Chapter 10 Muscle Function

23. What three processes are involved in repaying the oxygen debt during a muscle's recovery period?

(1) O2 for aerobic respiration is consumed by liver cells, which must make a great deal of ATP to convert lactate to pyruvate to glucose; (2) O2 for aerobic respiration is consumed by skeletal muscle fibers as they restore ATP, creatine phosphate, and glycogen concentrations to their former levels; and (3) the normal O2 con- centration in blood and peripheral tissues is replenished.

10. What three layers of connective tissue are part of each muscle? What functional role does each layer play?

(1) epimysium: surrounds entire muscle; (2) perimysium: surrounds muscle bundles (fascicles); and (3) endomysium: surrounds skel- etal muscle fibers

13. What five interlocking steps are involved in the contraction process?

(1) exposure of active sites; (2) attachment of cross-bridges; (3) pivoting of myosin heads (power stroke); (4) detachment of cross-bridges; and (5) activation of myosin heads (cocking)

1.Identify the structures in the following figure. a) b) c) d) e) f) g) h)

(a) sarcolemma (b) sarcoplasm (c) mitochondria (d) myofibril (e) thin filament (f) thick filament (g) sarcoplasmic reticulum (h) T tubules

9. List the three types of muscle tissue in the body.

1) skeletal muscle; (2) cardiac muscle; and (3) smooth muscle

Why does a muscle that has been over stretched produce less tension?

A muscle's ability to contract depends on the formation of cross-bridges between the myosin and actin myofila- ments in the muscle. In a muscle that is overstretched, the myofila- ments would overlap very little, so very few cross-bridges between myosin and actin could form, and thus the contraction would be weak. If the myofilaments did not overlap at all, then no cross- bridges would form and the muscle could not contract.

30. Bill broke his leg in a football game, and after six weeks the cast is fi- nally removed. As he steps down from the examination table, he loses his balance and falls. Why?

A skeletal muscle not regularly stimulated by a motor neuron will lose muscle tone and mass and become weak (it will atrophy). While his leg was immobilized, it did not receive sufficient stimulation to maintain proper muscle tone. It will take a while for Bill's muscles to build up enough to support his weight.

Why would a sprinter experience muscle fatigue before a marathon runner would?

A sprinter requires large amounts of energy for a short burst of activity. To supply this en- ergy, the sprinter's muscles rely on anaerobic metabolism. Anaero- bic metabolism is less efficient in producing energy than aerobic metabolism, and the process also produces acidic wastes; this com- bination contributes to muscle fatigue. Conversely, marathon run- ners derive most of their energy from aerobic metabolism, which is more efficient and produces fewer wastes than anaerobic metabo- lism does.

How would a drug that blocks acetylcholine release affect muscle contraction?

Acetylcholine (ACh) release is necessary for skeletal muscle contraction, because it serves as the first step in the process that en- ables the formation of cross-bridges in the sarcomeres. A muscle's ability to contract depends on the formation of cross-bridges be- tween the myosin heads and actin myofilaments. A drug that blocks ACh release would interfere with this cross-bridge formation and prevent muscle contraction.

Which activity would be more likely to create an oxygen debt: swimming laps or lifting weights?

Activities that require short periods of strenuous activity produce a greater oxygen debt, because such activities rely heavily on energy production by anaerobic metabolism. Because lifting weights is more strenuous over the short term than swim- ming laps, which is an aerobic activity, weight lifting would likely produce a greater oxygen debt than would swimming laps.

16. What two mechanisms are used to generate ATP from glucose in muscle cells?

Aerobic metabolism and glycolysis generate ATP from glucose in muscle cells.

28. Many potent insecticides contain toxins, called organophosphates, that interfere with the action of the enzyme acetylcholinesterase. Ivan is using an insecticide containing organophosphates and is very care- less. Because he does not use gloves or a dust mask, he absorbs some of the chemical through his skin and inhales a large amount as well. What signs would you expect to observe in Ivan as a result of organo- phosphate poisoning?

