Depends on the program. Generally no.
Generally grad schools only take their own grad classes for credit. But that's not always the case.
In any case, my grad school courses as an undergrad were a joke with regards to difficulty. I took pretty much the entire neuroscience PhD cirriculum there and aced it. Most of the grad students blew those courses off and got Bs. I put in some effort and easily aced it. When I got to grad school in biophysics, we had one student not even show up once to one of our quantum courses and he still got a C.
Haha so I'm guessing during your PhD years the required courses didn't take much time away from the lab/other work?
No. If I had taken them all at once it would have occupied about 2/3 of one semester.
Hmm, interesting.
If I may ask a kind of related question(don't want to start another thread),
What are some of the major factors that determine how long the PhD portion of one's program last?
I ask not because I want to rush through the program but I don't want to regret choosing the wrong lab/advisor who will keep me there longer than I'd need to be.
It's school and program specific. But most programs know that you're going to do most of your learning in the lab, not in a lecture hall/seminar room.
The length of your PhD will be determined by your advisor and you and will have little, if anything, to do with the amount of required coursework.
But what about the specific area of research?
I'm currently trying to decide on an area of medical research to actively explore and possibly take on if I decide on the MD/PhD, so it would be nice to know how some fare in terms of getting successful thesis work out in time.
The answer's still pretty much the same. The only thing I will say is that work that requires the use of animals will almost always take longer than work that doesn't.