The release of to muscle cell receptors triggers muscle contractions. ach serotonin dopamine adrenaline

The given blank can be filled with acetylcholine.  

Acetylcholine refers to a chemical messenger, that is, a neurotransmitter, discharged by the nerve cells in various segments of the peripheral nervous system. It monitors the contraction of all the voluntary muscles, like skeletal muscles. It also influences the contraction of the cardiac and smooth muscle.  

i think its control and coordinate because im pretty sure thats what tissue does and the joints are what control movement and then your skeletal/ bones provides portection. hope that .