Anticholinergic drugs act on the parasympathetic nervous system. Specifically, they block acetylcholine, the neurotransmitter that drives parasympathetic “rest and digest” functions. By shutting down parasympathetic activity, these drugs produce effects that look and feel a lot like sympathetic (“fight or flight”) activation, which is where the confusion often comes from.
How Anticholinergics Block the Parasympathetic System
Your autonomic nervous system has two main branches. The sympathetic branch speeds things up during stress: faster heart rate, dilated pupils, redirected blood flow to muscles. The parasympathetic branch does the opposite: it slows the heart, stimulates digestion, constricts the pupils, and increases secretions like saliva and mucus. These two branches constantly push against each other, and the balance between them determines what your body is doing at any given moment.
Anticholinergic drugs block acetylcholine from binding to muscarinic receptors, which are the receptors responsible for carrying out parasympathetic instructions throughout the body. These receptors sit on cardiac muscle, smooth muscle in the lungs and digestive tract, and glands that produce saliva, sweat, and other secretions. When an anticholinergic drug blocks these receptors, the parasympathetic brake is essentially released, and the sympathetic system runs with less opposition.
Why the Effects Mimic Sympathetic Activation
This is the key point that trips people up. Anticholinergics don’t stimulate the sympathetic nervous system directly. They remove the counterbalance. Think of it like a seesaw: instead of pushing the sympathetic side down harder, you’re lifting weight off the parasympathetic side. The result tips the same direction, but through a completely different mechanism.
That’s why anticholinergic effects overlap so heavily with what you’d expect from sympathetic stimulation. Your heart speeds up because the parasympathetic vagus nerve normally slows it down, and that slowing signal is now blocked. Your pupils dilate because the parasympathetic signal that constricts them is gone. Your airways relax because the parasympathetic tone that narrows them has been removed. The outcomes look sympathetic, but the drug is working entirely on the parasympathetic side.
Research on atropine, one of the oldest and most studied anticholinergic drugs, shows that the resulting fast heart rate comes from removing the parasympathetic brake on the heart’s natural pacemaker. Interestingly, some evidence suggests atropine may also boost heart rate through a second, receptor-independent pathway that increases a signaling molecule called cAMP inside heart cells. This could partly explain why the heart rate increase from atropine can sometimes be more dramatic than simple parasympathetic blockade alone would predict.
How Anticholinergics Differ From Sympathomimetics
Drugs that directly activate the sympathetic system are called sympathomimetics. They work by mimicking or boosting norepinephrine, the sympathetic neurotransmitter. The distinction matters clinically because the two drug types, while producing overlapping effects, hit different targets and carry different risk profiles.
A sympathomimetic drug can raise blood pressure by constricting blood vessels, something anticholinergics generally don’t do well. Anticholinergics, on the other hand, are much better at drying up secretions and relaxing the gut, effects that come specifically from silencing muscarinic receptors. The overlap in heart rate increase is real, but beyond that, the two drug classes diverge quite a bit in what they actually do to your body.
What Anticholinergic Effects Feel Like
The classic set of anticholinergic effects is easy to recognize once you know what to look for. Medical training uses the phrase “dry as a bone, blind as a bat, red as a beet, hot as a hare, mad as a hatter” to capture the full picture:
- Dry mouth and dry skin from suppressed salivary and sweat glands
- Blurred vision from dilated pupils and impaired focusing
- Flushed skin from impaired heat dissipation (without sweat, blood vessels in the skin dilate to try to cool you)
- Elevated body temperature for the same reason
- Fast heart rate from loss of parasympathetic slowing
- Confusion or agitation when central nervous system receptors are also affected
- Constipation and urinary retention from reduced gut motility and bladder function
All of these trace back to the same mechanism: parasympathetic functions being turned off. Digestion slows because the parasympathetic system normally drives it. Secretions dry up because the parasympathetic system normally triggers them. Bowel sounds may even disappear entirely when anticholinergic effects are strong.
Where Muscarinic Receptors Are Located
Understanding where the receptors sit helps explain why anticholinergic effects are so widespread. Muscarinic receptors come in several subtypes, each concentrated in different tissues. One subtype sits in the brain, stomach lining, and salivary glands. Another subtype dominates in cardiac muscle and smooth muscle. A third appears on smooth muscle, gastric glands, and salivary glands as well. This broad distribution is why anticholinergic effects touch so many organ systems at once: heart, lungs, gut, bladder, skin, eyes, and brain.
In the heart, muscarinic receptors receive input from the vagus nerve and act to slow the heart rate, decrease the force of contraction, and slow electrical conduction. Block those receptors and the heart speeds up. In the lungs, receptor activation normally narrows the airways and increases mucus production. Block them and the airways open, which is exactly why anticholinergic inhalers are used for conditions like COPD. In the digestive system, muscarinic receptor activation stimulates intestinal movement and digestive enzyme release. Block those receptors and everything slows to a crawl.
The Nicotinic Receptor Distinction
Acetylcholine doesn’t just work through muscarinic receptors. It also activates nicotinic receptors, which serve a very different role. Nicotinic receptors are found at the junction between nerves and skeletal muscles (controlling voluntary movement) and on the nerve cells that relay signals within both the sympathetic and parasympathetic systems. Every preganglionic nerve in both branches of the autonomic system uses acetylcholine at nicotinic receptors to pass signals along to the next nerve cell.
This is an important nuance. Acetylcholine is actually present in both the sympathetic and parasympathetic nervous systems at the relay points between nerve cells. What makes the parasympathetic system uniquely dependent on acetylcholine is that its final signal to organs also uses acetylcholine, acting on muscarinic receptors. The sympathetic system switches to norepinephrine for that final step. So when people say anticholinergics block the parasympathetic system, they’re referring to the muscarinic receptor blockade at the organ level, which is where the drug effects you actually notice come from.
Drugs that block nicotinic receptors (ganglionic blockers) would shut down signal relay in both autonomic branches simultaneously, which is a very different pharmacological situation and not what most anticholinergic medications do.