Tropane Alkaloids

Almost entirely the nightshade family, plus one conspicuous outlier:

Coca is the outlier: an unrelated family that arrived at the same tropane skeleton and used it for a completely different pharmacology — monoamine reuptake blockade rather than anticholinergic action.

The tropane ring on its own does nothing much. Tropine and pseudotropine — the free amino alcohols — are not anticholinergic.

What makes atropine and hyoscyamine antimuscarinic is the tropic acid esterified onto that ring; it is the ester, not the cage, that mimics acetylcholine. This trips up a lot of secondary writing about nightshade chemistry.

As competitive muscarinic antagonists these compounds do exactly what blocking parasympathetic tone predicts: pupils dilate, secretions dry, heart rate rises, gut and bladder smooth muscle relaxes. Every one of those became a clinical use — atropine in bradycardia and as the antidote to organophosphate poisoning, scopolamine for motion sickness and as an antisecretory premedication, hyoscine butylbromide for gut spasm.

Scopolamine crosses the blood-brain barrier readily and is sedative and amnestic; atropine does so less and is more peripherally stimulant. That difference in CNS penetration explains most of the clinical choices between them.