Review Question - QID 106489

The sarcomere is the basic unit of a myocyte. During contraction, the A bands made up of thick filaments do not change their length, whereas the I band and H zones shorten.

The sarcomere, the basic unit of muscle, is made up a number of different components as demonstrated in Illustration A.
(E) Z line - dark, thick protein band to which actin thin filaments attach and define the borders of adjacent sarcomeres
(D) I band - section of sarcomere that contains only actin thin filaments. This reduces in length during contraction.
(B) A band - section of sarcomere that contains the entire length of a single thick filament composed of myosin. This does not change in length during contraction.
(C) H zone - zone of the thick filaments that is not superimposed by the thin filaments. This reduces in length during contraction.

Kremneva et al. examine the protein that determines the length of the sarcomere. They demonstrate that the muscle-specific isoform cofilin-2 promotes actin filament disassembly in sarcomeres to control the precise length of thin filaments in the contractile apparatus.

Mansson et al. describe how errors and changes in sarcomeres produce myopathies such as familial hypertrophic cardiomyopathy (HCM). HCM occurs due to point mutations in genes for sarcomere proteins such as myosin. It occurs in 1/500 people and is the most common cause of sudden death in young individuals.

Figure A (unlabeled) and Illustration A (labeled) demonstrate a sarcomere. Illustration B demonstrates the difference in the lengths of different zones between a relaxed and contracted sarcomere.

Incorrect Answers:
Answer 1: The section labeled C is the H zone, not the A band. Furthermore, the H zone does decrease in length upon contraction.
Answer 2: The section labeled C is the H zone. However, the H zone does decrease in length upon contraction.
Answer 3: The section labeled B is the A band, not the I Band.
Answer 4: The section labeled D is the I band. This band decreases in length during contraction.