The cholinergic system utilizes acetylcholine as a neurotransmitter. Acetylcholine is synthesized from acetyl CoA and choline by the enzyme choline acetyltransferase and stored in vesicles. When an action potential reaches the presynaptic membrane, acetylcholine is released into the synaptic cleft and binds to cholinergic receptors on the postsynaptic membrane, leading to depolarization. There are two types of cholinergic receptors: muscarinic and nicotinic. Cholinergic drugs act on the same sites as acetylcholine and include esters of choline, cholinomimetic alkaloids, and anticholinesterases which inhibit the enzyme that breaks down acetylcholine.

1. CHOLINERGIC SYSTEM:
 Acetylcholine, an ester of choline is an important
neurotransmitter of the ANS.
 The nerves that synthesize, store and release Ach are
called cholinergics.
 The sites of release of acetyl choline are:
 Ganglia, post ganglionic parasympathetic nerve endings,
sweat glands, skeletal muscles, adrenal medulla, brain and
spinal cord.
 Synthesis of ACh: it is synthesized from acetyl CoA and
choline, catalyzed by enzyme choline acetyl transferase.
This ACh is stored in small oval vesicles in the cholinergic
nerve terminals

2.  Transmission of an impulse:
 When an action potential reaches the presynaptic
membrane, Ach is released in to the synaptic cleft.
 This ACh binds to and activates the cholinergic receptor on
the postsynaptic membrane leading to the depolarization of
this membrane.
 Thus the impulse is transmitted across the synapse.
 Two types of Cholinesterases are found: true
cholinesterase (neurons, ganglia and neuromuscular
junction) and pseudo cholinesterase (plasma, liver and
other organs)

3.  Cholinergic receptors:
 There are two types of cholinergic receptors
SUB TYPES LOCATION
MUSCARINIC M1 Autonomic ganglia, gastric glands, CNS
M2 Heart, nerves, smooth muscles
M3 Glands, smooth muscles
M4 CNS
M5 CNS
NICOTINIC Nm Neuromuscular junction
Nn Autonomic ganglia, adrenal medulla, CNS

4. CHOLINERGIC DRUGS:
 These acts on same site as acetylcholine and thereby
mimics its actions.
 They are therefore called parasympathomimetics or
cholinomimetics.
Classification:
 Ester of choline: acetyl choline, methacholine, carbachol.
 Cholinomimetic alkaloids: pilocarpine, muscarine.
 Anticholinesterases:
 Reversible: neostigmine, physostigmine.
 Irreversible: organophosphorus compounds.

5. ACETYLCHOLINE:
Muscarinic actions:
 Heart: it reduces heart rate and force of contraction.
 Blood vessels: relaxes vascular smooth muscles and dilates
blood vessels of the skin and mucous membrane.
 Smooth muscles: increases tone of all other smooth
muscles.
 Secretory glands: enhances secretions of all glands.
 Eye: brings about constriction of pupil by contracting
circular muscles of the iris.

6. Nicotinic actions:
 NMJ: brings about contraction of smooth muscles.
 Autonomic ganglia: stimulates sympathetic and para-
sympathetic ganglia and adrenal medulla.
 CNS: Ach is a neurotransmitter at several sites in CNS.
Uses:
 Carbachol is used in glaucoma.
 Methacholine is rerely used.
 Bethanechol may be used in some cases of postoperative
paralytic ileus and urinary retention.

7. CHOLINOMIMETIC ALKALOIDS:
 Pilocarpine is an alkaloid obtained from the leaves of
Pilocarpus microphyllus.
 Its actions on the eye are important.
 When appled to eye, it causes miosis, spasm of
accommodation and fall in intro ocular pressure.
 It also increases sweat and salivary secretions.
 Adverse effects: burning sensation and painful spasm.
 Uses:
 glaucoma.
 Also used alternatively with mydriatics like homatropine to
break adhesions between iris and lens.
 Used to overcome dryness of mouth

8. ANTICHOLINESTERASES:
 These are the drugs which inhibit the enzyme cholinesterase.
 Physostigmine is an alkaloid obtained from the plant
Physostigma venenosum.
 Uses: glaucoma, browache, long term use retinal detachment
and catarac. Myasthinia gravis, poisoning due to anti
cholinergic drugs, curare poisoning, cobra bite, alzheimer’s
disease
Acetylcholine
Choline + Acetic acid
AChE AntiAChE