Skeletal Muscle Tissue

1. Skeletal Muscle Tissue Chapter 10

2. Overview of Muscle Tissue • ‘Little mouse’ – 3 types of muscle tissue • Walls of the heart (cardiac muscle tissue) • Walls of hollow organs (smooth muscle tissue) • Skeletal muscle makes up 40% of body weight

3. Functions of Muscle Tissue • Movement • Skeletal muscle moves the body by moving the bones • Smooth muscle squeezes fluids and other substances through hollow organs • Maintenance of posture – enables the body to remain sitting or standing • Joint stabilization – muscle tone • Heat generation – muscle contractions produce heat that helps maintain normal body temperature

4. Functional Features of Muscle • Contractility – long cells shorten and generate pulling force • Excitability – electrical nerve impulse stimulates the muscle cells to contract • Extensibility – can be stretched back to its original length by contraction of an opposing muscle • Elasticity – can recoil passively and resume its resting length

5. Similarities of Muscle Tissue • Fibers: cells of smooth and skeletal muscles • Muscle contraction depends on two types of myofilaments (contractile proteins) • One type contains actin • Another type contains myosin • These two proteins generate contractile force • Sarcolemma (sarcos = flesh; lemma = sheath) – plasma membrane • Sarcoplasm = cytoplasm

6. Skeletal Muscle • Each muscle is an organ • Consists mostly of muscle tissue but also contains: • Connective tissue • Blood vessels • Nerves

7. Basic Features of a Skeletal Muscle • Connective tissue and fascicles – CT sheaths bind a skeletal muscle and its fibers together • Epimysium: dense regular CT surrounding entire muscle • Perimysium: surrounds each fascicle ( group of muscle fibers) • Endomysium: a fine sheath of CT wrapping each muscle cell • CT sheaths are continuous with tendons • When muscle fibers contract, pull is exerted on all layers of connective tissue and tendon • Sheaths provide elasticity and carry blood vessels and nerves

8. Connective Tissue Sheaths in Skeletal Muscle Epimysium Perimysium Epimysium Bone Endomysium Tendon Muscle fiber in middle of a fascicle (b) Blood vessel Fascicle (wrapped by perimysium) Endomysium (between individual muscle fibers) Muscle fiber Perimysium Fascicle (a) Figure 10.1

9. Nerves and Blood Vessels • Each skeletal muscle is supplied by branches of • One nerve, one artery, one or more veins • All enter or exit near the middle of its length • Nerves and vessels branch repeatedly • Smallest nerve branches serve individual muscle fibers • Neuromuscular junction (NMJ) signals the muscle to contract • Capillaries in the endomysium form a network • Wavy when muscle contracts and straight when muscle extends

10. Skeletal Muscle Attachments • Most muscles run from one bone to another • One bone will move, the other remains fixed • Origin: less movable attachment • Insertion: more movable attachment Muscle contracting Origin by direct attachment Brachialis Tendon Insertion by indirect attachment Figure 10.3

11. Skeletal Muscle Attachments • Muscles attach to origins and insertions by CT: • Fleshy attachments: CT fibers are short • Indirect attachments: CT forms a tendon or aponeurosis (a tendon sheet) • Most tendons and aponeuroses attach to bones • A few attach to skin, cartilage or sheets of fascia, or to a seam of fibrous tissue called a raphe (‘seam’) • Bone markings reflect where tendons meet bones • Tubercles, trochanters, and crests

12. Microscopic and Functional Anatomy • Skeletal muscle fibers are long and cylindrical • Are huge cells with a diameter from 10 – 100 μm • Length can be several centimeters (cm) to dozens of cm • Each cell formed by fusion of 100s of embryonic cells • Cells are multinucleate with nuclei peripherally located just deep to the sarcolemma

13. Diagram of Part of a Muscle Fiber Sarcolemma Mitochondrion Myofibril Dark A band Nucleus Light I band (b) Diagram of part of a muscle fiber showing the myofibrils. One myofibril is extended from the cut end of the fiber. Figure 10.4b

14. Myofibrils and Sarcomeres • Striations result from internal structure of myofibrils (functional units of skeletal muscle tissue) • Long rods within the cytoplasm (sarcoplasm) • Make up 80% of the sacroplasm • Are a specialized contractile organelle • Long row of repeating segments called sacromeres (‘muscle segments’)

15. Sarcomere • Basic unit of contraction of skeletal muscle • Z disc (Z line): boundaries of each sarcomere • Thin (actin) filaments: extend from Z disc toward the center of the sarcomere • Thick (myosin) filaments: located in the center of the sarcomere • Overlap inner ends of the thin filaments • Contain ATPase enzymes

