When any form of impulse strikes the nerve cell, it causes change in membrane potential by changing the ion permeability across the membrane.
If the membrane potential reach threshold potential (the minimum potential difference to elicit a response), then the action potential is generated. The action potential or nerve impulse travels along the axon and reach nerve terminals. Within the nerve terminals the neurotransmitters are stores, which are released out in response to the stimuli.
Thus, the blanks can be filled with action potential; nerve impulse.