Because organophosphates block the action of acetylcholines- terase (AChE), ACh released into the synaptic cleft would not be re- moved. It would continue to stimulate the motor end plate, causing a state of persistent contraction (spastic paralysis). If the muscles of respiration were affected (which is likely), Ivan would die of suffo- cation. Prior to death, the most obvious sign would be uncontrolled tetanic contractions of skeletal muscles.

How would severing the tendon attached to a muscle affect the muscle's ability to move a body part?

Because tendons attach muscles to bones, severing the tendon would disconnect the muscle from the bone, and so the muscle could not move a body part.

14. What two factors affect the amount of tension produced when a skeletal muscle contracts?

Both the frequency of motor unit stimulation and the number of motor units involved affect the amount of tension produced when a skeletal muscle contracts.

29. Linda's father suffers an apparent heart attack and is rushed to the emergency room of the local hospital. The doctor on call tells her that he has ordered some blood work and that he will be able to tell if her father actually had an attack by looking at the blood levels of CK and cardiac troponin. Why would knowing the level of CK and cardiac tro- ponin help to indicate if a person suffered a heart attack?

CK is the enzyme creatine kinase. It functions in the anaerobic reaction that transfers phosphate from creatine phosphate to ADP in muscle cells. The presence of cardiac troponin, a form found only in cardiac muscle cells, provides direct evidence that cardiac muscle cells have been se- verely damaged.

17. What is the calcium-binding protein in smooth muscle tissue?

Calmodulin is the calcium-binding protein in smooth muscle tissue.

What feature of cardiac muscle tissue allows the heart to act as a functional syncytium(single unit)?

Cardiac muscle cells are joined by gap junctions, which allow ions and small molecules to flow directly between cells. As a result, action potentials generated in one cell spread rapidly to adjacent cells. Thus, all the cells contract simultaneously, as if they were a single unit (a syncytium).

Compare and contrast skeletal muscle tissue and cardiac muscle tissue.

Compared to skeletal muscle, cardiac muscle (1) has relatively small cells; (2) has cells with a centrally located nucleus (some may contain two or more nuclei); (3) has T tubules that are short and broad and do not form triads; (4) has an SR that lacks terminal cisternae and has tubules that contact the plasma membrane as well as the T tubules; (5) has cells that are nearly totally dependent on aerobic metabolism as an energy source; and (6) contains intercalated discs that assist in stabilizing tissue structure and spreading action potentials.

7. Which of the following statements about myofibrils is not correct? (a) Each skeletal muscle fiber contains hundreds to thousands of myofibrils. (b) Myofibrils contain repeating units called sarcomeres. (c) Myofibrils extend the length of a skeletal muscle fiber. (d) Filaments consist of bundles of myofibrils. (e) Myofibrils are attached to the plasma membrane at both ends of a muscle fiber.

Filaments consist of bundles of myofibrils

25. Atracurium is a drug that blocks the binding of ACh to ACh receptors. Give an example of a site where such binding normally occurs, and predict the physiological effect of this drug.

If atracurium blocked the binding of ACh to ACh receptors at the motor end plates of neuromuscular junctions, the muscle's ability to contract would be inhibited.

What would happen to a resting skeletal muscle if the sarcolemma suddenly became very permeable to calcium ions?

If the sarcolemma of a resting skeletal muscle suddenly became permeable to Ca2+, the cytosolic concentration of Ca2+ would increase, and the muscle would contract. In addition, because the amount of calcium ions in the cytosol must decrease for relaxation to occur, the increased permeability of the sarcolemma to Ca2+ might prevent the muscle from relaxing completely.

22. For each portion of a myogram tracing a twitch in a stimulated calf muscle fiber, describe the events that occur within the muscle.

In an initial latent period (after the stimulus arrives and before tension begins to increase), an ac- tion potential generated in the muscle triggers the release of cal- cium ions from the SR. In the contraction phase, calcium ions bind to troponin (cross-bridges form) and tension begins to increase. In the relaxation phase, tension decreases because cross-bridges have detached and because calcium ion levels have decreased; the active sites are once again covered by the troponin-tropomyosin com- plex.

