CHAPTER 47: THE MUSCULOSKELETAL SYSTEM THE ESSENTIALS THEME ALERT ATP-induced phosphorylation and conformational change in proteins were principles learned earlier in the course the mechanism of muscle contractions is a specific application of these basic concepts. Students need to know: the sliding filament mechanism of muscle contraction. the role of ATP in muscle contraction. the role of calcium in muscle contraction. Key Terms striated muscles exoskeleton endoskeleton muscle fiber myofibril myofilament sarcomere sarcoplasmic reticulum sliding filament mechanism cross-bridges myosin actin tropomyosin troponin transverse tubules (T tubules) motor unit excitation-contraction coupling tetanus myoglobin slow-twitch fibers fast-twitch fibers muscle fatigue Strategy Class Time: The AP Acorn Book recommends devoting 32% of the course to the Structure and Function of Plants and Animals unit which includes 17 chapters: Chapters 36 42 (plants) and Chapters 43 54 (animals). This chapter can be covered in one to two days. Below is a suggested schedule based on a year-long class meeting three 45-minute periods every two days: Lecture 1 (40 45 minutes): Muscle Function Lecture 2 (40 45 minutes): Muscle Contraction Demonstration (optional) Approach: As with each of the chapters discussing the structure and function of a specific system in animals, the musculoskeletal system will contain information that is very familiar to students and other details that are surprisingly new to them. To enhance the connections in biology, muscle function is best taught as the means by which organisms meet their transport and locomotion needs whether it is moving food through the digestive system (smooth muscle), pumping blood through arteries (cardiac muscle), or propelling bodies through the environment (skeletal muscle). This system is one of the primary uses of the ATP 245
produced in cellular respiration (a working muscle recycles over 10 million ATP molecules per second). In this vein, the chapter can be presented earlier in the year directly after cellular respiration to better tie together cellular and whole organism systems. Students can easily relate to the need for muscle function in animal survival: finding food, hunting prey, escaping predators, and finding mates. Although there are a lot of new vocabulary words associated with muscle contraction, the concepts are quite straightforward and are best presented in a logical step-wise fashion which are explained well in the textbook. This is a good chapter to pull in students that are involved in sports and weight training applying scientific knowledge to what they do everyday at practice. Muscle contraction lends itself easily to demonstration and multimedia illustration. There is a Flash animation available through the OLC and many additional ones can be found through Web searches. Do not spend much time on the details and vocabulary of the skeletal system since it gets little attention on the AP exam. Concept Map motor function stimulated by enables nervous system movement motor neuron releases acetylcholine muscle skeleton smooth cardiac striated skeletal muscle sliding protein filament model sarcomere actin myosin Ca++ levels regulated by forming cross bridges 246
Student Misconceptions and Common Pitfalls Understanding muscle function can only be built on the foundational concept that a muscle only does work when contracting, or shortening. Students may think that the lengthening biceps lets the arm down as it straightens rather than it being the shortening of the triceps on the backside of the arm doing the work. Activities 1. Muscle Movement Demonstrations For demonstration, you can mimic the movement of myosin and actin with hand and arms: a. One arm can be the myosin filaments, held horizontal with wrist cocked and fist serving as the filament head. The other arm can be the actin filament, held horizontal below the first arm. The upper arm can demonstrate the ratchet system of the cross bridge cycle, moving along the lower arm as the cocked wrist myosin head binds and release the lower arm actin filament. b. Place a pencil (actin) on a smooth surface (even the overhead projector) walk your fingers (myosin heads) along its length so that it moves backward under your fingers. c. Outstretched fingers facing each other are actin, myosin is like holding short pencils between your fingers when they are placed tip to tip. As contraction proceeds, your finger tips get closer together, as do your hands (representing the Z lines). 2. Tissues & Organ Observations Obtain a number of animal organs to illustrate how they are made up of various tissues. Samples from the grocery/butcher/meat market are best. You usually need to order ahead of time: heart, kidney, liver, brains, and muscle tissue. The butcher can also save joints and large bone samples. Ask the butcher to bisect a long bone in both directions to show the internal structure. Have students make observations of differences in tissues. Prepared slides of tissue samples are also easily obtained from scientific supply catalogues. Web Resources Guided Tour of the Visible Human http://www.madsci.org/~lynn/vh/tour.html This Web site offers a guided tour of the human body using images and animations based on the Visible Human Project. The tour provides an introduction to human anatomy and physiology. How Muscles Work? http://health.howstuffworks.com/muscle.htm This Web page provides a detailed, clear account of how muscles work with useful animations scattered throughout. Cat Anatomy Tutorial http://biology.kenyon.edu/heithausp/cat-tutorial/ 247
