Rigor Mortis - Mechanism of Action

┌─────────────────────────────────────────────────┐
│                  💀 DEATH OCCURS                 │
│  Heart stops → No blood flow → No oxygen         │
│  delivered to muscles                            │
└───────────────────────┬─────────────────────────┘
                        │
                        ▼
┌─────────────────────────────────────────────────┐
│           ⚡ ATP PRODUCTION FAILS                │
│  The 3 ATP systems shut down in sequence:        │
│  1. Aerobic system → fails first (no O₂)         │
│  2. Glycogen-lactic acid system → uses up fast   │
│  3. Phosphagen system → last reserve depleted    │
│  (Only seconds-to-minutes of ATP remain)         │
└───────────────────────┬─────────────────────────┘
                        │
                        ▼
┌─────────────────────────────────────────────────┐
│         🔋 ALL ATP IS USED UP                    │
│  No ATP = no energy to run calcium pumps         │
│  Lactic acid builds up → muscle turns acidic     │
└───────────────────────┬─────────────────────────┘
                        │
                        ▼
┌─────────────────────────────────────────────────┐
│       🌊 CALCIUM (Ca²⁺) FLOODS INTO MUSCLE       │
│  Normally, Ca²⁺ is pumped back into the          │
│  sarcoplasmic reticulum after each contraction.  │
│  Without ATP, those pumps stop working.          │
│  Ca²⁺ leaks out uncontrolled into muscle fibers  │
└───────────────────────┬─────────────────────────┘
                        │
                        ▼
┌─────────────────────────────────────────────────┐
│     🔗 ACTIN + MYOSIN LOCK PERMANENTLY           │
│  Ca²⁺ triggers myosin heads to attach to actin   │
│  filaments (like they do during normal           │
│  contraction).                                   │
│                                                  │
│  NORMALLY: ATP breaks this bond so the muscle    │
│  can relax.                                      │
│                                                  │
│  NOW: No ATP → bond CANNOT be broken             │
│  → Actin and myosin fuse into a stiff gel        │
└───────────────────────┬─────────────────────────┘
                        │
                        ▼
┌─────────────────────────────────────────────────┐
│              🧊 RIGOR MORTIS SETS IN             │
│  Full body muscular stiffness                    │
│                                                  │
│  Timeline (progression):                         │
│  • Heart (involuntary): ~1 hr after death        │
│  • Eyelids/jaw: 3-4 hrs                          │
│  • Face: 4-5 hrs                                 │
│  • Neck & trunk: 5-7 hrs                         │
│  • Upper limbs: 7-9 hrs                          │
│  • Legs: 9-11 hrs                                │
│  • Fingers & toes: 11-12 hrs                     │
│  • Fully established: 6-12 hrs                   │
└───────────────────────┬─────────────────────────┘
                        │
                        ▼
┌─────────────────────────────────────────────────┐
│           🧫 RESOLUTION (Secondary Relaxation)  │
│  Decomposition enzymes (autolysis) break down    │
│  actin and myosin proteins → cross-bridges       │
│  dissolve → muscles soften → body relaxes again  │
│  (usually 24-48 hrs after death)                 │
└─────────────────────────────────────────────────┘

The Key Concept (Simple Summary)

Normal muscle relaxation requires ATP to detach myosin from actin. Death stops ATP production. No ATP = permanent locked cross-bridges = stiff muscles.

Factors That Speed Up Rigor Mortis

Factors That Slow Down Rigor Mortis

Special Cases

• Cadaveric Spasm: In rare cases of extreme exertion or terror right at the moment of death, ATP is so completely depleted that rigor appears instantaneously - the body freezes in the exact position of death.
• Breaking rigor mechanically: Forcibly bending a stiff joint physically tears the actin-myosin gel. The limb becomes permanently flaccid - rigor does not return once broken.