MP NHM CHO-2024 (Shift-2)
Applied Physiology
Easy
During muscle relaxation, the ____ covers the active site on actin.
Appeared in: MP NHM CHO-2024 (Shift-2)
Explanation
- In a relaxed muscle, the myosin-binding sites on the actin filaments must be blocked to prevent unwanted contraction.
- Tropomyosin is a long, fibrous protein that lies in the groove of the actin helix, directly covering these active sites.
- This blockage prevents the formation of cross-bridges between actin and myosin, thus keeping the muscle in a relaxed state.
- The position of tropomyosin is controlled by the troponin complex, which acts as a calcium-sensitive switch.
Why Other Options Were Wrong
- Option A: Myosin is the contractile protein that forms the thick filaments. Its heads bind to the active sites on actin to cause contraction; it does not cover them.
- Option B: Titin is a large, elastic structural protein. Its function is to provide elasticity and stabilize the position of the thick (myosin) filaments, not to regulate the interaction between actin and myosin.
- Option C: Troponin is a regulatory protein complex that binds to calcium ions. This binding causes a conformational change that moves tropomyosin away from the active sites. Troponin itself does not directly cover the active sites.
Related Visual
- Visual 1: Diagram of the Sarcomere. An illustration showing the arrangement of thin (actin) and thick (myosin) filaments. A detailed view should highlight the troponin-tropomyosin complex on the actin filament, showing how tropomyosin physically blocks the myosin-binding sites in the relaxed state (low calcium) and moves away in the contracted state (high calcium).
Clinical Relevance
- Nursing practice connection: This is primarily an exam-oriented knowledge point with limited direct bedside application, so retain Physiology of Muscle Contraction and Relaxation as background academic context rather than a clinical decision trigger.
- Understanding this mechanism is fundamental to comprehending pathologies like rigor mortis, where a lack of ATP prevents myosin heads from detaching from actin, leading to sustained muscle stiffness.
- The regulation of muscle contraction is a target for various drugs, including muscle relaxants, which may act by interfering with calcium release or the actin-myosin interaction cycle.
- What if? A mutation in the troponin complex that prevents it from properly controlling tropomyosin could lead to congenital myopathies, characterized by either muscle weakness or hyper-contractility, depending on the nature of the defect.
How to Approach the Question
- First, identify the key process in the question: 'muscle relaxation'. This implies a state where contraction is inhibited.
- Next, recall the main proteins involved in muscle contraction: contractile proteins (actin, myosin) and regulatory proteins (troponin, tropomyosin).
- Consider the function of each option in the context of relaxation. Relaxation requires preventing the interaction between actin and myosin.
- Analyze the roles: Myosin is the 'motor', Titin is a 'spring', Troponin is the 'calcium-sensor switch', and Tropomyosin is the 'physical blocker'.
- The question asks what 'covers the active site'. This directly corresponds to the function of tropomyosin, which acts as a physical barrier between actin and myosin in the resting state.
Concept Tested & Keywords
- Concept Tested: Physiology of Muscle Contraction and Relaxation
- Stem keywords: muscle relaxation, active site, actin
- Lead-in keywords: BEST, MOST RELEVANT CLUE
- Negative lead-in flag: false
Question ID
QZ2LfBgVTClAqZgaC0jexV
Reference Book
E6 Guide to Mental Health & PSYCHIATRIC NURSING R Sreevani 6th Edition — Part 2 (pp 290-564 of 579) p. 450-567
Practise the full MP NHM CHO-2024 (Shift-2)
Attempt every question from this paper in a timed mock, then review the full solution for each one.
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