But even more interesting, it was never made clear in TNG what would happen if a Galaxy-class saucer separated and the saucer's bridge were either damaged, captured or destroyed. How would a saucer's crew control the saucer without the bridge, particularly if the stardrive section were away on a mission?
Well I'd guess that there is an engineering control room for the primary hull. It is to all intents and purposes an independent spacecraft, so it would have its own main engineering section.
On a ship as vast as the Enterprise-D, there are probably plenty of auxiliary control rooms that can be brought online in the event of a catastrophe afflicting the main bridge, battle bridge and engineering sections.