Muscle Contraction Mechanisms
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Questions and Answers

What indicates that the I band has shortened during muscle contraction?

  • Absence of the A band
  • Lack of Z lines
  • Reduction in length of the I band (correct)
  • Presence of the H zone

Which of the following statements is true regarding the A band during muscle contraction?

  • It features a greater proportion of thin filaments
  • It shrinks to allow for filament sliding
  • It disappears entirely in a fully contracted muscle
  • It remains the same width (correct)

What visible feature would not be present in a contracted myofibril?

  • Visibility of the I band (correct)
  • Reduction of the H zone
  • Expansion of the A band
  • Presence of the Z line

How does the width of the H zone change during muscle contraction?

<p>It diminishes in width (B)</p> Signup and view all the answers

Which of the following adjustments occur in the sarcomere during muscle contraction?

<p>The I band decreases in size (A)</p> Signup and view all the answers

What role does ATP play in the sliding filament process?

<p>It causes myosin heads to detach from actin. (C)</p> Signup and view all the answers

During the power stroke phase, what change occurs in myosin heads?

<p>They change shape to pull actin filaments towards the center. (D)</p> Signup and view all the answers

What is the primary role of calcium ions during muscle contraction?

<p>To expose myosin-binding sites on actin filaments (D)</p> Signup and view all the answers

What prevents the binding of myosin heads to actin during muscle relaxation?

<p>Antagonistic muscle action. (D)</p> Signup and view all the answers

Which of the following correctly describes the sequence of events that occurs after calcium is released into the muscle cell?

<p>The troponin-tropomyosin complex shifts, exposing actin binding sites. (B)</p> Signup and view all the answers

What event directly follows the attachment of myosin heads to the actin filaments?

<p>Power stroke movement occurs (A)</p> Signup and view all the answers

What is the primary function of cross-bridges formed between myosin and actin?

<p>They allow for the sliding of filaments past each other. (B)</p> Signup and view all the answers

What occurs during the initial state of muscle contraction according to the sliding filament process?

<p>Myosin heads form connections with actin filaments. (D)</p> Signup and view all the answers

What is the consequence of ATP binding to myosin during muscle contraction?

<p>It prevents myosin from binding to actin. (C)</p> Signup and view all the answers

Which component is responsible for blocking the myosin-binding sites on actin when calcium is absent?

<p>Tropomyosin (C)</p> Signup and view all the answers

What primarily determines the length of the A-band in a sarcomere?

<p>The length of myosin filaments (A)</p> Signup and view all the answers

Which region of the sarcomere contains only myosin filaments?

<p>H-zone (B)</p> Signup and view all the answers

Which of the following statements about the Z-line is true?

<p>It defines the limits of the sarcomere. (A)</p> Signup and view all the answers

What is the role of the M-line in a sarcomere?

<p>It is found at the center of myosin filaments. (D)</p> Signup and view all the answers

What is the characteristic of the I-band in a sarcomere?

<p>It is the lightest colored band and contains only actin filaments. (C)</p> Signup and view all the answers

What occurs to the I band during muscle contraction?

<p>It gets shorter (B)</p> Signup and view all the answers

Which of the following statements about the A band is true during muscle contraction?

<p>The A band stays the same length (A)</p> Signup and view all the answers

In the sliding filament hypothesis, what happens to the sarcomere length during contraction?

<p>It decreases (C)</p> Signup and view all the answers

What is the primary function of sarcomeres in muscle fibers?

<p>They facilitate the sliding of thin and thick filaments during contraction. (B)</p> Signup and view all the answers

Which region experiences no change during muscle contraction?

<p>A band (D)</p> Signup and view all the answers

Which statement accurately describes myofibrils?

<p>They are structures that contain many repeating sarcomeres. (C)</p> Signup and view all the answers

What characteristic structure is formed by the arrangement of actin and myosin in sarcomeres?

<p>A distinct banding pattern visible under a microscope. (D)</p> Signup and view all the answers

What is the primary mechanism that shortens the muscle during contraction according to the sliding filament hypothesis?

<p>Actin filaments slide over myosin filaments (D)</p> Signup and view all the answers

How are muscle fibers structured in relation to connective tissue?

<p>Muscle fibers are grouped into bundles surrounded by connective tissue. (D)</p> Signup and view all the answers

What is the composition of muscle fibers in terms of nuclei?

