I think that the atropine blocks muscarinic acetylcholinergic receptors. These would be the ones on the smooth muscle and the heart. Acetylcholine speeds up the gut, but it slows down the heart.

The receptors that are problematic with MG are the nicotinic acetycholinergic receptors, which are the acetylcholine receptors on the muscles.

Prostigmine as well as pyridostigmine inhibit acetylcholinesterase. I'm not sure about prostigmine, but pyridostigmine does not cross the blood brain barrier, so it does not effect the nicotinic receptors in the brain. It does effect acetylcholinesterase all over the body. This has an effect on both nicotinic receptors as well as muscarinic receptors.

The good news: more acetylcholine at the nicotinic receptors means better muscle contractions.

The bad news: more acetylcholine at the muscarinic receptors means you crap your drawers.

Atropine should stop the muscarinic effects.

I think.