This tutorial was designed to be used by college students studying cat anatomy. It provides very good skeletal photos. On the Lighter Side 1. Why are dead people called stiffs? Have students work through what causes rigor mortis, based on what they now know of motor function. When you stop breathing, there is no oxygen for cellular respiration. Therefore the body is depleted of ATP. Without ATP, the Ca +2 pumps cannot remove Ca +2 from muscle cells which causes a continuous contraction. Muscles remain tense/contracted. Eventually the tissues breakdown and relax. 2. Skeletal System puzzle A large floor puzzle is available commercially depicting the human skeletal system, from the company Melissa & Doug. Give the students the puzzle without the aid of the cover image and have them put it together. Multiple Choice Questions 1. All of the following are types of connective tissue EXCEPT: a. bone b. blood c. ligaments and tendons d. muscle e. cartilage 2. Layered epithelial cells are called a. squamous. b. cuboidal. c. stratified. d. columnar. e. goblet cells. 3. Chondroitin is found in a. the extracellular matrix of cartilage. b. spongy bone. c. compact bone. d. within the sarcoplasmic reticulum of muscle fiber. e. squamous epithelial tissue. 248
4. Muscle contraction is accomplished within the sarcomere by a. shortening of thin actin filaments. b. shortening of thick myosin filaments. c. increasing the overlap of thick and thin filaments. d. formation of new microfilaments by the polymerization of actin. e. binding of tropomyosin to actin. 5. The role of myoglobin in muscle cells is to a. regulate the movement of Ca ++ from transport vesicles to myosin. b. bind to myosin to regulate muscle contraction. c. transport ATP to myosin. d. store glycogen for anaerobic respiration. e. improve the delivery of oxygen to slow-twitch muscle fibers. 6. Weight training builds bigger muscles by a. stimulating the growth of new muscle cells. b. increasing the amount of myoglobin in fast-twitch muscle fibers. c. increasing the size and number of myofibrils in fast-twitch muscle fibers. d. improving the metabolism of lactic acid. e. reducing the amount of glycogen carried by the blood. 7. Calcium ions (Ca ++ ) stimulate muscle contraction by a. causing voltage-gated ion channels in the plasma membrane to open. b. binding to troponin which causes tropomyosin to move away from sites on actin fibers where myosin heads bind. c. binding to actin which allows it to hydrolyze ATP. d. activating a G-protein which phosphorylates tropomyosin. e. binding to myosin which allows myosin to release tropomyosin. 8. Cardiac muscle is different from skeletal muscle in all of the following ways EXCEPT a. Cardiac muscle has more mitochondria and stores very little glycogen. b. Cardiac muscle cells have a single nucleus. c. The cytoplasm of cardiac muscle cells are interconnected by gap junctions. d. Cardiac muscle cells are not stimulated to contract by nerve impulses. e. Cardiac muscle is composed entirely of fast-twitch fibers. 9. The painful feeling of muscle fatigue is caused by the a. buildup of lactic acid due to anaerobic respiration. b. depletion of Ca ++ in the sarcoplasmic reticulum. c. death of muscle cells due to depletion of ATP reserves. d. depolymerization of actin fibers. e. buildup of high levels of adrenaline and cortisol in muscle cells. 249
10. Smooth muscle is found a. in the spinal cord. b. at joints. c. in the lungs. d. surrounding the organs of the digestive system. e. in the heart. Answers: 1. d, 2. c, 3. a, 4. c, 5. e, 6. b, 7. b, 8. e, 9. a, 10. d Essay Questions 1. How is it possible for broken bones to heal in adults (long after they have stopped growing taller)? Bone is a living tissue. Osteoblasts and osteocytes are living cells within the matrix of bone which continue to excrete collagen and calcium phosphate as long as a person is alive. These cells are stimulated to produce more bone matrix when the bone is broken. 2. What are the relative advantages and disadvantages of a chitinous exoskeleton vs. a bone endoskeleton for large and small animals? Chitin is stronger for its weight and more easily produced. Internal bones are not strong enough in very tiny creatures. The exoskeleton provides protection for the organism. A chitinous exoskeleton does not grow with an animal, forcing it to molt. An exoskeleton becomes very heavy as a creature grows larger. 3. Describe the difference between slow-twitch and fast-twitch muscle fibers. Slow-twitch fibers take longer to reach maximum contraction, but they can contract for a longer time. Slow-twitch fibers are able to generate more ATP by aerobic respiration because they have more capillaries, more mitochondria, and more myoglobin. Fasttwitch fibers reach maximum contraction faster but become fatigued sooner. Fast-twitch fibers store glycogen and generate more of their ATP by glycolysis. Fast-twitch fibers grow thicker and stronger in response to weight training. 250