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.  

what's the question?

sheep is a herbivore animal whereas grasshopper belongs to the class chordates but both of them feed on greens.  

if grasshopper and sheep feeding on a pasture land, i do not think both of them will have any impact on each other. however, the number of the species can make difference. since sheep feed on greens, there is a possibility there left no pasture land for the grasshoppers or they may get extinct.