In the structure of a myofibril, a sarcomere is defined as the distance between two successive which of the following structures?

Introduction:
Skeletal muscle fibres contain many myofibrils, which in turn are made up of repeating units called sarcomeres. Sarcomeres are the basic functional units responsible for muscle contraction and the characteristic striations seen under the microscope. This question asks you to recall the precise boundaries of a sarcomere in terms of the bands and lines seen in a myofibril and to select the correct structural definition.

Given Data / Assumptions:
- The term being defined is sarcomere. - Options refer to Z discs, H zones, A bands and I bands. - We assume a standard sarcomere pattern with alternating dark and light bands. - The question focuses on the structural boundaries, not the components inside.

Concept / Approach:
A sarcomere is the segment of a myofibril that runs from one Z disc (also called Z line) to the next Z disc. Within this segment, there is a central A band containing thick myosin filaments and overlapping thin actin filaments, flanked by half I bands that contain only thin filaments. The H zone is the central region within the A band where only thick filaments are present in a relaxed muscle. Because the Z discs serve as anchoring points for thin filaments and mark the repeating units, the correct definition of a sarcomere is the distance between two successive Z discs.

Step-by-Step Solution:
Step 1: Recall that myofibrils show a pattern of dark A bands and light I bands under the microscope. Step 2: Recognise that Z discs appear as dark lines in the middle of each I band and serve as boundaries between sarcomeres. Step 3: Define the sarcomere as the region running from one Z disc to the next along the length of the myofibril. Step 4: Identify the H zone as a lighter region in the centre of the A band and understand that it lies within a sarcomere, not at its boundaries. Step 5: Note that A bands and I bands are larger regions that span across parts of multiple sarcomeres and are not used to define sarcomere boundaries. Step 6: Conclude that the correct answer is the distance between two Z discs.

Verification / Alternative check:
Standard diagrams of muscle fibre ultrastructure in biology textbooks label the sarcomere as stretching from Z line to Z line. They show actin filaments anchored at the Z discs and extending through the I band into the A band, where they overlap with myosin filaments. The H zone is depicted within the A band and shrinks during contraction. These diagrams and explanations consistently use Z discs as the reference points for sarcomere length, not H zones, A bands or I bands. This confirms the definition used in the correct option.

Why Other Options Are Wrong:
H zones: These are central regions within the A band where only thick filaments are present; they are not the boundaries of sarcomeres. A bands: Dark bands containing the entire length of thick filaments; each A band spans across the centre of a sarcomere but does not define its endpoints. I bands: Light bands containing only thin filaments and overlapping adjacent sarcomeres; they are divided by a Z disc and therefore cannot define the full sarcomere length.

Common Pitfalls:
Because several bands and zones are visible in myofibril diagrams, students sometimes mix up which one marks the boundaries and which lie inside. Some may pick A band or I band simply because those names are more familiar. To avoid confusion, remember the simple phrase "Z to Z is a sarcomere", linking the letter Z with the end lines of each repeating unit. The H zone and A and I bands are internal regions within and across sarcomeres, not the defining boundaries.

Final Answer:
A sarcomere is defined as the distance between two successive Z discs in a myofibril.