Chapter 12 Answers: Muscular System

12.2 Introduction to the Muscular System: Review Questions and Answers

  1. What is the muscular system? The muscular system is the organ system that consists of all the muscles in the body.
  2. Describe muscle cells and their function. Muscle cells (or fibres) are long, thin cells that are specialized for the function of contracting. They contain protein filaments that slide over one another using energy in ATP. The sliding filaments increase the tension in, or shorten the length of, the muscle fibres and cause contractions. Muscle contractions are responsible for virtually all the movements of the body, both inside and out.
  3. Identify three types of muscle tissue and where each type is found. Three types of muscles are skeletal, smooth, and cardiac muscles. Skeletal muscle is attached to bones, cardiac muscle makes up the walls of the heart, and smooth muscle is found in the walls of internal organs and other internal structures.
  4. Define muscle hypertrophy and muscle atrophy. Muscle hypertrophy is an increase in the size of muscle. Muscle atrophy is a decrease in the size of muscle.
  5. What are possible causes of muscle hypertrophy? Possible causes of muscle hypertrophy include increased use (physical exercise) and hormones such as testosterone.
  6. Give three reasons that muscle atrophy may occur. Answers may vary. Sample answer: Three reasons that muscle atrophy may occur include lack of physical activity, such as might occur with immobility due to a broken bone or surgery; starvation; and certain diseases, such as AIDS or cancer.
  7. How do muscles change when they increase or decrease in size? When muscles increase or decrease in size, the individual muscle fibres grow wider or narrower, respectively.
  8. How do changes in muscle size affect strength? Muscle size is the main determinant of muscle strength. Therefore, an increase in muscle size generally causes an increase in strength, and a decrease in muscle size generally causes a decrease in strength.
  9. Explain why astronauts can easily lose muscle mass in space. Answers may vary. Sample answer: Astronauts can easily loss muscle mass in space because they are in a weightless environment. On Earth, muscle cells are continually challenged by gravity, and moving and lifting objects against gravity is a form of physical activity that helps maintain the size of muscle fibres. Without this constant challenge to the muscles, astronauts will lose muscle mass unless they proactively exercise.
  10. Describe how the terms muscle cells, muscle fibres, and myocytes relate to each other. Both muscle fibres and myocytes are muscle cells. The term muscle fibre is mainly used to describe muscle cells in skeletal and cardiac muscles. The term myocyte is mainly used to describe muscle cells in smooth muscles.
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  12. Name two systems in the body that work together with the muscular system to carry out movements. Answers will vary. Sample answer: The skeletal system and the nervous system.
  13. Describe one way in which the muscular system is involved in regulating body temperature. Answers may vary. Sample answer: Smooth muscles in the blood vessels can contract to cause vasoconstriction, or relax to cause vasodilation. This conserves body heat or dissipates it, respectively.

    12.3 Types of Muscle Tissue: Review Questions and Answers

    1. What is muscle tissue?Muscle tissue is a soft tissue that makes up most of the tissues in the muscles of the human muscular system. It is the only type of tissue that has cells with the ability to contract.
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    4. Where is skeletal muscle found, and what is its general function?Skeletal muscle is attached to bones by tendons. Its general function is to power voluntary body movements.
    5. Why do many skeletal muscles work in pairs?Many skeletal muscles work in opposing pairs to move bones back and forth at joints.
    6. Describe the structure of a skeletal muscle.A skeletal muscle consists of bundles of muscle fascicles, each of which in turn consists of bundles of muscle fibres. Skeletal muscles also have connective tissue supporting and protecting the muscle tissue.
    7. Relate muscle fibre structure to the functional units of muscles.Each muscle fibre consists of a bundle of myofibrils, which are bundles of protein filaments. The filaments are arranged in repeating units called sarcomeres, which are the basic functional units of skeletal muscles.
    8. Why is skeletal muscle tissue striated?Skeletal muscle tissue is striated because of the pattern of sarcomeres in its fibres.
    9. Where is smooth muscle found? What controls the contraction of smooth muscle?Smooth muscle is found in the walls of internal organs and vessels. Contractions of smooth muscles are not under conscious control. Instead, they are controlled by the autonomic nervous system, hormones, and other substances.
    10. Where is cardiac muscle found? What controls its contractions? Cardiac muscle is found only in the wall of the heart. Contractions of cardiac muscle are involuntary like those of smooth muscle. They are controlled by electrical impulses from specialized cardiac cells and may be influenced by hormones and other factors.
    11. The heart muscle is smaller and less powerful than some other muscles in the body. Why is the heart the muscle that performs the greatest amount of physical work in the course of a lifetime? How does the heart resist fatigue? The heart is the muscle that performs the greatest amount of physical work in the course of a lifetime because it beats continuously throughout life without rest. Its cells contains a great many mitochondria to produce ATP for energy and help the heart resist fatigue.
    12. Give one example of connective tissue that is found in muscles. Describe one of its functions. Answers will vary. Sample answer: The connective tissue called epimysium surrounds skeletal muscles and anchors the muscles to tendons.

