Which statement correctly describes the sarcomere and its major regions?

Which statement correctly describes the sarcomere and its major regions?

• A. The sarcomere is the contractile unit; Z-lines mark the ends; I-band contains actin; A-band contains actin and myosin; M-line is the center; H-zone is the myosin-only region.
• B. The sarcomere is the contractile unit; Z-lines mark the ends; I-band contains actin; A-band contains actin only; M-line is at the center; H-zone is not defined.
• C. The sarcomere is the non-contractile unit located in connective tissue.
• D. The sarcomere is the basic unit of mitochondria in muscle cells.

Answer: (B)

The sarcomere is the contractile unit of a muscle fiber, and its regions are defined by how thick (myosin) and thin (actin) filaments are arranged. Z-lines mark the boundaries of a sarcomere. The region between Z-lines that contains only actin is called the I-band. The A-band spans the full length of the thick filaments and includes areas where actin and myosin overlap. The center of the sarcomere contains the M-line, and inside the A-band there is a lighter area known as the H-zone where only thick filaments are present. The description that correctly captures these features states that actin and myosin are both present in the A-band, the I-band contains actin, Z-lines are at the ends, the M-line is at the center, and the H-zone is the myosin-only region. Descriptions that say the A-band contains actin only or leave the H-zone undefined don’t fit the actual anatomy, and statements calling the sarcomere a non-contractile unit in connective tissue or a basic unit of mitochondria are incorrect.

The sarcomere is the contractile unit of a muscle fiber, and its regions are defined by how thick (myosin) and thin (actin) filaments are arranged. Z-lines mark the boundaries of a sarcomere. The region between Z-lines that contains only actin is called the I-band. The A-band spans the full length of the thick filaments and includes areas where actin and myosin overlap. The center of the sarcomere contains the M-line, and inside the A-band there is a lighter area known as the H-zone where only thick filaments are present.

The description that correctly captures these features states that actin and myosin are both present in the A-band, the I-band contains actin, Z-lines are at the ends, the M-line is at the center, and the H-zone is the myosin-only region. Descriptions that say the A-band contains actin only or leave the H-zone undefined don’t fit the actual anatomy, and statements calling the sarcomere a non-contractile unit in connective tissue or a basic unit of mitochondria are incorrect.