<p>Muscle fibers exhibit multiple nuclei positioned under the membrane. (D)</p> Signup and view all the answers

What is the function of calcium ions in myofibril contraction?

<p>They bind to troponin, allowing myosin heads to attach to actin. (C)</p> Signup and view all the answers

Which statement best describes the role of ATP in the contraction process of a myofibril?

<p>Energy from ATP is used to change the shape of myosin heads during contraction. (D)</p> Signup and view all the answers

Which structure is primarily responsible for exposing binding sites on actin for myosin heads?

<p>Tropomyosin after binding with calcium ions. (D)</p> Signup and view all the answers

What happens during the power stroke in muscle contraction?

<p>Actin is pulled toward the center of the sarcomere by the myosin heads. (A)</p> Signup and view all the answers

What initiates the cross-bridge cycle in muscle contraction?

<p>Calcium binding to troponin, leading to myosin head attachment. (C)</p> Signup and view all the answers

Flashcards

Sliding Filament Hypothesis

During muscle contraction, actin filaments slide over myosin filaments, shortening the sarcomere.

H zone

The region of the sarcomere where only myosin filaments are present.

I band

The region of the sarcomere containing only actin filaments.

A band

The region of the sarcomere containing both actin and myosin filaments.

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Sarcomere

The basic functional unit of a muscle fiber, responsible for muscle contraction.

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Muscle Bundle

Groups of muscle fibers bundled together with connective tissue, blood vessels, and nerves.

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Muscle Fiber

Long, multinucleated cells formed by fused cells. They contain sarcoplasm and exhibit a striated pattern.

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Myofibril

Rod-like structures within muscle fibers, composed of repeating sarcomeres. They are the primary units responsible for muscle contraction.

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Actin and Myosin Filaments

Protein filaments that interact to cause muscle contraction. Actin is thin, myosin is thick.

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M-line

A thin, dark line at the centre of the sarcomere, found within the H-zone where myosin filaments are connected.

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Initial State

The state of a muscle before contraction, where actin and myosin filaments are separate.

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Muscle Contraction Begins

Myosin heads connect to actin filaments, initiating the muscle contraction process.

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Power Stroke

Myosin heads change shape, pulling actin filaments towards the center of the sarcomere, using energy.

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Detachment and Return to Original Position

ATP causes myosin heads to release actin, returning to their original position.

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Muscle Relaxation

The muscle relaxes, stopping myosin from binding to actin, and allowing the filaments to separate.

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What is the A band?

The region of a sarcomere that contains only thick filaments (myosin).

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What is the I band?

The region of a sarcomere that contains only thin filaments (actin).

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What is the H zone?

The region of a sarcomere that contains only thick filaments (myosin) and no thin filaments (actin).

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What is the sarcomere?

The region of a sarcomere that encompasses both the thick and thin filaments.

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What is the function of a sarcomere?

The distance between two Z lines, where the thick filaments attach, to form a functional unit.

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Depolarization in Muscle Contraction

The process where a nerve impulse triggers the release of calcium ions from the sarcoplasmic reticulum into the muscle cell.

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What does Calcium do in Muscle Contraction?

Calcium ions bind to the troponin-tropomyosin complex, causing it to shift and expose the binding sites on actin filaments.

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What happens when Myosin binds to Actin?

The myosin heads attach to the exposed binding sites on actin filaments, pulling the actin filaments towards the center of the sarcomere.

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What is ATP's role in Muscle Contraction?

ATP provides the energy for myosin to detach from actin, return to its original position, and prepare for another power stroke. It also fuels the whole muscle contraction process.

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What is the role of Tropomyosin in muscle contraction?

A structural protein that, in its relaxed state, blocks the binding sites on actin filaments, preventing myosin from attaching.

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Actin

A protein that makes up the thin filaments in a sarcomere, responsible for sliding past thick filaments during muscle contraction.

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Myosin Heads

The globular protein heads of the myosin filaments that attach to actin filaments and pull them inwards during muscle contraction.

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Calcium Ions' Role in Muscle Contraction

The process in muscle contraction where calcium ions bind to troponin, causing a conformational change in tropomyosin, and exposing binding sites on actin for myosin heads to attach.

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Myosin-Actin Cycle

The cycle of events involving the attachment of myosin heads to actin, the power stroke, and the detachment of myosin from actin, powered by ATP, leading to muscle contraction.

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