I never really thought too much about that, it is a good point to bring to the forefront; nonetheless. I am not speaking professionally, so please do not take anything i say as being etched in stone, ok?
I have come to believe a " nervous breakdown" is that point when there is so much stress in your life, you just can't handle anymore. You are on the edge of cracking, you feel as though you are going crazy, but this is the pressure caused by stress. Perhaps we are being pulled in different directions all of the time, deadlines at work, our spouce needing us, and our children needing us, on a constant day to day basis, no relief in sight.
It is also fair to say this is not a permanent situation, you see I was though to believe with seditives, and / or a breif stay in a hospital, we can get better in a reasonable period of time. I think, this is what is different between a breakdown and depression, depression is more long term, and a breakdown is a short term condition that will vanish once the stressors are lessened. Working less hours, taking time off from work, taking on less stainuous projects, etc. I further believe our diet has " A Lot " to do with our mental health state, and how stress will effect us.
I know depression is a way of thinking all of the time, that negative thinking, par se. It comes with the phobias and all, whereas, I am not certain but i do not think a nervous breakdown has this with it, but I could be wrong here.
Maybe others know better on this, but this is kind of what I always believed a nervous breakdown to be, more or less.
Bob