Which of the below is the true statement about the H zone of the sarcomere?

ESIC Nursing Officer -2019 (Shift -1)

Applied Physiology

Easy

Appeared in: ESIC Nursing Officer -2019 (Shift -1)

Explanation

The sarcomere is the basic contractile unit of striated muscle, composed of overlapping thick (myosin) and thin (actin) protein filaments.
The H zone (from the German 'heller', meaning 'brighter') is the lighter-appearing central region of the dark A-band.
As shown in the provided diagram and confirmed by muscle histology, the H zone is the specific area within the sarcomere that contains only thick (myosin) filaments.
In a relaxed muscle, the thin (actin) filaments do not extend into this central H zone.

Option A: The H zone is not empty; it is the region composed exclusively of thick (myosin) filaments.
Option B: This describes the parts of the A-band that are outside the H zone, where thick and thin filaments overlap.
Option C: This describes the I-band (isotropic band), which is the lighter band on either side of the Z-disc.

Visual 1: Diagram: A labeled diagram of a single sarcomere, clearly showing the A-band, I-band, H-zone, Z-disc, and M-line, and indicating the thick (myosin) and thin (actin) filaments.
Visual 2: Animation: A comparison of a sarcomere in a relaxed state versus a contracted state, showing how the H-zone and I-band shrink as the actin filaments slide over the myosin filaments.

Clinical Relevance

Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Anatomy of the muscle sarcomere as background academic context rather than a clinical decision trigger.
Understanding sarcomere structure is fundamental to comprehending the mechanism of muscle contraction and relaxation, which is essential for all nursing care involving patient mobility, respiratory function, and cardiac function.
Pathologies like Duchenne muscular dystrophy and certain cardiomyopathies involve structural or functional defects in sarcomeric and associated proteins, leading to progressive muscle weakness and failure.
What if? If a patient has a genetic mutation affecting titin, the protein that anchors myosin filaments, the sarcomere structure would be unstable. This would lead to impaired force generation and progressive muscle weakness, a condition known as titinopathy.

How to Approach the Question

First, identify the key term in the question: 'H zone of the sarcomere'.
Second, analyze the provided diagram or recall the structure of a sarcomere from memory.
Mentally map the different bands and zones: Z-disc (boundaries), I-band (light, thin filaments only), A-band (dark, contains thick filaments), and H-zone (center of A-band).
Focus on the specific composition of the H-zone. The 'H' can be remembered as standing for 'Heller' (German for 'bright'), as it appears brighter than the rest of the A-band because it lacks the overlapping thin filaments.
Evaluate each option against this knowledge: Option A (devoid) is wrong. Option B (thick and thin) describes the rest of the A-band. Option C (thin only) describes the I-band. Option D (thick only) correctly describes the H-zone.

Concept Tested & Keywords

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