Which of the following biochemical changes is the PRIMARY cause of rigor mortis?

Explanation

## Biochemical Mechanism of Rigor Mortis ### The ATP-Myosin Cross-Bridge Model **Key Point:** Rigor mortis is fundamentally a **failure of muscle relaxation** caused by depletion of adenosine triphosphate (ATP), not active contraction. ### Mechanism Step-by-Step 1. **At death:** Aerobic respiration ceases → no ATP production 2. **Glycogen depletion:** Anaerobic glycolysis continues briefly but exhausts muscle glycogen stores 3. **ATP depletion:** Without ATP, myosin heads cannot detach from actin filaments 4. **Cross-bridge locking:** Myosin remains bound to actin in a rigid, fixed state 5. **Result:** Muscle becomes inextensible and hard **High-Yield:** The key distinction is that rigor mortis is **passive stiffening** (inability to relax), not active contraction. The muscle does not contract forcefully; it simply locks in whatever position it was in at death. ### Why ATP Is Essential ```mermaid flowchart TD A[Death occurs]:::outcome --> B[Aerobic respiration stops]:::action B --> C[ATP production ceases]:::action C --> D{ATP available?}:::decision D -->|Yes| E[Myosin detaches from actin<br/>Muscle relaxes]:::action D -->|No| F[Myosin locked to actin<br/>Rigor mortis develops]:::urgent style A fill:#e8f4f8 style F fill:#fee2e2 ``` ### Timeline Correlation | Time After Death | ATP Status | Rigor Status | |------------------|------------|---------------| | 0–2 hours | Depleted | Absent | | 2–8 hours | Absent | Developing | | 8–12 hours | Absent | Complete | | 12–48 hours | Absent | Persists | | 48+ hours | Absent | Disappears (putrefaction) | **Clinical Pearl:** In cases of **heat-induced death** (e.g., fire victims), rigor mortis may appear very rapidly (within 1–2 hours) because heat accelerates ATP depletion and denatures muscle proteins simultaneously. [cite:Reddy Forensic Medicine 33e Ch 3]