Anticholinergics (anticholinergic agents) are substances that block the action of the acetylcholine (ACh) neurotransmitter at synapses in the central and peripheral nervous system. These agents inhibit the parasympathetic nervous system by selectively blocking the binding of ACh to its receptor in nerve cells. The nerve fibers of the parasympathetic system are responsible for the involuntary movement of smooth muscles present in the gastrointestinal tract, urinary tract, lungs, sweat glands, and many other parts of the body. In broad terms, anticholinergics are divided into two categories in accordance with their specific targets in the central and peripheral nervous system and at the neuromuscular junction: antimuscarinic agents and antinicotinic agents (ganglionic blockers, neuromuscular blockers). The term "anticholinergic" is typically used to refer to antimuscarinics that competitively inhibit the binding of ACh to muscarinic acetylcholine receptors; such agents do not antagonize the binding at nicotinic acetylcholine receptors at the neuromuscular junction, although the term is sometimes used to refer to agents that do so..