Final answer:
The undesirable effects of cholinergic drugs can come from stimulation of muscarinic receptors. They are part of the parasympathetic system and can be targeted by drugs to either enhance or inhibit their effects. There's a clear specificity with nicotine binding to nicotinic receptors, and muscarine binding to muscarinic receptors without cross-reactivity.
Step-by-step explanation:
The undesirable effects of cholinergic drugs may result from stimulation of muscarinic receptors. These receptors are targeted by parasympathetic drugs which can be either muscarinic agonists or antagonists, or they can affect the cholinergic system indirectly. Drugs that enhance cholinergic effects are called parasympathomimetic drugs, while those that inhibit cholinergic effects are referred to as anticholinergic drugs.
The cholinergic system includes two classes of receptors: the nicotinic receptor, which is a ligand-gated cation channel, and the muscarinic receptor, which is a G protein-coupled receptor. Acetylcholine (ACh) binds to both types of receptors, but there is specificity for other molecules binding to these receptors. For instance, nicotine binds to the nicotinic receptor and has no effect on the muscarinic receptor, while muscarine from certain mushrooms will bind to the muscarinic receptor but not to the nicotinic receptor.
All ganglionic neurons of the autonomic system are activated by ACh released from preganglionic fibers. The receptors on these ganglionic neurons are of the nicotinic type. When substances like nicotine bind to these nicotinic receptors, it mimics the action of ACh, causing depolarization of the ganglia.