Skeletal Muscles Lecture 13 RV Fall 2024 PDF

1 I. Skeletal Muscles © 2019 McGraw-Hill Education  2 Skeletal Muscles: introduction 1. A skeletal muscle is considered an organ of the muscular system. 2. Each...

1 I. Skeletal Muscles © 2019 McGraw-Hill Education  2 Skeletal Muscles: introduction 1. A skeletal muscle is considered an organ of the muscular system. 2. Each muscle consists of 1. skeletal muscle tissue, 2. connective tissue, 3. nerve tissue 4. blood or vascular tissue. 3. Are located throughout the body and their contraction and extension is under voluntary control. © 2019 McGraw-Hill Education  3 Skeletal Muscles: functions Skeletal muscle can contract producing voluntary movements of the body. Muscles also prevent excess movement of the bones and joints, maintaining skeletal stability and preventing skeletal structure damage or deformation. Protect internal organs (particularly abdominal and pelvic organs) by acting as an external barrier or shield to external trauma and by supporting the weight of the organs. They contribute to the maintenance of homeostasis in the body by generating heat. © 2019 McGraw-Hill Education  4 Skeletal Muscles: functions 1. Most muscles are attached to bones at both ends tough tendons. 2. When a muscle contracts, it shortens. a. This places tension on tendons connecting it to a bone. b. This moves the bone at a joint. c. The insertion is where it joins the moving bone. d. The origin is where the muscle joins the stationary bone. e. When a muscle contracts, the insertion moves towards the origin. 3. Different movements depend on the joint and how the muscles are attached a. Flexor muscles decrease the angle between two bones at a joint. b. Extensor muscles increase the angle between two bones at a joint. 4. The main muscle responsible for movement in a given direction is the agonist or prime mover. 5. Flexors and extensors that act on the same joint to produce opposite actions are antagonists. © 2019 McGraw-Hill Education  5 Structure of Skeletal Muscles a. Each skeletal muscle fiber is a single cylindrical muscle cell. They are multi- nucleated meaning that they have more than one nucleus. Individual muscle fiber, are surrounded by connective tissue called the endomysium. b. A bundle of muscle fibers form the fascicle. Each fascicle is surrounded by a layer of connective tissue called the perimysium. c. Multiple fascicles form the the skeletal muscle which is surrounded by a connective tissue sheath called the epimysium. d. Fascia, is connective tissue outside the epimysium, surrounds and separates the skeletal muscles. e. An individual skeletal muscle may be made up of hundreds, or even thousands, of muscle cells bundled together in fascicles and wrapped in a connective tissue. © 2019 McGraw-Hill Education  6 Structure of a skeletal muscle fiber - muscle cell a. Cylindrical cells that are multi-nucleated b. Membranes called sarcolemma c. The sarcoplasmic reticulum (SR) is a membrane-bound structure. The main function of the SR is to store calcium ions (Ca2+). d. Terminal cisternae are enlarged areas of the SR surrounding the transverse tubules. e. T-tubules (transverse tubules) are extensions of the cell membrane (sarcolemma) that penetrate into the center of the muscle cells. f. Myofibrils are long contractile fibers, groups of which run parallel to each other on the long axis of the muscle fiber. © 2019 McGraw-Hill Education  7 Structure of a myofibril A myofibril is formed by actin and myosin filaments and contain the contractile unit called sarcomeres. Each sarcomere consist of alternating thick (myosin) and thin (actin) protein filament giving the skeletal muscle its striated appearance (more on next slides). © 2019 McGraw-Hill Education  8 Actin and myosin Actin 1.F-actin is made of 300-400 G-actin subunits, arranged in a double row and twisted to form a helix. 