A muscle fiber relaxes when:a. The concentration of Ca2+ in the cytosol returns to resting levels.b. The supply of ATP is exhausted.c. Ca2+ flood the cytosol.d. Acetylcholine is released from the axon terminal and the sarcolemma depolarizes.

Ch. 10 Muscle Tissue and Physiology

Amerman - Human Anatomy & Physiology 2nd Edition

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Amerman 2nd Edition

Ch. 10 Muscle Tissue and Physiology

Problem 10

Chapter 10, Problem 10

A muscle fiber relaxes when:
a. The concentration of Ca²⁺ in the cytosol returns to resting levels.
b. The supply of ATP is exhausted.
c. Ca²⁺ flood the cytosol.
d. Acetylcholine is released from the axon terminal and the sarcolemma depolarizes.

Verified step by step guidance

Understand the process of muscle contraction and relaxation: Muscle contraction is initiated when calcium ions (Ca²⁺) are released into the cytosol, and relaxation occurs when these ions are removed from the cytosol and returned to the sarcoplasmic reticulum.

Recall the role of calcium ions (Ca²⁺): Calcium ions bind to troponin, causing a conformational change that allows actin and myosin to interact, leading to contraction. For relaxation, calcium ions must be removed from the cytosol.

Consider the role of ATP: ATP is required for both muscle contraction and relaxation. During relaxation, ATP powers the calcium pumps (Ca²⁺-ATPase) that actively transport calcium ions back into the sarcoplasmic reticulum.

Analyze the options provided: Option (a) aligns with the physiological process of muscle relaxation, as it involves the return of Ca²⁺ to resting levels. Option (b) is incorrect because ATP depletion would prevent both contraction and relaxation. Option (c) describes a condition that promotes contraction, not relaxation. Option (d) refers to the initiation of contraction, not relaxation.

Conclude that the correct answer is the option that describes the return of Ca²⁺ to resting levels, as this is the key event that allows the muscle fiber to relax.

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Key Concepts

Here are the essential concepts you must grasp in order to answer the question correctly.

Calcium Ion Regulation

Calcium ions (Ca2+) play a crucial role in muscle contraction and relaxation. When a muscle fiber is stimulated, Ca2+ is released from the sarcoplasmic reticulum into the cytosol, initiating contraction. For relaxation to occur, the concentration of Ca2+ must return to resting levels, which is achieved by reuptake into the sarcoplasmic reticulum.

ATP and Muscle Function

Adenosine triphosphate (ATP) is essential for muscle contraction and relaxation. It provides the energy required for the myosin heads to detach from actin filaments after a contraction. If ATP supply is exhausted, muscle fibers cannot relax properly, leading to a state of rigor or sustained contraction, known as rigor mortis in post-mortem muscles.

Neuromuscular Transmission

Neuromuscular transmission involves the release of neurotransmitters, such as acetylcholine, from the axon terminal of a motor neuron. This neurotransmitter binds to receptors on the sarcolemma, causing depolarization and initiating an action potential that leads to muscle contraction. The cessation of acetylcholine release is critical for muscle relaxation, as it stops the stimulation of the muscle fiber.

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Related Practice

Textbook Question

Mark the following statements as true or false. If a statement is false, correct it to make a true statement.

Muscle fibers generate more tension if the starting length of their sarcomeres is very short.

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Textbook Question

Which of the following statements accurately describes the role of ATP in a muscle contraction?

a. ATP is directly responsible for the power stroke.

b. ATP moves troponin and tropomyosin away from actin.

c. ATP breaks the actin/myosin attachment and 'cocks' the myosin head.

d. ATP causes the myofilaments to shorten.

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Textbook Question

Which of the following energy sources would provide the majority of the ATP for a person running a 26-mile marathon?

a. Stored ATP

b. Glycolytic catabolism

c. Oxidative catabolism

d. Creatine phosphate

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Textbook Question

Order the following events of excitation and excitation-contraction coupling. Put 1 by the first event, 2 by the second, and so on.

____ The motor end plate generates an end-plate potential.

____ The action potential spreads along the T-tubules, SR Ca²⁺ channels are pulled open, and Ca²⁺ floods the cytosol.

____ Acetylcholine binds to receptors on the motor end plate, and ligand-gated ion channels open.

____ Ca²⁺ bind troponin, which allows tropomyosin to move away from the actin active site, initiating a contraction cycle.

____ The action potential propagates through the sarcolemma and dives deeply into the cell along the T-tubules.

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Textbook Question

Mark the following statements as true or false. If a statement is false, correct it to make a true statement.

Stimulation by a motor neuron before a muscle fiber has fully relaxed results in a condition called wave summation.

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Textbook Question

What is the basic mechanism of contraction at the level of myofilaments?

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