MUSCLE CONTRACTIONS
The contraction of a muscle does not necessarily imply that the muscle shortens; it only means that tension has been generated. Muscles can contract in the following ways:
- ISOMETRIC CONTRACTION → no movement takes place, because the load on the muscle exceeds the tension generated by the contracting muscle → occurs when a muscle attempts to push or pull an immovable object.
- ISOTONIC CONTRACTIONS
- CONCENTRIC → the muscle decreases in length (shortens) against an opposing load, such as lifting a weight up
- ECCENTRIC → the muscle increases in length (lengthens) as it resists a load, such as pushing something down.
RIGOR MORTIS
When death occurs, circulation ceases and the skeletal muscles are deprived of nutrients and oxygen.
- Within a few hours, the skeletal muscle fibers have run out of ATP and the sarcoplasmic reticulum becomes unable to pump Ca2+ out of the sarcoplasm.
- Calcium ions diffusing into the sarcoplasm from the extra cellular fluid or leaking out of the sarcoplasmic reticulum then trigger a sustained contraction.
- Without ATP, the cross-bridges cannot detach from the active sites.
- Skeletal muscles throughout the body become locked in the contracted position.
Because all the skeletal muscles are involved, the individual becomes “stiff as a board” → This physical state, called RIGOR MORTIS → lasts until the lysosomal enzymes released by autolysis break down the myofilaments 15–25 hours later.
- The timing is dependent on environmental factors, such as temperature.
Forensic pathologists can estimate the time of death on the basis of the degree of rigor mortis and the characteristics of the local environment.