16. Sarcomere Structure • A bands: full length of the thick filament • Includes overlapping inner end of thin filaments • H zone: center part of A band, no thin filaments occur • A bands and I bands refract polarized light differently • A bands—anisotropic • I bands—isotropic • M line: center of H zone • Contains tiny rods that hold thick filaments together • I band: region with only thin filaments • Lies within two adjacent sarcomeres

17. Levels of Functional Organization in a Skeletal Muscle Muscle Fascicle Muscle Fiber Myofibril Sacromere

18. Sarcomere Structure Thin (actin) filament Z disc H zone Z disc Thick (myosin) filament I band A band I band M line Sarcomere Z disc Z disc (c) Small part of onemyofibril enlarged to show the myofilaments responsible for the banding pattern. Eachsarcomereextends from one Z disc to the next. I band I band Sarcomere M line Z disc Z disc Thin (actin) filament Elastic (titin) filaments Thick (myosin) filament Myosin heads (d) Enlargement of one sarcomere (sectioned lengthwise). Notice the myosin heads on the thick filaments. Figure 10.4c, d

19. Mechanism of Contraction • Two types of muscle contraction: • Eccentric contraction: muscle generates force as it lengthens • Essential for controlled movement and resistance to gravity • Muscles act as a ‘brake’ to resist gravity • ‘Down’ portion of a pushup • Concentric contraction: muscle shortens to do work • Sliding filament mechanism

20. Sliding Filament Mechanism • Explains concentric contraction • Myosin heads attach to thin filaments at both ends of a sarcomere • Then pull thin filaments toward the center of the sarcomere • Thin and thick filaments do not shorten • Initiated by release of calcium ions from the SR • Powered by ATP

21. Sliding Filament Mechanism Thick (myosin) filament Thin (actin) filament Thin (actin) filament Myosin heads Thick (myosin) filament Thin (actin) filament Movement Thick (myosin) filament Myosin head (b) Transmission electron micrograph of part of a sarcomere, showing myosin heads attached to the thin filaments (a) Myosin heads attach to actin in the thin filaments, then pivot to pull the thin filaments inward. Figure 10.7

22. Sliding Filament Mechanism • Contraction changes the striation pattern • Fully relaxed: thin filaments partially overlap thin filaments • Contraction: Z discs move closer together • Sarcomere shortens • I bands shorten, H zone disappears • A band remains the same length

23. Sliding Filament Mechanism • Myosin heads attach to actin in thin filaments • Pivot to pull the thin filaments inward toward the center of the sarcomere Z Z Z Z H A I A I I I 2 1 Fully contracted sarcomere of a muscle fiber Fully relaxed sarcomere of a muscle fiber Figure 10.8

24. Microscopic and Functional Anatomy of Skeletal Muscle Tissue • Muscle extension: muscle is stretched by a movement opposite that which contracts it • Muscle fiber length and force of contraction • Greatest force produced when a fiber starts out slightly stretched • Myosin heads can pull along the entire length of the thin filaments

25. The Role of Titin • Titin: spring-like molecule in sarcomeres • Resists overstretching • Holds thick filaments in place • Unfolds when muscle is stretched

27. Mechanism of Contraction • SR contains calcium ions – released when muscle is stimulated to contract • Ca2+ diffuse out triggering the sliding filament mechanism • After contraction ions pumped back into SR for storage • Contraction: controlled by nerve-generated impulses • Travel along the sarcolemma of the muscle fiber • Impulses further conducted by T-tubules • Each impulse promotes release of calcium ions from the terminal cisterns

28. Innervation of Skeletal Muscle • Motor neurons innervate skeletal muscle tissue • Each fiber served by a nerve ending that signals contraction • Neuromuscular junction (motor end plate) – point of contact between the nerve ending and muscle fiber • Axon terminals (ends of axons): store neurotransmitters • Synaptic cleft: space between axon terminal and sarcolemma of a muscle fiber • Acetylcholine: NT thatdiffuses across the synaptic cleft • Binds its receptor inducing an impulse that initiates fiber contraction

29. The Neuromuscular Junction Myelinated axon of motor neuron Nerve impulse Axon terminal of neuromuscular junction Nucleus Sarcolemma of the muscle fiber 1 1 Nerve impulse stimulates the release of the neurotransmitter acetylcholine (ACh) into the synaptic cleft. (a) Axon terminal of motor neuron 2 ACh stimulates changes in the sarcolemma that excite the muscle fiber. This stimulus is carried down the T tubules to initiate fiber contraction. Synaptic cleft Synaptic vesicle containing ACh Sarcolemma Terminal cisterna of SR Triad Muscle fiber 3 Enzymes in the synaptic cleft break down ACh and thus limit its action to a single muscle twitch. (b) Ca2+ Figure 10.9

