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Muscles and Muscle Tissue

Muscles and Muscle Tissue. P A R T A. 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.

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Muscles and Muscle Tissue

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  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

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