2.Have proteins called tropomyosin and troponin that prevent myosin binding at rest. Troponin complex binds to calcium. Tropomyosin physically blocks cross bridges. Myosin 1. Each protein has two globular heads with actin-binding sites and ATP-binding sites. © 2019 McGraw-Hill Education (a) © C. F. Armstrong/Science Source. Photo illustration by David  9 Structure of a myofibril A myofibril is formed by actin and myosin filaments and contain the contractile unit called sarcomeres. Each sarcomere consist of alternating thick (myosin) and thin (actin) protein filament giving the skeletal muscle its striated appearance (more on next slides). The sarcomere is the smallest functional unit of a skeletal muscle fiber and is a highly organized arrangement of contractile, regulatory, and structural proteins. It is the shortening of these individual sarcomeres that lead to the contraction of individual skeletal muscle fibers (and ultimately the whole muscle). © 2019 McGraw-Hill Education  1 Muscle Fiber Banding - striations 0 Produced by the alternation of thick (myosin) and thin (actin) filaments 1) I bands contain only thin filaments, primarily of the protein, actin 2) Z discs (lines) are found in the center of each I band. 3) A bands contain all of the thick filament with some thin filament overlap. 4) H bands are the center of the A band with no thin filament overlap. © 2019 McGraw-Hill Education  Skeletal muscle contraction - Sliding Filament 1 1 Theory 1. When a muscle contracts, sarcomeres shorten. a. A bands do not shorten. b. I bands do shorten, but thin filaments do not. c. Thin filaments slide toward the H band. d. H band shortens or disappears © 2019 McGraw-Hill Education (a) © C. F. Armstrong/Science Source. Photo illustration by David  1 Skeletal muscle contraction - Steps 2 1) Lower motor neuron axons, release ACh at the level of the neuromuscular junction – bind to nicotinic receptors. 2) Action potential is generated and propagated along the sarcolemma and down the T tubules. 3) Action potential triggers Ca2+ release from the terminal cisternae of the sarcoplasmic reticulum. 4) Ca2+ ions bind to troponin; troponin change shape, removing the blocking action of tropomyosin; actin active sites exposed 5) Contraction: myosin cross bridges attach to actin and pull (power stroke) the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers this process. 6) Removal of Ca2+ by calcium pumps into the sarcoplasmic reticulum. 7) Tropomyosin blockage restored blocking actin active site; contraction end and muscle fiber relax. © 2019 McGraw-Hill Education (a) © C. F. Armstrong/Science Source. Photo illustration by David  1 Skeletal muscle contraction - Steps 3 1) Lower motor neuron axons, release ACh at the level of the neuromuscular junction – bind to nicotinic receptors. 2) Action potential is generated and propagated along the sarcolemma and down the T tubules. 3) Action potential triggers Ca2+ release from the terminal cisternae of the sarcoplasmic reticulum. 4) Ca2+ ions bind to troponin; troponin change shape, removing the blocking action of tropomyosin; actin active sites exposed 5) Contraction: myosin cross bridges attach to actin and pull (power stroke) the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers this process. 6) Removal of Ca2+ by calcium pumps into the sarcoplasmic reticulum. 7) Tropomyosin blockage restored blocking actin active site; contraction end and muscle fiber relax. © 2019 McGraw-Hill Education (a) © C. F. Armstrong/Science Source. Photo illustration by David  1 Regulation of contraction 4 © 2019 McGraw-Hill Education  1 Regulation of Contraction 5 © 2019 McGraw-Hill Education  1 Skeletal muscle contraction - Steps 6 1) Lower motor neuron axons, release ACh at the level of the neuromuscular junction – bind to nicotinic receptors. 2) Action potential is generated and propagated along the sarcolemma and down the T tubules. 3) Action potential triggers Ca2+ release from the terminal cisternae of the sarcoplasmic reticulum (after the previous video we are here). 4) Ca2+ ions bind to troponin; troponin change shape, removing the blocking action of tropomyosin; actin active sites exposed 5) Contraction: myosin cross bridges attach to actin and pull (power stroke) the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers this process. 6) Removal of Ca2+ by calcium pumps into the sarcoplasmic reticulum. 7) Tropomyosin blockage restored blocking actin active site; contraction end and muscle fiber relax. © 2019 McGraw-Hill Education (a) © C. F. Armstrong/Science Source. Photo illustration by David  1 7 Regulation of Contraction - sarcoplasmic reticulum a. SR is modified endoplasmic reticulum that stores Ca2+ when muscle is at rest. b. Most is stored in terminal cisternae. c. When a muscle fiber is stimulated, Ca2+ diffuses out of calcium release channels. d. At the end of a contraction, Ca2+ is actively pumped back into the SR. © 2019 McGraw-Hill Education  1 Skeletal muscle contraction - Steps 8 1) Lower motor neuron axons, release ACh at the level of the neuromuscular junction – bind to nicotinic receptors. 