechanism • At higher intracellular Ca2+ concentrations: • Additional calcium binds to troponin (inactive troponin binds two Ca2+) • Calcium-activated troponin binds an additional two Ca2+ at a separate regulatory site Figure 9.10 (b)

43. Role of Ionic Calcium (Ca2+) in the Contraction Mechanism • Calcium-activated troponin undergoes a conformational change • This change moves tropomyosin away from actin’s binding sites Figure 9.10 (c)

44. Role of Ionic Calcium (Ca2+) in the Contraction Mechanism • Myosin head can now bind and cycle • This permits contraction (sliding of the thin filaments by the myosin cross bridges) to begin Figure 9.10 (d)

45. Sequential Events of Contraction • Cross bridge formation – myosin cross bridge attaches to actin filament • Working (power) stroke – myosin head pivots and pulls actin filament toward M line • Cross bridge detachment – ATP attaches to myosin head and the cross bridge detaches • “Cocking” of the myosin head – energy from hydrolysis of ATP cocks the myosin head into the high-energy state

46. Sequential Events of Contraction Myosin head (high-energy configuration) Myosin cross bridge attaches to the actin myofilament 1 Thin filament ADP and Pi (inorganic phosphate) released Thick filament Working stroke—the myosin head pivots and bends as it pulls on the actin filament, sliding it toward the M line As ATP is split into ADP and Pi, cocking of the myosin head occurs 2 4 Myosin head (low-energy configuration) As new ATP attaches to the myosin head, the cross bridge detaches 3 Figure 9.11

47. Motor Unit: The Nerve-Muscle Functional Unit • Large weight-bearing muscles (thighs, hips) have large motor units • Muscle fibers from a motor unit are spread throughout the muscle; therefore, contraction of a single motor unit causes weak contraction of the entire muscle

48. Motor Unit: The Nerve-Muscle Functional Unit • A motor unit is a motor neuron and all the muscle fibers it supplies • The number of muscle fibers per motor unit can vary from four to several hundred • Muscles that control fine movements (fingers, eyes) have small motor units

49. Motor Unit: The Nerve-Muscle Functional Unit Figure 9.12 (a)