Neuromuscular junction in cardiac and smooth muscles?

I was just doing an online quiz as part of my preparation for my upcoming exams. I came across a question which asked whether cardiac and smooth muscles have neuromuscular junctions and the answer was no. I'm not sure why this is. Don't they need a NMJ in order to receive signals to generate action potentials? I know that the cardiac muscle cells are electrially connected but how is an action potential generated in the first place without a NMJ present? I tried looking through my notes but it doesn't say anything about that. Can someone please help?