Fundamentals of Pharmacology Lecture 9 - The autonomic nervous system and drugs affecting the parasympathetic nervous system

Diagram of the main subdivisions of the nervous system

Diagram illustrating key aspects of the efferent nerves of the parasympathetic nervous system including: regions at which they exit spinal cord, relative lengths of pre and post synaptic elements, neurotransmitters and receptors present in the ganglia and neuroeffector junctions

Efferent nerves of the parasympathetic nervous system (PSNS) leave the spinal cord in the medullary and sacral regions

Chemical synaptic transmission occurs in the ganglia and at the “neuro-effector junction”

Acetylcholine (ACh) is the key neurotransmitter in the parasympathetic nervous system

Both nicotinic and muscarinic receptors are found in parasympathetic nervous system

Diagram illustrating key aspects of the efferent nerves of the sympathetic nervous system including: regions at which they exit spinal cord, relative lengths of pre and post synaptic elements, neurotransmitters and receptors present in the ganglia and neuroeffector junctions

Efferent nerves of the sympathetic nervous system (SNS) leave the spinal cord in the thoracic and lumbar regions

Both ACh and noradrenaline  (also called norepinephrine) act as neurotransmitters in the sympathetic nervous system

All ganglia have nicotinic AChRs

Sweat glands have muscarinic AChRs

Effects of the ANS on some key organs

Clinical uses of muscarinic receptor antagonists

These drugs will block the actions of the parasympathetic nervous system, and their effects include;

Dilation of the pupil (mydriasis); paralysis of ciliary muscle (cycloplegia); increased intraocular pressure (therefore dangerous in glaucoma)

Dry mouth and skin; increase in body temperature

Tachycardia (increase in heart rate)

Bronchodilation

High doses are required to inhibit gastric motility;

Inhibition of gastric acid secretion

Inhibition of bladder emptying (urinary retention)

Since these drugs show little selectivity for muscarinic receptor subtypes, and muscarinic receptors are found throughout the PSNS, these drugs produce side effects such as blurred vision, dry mouth, constipation, heart palpitations/arrhythmias.