Final answer:
Rigor mortis is a result of the cessation of ATP production after death, which prevents the detachment of myosin heads from actin-binding sites, leading to sustained muscle contraction. In contrast, writer's cramp is a temporary condition caused by a temporary decrease in ATP in living individuals.
Step-by-step explanation:
Rigor mortis, the muscle stiffness following death, is due to several physiological factors linked to the molecular processes that manage muscle contraction and relaxation. During life, the contraction cycle of muscles involves the head of the protein myosin binding to the protein actin, pulling the actin filaments together and causing the muscle to contract. ATP (Adenosine Triphosphate) is required for the myosin heads to detach from the actin-binding sites after contraction, allowing the muscle to relax.
Upon death, ATP production ceases, which means there is no longer ATP available to release the myosin from the actin. As a result, the myosin heads remain stuck to actin, maintaining muscle contraction and causing rigor mortis. This condition persists until enzymatic activity in the body leads to a breakdown of the muscle tissue, allowing it to relax again.
In contrast, in a live person, a condition called "writer's cramp" can occur when muscles are overused and the ATP levels temporarily decrease, preventing the normal cycling of contraction and relaxation. However, this is temporary as the body continues to produce ATP, unlike the permanent cessation of ATP production seen with rigor mortis after death.