Chapter 12: Physiology of the Muscular System

1. Chapter 12: Physiology of the Muscular System Anatomy & Physiology

2. Introduction • Muscular system is responsible for moving the framework of the body • In addition to movement, muscle tissue performs various other functions 2

3. General Functions • Movement of the body as a whole or movement of its parts • Heat production • Posture 3

4. Function of Skeletal Muscle Tissue • Characteristics of skeletal muscle cells • Excitability (irritability)—ability to be stimulated • Contractility—ability to contract, or shorten, and produce body movement • Extensibility—ability to extend, or stretch, thereby allowing muscles to return to their resting length 4

5. Function of Skeletal Muscle Tissue • Overview of the muscle cell (Figures 12-1 and 12-2) • Muscle cells are called fibers because of their threadlike shape • Sarcolemma—plasma membrane of muscle fibers • Sarcoplasmic reticulum (SR) • T tubules—network of tubules and sacs found within muscle fibers • Membrane of the SR continually pumps calcium ions from the sarcoplasm and stores the ions within its sacs for later release (Figure 12-3) 5

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9. Function of Skeletal Muscle Tissue • Overview of the muscle cell (cont) • Muscle fibers contain many mitochondria and several nuclei • Myofibrils—numerous fine fibers packed close together in sarcoplasm • Sarcomere • Segment of myofibril between two successive Z disks • Each myofibril consists of many sarcomeres • Contractile unit of muscle fibers 9

10. Function of Skeletal Muscle Tissue • Overview of the muscle cell (cont) • T tubules • Transverse tubules extend across the sarcoplasm at right angles to the long axis of the muscle fiber • Formed by inward extensions of the sarcolemma • Membrane has ion pumps that continually transport Ca++ ions inward from the sarcoplasm • Allow electrical impulses traveling along the sarcolemma to move deeper into the cell 10

11. Function of Skeletal Muscle Tissue • Overview of the muscle cell (cont) • Striated muscle (Figure 12-4) • Dark stripes called A bands; light H band runs across the midsection of each dark A band • Light stripes called I bands; dark Z disk extends across the center of each light I band • Triad • Triplet of tubules; a T tubule sandwiched between two sacs of sarcoplasmic reticulum • Allows an electrical impulse traveling along a T tubule to stimulate the membranes of adjacent sacs of the sarcoplasmic reticulum 11

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13. Function of Skeletal Muscle Tissue • Myofilaments (Figures 12-5 and 12-6) • Each myofibril contains thousands of thick and thin myofilaments • Four different kinds of protein molecules make up myofilaments • Myosin • Makes up almost all the thick filament • Myosin “heads” are chemically attracted to actin molecules • Myosin “heads” are known as cross bridges when attached to actin 13

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16. Function of Skeletal Muscle Tissue • Four different kinds of protein molecules make up myofilaments (cont) • Actin—globular protein that forms two fibrous strands twisted around each other to form the bulk of the thin filament • Tropomyosin—protein that blocks the active sites on actin molecules • Troponin—protein that holds tropomyosin molecules in place 16

17. Function of Skeletal Muscle Tissue • Mechanism of contraction • Excitation and contraction (Figures 12-7 through 12-13; Table 12-1) • A skeletal muscle fiber remains at rest until stimulated by a motor neuron • Neuromuscular junction—motor neurons connect to the sarcolemma at the motor endplate (Figure 12-7) • Neuromuscular junction is a synapse where neurotransmitter molecules transmit signals 17

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25. Function of Skeletal Muscle Tissue • Excitation and contraction (cont) • Acetylcholine—the neurotransmitter released into the synaptic cleft that diffuses across the gap, stimulates the receptors, and initiates an impulse in the sarcolemma • Nerve impulse travels over the sarcolemma and inward along the T tubules, which triggers the release of calcium ions • Calcium binds to troponin, which causes tropomyosin to shift and expose active sites on actin 25

26. Function of Skeletal Muscle Tissue • Excitation and contraction (cont) • Sliding filament model (Figures 12-12 and 12-13) • When active sites on actin are exposed, myosin heads bind to them • Myosin heads bend and pull the thin filaments past them • Each head releases, binds to the next active site, and pulls again • The entire myofibril shortens 26

