Final answer:
The falling phase of nerve and skeletal muscles is due to the outflow of K+ ions during repolarization, and in skeletal muscles, it can also occur due to muscle fatigue, ATP depletion, and accumulation of lactic acid.
Step-by-step explanation:
The falling phase of action potentials in nerve and skeletal muscles is primarily caused by the outflow of K+ ions from the cell after a peak in depolarization is reached. During muscle contraction, a similar reduction in tension occurs due to factors like muscle fatigue, ATP depletion, and lactic acid buildup affecting muscle function.
Neural control triggers muscle contraction through the formation of actin-myosin cross-bridges and sarcomere shortening. Tension created in muscles can vary, which is a crucial aspect of skeletal movement. However, during the falling phase or muscle relaxation, the inflow of Na+ ions ceases, and K+ channels open, leading to an efflux of K+ ions, which causes repolarization of the nerve or muscle cell. In skeletal muscles, when ATP levels are reduced, or there's a build-up of lactic acid, muscle function declines. This process is evident in conditions like sarcopenia, where there's a decrease in the ability to maintain muscle mass and function. Additionally, damage to components of the central nervous system can lead to hypotonia, characterized by decreased muscle tone and weakened reflexes.