2) Action potential is generated and propagated along the sarcolemma and down the T tubules. 3) Action potential triggers Ca2+ release from the terminal cisternae of the sarcoplasmic reticulum. 4) Ca2+ ions bind to troponin; troponin change shape, removing the blocking action of tropomyosin; actin active sites exposed; formation of cross-bridges. 5) Contraction: myosin cross bridges attach to actin and pull (power stroke) the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers this process. 6) Removal of Ca2+ by calcium pumps into the sarcoplasmic reticulum. 7) Tropomyosin blockage restored blocking actin active site; contraction end and muscle fiber relax. © 2019 McGraw-Hill Education (a) © C. F. Armstrong/Science Source. Photo illustration by David  1 Actin and myosin 9 Actin 1.F-actin is made of 300-400 G-actin subunits, arranged in a double row and twisted to form a helix. 2.Have proteins called tropomyosin and troponin that prevent myosin binding at rest. Troponin complex binds to calcium. Tropomyosin physically blocks cross bridges. Myosin 1. Each protein has two globular heads with actin-binding sites and ATP-binding sites. © 2019 McGraw-Hill Education (a) © C. F. Armstrong/Science Source. Photo illustration by David  2 Regulation of Contraction 0 © 2019 McGraw-Hill Education  2 Cross-bridges actin/myosin 1 Actin 1.F-actin is made of 300-400 G-actin subunits, arranged in a double row and twisted to form a helix. 2.Have proteins called tropomyosin and troponin that prevent myosin binding at rest. Troponin complex binds to calcium. Tropomyosin physically blocks cross bridges. Myosin 1. Each protein has two globular heads with actin-binding sites and ATP-binding sites. © 2019 McGraw-Hill Education (a) © C. F. Armstrong/Science Source. Photo illustration by David  2 Skeletal muscle contraction - Steps 2 1) Lower motor neuron axons, release ACh at the level of the neuromuscular junction – bind to nicotinic receptors. 2) Action potential is generated and propagated along the sarcolemma and down the T tubules. 3) Action potential triggers Ca2+ release from the terminal cisternae of the sarcoplasmic reticulum. 4) Ca2+ ions bind to troponin; troponin change shape, removing the blocking action of tropomyosin; actin active sites exposed; formation of cross-bridges. (after the previous video we are here). 5) Contraction: myosin cross bridges attach to actin and pull (power stroke) the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers this process. 6) Removal of Ca2+ by calcium pumps into the sarcoplasmic reticulum. 7) Tropomyosin blockage restored blocking actin active site; contraction end and muscle fiber relax. © 2019 McGraw-Hill Education (a) © C. F. Armstrong/Science Source. Photo illustration by David  2 Regulation of Contraction 3 © 2019 McGraw-Hill Education  2 Regulation of Contraction - actin and myosin 4 filament 1) Each myosin head has an ATP- and an actin-binding site. 2) Adjacent to the ATP-binding site there is an enzyme (myosin ATPase) that hydrolyzes ATP to ADP plus Pi and in the process releases energy for the crossbridge cycle. 3) After the cross-bridge formation Pi is released, lead a conformational change in the myosin. 4) The power stroke of the crossbridge cycle (the rotation of the myosin head) occurs with the release of the ADP 5) The binding of ATP to the myosin heads enables the detachment of myosin from actin. © 2019 McGraw-Hill Education  2 Skeletal muscle contraction - Steps 5 1) Lower motor neuron axons, release ACh at the level of the neuromuscular junction – bind to nicotinic receptors. 2) Action potential is generated and propagated along the sarcolemma and down the T tubules. 3) Action potential triggers Ca2+ release from the terminal cisternae of the sarcoplasmic reticulum. 4) Ca2+ ions bind to troponin; troponin change shape, removing the blocking action of tropomyosin; actin active sites exposed; formation of cross-bridges. (after the previous video we are here). 5) Contraction: myosin cross bridges attach to actin and detach, pulling the actin filaments toward the center of the sarcomere; release of energy by ATP hydrolysis powers the cycling process. 