30. Motor Units Axon terminals at neuromuscular junctions Spinal cord Branching axon to motor unit Motor unit 1 Motor unit 2 Nerve Motor neuron cell body Motor neuron axon Muscle Muscle fibers (b) Branching axon terminals form neuromuscular junctions, one per muscle fiber (photomicrograph 110). (a) Axons of motor neurons extend from the spinal cord to the muscle. There each axon divides into a number of axon terminals that form neuromuscular junctions with muscle fibers scattered throughout the muscle. Fig 10.10

31. Skeletal Muscle: Relaxation and Contraction Transverse tubules Terminal cistern of SR Muscle action potential Sarcolemma Ca2+ release channels open Ca2+ release channels closed Thin filament Myosin-binding site on actin Myosin Troponin Tropomyosin Key: Ca2+ binds to troponin, which changes the shape of the troponin–tropomyosin complex and uncovers the myosin-binding sites on actin. = Ca2+ = Ca2+ active transport pumps = Ca2+ release channels Troponin holds tropomyosin in position to block myosin-binding sites on actin. (a) Relaxation (b) Contraction

32. The Contraction Cycle: myosin heads attach to actin (cross-bridges), rotate, and detach Myosin heads break down ATP and become reoriented and energized 1 Key: = Ca2+ 2 Myosin heads bind to actin, forming cross–bridges ADP P P ATP Contraction cycle continues if ATP is available and Ca2+ level in the sarcoplasm is high ADP ATP As myosin heads bind ATP, the cross–bridges detach from actin 4 ADP 3 Myosin cross–bridges rotate toward center of the sarcomere (power stroke)

33. Types of Skeletal Muscle Fibers • Skeletal muscle fibers are categorized according to two characteristics • How they manufacture energy (ATP) • How quickly they contract • Oxidative fibers—produce ATP aerobically • Glycolytic fibers—produce ATP anaerobically by glycolysis

34. Types of Skeletal Muscle Fibers • Skeletal muscle fibers divided into three classes • Slow oxidative fibers: red slow oxidative fibers • Fast glycolytic fibers: white fast glycolytic fibers • Fast oxidative fibers: intermediate fibers • Slow Oxidative Fibers: red color due to abundant myoglobin • Obtain energy from aerobic metabolic reactions • Contain a large number of mitochondria • Richly supplied with capillaries • Contract slowly and resistant to fatigue • Fibers are small in diameter

35. Types of Skeletal Muscle Fibers • Fast Glycolytic Fibers: contain little myoglobin and few mitochondria • About twice the diameter of slow-oxidative fibers • Contain more myofilaments and generate more power • Depend on anaerobic pathways • Contract rapidly and tire quickly • Fast Oxidative Fibers: have an intermediate diameter • Contract quickly like fast glycolytic fibers • Are oxygen-dependent • Have high myoglobin content and rich supply of capillaries • Somewhat fatigue-resistant • More powerful than slow oxidative fibers

36. Exercise and Skeletal Muscle Tissue • The relative ratio of FG and SO fibers is genetically determined and helps account for individual differences in physical performance

41. Disorders of Muscle Tissue • Muscle tissues experience few disorders • Heart muscle is the exception • Skeletal muscle remarkably resistant to infection • Smooth muscle problems stem from external irritants • Muscular dystrophy: group of inherited muscle destroying disease • Affected muscles enlarge with fat and connective tissue and muscles degenerate • Types of muscular dystrophy: Duchenne muscular dystrophy and myotonic dystrophy

42. Disorders of Muscle Tissue • Myofascial pain syndrome: pain caused by tightened bands of muscle fibers • Fibromyalgia: mysterious chronic-pain syndrome • Affects mostly women • Symptoms include fatigue, sleep abnormalities, severe musculoskeletal pain, and headache

43. Muscle Tissue Throughout Life • Muscle tissue develops from myoblasts • Myoblasts fuse to form skeletal muscle fibers • Skeletal muscles contract by the seventh week of development Myotube (immature multinucleate muscle fiber) Embryonic mesoderm cells Myoblasts Satellite cell Mature skeletal muscle fiber Embryonic mesoderm cells undergo cell division (to increase number) and enlarge. Several myoblasts fuse together to form a myotube. 2 1 Myotube matures into skeletal muscle fiber. 3

44. Muscle Tissue Throughout Life • Cardiac muscle: pumps blood three weeks after fertilization • Satellite cells surround skeletal muscle fibers • Resemble undifferentiated myoblasts • Fuse into existing muscle fibers to help them grow • With increased age amount of connective tissue increases in muscles and number of muscle fibers decreases • Loss of muscle mass with aging • Decrease in muscular strength is 50% by age 80 • Sarcopenia: muscle wasting