## Rigor Mortis: Site of Onset **Key Point:** Rigor mortis typically begins in the smaller muscles and progresses to larger ones, following a cephalocaudal (head-to-toe) pattern. ### Sequence of Onset The classical progression of rigor mortis follows this order: 1. **Muscles of mastication** (jaw muscles) — **EARLIEST** 2. Neck muscles 3. Trunk muscles 4. Upper limb muscles 5. Lower limb muscles — **LATEST** ### Why Muscles of Mastication First? - Smaller muscles have **lower ATP reserves** and deplete faster - Muscles of mastication are among the **smallest voluntary muscles** in the body - They are metabolically active even at rest (jaw tone) - The cephalocaudal progression is consistent across all bodies **High-Yield:** The **jaw becomes rigid first**, making it one of the earliest clinical signs of rigor mortis. This is why forensic pathologists often examine the jaw to determine if rigor has begun. ### Timeline | Time After Death | Location | Clinical Sign | |---|---|---| | 2–6 hours | Muscles of mastication, eyelids | Jaw locked, eyes partially closed | | 6–12 hours | Neck, trunk | Trunk stiffness | | 12–24 hours | Limbs | Full body rigidity | | 24–48 hours | All muscles | Maximum rigidity | **Clinical Pearl:** In cases of suspected poisoning or sudden death, the **absence of rigor in the jaw despite body rigidity elsewhere** should raise suspicion for postmortem manipulation or unusual circumstances. ### Mechanism Rigor mortis results from: - **ATP depletion** → actin-myosin cross-bridges cannot dissociate - **Anaerobic metabolism** → lactic acid accumulation → muscle contraction - Smaller muscles exhaust ATP first due to lower reserves **Mnemonic:** **"JAM"** — **J**aw first, **A**rms next, **M**assive muscles last.
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