Review of Pharmacology - 9E (2015)

Autonomic Nervous System
Functions of cholinergic system
Sympathetic and parasympathetic systems have opposite actions on most of the organs.
At almost all organs except heart, cholinergic system has excitatory activity and
adrenergic system has relaxing properties.
Muscarinic Actions
• Heart: Parasympathetic system has inhibitory effect on the heart (M 2
) and is responsible
for the negative chronotropic (decreased heart rate) and dromotropic (decreased
conduction) effects. Anticholinergic drugs stimulate the heart by decreasing the
inhibitory effect of ACh on heart.
• Blood vessels: No direct cholinergic supply is present in blood vessels but cholinergic
receptors (M 3
) are present on endothelium of blood vessels. Stimulation of these
receptors causes release of NO from endothelium resulting in vasodilation. Additional
mechanism of vasodilation is inhibitory action of ACh on nor-adrenaline release
from tonically active vasoconstrictor nerve endings. However, if endothelium is
damaged, ACh can stimulate M 3
receptors in the vascular smooth muscle leading
to vasoconstriction.
• Eye: Cholinergic system stimulates sphincter pupillae (circular muscle of eye) and
thus results in miosis (M 3
). ACh also causes contraction of ciliary muscle of the eye
and thus accommodation is possible. Anticholinergic drugs result in mydriasis and
loss of accommodation (blurred vision).
• Glands: Cholinergic system stimulates the secretion of glands and results in
the increased salivation, lacrimation as well as sweating (M 3
). On the other hand
anticholinergic drugs will result in dry mouth, dry eyes and difficulty in swallowing
(due to decreased saliva).
• Urinary bladder: Cholinergic drugs stimulate detrussor and relax the trigone (sphincter)
of urinary bladder resulting in increased micturition (M 3
). Anticholinergic drugs
may result in urinary retention.
• Gastro-intestinal tract: Hydrochloric acid secretion in the stomach (M 1
and M 3
)
is stimulated by parasympathetic system and thus increases the risk of peptic ulcer
disease. Peristalsis of GIT is increased and sphincters are relaxed by the cholinergic
drugs. Anticholinergic drugs can be used as spasmolytic agents for intestinal colic.
• Bronchus: Cholinergic system causes bronchoconstriction (M 3
) and anticholinergic
drugs may lead to bronchodilation.
• Male sex organs: Due to vasodilation, cholinergic system is responsible for erection
of the male organ.
To remember location of
cholinergic receptors, read in
Hindi
- Pehle khao, Phir dil lagao, Baki
kaam bad mein
i.e. M 1
in stomach, M 2
in heart
and M 3
at all other places.
No direct cholinergic supply is
present in blood vessels but
cholinergic receptors (M3) are
present on endothelium of blood
vessels.
Autonomic General Pharmacology
Nervous System
Nicotinic Actions
• Autonomic ganglia: Both sympathetic and parasympathetic ganglia are stimulated
by ACh through the stimulation of N N
receptors.
• Neuromuscular junction: ACh stimulates skeletal muscle contraction by its action on
NMJ (N M
receptors).
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