Muscles and Muscle Tissue

1. Muscles and Muscle Tissue P A R T A

2. Muscle Overview • The three types of muscle tissue are skeletal, cardiac, and smooth • Skeletal and smooth muscle cells are elongated and are called muscle fibers • Muscle contraction depends on two kinds of myofilaments – actin and myosin

3. Muscle Similarities • Muscle terminology is similar • Sarcolemma – muscle plasma membrane • Sarcoplasm – cytoplasm of a muscle cell • Prefixes – myo, mys, and sarco all refer to muscle

4. Skeletal Muscle Tissue • Packaged in skeletal muscles that attach to and cover the bony skeleton • It is striated • Is controlled voluntarily • Contracts rapidly but tires easily • Is responsible for all locomotion, posture, joint stabilization and heat generation

5. Cardiac Muscle Tissue • Occurs only in the heart • Is striated like skeletal muscle but involuntary • Contracts at a fairly steady rate set by the heart’s pacemaker

6. Smooth Muscle Tissue • Found in the walls of hollow visceral organs • Propels food and other substances through organs • It is not striated and involuntary

7. 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

8. Structure and Organization of Skeletal Muscle Table 9.1a

9. Structure and Organization of Skeletal Muscle

10. Skeletal Muscle • Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue

11. Skeletal Muscle • The three connective tissue sheaths are: • Endomysium – fine sheath of connective tissue surrounding each muscle fiber • Perimysium – fibrous connective tissue that surrounds groups of muscle fibers • Fascicles

12. Skeletal Muscle • Epimysium • A layer of fibrous connective tissue that surrounds the entire muscle

13. Skeletal Muscle Figure 9.2a

14. Skeletal Muscle: Nerve and Blood Supply • Each muscle is served by one nerve, an artery, and one or more veins • Each skeletal muscle fiber is supplied with a nerve ending that controls contraction • Contracting fibers require continuous delivery of oxygen and nutrients via arteries • Wastes must be removed via veins

15. Skeletal Muscle: Attachments • Most skeletal muscles span joints and are attached to bone in at least two places • When muscles contract the movable bone, the muscle’s insertion moves toward the immovable bone, the muscle’s origin

16. Skeletal Muscle: Attachments • Muscles attach: • Directly – epimysium of the muscle is fused to the periosteum of a bone • Indirectly – connective tissue wrappings extend beyond the muscle as a tendon or aponeurosis

17. Microscopic Anatomy of a Skeletal Muscle Fiber • Fibers are up to hundreds of centimeters long • Each cell is a syncytium produced by fusion of embryonic cells

18. Microscopic Anatomy of a Skeletal Muscle Fiber • Sarcoplasm has numerous glycosomes and a unique oxygen-binding protein called myoglobin • Fibers contain the usual organelles, myofibrils, sarcoplasmic reticulum, and T tubules

19. Skeletal muscle fibers • Sarcoplasmic reticulum (modified ER) • T-tubules and myofibrilsaid in contraction • T-tubules are continuous with the sarcolemma and filled with extracellular fluid

20. Sarcoplasmic Reticulum (SR) • SR - smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril • Terminal cisterna • Terminal enlargement of the SR • Functions in the regulation of intracellular calcium levels

21. Sarcoplasmic Reticulum (SR) Figure 9.5

22. T Tubules • T tubules are continuous with the sarcolemma • T tubule proteins act as voltage sensors • They conduct impulses to the deepest regions of the muscle • These impulses signal for the release of Ca2+ from adjacent terminal cisternae

23. T Tubules • They penetrate into the cell’s interior at each A band–I band junction • Filled with extracellular fluid • Triad • Pair of terminal cisterna • T-tubule

24. Myofibrils • Myofibrils are densely packed, rodlike contractile elements • They make up most of the muscle volume • The arrangement of myofibrils within a fiber is such that a perfectly aligned repeating series of dark A bands and light I bands is evident

