Skeletal muscles under microscope have cross-striations formed by dark and light bands. The dark bands are fo… - ESIC Nursing Officer 2016 (shift-1)

ESIC Nursing Officer 2016 (shift-1)

Applied Physiology

Easy

Appeared in: ESIC Nursing Officer 2016 (shift-1)

Explanation

Skeletal muscle appears striated due to the arrangement of contractile proteins into alternating dark (A-bands) and light (I-bands) bands within units called sarcomeres.
The dark A-bands (Anisotropic bands) are primarily composed of thick filaments made of the motor protein myosin.
These bands appear dark because the thick myosin filaments are denser and overlap with thin actin filaments, which affects how they polarize light.
The image provided clearly illustrates the A-band as the dark region containing the thick myosin filaments.

Option B: The term 'mysium' refers to the connective tissue coverings of muscle (e.g., epimysium, perimysium), not a contractile protein.
Option C: Coronin is a regulatory protein that binds to actin and is involved in processes like cell motility, but it is not the primary structural protein that forms the dark bands.
Option D: Actin is the protein that forms the thin filaments. These filaments make up the light-colored I-bands, not the dark A-bands.

Visual 1: Diagram - A labeled diagram of a single sarcomere, clearly showing the A-band, I-band, H-zone, M-line, and Z-disk, and the arrangement of thick (myosin) and thin (actin) filaments. This helps visualize the structural basis of the striations.
Visual 2: Animation - The sliding filament model of muscle contraction, showing how the actin and myosin filaments slide past each other, causing the I-band and H-zone to shorten while the A-band remains constant in length.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Structure of the sarcomere and muscle filament proteins as background academic context rather than a clinical decision trigger.
Understanding the roles of actin and myosin is fundamental to comprehending the physiology of muscle contraction and relaxation, which is essential for assessing motor function, strength, and mobility in patients.
Many neuromuscular diseases, such as certain types of muscular dystrophy, involve genetic defects in the proteins that make up or support the sarcomere, leading to progressive muscle weakness.
Rigor mortis, the stiffening of muscles after death, occurs because ATP is no longer available to detach the myosin heads from the actin filaments, leaving the muscles in a contracted state.

How to Approach the Question

First, identify the key terms in the question: 'skeletal muscles', 'cross-striations', and 'dark bands'.
Recall the microscopic anatomy of a muscle fiber, specifically the structure of a sarcomere, which is the basic contractile unit.
Remember the mnemonic: A-band for Anisotropic (dArk) and I-band for Isotropic (lIght).
Associate the dark A-band with the thick filaments, which are made of the protein myosin.
Associate the light I-band with the thin filaments, which are made of the protein actin.
Evaluate the options based on this knowledge. Myosin correctly corresponds to the protein forming the dark bands. This process of elimination and association confirms the answer.

Concept Tested & Keywords

Concept Tested: Structure of the sarcomere and muscle filament proteins
Stem keywords: Skeletal muscles, cross-striations, dark bands, light bands, protein
Lead-in keywords: formed by

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