Question

Sarcomeres are the smallest contractile units of a striated muscle cell. They are segments of myofibrils. Each sarcomere has a dark A band and light I bands. The A band contains the M line, the H band, and the zone of overlap. Each I band contains thin filaments, but not thick filaments. Z lines mark the boundaries between adjacent sarcomeres. What are the components of the A band in a sarcomere?

1) M line, H band, and zone of overlap
2) thin filaments
3) thick filaments
4) Z lines

Answer 1

The components of the A band in a sarcomere are the M line, the H band, and the zone of overlap. It also contains thick (myosin) filaments. The Z lines are not part of the A band, they mark the boundaries between adjacent sarcomeres.

Step-by-step explanation:

Components of the A Band in a Sarcomere

The A band within a sarcomere contains several key components that are critical for muscle contraction. These components are the M line, the H band, and the zone of overlap. The M line is located at the center of the A band and marks the alignment of the myosin filaments. Surrounding the M line is the H band, which is the region of the A band where only thick (myosin) filaments are present without overlapping thin (actin) filaments. This area appears lighter than the rest of the A band under a microscope. The zone of overlap is where thick and thin filaments overlap, consisting of myosin and actin respectively. During muscle contraction, the zone of overlap increases as the thin filaments are pulled towards the center of the sarcomere.

To answer the student's question, the components of the A band in a sarcomere are:

1. M line
2. H band
3. Zone of overlap

The thick filaments (myosin) are also present throughout the A band, including within the H zone and zone of overlap, while the Z lines serve as the boundaries between adjacent sarcomeres but are not part of the A band itself.