26. Explain why a murder victim's time of death can be estimated accord- ing to the flexibility or rigidity of the body.

In rigor mortis, the membranes of the dead cells are no longer selectively permeable; the SR is no longer able to retain calcium ions. As calcium ions enter the cytosol, a sustained contraction develops, making the body extremely stiff. Contraction persists because the dead muscle cells can no longer make the ATP required for cross-bridge detachment from the active sites. Rigor mortis begins a few hours after death and ends after one to six days, or when decomposition begins.

Which type of muscle fibers would you expect to predominate in the leg muscles of someone who excels at endurance activities, such as cycling or long-distance running?

In- dividuals who excel at endurance activities have a higher than nor- mal percentage of slow fibers. Slow fibers are physiologically better adapted to this type of activity than are fast fibers, which are less vascular and fatigue faster.

How do muscle cells continuously synthesize ATP?

Muscle cells synthesize ATP continuously by utiliz- ing creatine phosphate (CP) and metabolizing glycogen and fatty acids. Most cells generate ATP only through aerobic metabolism in the mitochondria and through glycolysis in the cytoplasm.

What is muscle fatigue?

Muscle fatigue is a muscle's reduced ability to contract due to low pH (lactic acid buildup and its dissociation to lactate and a hy- drogen ion), low ATP levels, or other problems.

Define oxygen debt.

Oxygen debt is the amount of oxygen required to restore normal, pre-exertion conditions in muscle tissue.

15. What forms of energy reserves do resting skeletal muscle fibers contain?

Resting skeletal muscle fibers contain ATP, creatine phosphate, and glycogen.

Describe the structural components of a sarcomere.

Sarcomeres, the smallest contractile units of a striated muscle cell, are segments of myofibrils. Each sarcomere has dark A bands and light I bands. The A band contains the M line, the H band, and the zone of overlap. Each I band contains thin filaments, but not thick filaments. Z lines bisect the I bands and mark the boundaries between adjacent sar- comeres.

Why do skeletal muscle fibers appear striated when viewed through a light microscope?

Skeletal muscle appears striated when viewed through a light microscope because the Z lines and thick filaments of the myofibrils within the muscle fibers are aligned.

Which type of muscle tissue is least affected by changes in extracellular Ca2 concentration during contraction?

Skeletal muscle contractions are least affected by changes in extracellular Ca2+ concentrations. In skeletal muscle, most of the calcium ions come from the sarcoplasmic reticulum. Most of the calcium ions that trigger a contraction in cardiac and smooth mus- cles come from the extracellular fluid.

Identify the six major functions of skeletal muscle.

Skeletal muscles 1.produce skeletal movement, 2.maintain posture and body position, 3.support soft tissues, 4.guard body entrances and exits, 5.maintain body temperature, and 6.store nutrient reserves.

Identify the structural characteristics of smooth muscle tissue.

Smooth muscle cells lack sarcomeres, and thus smooth muscle tissue is nonstriated. Additionally, the thin filaments are anchored to dense bodies.

Describe the three connective tissue layers associated with skeletal muscle tissue.

The epimysium is a dense layer of collagen fibers that surrounds the entire muscle; the perimysium divides the skeletal muscle into a series of compart- ments, each containing a bundle of muscle fibers called a fascicle; and the endomysium surrounds individual skeletal muscle cells (muscle fibers). The collagen fibers of the epimysium, peri- mysium, and endomysium come together to form either bundles known as tendons, or broad sheets called aponeuroses. Tendons and aponeuroses generally attach skeletal muscles to bones.

Why can smooth muscle contract over a wider range of resting lengths than skeletal muscle can?

The looser organization of actin and myosin filaments in smooth muscle allows smooth muscle to contract over a wider range of resting lengths.

Describe the neuromuscular junction.

The neuromuscular junction, also known as the myoneural junction, is the synapse between a motor neuron and a skeletal muscle cell (fiber). This connection enables com- munication between the nervous system and a skeletal muscle fi- ber.

Identify the three types of muscle tissue.

The three types of muscle tissue are skeletal muscle, cardiac muscle, and smooth muscle.

