Human Physiology
15th Edition
ISBN: 9781259864629
Author: Fox, Stuart Ira
Publisher: Mcgraw-hill Education,
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Chapter 12, Problem 21RA
Explain why the myosin heads don't bind to the actin when the muscle is at rest. Then, provide a step-by-step explanation of how depolarization of the muscle fiber plasma membrane by ACh leads to the binding of the myosin heads to actin. (That is, explain excitation-contraction coupling.)
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Describe the contraction of a muscle fiber, starting with what events take place at the neuromuscular junction, explaining how depolarization along the length of the sarcolemma occurs, the role of calcium (where it is released, what it binds to, etc.), and the events that occur after calcium binds to troponin. Draw a few simple, labelled diagrams of a neuromuscular junction and sarcomere and outline the steps of excitation-contraction coupling, including power stroke, recovery stroke, formation of cross bridges, role of ATP, etc.
As mentioned in class, one additional major use of ATP in skeletal muscle (besides
powering the myosin heads) is the recycling of calcium ions back into the sarcoplasmic
reticulum after depolarization. The resting concentration of Ca++ in the muscle cell
cytoplasm is about 50-100 nM, and the spike concentration after depolarization is about
10-20 μΜ.
a) Consider a single sarcomere. What is the number of free calcium ions within the
sarcomere at rest? What is the number of free calcium ions after depolarization?
b) The major ion pump responsible for calcium ion recycling is SERCA (sarco/endoplasmic
reticulum calcium ATPase). SERCA uses one molecule of ATP to pump two calcium ions,
and the resting level can be restored in about 10-20 ms. How many molecules of ATP are
used in a single sarcomere for pumping calcium in a single "twitch"?
c) Assume that a single "twitch" is sufficient to drive one sarcomere from its fully extended
length (about 2.5 µm) to its fully contracted length (about 1…
Briefly describe why contraction of the muscle cell is called “sliding filament theory” (what is sliding?) and what do we mean by “excitation-contraction coupling?” (What is being excited, and what is coupled?)
Chapter 12 Solutions
Human Physiology
Ch. 12 - Describe the actions of muscles when they...Ch. 12 - Describe the different levels of muscle structure,...Ch. 12 - Define the terms motor unit and innervation ratio...Ch. 12 - Using the concept of recruitment, explain how...Ch. 12 - With reference to the sliding filament theory,...Ch. 12 - Prob. 3bCPCh. 12 - Describe a cycle of cross-bridge activity during...Ch. 12 - Prob. 4bCPCh. 12 - Use a flowchart to show the sequence of events...Ch. 12 - Prob. 5bCP
Ch. 12 - Explain how graded contractions and smooth,...Ch. 12 - Prob. 6bCPCh. 12 - Prob. 7aCPCh. 12 - Describe the relationship between the resting...Ch. 12 - Prob. 8CPCh. 12 - Prob. 9CPCh. 12 - Explain the different causes of muscle fatigue...Ch. 12 - Prob. 11CPCh. 12 - Prob. 12aCPCh. 12 - Describe all of the events that occur from the...Ch. 12 - Prob. 13CPCh. 12 - Explain the significance of reciprocal innervation...Ch. 12 - Describe the functions of gamma motor neurons and...Ch. 12 - Explain how cardiac muscle differs from skeletal...Ch. 12 - Contrast the structure of a smooth muscle cell...Ch. 12 - Distinguish between single-unit and multiunit...Ch. 12 - Describe the events by which depolarization of a...Ch. 12 - Prob. 1RACh. 12 - Prob. 2RACh. 12 - Prob. 3RACh. 12 - Prob. 4RACh. 12 - Prob. 5RACh. 12 - Prob. 6RACh. 12 - Prob. 7RACh. 12 - Electrical excitation of a muscle fiber most...Ch. 12 - The energy for muscle contraction is most directly...Ch. 12 - Prob. 10RACh. 12 - Prob. 11RACh. 12 - Prob. 12RACh. 12 - Which of these types of muscle cells are not...Ch. 12 - Prob. 14RACh. 12 - In an isotonic muscle contraction,...Ch. 12 - Prob. 16RACh. 12 - Prob. 17RACh. 12 - Prob. 18RACh. 12 - Using the concept of motor units, explain how...Ch. 12 - Prob. 20RACh. 12 - Explain why the myosin heads don't bind to the...Ch. 12 - Using the sliding filament theory of contraction,...Ch. 12 - Prob. 23RACh. 12 - Prob. 24RACh. 12 - Prob. 25RACh. 12 - What changes occur in muscle metabolism as the...Ch. 12 - Prob. 27RACh. 12 - Compare cardiac muscle, single-unit smooth muscle,...Ch. 12 - Your friend eats huge helpings or pasta for two...Ch. 12 - Prob. 30RACh. 12 - Prob. 31RACh. 12 - How is electrical excitation of a skeletal muscle...Ch. 12 - Prob. 33RACh. 12 - Prob. 34RACh. 12 - Prob. 35RACh. 12 - Prob. 36RACh. 12 - Refer to figure 12.22 to answer the following...Ch. 12 - Refer to figure 12.22 to answer the following...Ch. 12 - Prob. 39RACh. 12 - Refer to figure 12.22 to answer the following...
