Answer:
What tells a muscle when to contract?
Step-by-step explanation:
(1) A message travels from the nervous system to the muscular system, triggering chemical reactions.
(2) The chemical reactions lead to the muscle fibers reorganizing themselves in a way that shortens the muscle--that’s the contraction.
(3) When the nervous system signal is no longer present, the chemical process reverses, and the muscle fibers rearrange again and the muscle relaxes.