Chapter 9 Muscles and Muscle Tissue Part A

2 Muscle Overview The three types of muscle tissue are skeletal, cardiac, and smooth These types differ in structure, location, function, and means of activation

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5 Myo tendon Striated Muscle Tendon

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7 Skeletal Muscle compared to smooth

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12 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

13 Skeletal Muscle Tissues Packaged in skeletal muscles that attach to and cover the bony skeleton Has obvious stripes called striations Is controlled voluntarily (i.e., by conscious control) Contracts rapidly but tires easily Is responsible for overall body motility Is extremely adaptable and can exert forces over a range from a fraction of an ounce to over 70 pounds

14 Smooth Muscle Tissue Found in the walls of hollow visceral organs, such as the stomach, urinary bladder, and respiratory passages Forces food and other substances through internal body channels It is not striated and is involuntary

15 Cardiac Muscle Tissue Occurs only in the heart Is striated like skeletal muscle but is not voluntary Contracts at a fairly steady rate set by the heart’s pacemaker Neural controls allow the heart to respond to changes in bodily needs

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17 Muscle Function Skeletal muscles are responsible for all locomotion Cardiac muscle is responsible for coursing (pumping) 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

18 Functional Characteristics of Muscles Excitability, or irritability –ability to receive/respond to stimuli-nerves Contractility –ability to shorten forcibly Extensibility –ability to be stretched or extended Elasticity –ability to recoil and resume the original resting length

Structure and Organization of Skeletal Muscle 19

Structure and Organization of Skeletal Muscle 20

21 Skeletal Muscle Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue The three connective tissue wrappings are: – Endomysium – fine sheath of CT composed of reticular fibers surrounding each muscle fiber and interwoven w/ perimysium. Also contains muscle stem cells called satellite cells – Perimysium – dense irregular CT that surrounds groups of muscle fibers called fascicles – Epimysium –overcoat of dense irregular CT that surrounds the entire muscle

22 Skeletal Muscle The three connective tissue wrappings provide: – Transmission of the tensive force to the tendon – Electrical isolation of each compartment (fiber, fascicle and muscle). This prevents adjacent compartments from unwanted contraction.

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24 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 – neuromuscular junction Contracting fibers require continuous delivery of oxygen and nutrients via arteries Wastes (CO 2 and lactic acid) must be removed via veins

25 Neuromuscular Junction Figure 9.8a, b

26 Skeletal Muscle: Attachments Muscles span joints and are attached to bone in at least two places (usually) When muscles contract, the movable bone (insertion) moves toward the immovable bone (origin) Muscles attach: – Directly – epimysium of the muscle is fused to the periosteum of a bone – Indirectly – CT wrappings extend beyond the muscle as a tendon or aponeurosis

27 Microscopic Anatomy of a Skeletal Muscle Fiber Each fiber is a long, cylindrical cell w/ multiple nuclei just beneath the sarcolemma (plasma membrane) Fibers are 10 to 100  m in diameter; up to hundreds of centimeters long Each cell is a syncytium produced by fusion of embryonic cells Sarcoplasm has numerous glycosomes and a unique oxygen-binding protein called myoglobin Fibers contain the usual organelles, and myofibrils, sarcoplasmic reticulum, and T tubules

myoglobin Myoglobin's function is similar to that of hemoglobin Myoglobin has even higher affinity for oxygen than hemoglobin and is specific to muscle cells. Myoglobin thus acts as a storage of oxygen, as it holds oxygen inside heart and skeletal muscles. 28

29 Myofibrils Myofibrils : 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 Figure 9.2b

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32 LM 7 - Muscle fiber

33 Sarcomeres The smallest contractile unit of a muscle The region of a myofibril between two successive Z discs Composed of myofilaments made up of contractile proteins – Myofilaments are of two types – thick and thin Figure 9.2c

34 Sarcomeres Figure 9.2c

35 Myof ibril XS w/ T- tube

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37 Scanning EM

38 Myofilaments: Banding Pattern Thick filaments – extend the entire length of an A band Thin filaments – extend across the I band and partway into the A band Z-disc – coin-shaped sheet of proteins (connectins) that anchors the thin filaments and connects myofibrils to one another

39 Myofilaments: Banding Pattern Thin filaments do not overlap thick filaments in the lighter H zone M lines appear darker due to the presence of the protein desmin Figure 9.2d

40 Myofilaments: Banding Pattern Figure 9.2d

41 Ultrastructure of Myofilaments: Thick Filaments Thick filaments are composed of the protein myosin Figure 9.3a, b

42 Ultrastructure of Myofilaments: Thick Filaments Each myosin molecule has a rod-like tail and two globular heads – Tails – two interwoven, heavy polypeptide chains – Heads – two smaller, light polypeptide chains called cross bridges Figure 9.3a, b

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44 Ultrastructure of Myofilaments: Thin Filaments Thin filaments are chiefly composed of the protein actin Each actin molecule is a helical polymer of globular subunits called G actin The subunits contain the active sites to which myosin heads attach during contraction Tropomyosin(_____________) and troponin(_______________) are regulatory subunits bound to actin Figure 9.3c

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Other proteins Elastic filament: giant protein titan Z disc to thick filament – Holds thick filaments in place – Assists muscle cell to spring back after being stretched or shortened Dystrophin: links thin filaments to the integral proteins of the sarcolemma; does NOT function properly in muscular dystrophy Nebulin, myomesin, vimentin 46

47 Arrangement of the Filaments in a Sarcomere Longitudinal section within one sarcomere Figure 9.3d

49 Sarcoplasmic Reticulum (SR) SR is an elaborate smooth endoplasmic reticulum that mostly runs longitudinally and surrounds each myofibril Paired terminal cisternae form perpendicular cross channels Functions in the regulation of intracellular calcium ion levels Elongated tubes called T tubules penetrate into the cell’s interior at each A band–I band junction T tubules associate with the paired terminal cisternae to form triads

50 Sarcoplasmic Reticulum (SR) Figure 9.4

51 T Tubules T tubules are continuous with the sarcolemma They conduct impulses to the deepest regions of the muscle These impulses signal for the release of Ca 2+ from adjacent terminal cisternae

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Triad Relationships T tubules and SR provide tightly linked signals for muscle contraction A double zipper of integral membrane proteins protrudes into the intermembrane space T tubule proteins act as voltage sensors SR foot proteins are receptors that regulate Ca 2+ release from the SR cisternae 53