Pharmacology
Autonomic Nervous System Pharmacology
Cholinergic and adrenergic drugs for MBBS and NEET-PG: atropine, adrenaline, beta blockers, alpha blockers and organophosphate poisoning, mapped to NMC codes.
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Autonomic Nervous System Pharmacology
Cholinergic and adrenergic drugs for MBBS and NEET-PG: atropine, adrenaline, beta blockers, alpha blockers and organophosphate poisoning, mapped to NMC codes.
This chapter covers the drugs that act on the sympathetic and parasympathetic divisions of the autonomic nervous system. It maps the cholinergic and adrenergic receptor families and the agonists and antagonists that target them, which underpins the therapeutics of glaucoma, asthma, hypertension, shock and poisoning.
High-yield: Autonomic Nervous System Pharmacology
- Acetylcholine acts on nicotinic receptors at ganglia and the neuromuscular junction and on muscarinic receptors at parasympathetic effector organs.
- