The phenomenon of multiple, overlapping action potentials gradually building muscle tension is called:

The answer is graded potential. Its made of various action potentials that depolarise the neuron incrementaly, in short interspersed intervals that do not give time for the neurons to return to resting potential. Therefore, the actions potentials, of varying sizes, grade up in  sum and when they cumulatively reach the threshold level, they invoke an impulse in the nerve

A. mitochondrion b. nucleus