At what interval after death does rigor mortis typically become apparent in the muscles of the jaw and neck under normal ambient temperature conditions?

Explanation

## Post-mortem Rigidity (Rigor Mortis) — Onset Timeline **Key Point:** Rigor mortis is a post-mortem change caused by depletion of ATP and cross-linking of actin and myosin filaments, resulting in muscle stiffening. ### Timeline of Appearance | Time Interval | Muscles Affected | Ambient Conditions | | --- | --- | --- | | 2–6 hours | Jaw, neck, eyelids (small muscles first) | Normal temperature (15–25°C) | | 8–12 hours | Spreads to trunk and limbs | Continues in normal conditions | | 12–24 hours | Complete body involvement | Peak rigidity | | 24–48 hours | Begins to disappear (rigor mortis passes off) | Decomposition accelerates | **High-Yield:** The onset of rigor mortis follows a **craniocaudal (head-to-toe) progression**. Small muscles of the jaw and eyelids stiffen first, followed by the neck, trunk, and finally the limbs. **Mnemonic:** **JENT** — **J**aw, **E**ye, **N**eck, **T**runk (order of rigor mortis spread). ### Factors Affecting Onset - **Temperature:** Cold delays onset; heat accelerates it. - **Physical exertion before death:** Hastens rigor (ATP depleted faster). - **Muscular build:** Well-developed muscles show rigor sooner. - **Cause of death:** Asphyxia and electrocution accelerate rigor. **Clinical Pearl:** Rigor mortis is NOT a reliable indicator of time of death because of wide individual and environmental variation, but early onset (< 2 hours) may suggest ante-mortem exertion or certain causes of death. **Warning:** Do NOT confuse rigor mortis with heat stiffening (cadaveric spasm) — the latter occurs immediately at death in the muscles of the face and hands and is localized, not progressive.