1. Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. Chapter 8 Histology and Physiology of Muscles Skeletal Muscle Fibers

2. Functions of the Muscular System 1.Body movement (Skeletal Muscle) 2.Maintenance of posture (Skeletal Muscle) 3.Respiration (Skeletal Muscle) 4.Production of body heat (Skeletal Muscle) 5.Communication (Skeletal Muscle) 6.Constriction of organs and vessels (Smooth Muscle) 7.Heartbeat (Cardiac Muscle)

3. Functional Characteristics of Muscle 1.Contractility: the ability to shorten forcibly 2.Excitability: the ability to receive and respond to stimuli 3.Extensibility: the ability to be stretched or extended 4.Elasticity: the ability to recoil and resume the original resting length

4. Types of Muscle Tissue The three types of muscle tissue are skeletal, smooth, and cardiac These types differ in –Structure –Location –Function –Means of activation Each muscle is a discrete organ composed of muscle tissue, blood vessels, nerve fibers, and connective tissue

5. Types of Muscle Tissue Skeletal muscles are responsible for most body movements –Maintain posture, stabilize joints, and generate heat Smooth muscle is found in the walls of hollow organs and tubes, and moves substances through them –Helps maintain blood pressure –Squeezes or propels substances (i.e., food, feces) through organs Cardiac muscle is found in the heart and pumps blood throughout the body

6. Tab. 8.1

7. Skeletal Muscle Structure Skeletal muscle cells are elongated and are often called skeletal muscle fibers Each skeletal muscle cell contains several nuclei located around the periphery of the fiber near the plasma membrane Fibers appear striated due to the actin and myosin myofilaments A single fiber can extend from one end of a muscle to the other Contracts rapidly but tires easily Is controlled voluntarily (i.e., by conscious control)

8. Skeletal Muscle Structure Fascia is a general term for connective tissue sheets The three muscular fascia, which separate and compartmentalize individual muscles or groups of muscles are: –Epimysium: an overcoat of dense collagenous connective tissue that surrounds the entire muscle –Perimysium: fibrous connective tissue that surrounds groups of muscle fibers called fascicles (bundles) –Endomysium: fine sheath of connective tissue composed of reticular fibers surrounding each muscle fiber

9. Skeletal Muscle Structure Muscle Fibers –Terminology Sarcolemma: muscle cell plasma membrane Sarcoplasm: cytoplasm of a muscle cell Myo, mys, and sarco: prefixes used to refer to muscle –Muscle contraction depends on two kinds of myofilaments: actin and myosin Myofibrils are densely packed, rod-like contractile elements They make up most of the muscle volume

10. Fig. 8.2

11. Skeletal Muscle Structure Actin (thin) myofilaments consist of two helical polymer strands of F actin (composed of G actin), tropomyosin, and troponin The G actin contains the active sites to which myosin heads attach during contraction Tropomyosin and troponin are regulatory subunits bound to actin Fig. 8.2

12. Skeletal Muscle Structure Myosin (thick) myofilaments consist of myosin molecules Each myosin molecule has –A head with an ATPase, which breaks down ATP –A hinge region, which enables the head to move –A rod A cross-bridge is formed when a myosin head binds to the active site on G actin Fig. 8.2

13. Skeletal Muscle Structure Sarcomeres –The smallest contractile unit of a muscle –Sarcomeres are bound by Z disks that hold actin myofilaments –Six actin myofilaments surround a myosin myofilament –Myofibrils appear striated because of A bands and I bands Fig. 8.2

14. Skeletal Muscle Structure 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: a coin-shaped sheet of proteins (connectins) that anchors the thin filaments and connects myofibrils to one another Thin filaments do not overlap thick filaments in the lighter H zone M lines appear darker due to the presence of the protein desmin The arrangement of myofibrils within a fiber is so organized a perfectly aligned repeating series of dark A bands and light I bands is evident

15. Fig. 8.3bc

16. Sliding Filament Model Actin and myosin myofilaments do not change in length during contraction Thin filaments slide past the thick ones so that the actin and myosin filaments overlap to a greater degree –Upon stimulation, myosin heads bind to actin and sliding begins –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) In the relaxed state, thin and thick filaments overlap only slightly As this event occurs throughout the sarcomeres, the muscle shortens The I band and H zones become narrower during contraction, and the A band remains constant in length

17. Sliding Filament Model Actin and myosin myofilaments in a relaxed muscle and a contracted muscle are the same length. Myofilaments do not change length during muscle contraction Fig. 8.4 During contraction, actin myofilaments at each end of the sarcomere slide past the myosin myofilaments toward each other. As a result, the Z disks are brought closer together, and the sarcomere shortens

