4-2-3-1 Soccer Formation?

I love the 4-2-3-1 formation, but am unsure on how to play it defensively. What are the defensive responsibilities of the outside mids? Do they track the opposing mids or outside backs? Do your defensive center mids play all the way across the field left and right or stay more centrally leaving the wings to the mids and backs to pick up defensively? Thanks for the help!