0907T_c13_205-218.qxd 1/25/05 11:05 Page 209 EXERCISE 13 CONTRACTION OF SKELETAL MUSCLE 209 Aerobic cellular respiration produces ATP slowly, but can produce large amounts of ATP over time if there is a sufficient blood supply and myoglobin (red pigment that stores oxygen) stores. Therefore, muscle fibers that use aerobic respiration take longer to fatigue. These muscle fibers have lower glycogen stores, are darker due to higher myoglobin content, and have a greater blood supply. These fibers also have greater numbers of mitochondria because the enzymes for aerobic cellular respiration are located in the mitochondria.? DISCUSSION QUESTIONS: INFLUENCE OF MUSCLE FIBER TYPE ON MUSCLE CONTRACTION 1 Discuss why slow oxidative fibers are more fatigueresistant than fast glycolytic fibers. 2 Discuss why slow oxidative fibers have more capillaries and mitochondria than fast glycolytic fibers. ACTIVITY 2 Influence of Muscle Fiber Type on Muscle Contraction 1 Label the muscle fiber types in Figure 13.1. 2 Complete Table 13.2 by choosing the correct choice in parentheses. 3 Answer the discussion questions with your lab partners. 1 3 Discuss why fast glycolytic fibers have more glycogen stores than slow oxidative fibers. 4 Weight training causes fast glycolytic fibers to hypertrophy (increase in size). Explain why this increases strength. 2 FIGURE 13.1 Cross-section of skeletal muscle showing all three fiber types. fast glycolytic fiber fast oxidative-glycolytic fiber slow oxidative fiber 1 2 3 LM 440 3 5 Identify which muscle fiber type would be primarily involved in each of the following functions. a. walking b. standing c. lifting a heavy object and immediately putting it down.
0907T_c13_205-218.qxd 1/11/05 10:19 Page 210 210 EXERCISE 13 CONTRACTION OF SKELETAL MUSCLE TABLE 13.2 Characteristics of Skeletal Muscle Fiber Types SLOW OXIDATIVE FAST OXIDATIVE-GLYCOLYTIC FAST GLYCOLYTIC Fiber diameter (smallest, intermediate, largest) Force (lowest, intermediate, greatest) Myosin ATPase (slowest, fastest) Contraction velocity (slow, fast) Methods of ATP generation (glycolysis, aerobic respiration) Glycogen stores (low, intermediate, high) Capillaries (many, few) Myoglobin content (low, high) Color (white, red-pink, red-brown) C. Contraction of Skeletal Muscle Fibers A motor neuron stimulates all the skeletal muscle fibers in a motor unit simultaneously. The amount of force generated by a motor unit is determined by the number of muscle fibers within the motor unit, the size of the individual muscle cells, and the number of crossbridges attached in the muscle fibers. A twitch contraction is the quick shortening observed in a skeletal muscle when a single action potential traveling down a motor neuron stimulates the skeletal muscle fibers of the motor unit to contract. The minimal stimulus that results in a muscle twitch is called the threshold stimulus. The three phases of a twitch contraction are the latent period, the contraction period, and the relaxation period. The latent period lasts about 2 msec (milliseconds) and is the time between stimulation of muscle cells and force generation. The contraction period lasts about 10 100 msec and is the period during which force (measured in grams) is increasing, whereas the relaxation period, which lasts 10 100 msec, is the period when force is decreasing. Normal muscle contractions are not twitch contractions, but are sustained contractions of varying force. If the muscle fibers of a motor unit are stimulated before the relaxation phase of a muscle twitch is complete, then the next contraction will produce a greater force. This is called wave summation. Increasing the frequency of muscle stimulation produces sustained force generation. Unfused tetanus (tetan rigid) occurs when there is a partial relaxation between muscle twitches. Fused tetanus is a sustained contraction with no relaxation observed between twitches. Increasing the number of motor units contracting at the same time, motor unit recruitment, also increases force generated. Lifting a feather requires fewer motor units than lifting your anatomy and physiology textbook. Maximal force development occurs when all motor units of a muscle are stimulated and all muscle fibers are contracting. In the lab, the stimulus that produces maximal force is called the maximal stimulus. Therefore, a stimulus to the muscle greater than maximal does not produce agreater force. Most sustained voluntary skeletal muscle contractions are unfused tetanic contractions with different motor units stimulated at different times (asynchronous contractions). The asynchronous contractions delay muscle fatigue, which is an inability to contract caused by long periods of muscle contraction. Muscle contraction results in development of tension or force usually measured in grams. Muscles will shorten if they develop more force than the force that is opposing them. For example, contracting muscles in our arm will shorten and allow us to lift a book if the force developed by the muscles is greater than the weight (force) of the book. The weight of the book is also called load because it is a force that the muscle is contracting against.