27. Function of Skeletal Muscle Tissue • Mechanism of contraction (cont) • Relaxation • Immediately after the Ca++ ions are released, the sarcoplasmic reticulum begins actively pumping them back into the sacs (Figure 12-3) • Ca++ ions are removed from the troponin molecules, thereby shutting down the contraction 27

28. Function of Skeletal Muscle Tissue • Mechanism of contraction (cont) • Energy sources for muscle contraction (Figure 12-14) • Hydrolysis of ATP yields the energy required for muscular contraction • ATP binds to the myosin head and then transfers its energy to the myosin head to perform the work of pulling the thin filament during contraction • Muscle fibers continually resynthesize ATP from the breakdown of creatine phosphate (CP) 28

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30. Function of Skeletal Muscle Tissue • Energy sources for muscle contraction (cont) • Catabolic pathways • Aerobic pathway • Occurs when adequate O2 is available from blood (Figure 12-15) • Slower than anaerobic pathway, thus supplies energy for the long term rather than the short term 30

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32. Function of Skeletal Muscle Tissue • Energy sources for muscle contraction (cont) • Catabolic pathways (cont) • Anaerobic pathway (Figure 12-16) • Very rapid, providing energy during first minutes of maximal exercise (Figure 12-16) • May occur when low levels of O2 are available • Results in the formation of lactic acid, which requires oxygen to convert back to glucose, the producing of an “oxygen debt” or excess postexercise oxygen consumption (EPOC) 32

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34. Function of Skeletal Muscle Tissue • Energy sources for muscle contraction (cont) • Skeletal muscle contraction produces waste heat that can be used to help maintain the set point body temperature (Figure 12-17) 34

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36. Function of Skeletal Muscle Organs • Muscles are composed of bundles of muscle fibers held together by fibrous connective tissue • Motor unit (Figure 12-18) • Motor unit—motor neuron plus the muscle fibers to which it attaches • Some motor units consist of only a few muscle fibers, whereas others consist of numerous fibers • Generally, the smaller the number of fibers in a motor unit, the more precise are the available movements; the larger the number of fibers in a motor unit, the more powerful the contraction available 36

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38. Function of Skeletal Muscle Organs • Myography—method of graphing the changing tension of a muscle as it contracts (Figure 12-19) • Twitch contraction (Figure 12-20) • A quick jerk of a muscle that is produced as a result of a single, brief threshold stimulus (generally occurs only in experimental situations) 38

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41. Function of Skeletal Muscle Organs • Twitch contraction (cont) • The twitch contraction has three phases • Latent phase—nerve impulse travels to the sarcoplasmic reticulum to trigger release of Ca++ • Contraction phase—Ca++ binds to troponin and sliding of filaments occurs • Relaxation phase—sliding of filaments ceases 41

42. Function of Skeletal Muscle Organs • Treppe—the staircase phenomenon (Figure 12-21, B) • Gradual, steplike increase in the strength of contraction that is seen in a series of twitch contractions that occur 1 second apart • Eventually, the muscle responds with less forceful contractions, and the relaxation phase becomes shorter • If the relaxation phase disappears completely, a contracture occurs 42

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44. Function of Skeletal Muscle Organs • Tetanus—smooth, sustained contractions • Multiple wave summation—multiple twitch waves are added together to sustain muscle tension for a longer time • Incomplete tetanus—very short periods of relaxation occur between peaks of tension (Figure 12-21, C) 44

45. Function of Skeletal Muscle Organs • Tetanus—smooth, sustained contractions (cont) • Complete tetanus—the stimulation is such that twitch waves fuse into a single, sustained peak (Figure 12-21, D) • The availability of calcium determines whether a muscle will contract; if the calcium is continuously available, then contraction will be sustained (Figure 12-22) 45

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47. Function of Skeletal Muscle Organs • Muscle tone • Tonic contraction—continual, partial contraction of a muscle • At any one time, a small number of muscle fibers within a muscle contract and produce a tightness or muscle tone • Muscles with less tone than normal are flaccid • Muscles with more tone than normal are spastic • Muscle tone is maintained by negative feedback mechanisms 47

48. Graded Strength Principle • Graded strength principle—skeletal muscles contract with varying degrees of strength at different times • Factors that contribute to the phenomenon of graded strength (Figure 12-26) • Metabolic condition of individual fibers • Number of muscle fibers contracting simultaneously; the greater the number of fibers contracting, the stronger the contraction • Number of motor units recruited • Intensity and frequency of stimulation (Figure 12-23) 48

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