Question

In the muscular system, which of the following statement is true?

• A. The H zone and the area in which the actin and myosin overlap form the I band
• B. The H zone and the area in which the actin and myosin overlap form the A band
• C. The I band consists exclusively of thick myosin filaments
• D. The H zone contains both thin and thick structural filaments

Hint:

Sarcomere Anatomy Cheat-Sheet: - A Band: Total length of thick Myosin filaments. (Does not change width during muscle contraction). - I Band: Contains Inly thin Actin filaments. (Narrows during contraction). - H Zone: Contains Honly thick Myosin filaments. (Narrows and can disappear during full contraction). - M Line: Anchor point located directly in the center of the H zone. [Image structure of a muscle sarcomere detailing the A band, I band, and H zone filaments]

Answer 1

The Correct Option is B

Concept: Skeletal muscle fibers contain cylindrical organelles called myofibrils, which are divided into repeating structural units called sarcomeres. The sarcomere is the fundamental functional unit of muscle contraction, bounded on each end by a Z-line. The characteristic striations of skeletal muscle are created by the organized arrangement of two primary overlapping protein myofilaments:
• Thin Filaments: Composed primarily of the protein actin.
• Thick Filaments: Composed primarily of the protein myosin.

Step 1: Breaking down the structural zones of a sarcomere.
To evaluate the choices, let us define each structural band clearly:
• A Band (Anisotropic Band): Represents the entire length of the thick myosin filaments. Within the A band, two sub-regions exist:
• A central region containing exclusively thick filaments (H zone).
• Outer zones where the thick filaments overlap with thin actin filaments. Therefore, the complete A band is formed by combining the H zone and the regions of actin-myosin overlap. This matches statement (B).
• I Band (Isotropic Band): The light region centered around the Z-line. It contains thin actin filaments exclusively, with no thick filaments present.
• H Zone (Hensens Zone): The narrow, lighter region right in the middle of the dark A band. It contains thick myosin filaments exclusively, with no thin filaments reaching this zone when the muscle is at rest.

Step 2: Fact-checking the remaining options.

• Option (A): Incorrect; the I band contains only thin filaments and does not feature myosin overlap or the H zone.
• Option (C): Incorrect; the I band consists entirely of *thin* actin filaments, not thick myosin.
• Option (D): Incorrect; the H zone contains *only* thick filaments under resting conditions, with no thin filaments present. Thus, Statement (B) stands as the accurate structural description of muscle sarcomere anatomy.