Final answer:
Rigor mortis occurs due to the lack of ATP after death, which prevents myosin heads from detaching from actin-binding sites, causing muscle stiffness. This rigidity continues until the breakdown of muscle proteins allows for relaxation.
Step-by-step explanation:
Rigor mortis occurs because no ATP is available to resequester the leaking calcium ions and release the now attached actin and myosin molecules.
ATP is essential for muscle contractions, as it supplies the energy necessary for myosin heads to attach and detach from the actin-binding sites during the cross-bridge cycle. After death, ATP production ceases, resulting in myosin heads remaining bound to the actin, causing muscle stiffness known as rigor mortis. This state of rigidity lasts until other processes, such as the breakdown of muscle proteins, start to occur, leading to the eventual relaxation of muscles.
During life, muscle contractions are facilitated by ATP and the regulated movement of calcium ions. When the body runs out of ATP, due to depletion or because of death, calcium ions are not pumped back into the sarcoplasmic reticulum, and tropomyosin does not cover the actin-binding sites, leading to a sustained contraction, known as rigor mortis.