If the vertebrate of a reflex arc controlling leg movement is broken, what part of the nerve pathway may temporarily block the nerve impulse?

The broken vertebrae controlling the reflex arc of the leg movement can be temporarily blocked by the interneurons. The reflexes are controlled by the central nervous system, and interneurons are a part of it (the brain and the spinal cord). The pathway of the reflex arc of the leg movement from the sensory neurons and then back to the motor neurons will be blocked by the interneurons as the vertebrae is broken.

Hence, the answer is 'interneurons'.

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.