Cholinergic antagonist

Cholinergic antagonist is a general term for agents that bind to cholinoceptors (muscarinic or nicotinic) and prevent the effects of acetylcholine (ACh) and other cholinergic agonists. The most clinically useful of these agents are selective blockers of muscarinic receptors.
Cholinergic antagonist classified as:
1- Antimuscarinic agents(they antagonize only muscarinic receptors)
2- Ganglionic blockers, shows a preference for the nicotinic receptors of the sympathetic and parasympathetic ganglia.
3- The neuromuscular-blocking agents (mostly nicotinic antagonists), interfere with transmission of efferent impulses to skeletal muscles.

1-Antimuscarinic drugs(parasympatholytics).
A. Atropine
Atropine is a tertiary amine belladonna alkaloid with a high affinity for muscarinic receptors. It binds competitively and prevents ACh from binding to those sites
Actions:
a. Eye: Atropine blocks muscarinic activity in the eye, resulting in mydriasis (dilation of the pupil), unresponsiveness to light, and cycloplegia (inability to focus for near vision). In patients with angle-closure glaucoma, intraocular pressure may rise dangerously.
b. Gastrointestinal (GI): Atropine can be used as an antispasmodic to reduce activity of the GI tract.
c. Cardiovascular: Atropine produces divergent effects on the cardiovascular system, depending on the dose. At low doses, the predominant effect is a slight decrease in heart rate. This effect results from blockade of the M1 receptors on the inhibitory prejunctional (or presynaptic) neurons. Higher doses of atropine cause a progressive increase in heart rate by blocking the M2 receptors on the sinoatrial node.
d. Secretions: Atropine blocks muscarinic receptors in the salivary glands, producing dryness of the mouth (xerostomia). The salivary glands are exquisitely sensitive to atropine. Sweat and lacrimal glands are similarly affected.