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 guidance

1. Step 1: Understand the structural components of a skeletal muscle fiber. Skeletal muscle fibers are composed of repeating units called sarcomeres, which are the functional units of contraction. Sarcomeres contain Z discs, A bands, I bands, and H zones.
2. Step 2: Recall the changes that occur during muscle contraction. When a muscle fiber contracts, the sarcomere shortens, and certain regions within the sarcomere change in appearance. Specifically, the I bands and H zones become narrower or disappear entirely, while the A bands remain unchanged.
3. Step 3: Compare the relaxed and contracted states of the muscle fiber. In a relaxed muscle fiber, the I bands and H zones are visible because the sarcomere is not shortened. In a fully contracted muscle fiber, these regions are no longer visible due to the overlap of actin and myosin filaments.
4. Step 4: Analyze the options provided in the question. Z discs and sarcomeres are present in both relaxed and contracted muscle fibers, while A bands remain constant regardless of contraction. I bands and H zones, however, are visible only in the relaxed state.
5. Step 5: Conclude that the correct choices for structures visible only in the relaxed muscle fiber are I bands and H zones, based on their disappearance during contraction.

Key Concepts

Skeletal Muscle Structure

Skeletal muscle fibers are composed of myofibrils, which are further divided into sarcomeres. Each sarcomere contains specific regions that are visible under a microscope, including Z discs, I bands, A bands, and H zones, which play crucial roles in muscle contraction and relaxation.

Sarcomeres

Sarcomeres are the basic contractile units of muscle fibers, defined by the area between two Z discs. They contain overlapping thick (myosin) and thin (actin) filaments, which slide past each other during contraction, altering the lengths of the I band and H zone.

I Bands and H Zones

The I band is the region of a sarcomere that contains only thin filaments and is visible in relaxed muscle fibers, while the H zone is the area within the A band where only thick filaments are present. During contraction, the I band shortens, and the H zone may disappear, making these regions key indicators of muscle fiber state.