Question

Rigor mortis is due to
(a) Depletion of ATP
(b) Excess ATP
(c) Excess availability of calcium
(d) Release of magnesium

Answer 1

Hint: Rigor means stiffness and Mortis means death thus rigor mortis is referred to the process which occurs in the muscles of the body after the death of a person which causes the muscles to become stiff. Rigor mortis is seen in all the muscles of the body.

Complete Answer:
Rigor mortis sets in due to the depletion of ATP which is essential for the breaking of actin-myosin cross-bridges that happens during the relaxation of muscles. As the dead person has stopped breathing, oxygen required for aerobic respiration is not available resulting in the depletion of ATP.
In Addition to this, calcium ions stored in the sarcoplasmic reticulum are released in the cytoplasm which causes more actin-myosin bridges to be formed thus stiffening the muscles even more.

Additional information: Let us look at the mechanism of muscle contraction.
- The active sites on actin filament are exposed because of calcium binding to troponin.
- This allows myosin to form a cross- bridge with actin through these active sites.
- With the hydrolysis of ATP, myosin bends causing the muscle to contract.
- Now a new molecule of ATP binds to myosin causing the cross- bridge to break.
- This new molecule of ATP is hydrolyzed by which myosin returns back to its original position ready to bind to a new site of actin.
So, the correct option is ‘(a) Depletion of ATP’.

Note:
- The cells of muscles are known as muscle fibers as they are modified into long pieces of fibers.
- These muscle fibers are lined throughout their length by fibrils known as myofibrils and are responsible for the contraction of muscles.
- Calcium is stored in the sarcoplasmic reticulum of muscle fibers which is similar in nature to the endoplasmic reticulum.