Chapter 6, Problem 1MC

If you compare electron micrographs 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

Summary Introduction

Introduction:

The skeletal muscle fibers are made of striated muscle tissues, which consists of alternating light (I) and dark bands (A). I-band consists of a dark region known as Z disc. On the other hand, A band consists of a light region known as H-zone. I-band consists only of thin actin filaments, while the A-band consists of both actin and myosin filaments. All these units of a muscle fiber makes sarcomere. The micrograph of a sarcomere is shown as below:

Answer to Problem 1MC

Correct answer:

I bands and the H zones of the skeletal muscle fiber will be visible only in the micrograph of the relaxed fiber.

Explanation of Solution

Explanation for the correct answer:

Option (c) and option (e) are given as I bands, and H zones, respectively. A sarcomere is basically a unit of the striated muscle tissue. Repeating units of Z lines are present in this and the actin molecules are bounded to these lines forming a border in the sarcomere. Alternating I which are light and A which are dark bands are present. When looking closer, I band comprises of a midline interruption which is known as Z disc while in the A band there is a lighter middle section known as H zone. During the muscle contraction, the actin filaments move over the myosin filaments according to the sliding filament theory. I-band and the H zone tends to overlap when a muscle is contracted. When they overlap it becomes difficult to observe I and H bands. Therefore, these will only be visible in the relaxed position. Hence, option (c) and option (e) are correct.

Explanation for the incorrect answers:

Option (a) is given as Z discs. The Z discs are dark stripes, which marks the ending of a sarcomere. These dark zones can be clearly seen in both the contracted and relaxed electron micrographs of the skeletal muscles. So, it is an incorrect option.

Option (b) is given as sarcomere. This is the part of the muscle composing of the thin and thick filaments. These thick and thin filaments forming the sarcomere are the basic unit of the muscle tissue present between the two Z discs. Therefore, sarcomere will be visible in both the contracted as well as a relaxed muscle fiber. So, it is an incorrect option.

Option (d) is given as A bands. These are the darker areas of the sarcomere. These bands do not shrink, when the muscle contracts, and hence, can be seen both in relaxed and contracted position. So, it is an incorrect option.

Hence, options (a), (b), and (d) are incorrect.

Conclusion

Thus, in a relaxed state, I-band and H-zone will be visible, which will disappear during contraction of the muscle.