0907T_c13_205-218.qxd 1/11/05 10:19 Page 211 EXERCISE 13 CONTRACTION OF SKELETAL MUSCLE 211 If a muscle is generating a constant force to move a load, that contraction is called an isotonic contraction. There are two types of isotonic contraction, concentric and eccentric isotonic contractions. In concentric isotonic contractions, the muscle is shortening while it is contracting. An example of this is using the biceps brachii (large anterior muscle of arm) to lift a book off the table. In eccentric isotonic contractions, the muscle is lengthening while it is contracting. This occurs when you slowly lower your arm to return the book to the table. The biceps brachii is still contracting, but it is lengthening while it is contracting. This enables you to lower the book in a controlled manner. Isometric contractions are contractions in which the muscle is developing force but not shortening and no visible movement is seen. In this case, the force developed by the muscle equals the force (load) it is contracting against. An example of this would be holding a book in the same position. Isometric muscle contractions are maintaining the position against the weight (load) of the book, but the biceps brachii muscle is not moving the book because it is not generating a force greater than the weight (force) of the book. ACTIVITY 3 Contraction of Skeletal Muscle Fibers 1 Draw a twitch contraction that has a latent period of 2 msec, a contractile period of 10 msec with a maximum force of 2 g, and a relaxation period of 10 msec. Use the graph in Figure 13.2. 2 Observe unfused tetanus, motor unit recruitment, fatigue, isotonic, and isometric contractions in the muscles that cause bending of the knee. In your lab group, decide who will be the subject, observer, and recorder. Have the subject stand while holding onto the lab bench for support. Have subject bend the left knee to a 90-degree angle and hold this position. The hamstring muscles are used to bend the knee. Observe the unfused tetanic contraction. Start timing. Time how long it takes for the muscle to fatigue, which is demonstrated by any vertical movement in the leg. Time until fatigue: min. Place a 5-lb ankle weight on the subject s right ankle and repeat the steps. Time until fatigue: min. 3 Answer discussion questions with your lab partners.? DISCUSSION QUESTIONS: OBSERVATION OF UNFUSED TETANUS, RECRUITMENT, AND FATIGUE IN HAMSTRING MUSCLES 1 Explain why you can maintain contraction of the hamstring muscles over time. 2 Explain why you can sustain the same contraction with a 5-lb weight attached to the ankle. 3 Explain why the leg with the 5-lb ankle weight fatigued faster. 4 As the subject bends the knee and holds it at 90 degrees, state which muscle movement causes isometric contractions and which causes isotonic contractions. force (grams) 5 Is the isotonic contraction observed when the hamstrings contract a concentric isotonic contraction or an eccentric isotonic contraction? What would you ask the subject to do to observe the other type of isotonic contraction? time (msec) FIGURE 13.2 Student drawing of twitch contraction.
0907T_c13_205-218.qxd 1/25/05 08:33 Page 215 Name Date Section Reviewing Your Knowledge 13 EXERCISE A. Role of ATP in Muscle Contraction Write T for true or F for false for the following statements. 1. There is only one type of myosin ATPase involved in muscle contraction. 2. ATP causes the detachment of myosin from actin. 3. Glycerinated muscle fibers need calcium to couple the electrical excitation with the contraction cycle. 4. Skeletal muscle fibers need ATP in order to contract. 5. A solution of 0.05% ATP causes more skeletal muscle contraction than a solution of 0.25% ATP. B. Influence of Muscle Fiber Type on Muscle Contraction Match the correct fiber type to the fiber characteristic. Questions may have more than one answer. a. fast glycolytic b. fast oxidative-glycolytic c. slow oxidative 1. largest diameter 2. is pink in color 3. myoglobin content is low 4. has the fastest contraction velocity 5. has many capillaries 6. is red in color 7. generates ATP by aerobic respiration 8. has the slowest myosin ATPase 9. has glycogen stores 10. lowest force 215
0907T_c13_205-218.qxd 1/11/05 10:19 Page 216 216 EXERCISE 13 CONTRACTION OF SKELETAL MUSCLE C. Contraction of Skeletal Muscle Fibers Match structure(s) to definition. a. contractile period g. muscle fatigue b. fused tetanus h. relaxation period c. latent period i. threshold stimulus d. maximal stimulus j. twitch e. motor unit k. unfused tetanus f. motor unit recruitment 1. Inability of muscle fibers to contract after a long period of contraction. 2. Phase of contraction; time between muscle fiber stimulation and measurement of force generation. 3. Motor neuron and all the muscle fibers it innervates. 4. Phase of contraction; period of force generation. 5. Type of wave summation with partial relaxation observed between twitches. 6. Increasing the number of motor units that are stimulated to contract. 7. Stimulus that results in maximal force generation. 8. Lowest stimulus that results in force generation. 9. Single contractile event in response to single action potential. 10. Phase of contraction; period during which more crossbridges detach than reattach to thin filaments. 11. Type of wave summation with no observable relaxation between twitches. D. Recording Force Generated by Muscle Contractions Match the term to the definition. a. amplifier e. sensors b. myograph f. stimulator c. polygraph g. transducer d. recorder 1. Examples are: ph, pressure, temperature. 2. Increases the amplitude to the transducer signal and transmits it to a recorder channel. 3. Makes skeletal muscle fibers contract. 4. A machine that amplifies and records physiological changes detected by a variety of sensors. 5. Uses the electric signal from transducer to make a tracing on moving paper. 6. A machine that records force generated by muscle contractions. 7. Sensor that converts force generated by a contracting muscle into an electric signal.
0907T_c13_205-218.qxd 1/11/05 10:19 Page 217 Name Date Section Using Your Knowledge 13 EXERCISE Answer the following questions with a short answer. 1. Explain why muscles are stiff (contracted) when rigor mortis occurs. 2. Large muscles, such as the muscles of the leg, have more muscle fibers than small muscles, such as the muscles of the finger. Explain why the muscles of the finger cannot develop as much force as the muscles of the leg. 3. Which type of muscle would fatigue faster, one that has many blood vessels, or one that has fewer blood vessels? Explain. Using your textbook or another reference, for each contraction below, indicate whether it is an example of a muscle twitch (mt), unfused tetanus (ut), or fused tetanus (ft). 4. muscle spasm 5. facial muscle tic 6. cardiac fibrillation 7. smiling The percentage of each muscle fiber type in any given muscle of your body is determined genetically. Also, physical activity can cause slight changes in muscle fiber type. Discuss the relative amount of each muscle type in the thigh muscles of Olympic athletes participating in the following events: 8. marathon 9. weight lifting 10. 100 meter dash 217