18. Sliding Filament Model As the actin myofilaments slide over the myosin myofilaments, the H zones and the I bands narrow. The A bands, which are equal to the length of the myosin myofilaments, do not narrow because the length of the myosin myofilaments does not change Fig. 8.4 In a fully contracted muscle, the ends of the actin myofilaments overlap at the center of the sarcomere and the H zone disappears

19. Physiology of Skeletal Muscle Fibers Membrane Potentials –The nervous system stimulates muscles to contract through electric signals called action potentials –Plasma membranes are polarized, which means there is a charge difference (resting membrane potential) across the plasma membrane –The inside of the plasma membrane is negative as compared to the outside in a resting cell –An action potential is a reversal of the resting membrane potential so that the inside of the plasma membrane becomes positive

20. Physiology of Skeletal Muscle Fibers Action Potentials –Depolarization results from an increase in the permeability of the plasma membrane to Na + –If depolarization reaches threshold, an action potential is produced –The depolarization phase of the action potential results from the opening of many Na + channels Fig. 8.6 –The repolarization phase of the action potential occurs when the Na+ channels close and K+ channels open briefly

21. Physiology of Skeletal Muscle Fibers Action Potentials –Occur in an all-or-none fashion A stimulus below threshold produces no action potential A stimulus at threshold or stronger will produce an action potential –Propagate (travel) across plasma membranes

22. Physiology of Skeletal Muscle Fibers 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

23. Physiology of Skeletal Muscle Fibers The neuromuscular junction is formed from: –Axonal endings Have small membranous sacs (synaptic vesicles) Contain the neurotransmitter acetylcholine (ACh) –Motor end plate of a muscle Specific part of the sarcolemma Contains ACh receptors Though exceedingly close, axonal ends and muscle fibers are always separated by a space called the synaptic cleft

24. Fig. 8.7

25. Fig. 8.8

26. Excitation-Contraction Coupling 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 Ca 2+ levels, the final trigger for contraction Linking the electrical signal to the contraction is excitation-contraction coupling

27. Excitation-Contraction Coupling Invaginations of the sarcolemma form T tubules, which wrap around the sarcomeres and penetrate into the cell’s interior at each A band –I band junction Sarcoplasmic reticulum (SR) is an elaborate, smooth endoplasmic reticulum that mostly runs longitudinal and surrounds each myofibril –Paired terminal cisternae form perpendicular cross channels –Functions in the regulation of intracellular calcium levels A triad is a T tubule and two terminal cisternae Fig. 8.9

28. Excitation-Contraction Coupling 1.An action potential produced at the presynaptic terminal in the neuromuscular junction is propagated along the sarcolemma of the skeletal muscle. The depolarization also spreads along the membrane of the T tubules 2.The depolarization of the T tubule causes gated Ca 2+ channels in the SR to open, resulting in an increase in the permeability of the SR to Ca 2+, especially in the terminal cisternae. Calcium ions then diffuse from the SR into the sarcoplasm 3.Calcium ions released from the SR bind to troponin molecules. The troponin molecules bound to G actin molecules are released, causing tropomyosin to move, exposing the active sites on G actin 4.Once active sites on G actin molecules are exposed, the heads of the myosin myofilaments bind to them to form cross-bridges

29. Muscle Relaxation Calcium ions are transported back into the sarcoplasmic reticulum Calcium ions diffuse away from troponin and tropomyosin moves, preventing further cross-bridge formation

30. Muscle Twitch The contraction of a muscle as a result of one or more muscle fibers contracting Has lag, contraction, and relaxation phases Table 8.2

32. Strength of Muscle Contraction For a given condition, a muscle fiber or motor unit contracts with a consistent force in response to each action potential For a whole muscle, stimuli of increasing strength result in graded contractions of increased force as more motor units are recruited (multiple motor unit summation) Stimulus of increasing frequency increase the force of contraction (multiple-wave summation)

33. Motor Unit Fig. 8.13

34. Strength of Muscle Contraction Incomplete tetanus is partial relaxation between contractions, and complete tetanus is no relaxation between contractions The force of contraction of a whole muscle increases with increased frequency of stimulation because of an increasing concentration of Ca 2+ around the myofibrils Treppe is an increase in the force of contraction during the first few contractions of a rested muscle

35. Types of Muscle Contraction Isometric contractions cause a change in muscle tension but no change in muscle length Isotonic contractions cause a change in muscle length but no change in muscle tension Concentric contractions are isotonic contractions that cause muscles to shorten Eccentric contractions are isotonic contractions that enable muscles to shorten Muscle tone is the maintenance of a steady tension for long periods Asynchronous contractions of motor units produce smooth, steady muscle contractions

