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 discsb. Sarcomeresc. I bandsd. A bandse. H zones

Accepted Answer

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.

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 video answer for a similar problem:

Key Concepts

Sarcomere Structure

Muscle Contraction and Sliding Filament Theory

Identification of Sarcomere Bands and Zones