      12.4 Muscle Contraction: Review Questions and Answers

      1. What is a skeletal muscle contraction? A skeletal muscle contraction is an increase in the tension or a decrease in the length of a skeletal muscle.
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      3. Explain sliding filament theory and describe crossbridge cycling. The sliding filament theory is the most widely accepted explanation for how a muscle contraction occurs. According to this theory, thick myosin filaments repeatedly attach to and pull on thin myosin filaments. This shortens sarcomeres and thus causes contractions.
      4. If the acetylcholine receptors on muscle fibres were blocked by a drug, what do you think this would do to muscle contraction? Explain your answer. Answers may vary. Sample answer: If the acetylcholine receptors were blocked, muscle contraction would be prevented or at least inhibited. This is because the neurotransmitter acetylcholine is necessary to trigger muscle contractions at the neuromuscular junction by binding to its receptors on the muscle fibres.
      5. Explain how crossbridge cycling and sliding filament theory are related to each other. Sliding filament theory describes how actin and myosin filaments slide past each other during muscle contraction. Crossbridge cycling is the specific mechanism by which the filaments slide past each other, which involves the use of ATP.
      6. When does anaerobic respiration typically occur in human muscle cells? Anaerobic respiration typically only occurs in human muscle cells during strenuous exercise when sufficient oxygen cannot be delivered to the muscle to keep up with the demand for ATP.
      7. If there were no ATP available in a muscle, how would this affect crossbridge cycling? What would this do to muscle contraction? Answers may vary. Sample answer: ATP is required to move the myosin head into the cocked position. If this does not occur, the myosin head cannot attach to the actin filament and the “power stroke” cannot occur. The filaments would not slide past each other and therefore muscle contraction would not occur.

        12.5 Physical Exercise: Review Questions and Answers

        1. How do we define physical exercise?Physical exercise is defined as any bodily activity that enhances or maintains physical fitness and overall health even if it is not done for its health benefits.
        2. What are current recommendations for physical exercise for adults? Current recommendations for physical exercise for adults are 30 minutes a day of moderate exercise.
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        4. Define flexibility exercise, and state its benefits. What are two examples of flexibility exercises? Flexibility exercise is any physical activity that stretches and lengthens muscles. Benefits of flexibility exercise include improving range of motion and reducing risk of injury. Examples may vary. Sample answer: Two examples of flexibility exercises include stretching and yoga.
        5. In general, how does physical exercise affect health, quality of life, and longevity? In general, physical exercise improves physical, mental, and emotional health. It also increases quality of life and longevity.
        6. What mechanism may underlie many of the general health benefits of physical exercise? The mechanism that may underlie many of the general health benefits of physical exercise is the release of hormones called myokines from contracting muscles. Myokines are endocrine hormones that promote tissue repair and growth and have anti-inflammatory effects.
        7. Relate physical exercise to cardiovascular disease risk. Physical exercise can reduce risk factors for cardiovascular disease, including hypertension, high levels of “bad” and total cholesterol, and excess body weight. Physical exercise can also increase factors associated with good cardiovascular health, such as “good” cholesterol level and the mechanical efficiency of the heart.
        8. What may explain the positive benefits of physical exercise on cognition? Positive benefits of physical exercise on cognition may be explained by an increase in blood flow to the brain, which brings more oxygen to brain cells; an increase in growth factors that promote growth of brain cells and neuronal pathways in the brain; and an increase in neurotransmitters in the brain.
        9. How does physical exercise compare with antidepressant drugs in the treatment of depression? Numerous studies suggest that regular aerobic exercise works as well as pharmaceutical antidepressants in treating mild-to-moderate depression, possibly because it increases synthesis of natural euphoriants in the brain.
        10. Identify several other health benefits of physical exercise. Other health benefits of physical exercise include improved sleep, better immune system function, and reduced risk of type 2 diabetes and obesity.
        11. Explain how genetics may influence the way individuals respond to physical exercise. Genetic differences in proportions of slow-twitch and fast-twitch skeletal muscle fibres may influence how people respond to physical exercise. People with more slow-twitch fibres may be able to develop greater endurance from aerobic exercise, whereas people with more fast-twitch fibres may be able to develop greater muscle size and strength from anaerobic exercise.
        12. Can too much physical exercise be harmful? Some adverse effects may occur if exercise is extremely intense and the body is not given proper rest between exercise sessions. Many people who overwork their muscles develop delayed onset muscle soreness (DOMS), which may be caused by tiny tears in muscle fibres.