6) Removal of Ca2+ by calcium pumps into the sarcoplasmic reticulum. 7) Tropomyosin blockage restored blocking actin active site; contraction end and muscle fiber relax. © 2019 McGraw-Hill Education (a) © C. F. Armstrong/Science Source. Photo illustration by David  2 Stimulating a Muscle Fiber: muscle relaxation 6 a. Action potentials cease. b. Calcium release channels close c. Ca2+-pumps move Ca2+ back into SR (active transport). d. No more Ca2+ is available to bind to troponin C e. Tropomyosin moves to block the myosin heads from binding to actin © 2019 McGraw-Hill Education  2 Motor Units 7 1. A motor unit is a single motor neuron and all the muscle fibers it innervates; all the muscle fibers in a motor unit contract at once. 2. Graded contractions – varied contraction strength due to different numbers of motor units being stimulated 3. Neuromuscular junction or motor endplate: site where a motor neuron stimulates a muscle fiber using the neurotransmitter, acetylcholine © 2019 McGraw-Hill Education  2 8 III. Contractions of Skeletal Muscles © 2019 McGraw-Hill Education  2 Twitch, Summation, and Tetanus 9 1. Twitch: when a muscle fiber quickly contracts and relaxes after a single electrical shock of sufficient voltage a. Increasing the voltage increases the strength of the twitch up to a maximum. b. When a second shock is applied immediately after the first, a second twitch will partially piggyback the first. This is called summation. c. Latent period – time between the stimulus and the contraction (excitation-contraction coupling to the attachment of myosin cross bridges to actin) d. Graded contractions – stronger contractions result in recruitment of more fibers, until all fibers are contracting. 2. Tetanus a. Increasing the frequency of electrical shocks decreases the relaxation time between twitches. This is called incomplete tetanus. b. At a certain frequency, there will be no relaxation. This is called complete tetanus, a smooth, sustained contraction. © 2019 McGraw-Hill Education  3 Twitch, Summation, and Tetanus 0 Tetanus In Vivo 1) Asynchronous activation of motor units 2) Some motor units start to twitch when others start to relax. 3) This produces continuous contraction of the whole muscle. 4) Recruitment makes contractions stronger. © 2019 McGraw-Hill Education  3 Types of Muscle Contractions 1 Force Velocity Curve a. For muscles to contract, they must generate force that is greater than the opposing forces. b. The greater the force, the slower the contraction. c. The force generated by the contraction of the muscle (or shortening of the sarcomeres) is called muscle tension. d. Muscle tension also is generated when the muscle is contracting against a load that does not move, resulting in two main types of skeletal muscle contractions: isotonic contractions and isometric contractions.  3 2 IV. Energy Requirements of Skeletal Muscles © 2019 McGraw-Hill Education  3 Energy Needed For: 3 Myosin ATPase (70%) Ca2+ pump to actively return calcium to the SR (30%) 1. At rest and for mild exercise: from the aerobic respiration of fatty acids 2. For moderate exercise: from glycogen stores 3. For heavy exercise: from blood glucose a. As exercise intensity and duration increase, GLUT4 channels are inserted into the sarcolemma to allow more glucose into cells. © 2019 McGraw-Hill Education  3 Slow- and Fast-Twitch Fibers 4 1. Slow-twitch (type I): slower contraction speed; can sustain contraction for long periods without fatigue; rich capillary supply; more mitochondria; more respiratory enzymes; more myoglobin a. Said to have high oxidative capacity, so are called slow oxidative fibers b. Due to high myoglobin content (which has a red pigment), these are also called red fibers c. Found in postural muscles 2. Fast (type IIx): faster contraction speed, fatigue fast, fewer capillaries, mitochondria, respiratory enzymes, and less myoglobin a. Also called white fibers b. Have more glycogen stores and are called fast glycolytic fibers c. Found in stronger muscles 3. Intermediate (type IIA): fast-twitch but with high oxidative capacity; called fast oxidative fibers 4. People vary greatly in the percentage of fast- or slow-twitch fibers in their muscles; result of genetics and training © 2019 McGraw-Hill Education  3 Relative Abundance of Fiber Types 5 © 2019 McGraw-Hill Education