Question

Rigor mortis occurs when a body dies. A few hours post-death, the muscles will contract somewhat and the body will feel stiff. Get a sense of how this feels – have a friend tense up the muscles in their arm without fully bending the joints, then feel their arm – this is a bit like what rigor mortis feels like. The stiffening reduces after 24-72 hours (depending on temperature and other factors).

Why does rigor mortis happen? Once a person dies, they are no longer taking in oxygen (O2).

1- The process of cellular respiration, which requires O2, is performed for what purpose? So, without O2, what do the muscle cells not have?

2- Explain how the lack of this substance will make the muscles stiff.

Answer 1

Rigor Mortis is a temporary condition after death, which is characterised by stiffening of the muscles and it happens due to lack of ATP generation.

Cellular respiration is aimed to generate ATP. ATP plays the central role in regulating the muscle contraction and relaxation.

Muscle contraction happens in different stages beginning from generation of action potential and finally to the formation of cross bridging between actin and myosin filament. Calcium ions also plays the important role in promoting muscle contraction. In contracted state the muscle is stiffened and is characterised by cross bridging of actin and myosin filament. ATP is the only molecule which is able to break this cross linking and to release Myosin from actin.

In muscles ATP can be produced by three pathways

Aerobic respiration

Glycogen - lactic acid system

Creatine Phosphate - creatine Kinase system.

Among all these pathways ATP generation is maximum in the aerobic respiration and is the major one. But when death happens this pathway ends and muscle will depend on the other two pathways for ATP generation. But the generation of ATP is very less and depleted fast in these alternative pathways. Thus due to lack of ATP generation muscle are not able to relax . At the same time the cells fails to identify the death and there is continuous release of calcium ions from sarcoplasmic reticulum which also promote muscle contraction and rigor Mortis.

This contracted state of muscle is a temporary condition because the continuous generation of lactic acid in the body by anaerobic respiration after death will reduce the body pH and will decay or detoriate the muscle.

To conclude, rigur mortis can be defined as temporary stiffening of body muscle due to muscle contraction , which is happening due to absence of ATP which is necessary to relax the muscle by breaking actin myosin cross linking.