Identify the three types of skeletal muscle fibers.

The three types of skeletal muscle fibers are (1) fast fibers (2) slow fibers and (3) intermediate fibers.

24. How does cardiac muscle tissue contract without neural stimulation?

The timing of cardiac muscle contractions is determined by specialized cardiac muscle fibers called pacemaker cells; this property of cardiac muscle tissue is termed automaticity.

12. What structural feature of a skeletal muscle fiber propagates action potentials into the interior of the cell?

The transverse (T) tubules propagate action potentials into the interior of the cell.

Predict what would happen to a muscle if the motor end plate failed to produce acetylcholinesterase (AChE).

Without acetylcholinesterase (AChE), the motor end plate would be continuously stimulated by acetylcholine, locking the muscle in a state of contraction.

Can a skeletal muscle contract without shortening?

Yes, a skeletal muscle can contract without shortening. The muscle can shorten (isotonic, concentric contraction), elongate (isotonic, ec- centric contraction), or remain the same length (isometric contrac- tion), depending on the relationship between the load (resistance) and the tension produced by actin-myosin interactions.

Where would you expect to find the greatest concentration of Ca2+ in resting skeletal muscle?

You would expect the greatest concentration of calcium ions in resting skeletal muscle to be in the terminal cisternae of the sarcoplasmic reticu- lum.

20. During relaxation, muscles return to their original length because of all of the following except (a) actin and myosin actively pushing away from one another, (b) the contraction of opposing muscles, (c) the pull of gravity, (d) the elastic nature of the sarcolemma, (e) elastic forces.

actin and myosin actively pushing away from one another

18. An activity that would require anaerobic endurance is (a) a 50-meter dash, (b) a pole vault, (c) a weight-lifting competition, (d) all of these.

all of these

6. The type of contraction in which the tension rises, but the load does not move, is (a) a wave summation, (b) a twitch, (c) an isotonic contraction, (d) an isometric contraction.

an isometric contraction

11. The _____ contains vesicles filled with acetylcholine. (a) axon termi- nal, (b) motor end plate, (c) neuromuscular junction, (d) synaptic cleft, (e) transverse tubule.

axon terminal

8. An action potential can travel quickly from one cardiac muscle cell to another because of the presence of (a) gap junctions, (b) tight junctions, (c) intercalated discs, (d) both a and c.

both a and c

21. According to the length-tension relationship, (a) longer muscles can generate more tension than shorter muscles, (b) the greater the zone of overlap in the sarcomere, the greater the tension the muscle can develop, (c) the greatest tension is achieved in sarcomeres where actin and myosin initially do not overlap, (d) there is an optimum range of actin and myosin overlap that will produce the greatest amount of tension, (e) both b and d are correct.

both b and d are correct

2.The connective tissue coverings of a skeletal muscle, listed from superficial to deep, are (a) endomysium, perimysium, and epimysium, (b) endomysium, epimysium, and perimysium, (c) epimysium, endomysium, and perimysium, (d) epimysium, perimysium, and endomysium.

epimysium, perimysium, endomysium

19. Areas of the body where you would not expect to find slow fibers include the (a) back and calf muscles, (b) eye and hand, (c) chest and abdomen, (d) a, b, and c.

eye and hand

5. A muscle producing near-peak tension during rapid cycles of contraction and relaxation is said to be in (a) incomplete tetanus, (b) treppe, (c) complete tetanus, (d) a twitch.

incomplete tetanus

27. Which of the following activities would employ isometric contrac- tions? (a) flexing the elbow, (b) chewing food, (c) maintaining an upright posture, (d) running, (e) writing.

maintaining upright posture

4. The detachment of the myosin cross-bridges is directly triggered by (a) the repolarization of T tubules, (b) the attachment of ATP to myosin heads, (c) the hydrolysis of ATP, (d) calcium ions.

the hydrolysis of ATP

3. The signal to contract is distributed deep into a muscle fiber by the (a) sarcolemma, (b) sarcomere, (c) transverse tubules, (d) myotubules, (e) myofibrils.

transverse tubules