Dr Oliver Pratt, Specialist Registrar
Dr Carl Gwinnutt, Consultant
Department of Anaesthesia, Hope Hospital, Salford M6 8HD, UK.
e mail: firstname.lastname@example.org
Tutorial Of The Week
The Autonomic Nervous System
Part II - Basic Pharmacology
In the first of our autonomic nervous system tutorials, we saw that:
• The autonomic nervous system (ANS) reflexes are instrumental in the control of
most of the body’s organ systems
• The afferent limb of these reflexes can be from the ANS or central nervous
system (CNS). The efferent limb is mediated by the sympathetic (SNS) or
parasympathetic (PNS) divisions, which are functionally and structurally distinct.
• The observed physiological effect will depend upon which neurotransmitter and
types of receptors are involved.
In the normal, resting situation equilibrium exists between sympathetic and
parasympathetic activity. Drugs that stimulate or inhibit activity of either the
parasympathetic or sympathetic division affect this balance.
Some common terminology is used to describe the action of drugs on the autonomic
Sympathomimetics are drugs with similar actions to the postganglionic fibres of the SNS.
They resemble adrenaline (epinephrine) in their actions and are also referred to as
adrenergics. (Sometimes called sympathetic agonists).
Sympatholytics are drugs that oppose the actions of the postganglionic fibres of the SNS.
They are also referred to as antiadrenergics or sympathetic (adrenergic) antagonists.
Both groups are subdivided further depending on their actions on either alpha or beta-
Parasympathomimetics are drugs that stimulate postsynaptic muscarinic receptors. Their
actions resemble acetylcholine and they are also referred to as cholinergics or
Parasympatholytics are drugs that oppose the actions of the PNS at the muscarinic
receptors by blocking the actions of acetylcholine. They are also referred to as