Review of Pharmacology - 9E (2015)

Review of Pharmacology
Most of the actions of sympathetic and parasympathetic systems are opposite. To
remember major actions, we can assume that sympathy is related to heart, so sympathetic
system stimulates it (i.e. tachycardia, positive inotropic action etc.). On the other hand,
parasympathetic system has opposite action, so depress heart. At most other parts action
is reverse i.e. sympathetic system inhibits and parasympathetic system stimulates.
Site Action of Sympathetic System Action of Parasympathetic System
Heart Stimulates (↑ HR, ↑ Contractility, ↑ Conduction) Depresses (↓ HR, ↓ Conduction)
Bronchus Relax (Bronchodilation) Stimulates (Bronchoconstriction)
GIT Relax (↓ Movements) Stimulates (↑ Movements)
Bladder Relax (↓ Urine outflow) Stimulates (↑ Urine outflow)
Pupil Relax (Mydriasis) Stimulates (Miosis)
Glands Depress (↓ Secretions except sweating) Stimulates (↑ Secretions)
PARASYMPATHETIC NERVOUS SYSTEM
Autonomic Nervous System
In parasympathetic system, acetylcholine is the principal NT secreted by preganglionic as
well as postganglionic fibres. Therefore, it is also known as cholinergic nervous system. ACh
is synthesized (from acetyl Co-A and choline) and stored within the cholinergic neurons.
Fig. 3.2: Drugs acting at
cholinergic neurons
Uptake of choline by the neurons
is the rate limiting step in the
biosynthesis of acetylcholine.
Uptake of choline by the neurons is the rate limiting step in the biosynthesis of this NT. After
its synthesis, ACh is stored in the vesicles. It is released in the synaptic cleft (by exocytosis)
when nerve impulse stimulates the neuron. Here, it stimulates post-ganglionic as well as preganglionic
cholinergic receptors and produces the response.
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