The answer is Acetylcholine. Acetycholine is a chemical messenger, a neurotransmitter that is released by nerve cells in many parts of the peripheral nervous system. It controls the contraction of skeletal muscles or voluntary muscles and also affects the contraction of smooth and cardiac muscle. It binds to the nicotinic ion channel receptors on the muscle cell membrane, causing the ion channels to open. Then calcium ions flows into the muscle cel, initiating a sequence of steps that finally produce muscle contraction.