Case Study
Passage with linked questions
Case Set 1
Case AnalysisPassage
During a biology practical, students observed stained slides of skeletal muscle fibres under a high-power microscope. They noticed alternating dark and light bands running across each fibre. The teacher explained that these bands arise due to the organised arrangement of two key proteins within the myofibrils. She further pointed out a thin dark line bisecting each light band, and a lighter central zone within the dark band. She asked students to identify the structural components and explain how the arrangement of these proteins gives rise to the banding pattern. One student correctly noted that the functional unit of contraction lies between two of these lines, and that this unit shortens during muscle contraction while one band retains its original length.
Question 1: Name the two proteins responsible for the striated appearance of skeletal muscle and identify which band each occupies.
- Actin (thin filament) is responsible for the light I band (Isotropic band), which appears lighter because it contains only thin filaments.
- Myosin (thick filament) is responsible for the dark A band (Anisotropic band), which appears darker because it contains thick filaments along with overlapping portions of thin filaments.
- Both proteins are arranged as rod-like structures parallel to each other and to the longitudinal axis of the myofibril.
Question 2: Identify the 'thin dark line' and the 'lighter central zone' mentioned in the passage and state the function of each.
- The thin dark line bisecting each I band is the Z line — it is an elastic fibre to which the thin actin filaments are firmly attached, anchoring them in place.
- The lighter central zone within the A band is the H zone — it represents the central portion of thick myosin filaments that is not overlapped by thin actin filaments in the resting state.
- The M line, found in the middle of the A band, holds the thick myosin filaments together within the H zone region.
Question 3: The student mentioned that one band retains its length during contraction. Explain what happens to the I band, A band, H zone, and Z lines during muscle contraction, and relate these changes to the sliding filament theory.
- During contraction, thin actin filaments slide over thick myosin filaments towards the centre of the A band — this is the basis of the sliding filament theory.
- The I bands shorten because the actin filaments slide inward, reducing the portion of the sarcomere occupied solely by thin filaments.
- The A band retains its original length because the thick myosin filaments themselves do not shorten or move — only the thin filaments slide over them.
- The H zone reduces and may disappear at maximum contraction as thin filaments from both sides meet at the centre of the A band, fully overlapping the previously uncovered region of the thick filament.
- The Z lines are pulled inward (towards each other) as the actin filaments to which they are attached slide inward, causing shortening of the sarcomere — the functional unit of contraction.
- The net result is a shorter sarcomere, and since sarcomeres are arranged in series along each myofibril, the entire muscle fibre and muscle shorten.