36. Fatigue The decreased ability to do work Can be caused by –The central nervous system (psychologic fatigue) –Depletion of ATP in muscles (muscular fatigue) Physiologic contracture (the inability of muscles to contract or relax) and rigor mortis (stiff muscles after death) result from inadequate amounts of ATP

37. Energy Sources Creatine phosphate –ATP is synthesized when ADP reacts with creatine phosphate to form creatine and ATP –ATP from this source provides energy for a short time Fig. 8.18

38. Energy Sources Anaerobic respiration –ATP synthesized provides energy for a short time at the beginning of exercise and during intense exercise –Produces ATP less efficiently but more rapidly than aerobic respiration –Lactic acid levels increase because of anaerobic respiration Fig. 8.18

39. Energy Sources Aerobic respiration –Requires oxygen –Produces energy for muscle contractions under resting conditions or during endurance exercise Fig. 8.18

40. Speed of Contraction The three main types of skeletal muscle fibers are –Slow-twitch oxidative (SO) fibers –Fast-twitch glycolytic (FG) fibers –Fast-twitch oxidative glycolytic (FOG) fibers SO fibers contract more slowly than FG and FOG fibers because they have slower myosin ATPases than FG and FOG fibers

41. Fatigue Resistance SO fibers are fatigue-resistant and rely on aerobic respiration –Many mitochondria, a rich blood supply, and myoglobin FG fibers are fatigable –Rely on anaerobic respiration and have a high concentration of glycogen FOG fibers have fatigue resistance intermediate between SO and FG fibers –Rely on aerobic and anaerobic respiration

42. Functions SO fibers maintain posture and are involved with prolonged exercise –Long-distance runners have a higher percentage of SO fibers FG fibers produce powerful contractions of short duration –Sprinters have a higher percentage of FG fibers FOG fibers support moderate-intensity endurance exercises –Aerobic exercise can result in the conversion of FG fibers to FOG fibers

43. Tab. 8.3

44. Muscular Hypertrophy and Atrophy Hypertrophy is an increase in the size of muscles –Due to an increase in the size of muscle fibers resulting from an increase in the number of myofibrils in the muscle fibers Aerobic exercise –Increases the vascularity of muscle –Greater hypertrophy of slow-twitch fibers than fast-twitch fibers Intense anaerobic exercise –Greater hypertrophy of fast-twitch fibers than slow-twitch Atrophy is a decrease in the size of muscle –Due to a decrease in the size of muscle fibers or a loss of muscle fibers

45. Effects of Aging on Skeletal Muscle By 80 years of age 50% of the muscle mass is gone –Due to a loss in muscle fibers –Fast-twitch muscle fibers decrease in number more rapidly than slow-twitch fibers Can be dramatically slowed if people remain physically active

46. Types of Smooth Muscle Visceral smooth muscle fibers have many gap junctions and contract as a single unit Multiunit smooth muscle fibers have few gap junctions and function independently –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

47. Regulation of Smooth Muscle Contraction is involuntary –Multiunit smooth muscle contracts when externally stimulated by nerves, hormones, or other substances –Visceral smooth muscle contracts autorhythmically or when stimulated externally Hormones are important in regulating smooth muscle

48. Structure of Smooth Muscle Cells Spindle-shaped with a single nucleus Have actin and myosin myofilaments –Actin myofilaments are connected to dense bodies and dense areas Not striated No T tubule system and most have less SR than skeletal muscle No troponin

49. Contraction and Relaxation of Smooth Muscle Calcium ions enter the cell to initiate contraction –Bind to calmodulin –Activate myosin kinase, which transfers a phosphate group from ATP to myosin –When phosphate groups are attached to myosin, cross-bridges form Relaxation results when myosin phosphatase removes a phosphate group from the myosin molecule

50. Fig. 8.19

51. Functional Properties of Smooth Muscle Pacemaker cells are autorhythmic smooth muscle cells that control the contraction of other smooth muscle cells Smooth muscle cells contract more slowly than skeletal muscle cells Smooth muscle tone is the ability of smooth muscle to maintain a steady tension for long periods with little expenditure of energy Smooth muscle in the walls of hollow organs maintain a relatively constant pressure on the contents of the organ despite changes in content volume The force of smooth muscle contraction remains nearly constant despite changes in muscle length

52. Cardiac Muscle Cells Occurs only in the heart Is striated like skeletal muscle but is not voluntary Have a single nucleus Connected by intercalated disks that allowing them to function as a single unit Capable of autorhythmicity Contracts at a fairly steady rate set by the heart’s pacemaker Neural controls allow the heart to respond to changes in bodily needs