Several hours after death, a body becomes rigid and immovable. This state persists until lysosomal proteases degrade the muscle proteins. The immediate molecular basis of this postmortem rigidity is:

• A Depletion of acetylcholine at the neuromuscular junction
• B Loss of ATP, which prevents detachment of myosin heads from actin ✓
• C Sustained firing of the gamma motor neurons
• D Failure of the sarcoplasmic reticulum calcium pump alone

Explanation

Cross-bridge cycling requires ATP for two steps: energizing the myosin head and detaching it from actin. After death, ATP concentration falls below the level required for detachment, so actomyosin bridges remain locked and the muscle stiffens in rigor mortis. The SR calcium pump also fails and cytosolic calcium rises, which initiates bridging, but the defining event of rigor is the absence of detachment. Depletion of creatine phosphate occurs early but cannot maintain ATP supply once glycolysis stops.

Reference: Guyton and Hall Textbook of Medical Physiology, 14th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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