Autonomic Drugs
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AUTONOMIC NERVOUS SYSTEM
Autonomic pathways
The autonomic nervous system regulates smooth muscle, cardiac muscle and glands.
| Pathway | Preganglionic neurotransmitter | Postganglionic neurotransmitter |
|---|---|---|
| Sympathetic | ACh | Usually noradrenaline |
| Parasympathetic | ACh | ACh |
| Sympathetic to sweat glands | ACh | ACh |
| Adrenal medulla | ACh at nicotinic receptors | Releases mainly epinephrine and some norepinephrine into blood |
All autonomic preganglionic neurons release acetylcholine onto nicotinic neuronal (Nn) receptors.
Receptor summary
| Receptor | Main location/effect | Memory Aid |
|---|---|---|
| α₁ | Vascular smooth-muscle contraction; mydriasis via radial iris muscle | |
| α₂ | Presynaptic inhibition of noradrenaline release; reduced sympathetic outflow | |
| β₁ | Increased heart rate, contractility and renin release | B1 = 1 Heart |
| β₂ | Bronchodilation and relaxation of selected smooth muscles | B2 = 2 Lungs |
| β₃ | Bladder detrusor relaxation; lipolysis | |
| M₂ | Slows heart rate and AV nodal conduction | |
| M₃ | Glandular secretion, smooth-muscle contraction and endothelial nitric oxide release | |
| Nn | Autonomic ganglia and adrenal medulla | |
| Nm | Neuromuscular junction of skeletal muscle |
High-yield receptor effects
- α₁ activation → vasoconstriction.
- α₂ activation → reduced presynaptic neurotransmitter release.
- β₁ activation → increased heart rate and contractility.
- β₂ activation → bronchodilation.
- M₂ activation → reduced heart rate.
- M₃ activation → increased glandular secretion and smooth-muscle contraction.
Cholinergic drugs
| Drug | Mechanism/use | Important adverse effects |
|---|---|---|
| Bethanechol | Muscarinic agonist; urinary retention in selected patients | Bradycardia, hypotension, diarrhea |
| Pilocarpine | Muscarinic agonist; xerostomia and glaucoma | Sweating, salivation, bronchospasm |
| Neostigmine | Acetylcholinesterase inhibitor; myasthenia gravis and reversal of selected neuromuscular blockade | Bradycardia, secretions, diarrhea |
| Atropine | Muscarinic antagonist | Dry mouth, tachycardia, blurred vision, urinary retention |
| Ipratropium | Inhaled muscarinic antagonist | Dry mouth, possible ocular effects if aerosol enters eyes |
Organophosphate poisoning
Organophosphates inhibit acetylcholinesterase, causing excess acetylcholine at muscarinic, nicotinic and central nervous system sites.
| Feature | Findings |
|---|---|
| Muscarinic | Salivation, lacrimation, urination, diarrhea, bronchorrhea, bronchospasm, bradycardia |
| Nicotinic | Fasciculations, weakness, paralysis |
| CNS | Confusion, seizures, reduced consciousness |
Treatment includes appropriate decontamination and supportive care, atropine for muscarinic manifestations and pralidoxime when indicated, particularly when given early. Severe poisoning is an emergency.
Adrenergic drugs
| Drug | Main action | Common clinical use |
|---|---|---|
| Epinephrine | α₁, α₂, β₁, β₂ agonist | Anaphylaxis; cardiac arrest |
| Norepinephrine | Predominantly α₁/α₂, with β₁ activity | Vasopressor support |
| Phenylephrine | α₁ agonist | Vasoconstriction; selected hypotensive states |
| Salbutamol | β₂ agonist | Bronchospasm |
| Clonidine | Central α₂ agonist | Selected hypertension indications |
| Propranolol | Nonselective β blocker | Selected cardiovascular and other indications |
| Metoprolol | Relatively β₁-selective blocker | Selected cardiovascular conditions |
| Prazosin | α₁ blocker | Selected hypertension and urinary symptoms |
Exam tip: Epinephrine is the first-line medication for anaphylaxis.