Give the mechanistic explanation as to why tetanus does not occur in myocardial contractile cells?

Cardiac muscle cells also known as myocardial cells present in the heart. The myocardial cells contain the myofibrils that contains long chain of sarcomeres.  These cells also shows the striations like skeletal muscle.

Teatnus infection do not occur in the myocardial cells because as the calcium enters in the cell, it causes cell depolarization and also increases the length of the refractory period. This prevents the summation of the tension that are developed from the repeated action potentials.

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.