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- What is the function of the sarcoplasmic reticulum in muscle cell contraction?arrow_forwardIn the diagram below, label the fine structure of a muscle, down to one of its myofibrils. Identify the basic unit of contraction in a myofibril.arrow_forwardExplain why (a) calcium ions and (b) ACh are vital for muscle contraction.arrow_forward
- For the following five events, which is the correct sequence that describes the excitation and contraction of a skeletal muscle fiber? 1. Tropomyosin shifts, exposing the myosin-binding sites on actin. 2. Calcium is released from the sarcoplasmic reticulum and binds to the troponin complex. 3. An action potential is propagated down the transverse tubules. 4. Cycles of myosin cross-bridge formation and breakdown cause the thin filaments to slide toward the center of the sarcomere. 5. An action potential in a motor neuron causes the axon to release acetylcholine, which triggers an action potential in a muscle fiber. 1 → 2 → 3 → 4 → 5 3 → 2 → 4 → 1 → 5 2 → 1 → 3 → 5 → 4 5 → 3 → 2 → 1 → 4 4 → 3 → 1 → 2 → 5arrow_forwardSkeletal muscle cells undergo contractions based on a molecular mechanism involving: 1)the sliding of actin and myosin filaments past each other without any change in filament length 2) the shortening of actin filaments following direct interaction with myosin 3) dynamic instability of microtubules 4)nonearrow_forwardGive an account of the excitation-contraction coupling (Figure 3) in skeletal muscle fibers. Highlight the role of dihydropyridine receptors and ryanodine receptors and the fate of A detailed description of the cross bridge cycle is not calcium ions after contraction. necessary. -Axon teminal of (=) somatic motor neuron ACh Muscle fiber potential Pion ++ Action potential- Na Motor end plate- T-tubule Sarcoptasmic reticulum lellll Ca2+ DHP receptor Tropomyosin Troponin z disk Actin M line Myosin head Myosin thick filamentarrow_forward
- Explain how optimal skeletal muscle force (high muscle tension) can be achieved during a high intensity exercise. (Mention and explain how wave summation, motor recruitment summation, muscle length, and type of muscle fibers activated can affect muscle force).arrow_forwardOne of the primary characteristics of all muscle tissue is contractility, the ability to shorten (contract) and lengthen (relax). You've identified the various components of a myofibril above. Now, use the table below to indicate what occurs to each band, line, or zone when the muscle contracts vs. relaxes. What occurs to this region / structure during contraction/relaxation? Region / Structure A-Band H-Band I-Band M-Line Z-Line Zone of Overlap Sarcomere No change Shortens Lengthens Muscle Contraction Muscle Relaxationarrow_forwardPut these events that occur in skeletal muscle in the correct chronological sequence: 1. Opening of mechanically-gated ryanodine receptors (RyR) on the sarcoplasmic reticulum 2. Ca²+ binding to troponin to initiate contraction 3. Activation of voltage-gated dihydropyridine (DHP) receptor in the T-tubule 4. Na+ influx through ligand-gated ion channels on the motor end plate 4, 3, 2, 1 2, 1, 4, 3 3, 1, 2, 4 4, 3, 1, 2 3, 1, 4, 2arrow_forward
- Hill's equation gives a relation between muscle contraction rates νν and muscle tension TT (T+α)(ν+β)=(T0+α)β(T+α)(ν+β)=(T0+α)β for positive parameters αα and ββ and resting tension T0T0. Rewrite the equation so that the contraction rate is a function of tension.arrow_forwardMuscle motor unit recruitment follows the pattern of the SO (I) fibers being recruited first followed by the FOG (IIa) and then ultimately by the FG (IIb) if and when workloads require this. Muscle fatigue follows the order that the FG fibers fatigue first, followed by the FOG fibers and finally (maybe never) the SO fibers. Explain how it makes sense physiologically that the last fibers to be recruited to contract are the first to fatigue, while the first fibers to be recruited to contract would be the last to fatigue.arrow_forwardSkeletal muscle is described as striated. What are the lighter, thinner stripes noted under the microscope? Group of answer choices troponin actin myosin dystrophin Rather than innervating individual muscle fibers, motor neurons often innervate two or more muscle fibers. The motor neuron and the muscle fibers it innervates are together called a: Group of answer choices motor unit myofibril muscle fascicle motor fascia Fast glycolytic muscle fibers allow for relatively quick ATP energy production by splitting glucose (but eventually leads to fatigue), a process described as: Group of answer choices fermentation aerobic pathway anaerobic pathway ATP reserve usearrow_forward
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