Multiple Choice
Which region is the same length as the actin filament? An image is shown for reference.
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9. Muscle Tissue
Sliding Filament Theory and the Sarcomere
1:46 minutesProblem 1
Textbook Question
Compare these images of a relaxed skeletal muscle fiber and a fully contracted muscle fiber. Which would you see only in the relaxed fiber?
a. Z discs
b. Sarcomeres
c. I bands
d. A bands
e. H zones
Verified step by step guidance1
Step 1: Identify the structural components in the relaxed sarcomere image. Notice the Z discs (dark vertical lines), I bands (light regions with thin filaments only), A bands (dark regions with thick filaments), and the H zone (lighter central region within the A band where there are only thick filaments).
Step 2: Compare these components to the fully contracted sarcomere image. Observe that the Z discs move closer together, the I bands and H zones become narrower or disappear, while the A bands remain the same length.
Step 3: Understand that the H zone is the region in the center of the A band where only thick filaments are present and no thin filaments overlap. During contraction, thin filaments slide over thick filaments, reducing or eliminating the H zone.
Step 4: Recognize that the I bands, which contain only thin filaments, also shorten during contraction as the thin filaments slide inward.
Step 5: Conclude that the H zone is visible only in the relaxed sarcomere and disappears in the fully contracted sarcomere, making option (e) H zones the correct answer.
Verified video answer for a similar problem:This video solution was recommended by our tutors as helpful for the problem above
Video duration:
1mKey Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Sarcomere Structure
The sarcomere is the basic contractile unit of a skeletal muscle fiber, defined by the area between two Z discs. It contains thick filaments (myosin) and thin filaments (actin), organized into distinct bands and zones that change during muscle contraction.
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Proteins of the Sarcomere
Muscle Contraction and Sliding Filament Theory
Muscle contraction occurs when thin actin filaments slide over thick myosin filaments, shortening the sarcomere. This sliding reduces the width of the I bands and H zones, while the A band remains constant, reflecting the overlap of filaments.
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6:26Sliding Filament Theory
Identification of Sarcomere Bands and Zones
The Z discs mark sarcomere boundaries; I bands contain only thin filaments; A bands contain thick filaments and overlapping thin filaments; H zones are central regions of thick filaments without overlap. The H zone disappears during full contraction.
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6:01Structure of the Sarcomere: Bands, Zones, Discs & Lines
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