25. Myofibrils Figure 9.3b

26. Sarcomeres • The smallest contractile unit of a muscle • It goes from Z disc to Z disc • Composed of thin (actin) and thick (myosin) myofilaments

27. Sarcomeres Figure 9.3c

28. Sarcomere • A bands • Formed by thin and thick filaments • M line • Central part of the thick filament • Protein that keeps the thick filaments together • H zone • On each side of the M line • Formed by thick filament • Zone of overlap • Thin filament between thick filaments

29. Sarcomere • I band • Formed by thin filaments • Z line • actinin proteins that anchors the thin filaments • Titin keep the thick and thin filaments aligned and prevents the muscle fiber from overstretching • From Z line to Z line

30. Titin • Keep the thick and thin filaments aligned • Prevents the muscle fiber from overstretching • Helps the sarcomere to restore to resting length after contraction

31. Myofilaments: Banding Pattern Figure 9.3c,d

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

33. Thick Filaments Figure 9.4a,b

34. Thin Filaments • Thin filaments are chiefly composed of • G actin - globular subunits • The G actin contain the active sites to which myosin heads attach during contraction

35. Thin Filaments • Tropomyosin • Double stranded protein that covers the active sites of the G protein • Troponin – composed of 3 globular subunits • One subunit binds Ca • One subunit binds to tropomyosin • One subunit binds to G actin

36. Ultrastructure of Myofilaments: Thin Filaments Figure 9.4c

37. Arrangement of the Filaments in a Sarcomere • Longitudinal section within one sarcomere Figure 9.4d

38. Sliding Filament Model of Contraction • Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree • In the relaxed state, thin and thick filaments overlap only slightly • Upon stimulation, myosin heads bind to actin and sliding begins

39. Sliding Filament Model of Contraction • Each myosin head binds and detaches several times during contraction, acting like a ratchet to generate tension and propel the thin filaments to the center of the sarcomere • As this event occurs throughout the sarcomeres, the muscle shortens

40. Skeletal Muscle Contraction • In order to contract, a skeletal muscle must: • Be stimulated by a nerve ending • Propagate an electrical current, or action potential, along its sarcolemma • Have a rise in intracellular Ca2+ levels, the final trigger for contraction

41. Skeletal Muscle Contraction • Linking the electrical signal to the contraction is excitation-contraction coupling

42. Nerve Stimulus of Skeletal Muscle • Skeletal muscles are stimulated by motor neurons of the somatic nervous system • Axons of these neurons travel in nerves to muscle cells • Axons of motor neurons branch profusely as they enter muscles • Each axonal branch forms a neuromuscular junction with a single muscle fiber

43. Neuromuscular Junction • Formed from: • Axonal endings whith synaptic vesicles that contain acetylcholine • The motor end plate of a muscle, which is a specific part of the sarcolemma that contains ACh receptors and helps form the neuromuscular junction

44. Neuromuscular Junction • Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

45. Neuromuscular Junction Figure 9.7 (a-c)

46. Neuromuscular Junction • When a nerve impulse reaches the end of an axon at the neuromuscular junction: • Voltage-regulated calcium channels open and allow Ca2+ to enter the axon • Ca2+ inside the axon terminal causes axonal vesicles to fuse with the axonal membrane

47. Neuromuscular Junction • This fusion releases ACh into the synaptic cleft via exocytosis • ACh diffuses across the synaptic cleft to ACh receptors on the sarcolemma • Binding of ACh to its receptors initiates an action potential in the muscle

48. Role of Acetylcholine (Ach) • ACh binds its receptors at the motor end plate • Binding opens chemically (ligand) gated channels that allows Na+ and K+ movements • Na+ diffuses in, K+ diffuse out and the interior of the sarcolemma becomes less negative

49. Depolarization • Initially, this is a local electrical event called end plate potential • Later, it ignites an action potential that spreads in all directions across the sarcolemma

50. Destruction of Acetylcholine • ACh bound to ACh receptors is quickly destroyed by the enzyme acetylcholinesterase • This destruction prevents continued muscle fiber contraction in the absence of additional stimuli