Question

When a muscle contracts, what happens to the H zones? What is a rigor mortis and why does it occur?

Answer 1

Answer of 1st Question:
H-zone is contained by thick filament only. It appears as a lighter band in the middle of the dark A band at the center of a sarcomere.
According to sliding filament model of muscle contraction: When muscles contract then Z-lines come close each other, I-band shortens and `tt(color(orange)"H-zone disappears"`.

Answer of 2nd Question:

Definition:
The stiffening of body after death is termed as rigor mortis.

Cause:
ATP is needed to break the link between actin and myosin bridges that are formed as a result of contraction. But the amount of `ATP` in the body falls after death. And the links or cross-bridges can't be broken. Thus these bridges remain firmly bound. Consequently, the body becomes stiff after death and this condition is termed as rigor mortis.

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