SN2
SHORT NOTE
5M
★ High-yield
Write a short note on rigor mortis.
Definition
Rigor mortis = the stiffening of muscles that develops after death, owing to depletion of ATP, which leaves the myosin heads permanently attached to actin.
Mechanism
Death → Circulation and oxygenation cease → Anaerobic glycolysis, then ATP exhausted → No ATP to detach myosin from actin → Ca2+ also leaks out of SR (pump needs ATP) → Permanent actin–myosin cross-bridges → rigidity
- Recall that ATP is required for relaxation, not only for contraction — this is the whole basis of rigor mortis
- Lactic acid accumulation lowers muscle pH and contributes
Time Course (temperate Conditions)
| Event | Time after death |
|---|---|
| Onset | 1–2 hours |
| Complete | 12 hours |
| Persists | 12–24 hours |
| Passes off | 36 hours (by autolysis and putrefaction) |
- Order — proceeds from above downwards: eyelids, face and jaw, neck, trunk, upper limbs, lower limbs. Disappears in the same order
Factors Affecting Onset
| Rapid onset | Delayed onset |
|---|---|
| High environmental temperature | Cold environment |
| Violent exercise before death (glycogen already used) | Rest before death |
| Convulsions, electrocution, strychnine | Death from haemorrhage |
| Children and the elderly (less muscle) | Muscular adults |
Conditions to Distinguish
- Cadaveric spasm — instantaneous rigidity at the moment of death, affecting one group of muscles; no preceding primary flaccidity; of great medicolegal value as it indicates the last act
- Heat stiffening — from burns; muscle protein coagulates → "pugilistic attitude"
- Cold stiffening — freezing of body fluids
Applied Aspects
- Estimation of time since death — the chief medicolegal use
- Position of the body in rigor may indicate whether it was moved after death
All questions in Nerve & Muscle Physiology
- Q1Define action potential. Describe its phases and ionic basis. Add a note on the refractory period. ★
- Q2Describe the structure of the neuromuscular junction and the steps of neuromuscular transmission. Add a note on the drugs acting on it. ★
- Q3Describe the sliding filament mechanism of skeletal muscle contraction. Explain excitation–contraction coupling and the role of calcium. ★
- Q4Classify nerve fibres. Describe the mechanism of impulse conduction and compare saltatory with continuous conduction. Add a note on nerve degeneration and regeneration. ★
- Q5Compare the structure and properties of skeletal, cardiac and smooth muscle. Describe the mechanism of smooth muscle contraction. ★
- SN1Write a short note on myasthenia gravis. ★
- SN2Write a short note on rigor mortis. ★
- SN3Define motor unit. Describe the innervation ratio and the size principle. ★
- SN4Differentiate between isotonic and isometric contraction.
- SN5Describe the changes following section of a peripheral nerve. Add a note on regeneration. ★
- SN6Describe the length–tension relationship of skeletal muscle and give its significance.
- SN7Write a short note on muscle fatigue. Add a note on oxygen debt.