Cholinergics mimic the parasympathetic neurotransmitter acetylcholine. Acetylcholine (Ach) is a neurotransmitter located in the ganglions and terminal nerve endings of parasympathetic nerves that connect to receptors in organs, tissues, and glands.
There are two types of cholinergic receptors. These are muscarinic receptors and nicotinic receptors. Muscarinic receptors stimulate smooth muscles and slow the heart rate. Nicotinic receptors affect skeletal muscles. Some cholinergic medications are selective and affect either muscarinic receptor or nicotinic receptors while other cholinergic medications are non-specific and affect both receptors.
Direct-acting cholinergics act on the receptors to activate a tissue response. Indirect-acting cholinergic drugs inhibit the action of cholineresterase (acetylcholinesterase) by forming a chemical complex that permits acetylcholine to persist and attach to the receptor. These drugs are called cholinesterase inhibitors or anticholinesterase drugs.
Pilocarpine is a commonly used direct-acting cholinergic that is used to treat glaucoma. Pilocarpine reduces intraocular pressure by constricting pupils and opening the Canal of Schlemm enabling aqueous humor (fluid) to drain.
There are two types of cholinesterase inhibitors. These are reversible inhibitors and irreversible inhibitors. A reversible inhibitor binds to the cholinesterase enzyme for a period of time and then unbinds enabling the cholinesterase enzyme to properly function. An irreversible inhibitor permanently binds to the cholinesterase enzyme.
Cholinergics stimulate (see chart) the bladder, constrict pupils (miosis), increase neuromuscular transmission, and provide muscle tone to the GI tract. Other effects include a decreased heart rate and blood pressures while increasing secretion of the salivary glands.
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