Originally Posted by mrsD

Yes, it could be neurological.
But I would not be able to help you with that.

Serotonin is a typical player in GI transit. Low Serotonin levels lead to constipation, and higher ones cause diarrhea.
SSRI drugs may cause diarrhea in some people because of this effect.

There is just a subset of people who as they age become constipated.

There is a drug out now, called Amitiza, which GI specialists use for chronic IBS which is constipation oriented. But I think you should check with a gastroenterologist first to see if you qualify for it.