          12.6 Disorders of the Muscular System: Review Questions and Answers

          1. What are musculoskeletal disorders? What causes them? Musculoskeletal disorders are injuries that occur in muscles or associated tissues such as tendons because of biomechanical stresses. The disorders may be caused by sudden exertion, over-exertion, repetitive motions, and similar stresses.
          2. How does a muscle strain occur? A muscle strain occurs when muscle fibres tear as a result of overstretching.
          3. Define tendinitis. Why does it occur? Tendinitis is inflammation of a tendon. It occurs when a tendon is over-extended or worked too hard without rest.
          4. Identify first-aid steps for treating musculoskeletal disorders, such as muscle strains and tendinitis. First-aid steps for treating musculoskeletal disorders such as muscle strains and tendinitis include protection, rest, ice, compression, and elevation.
          5. Describe carpal tunnel syndrome and how it may be treated. Carpal tunnel syndrome is a biomechanical problem that occurs in the wrist when the median nerve becomes compressed between carpal bones, often due to repetitive use of the wrist and typically causing pain, numbness, and eventually muscle wasting in the thumb and first two fingers of the hand if untreated. Carpal tunnel syndrome may be treated by wearing a wrist splint, receiving corticosteroid injections, or undergoing surgery to cut the carpal ligament and reduce pressure on the median nerve.
          6. Define neuromuscular disorders. Neuromuscular disorders are systemic disorders that occur because of problems with the nervous control of muscle contractions or with the muscle cells themselves.
          7. Identify the cause and symptoms of muscular dystrophy. Muscular dystrophy is a genetic disorder caused by defective proteins in muscle cells. Its symptoms include progressive skeletal muscle weakness due to the death of muscle cells and tissues.
          8. Outline the cause and progression of myasthenia gravis. Myasthenia gravis is a genetic neuromuscular disorder most often caused by immune system antibodies blocking acetylcholine receptors on muscle cells and the actual loss of acetylcholine receptors. It is characterized by fluctuating muscle weakness and fatigue, with more muscles becoming affected and muscles becoming increasingly weakened as the disorder progresses.
          9. What is Parkinson’s disease? List four characteristic signs of the disorder. Parkinson’s disease is a degenerative disorder of the central nervous system that mainly affects the muscular system and movement. Four characteristic signs of the disorder are muscle tremor, rigidity, slowness of movement, and postural instability.
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          11. What are the main differences between musculoskeletal disorders and neuromuscular disorders? Answers may vary. Sample answer: Musculoskeletal disorders are due to biomechanical stresses; typically only affect just one or a few muscles; and are often fully treatable. Neuromuscular disorders are not due to biomechanical stresses (they often have a genetic cause); they usually affect most or all of the muscles in the body; and they are often progressive and incurable.
          12. Why is padding of a strained muscle part of the typical treatment? A strained muscle is caused by the tearing of muscle fibres. Padding of a strained muscle protects it from further impact.
          13. What are two tissues — other than muscle tissue — that can experience problems that result in muscular system disorders? Answers may vary. Sample answer: Tendons and nervous system tissue.

            12.7 Case Study Conclusion and Chapter Summary: Review Questions and Answers

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            2. What are tendons? Name a muscular system disorder involving tendons. Tendons are bundles of collagen fibres that attach skeletal muscles to bone.  Answers may vary. Sample answer. Tendonitis.
            3. Describe the relationship between muscles, muscle fibres, and fascicles. Muscle fibres are the cells that make up skeletal muscle tissue. Muscle fibres are bundled together in fascicles. In turn, bundles of fascicles make up individual muscles.
            4. The biceps and triceps muscles are shown above. Answer the following questions about these arm muscles.
              1. When the biceps contract and become shorter (as in the picture above), what kind of motion does this produce in the arm? The arm bends at the elbow and the forearm will move up.
              2. Is the situation described in part (a) more likely to be an isometric or isotonic contraction? Explain your answer. It is more likely to be an isotonic contraction because the muscle is shortening and isotonic contractions involve a change in muscle length. Isometric contractions do not involve a change in muscle length.
              3. If the triceps were to then contract, which way would the arm move? The arm would straighten out.
            5. What are Z discs? What happens to them during muscle contraction? Z discs are structures that mark the end of a sarcomere in a muscle fibre. They are attached to actin filaments. During muscle contraction, the sliding of the actin and myosin filaments pulls the Z discs closer together, shortening the sarcomere.
            6. What is the function of mitochondria in muscle cells? Which type of muscle fibre has more mitochondria ­— slow-twitch or fast-twitch? The function of mitochondria in muscle cells is to provide energy for the muscles in the form of ATP, through aerobic respiration. Slow-twitch.
            7. What is the difference between primary and secondary Parkinson’s disease? Primary Parkinson’s disease occurs mostly in older people, for no known reason. Secondary Parkinson’s disease occurs due to some kind of known or suspected cause, such as repeated head trauma or exposure to toxins.
            8. Why can carpal tunnel syndrome cause muscle weakness in the hands? Answers may vary. Sample answer: Carpal tunnel syndrome is due to the compression of the median nerve in the wrist. This nerve is then unable to adequately stimulate the muscles that it innervates, causing muscle weakness.


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            Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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