an action potential is commonly called an electrical impulse or a nerve impulse.
Action potentials are cascading, temporary shifts in a neuron's resting membrane potential. This is caused by the movement of Na+/K+ ions into/out of the cell.

action potential triggering contraction of muscle fiber

so what causes the Na+  channels to open permitting Na+  initiating the
action potential?

neuron/neuron 

junction w/
pre-synapatic and
post-synaptic neurons

action potential

neuromuscular junction 

w/ neuron and muscle fiber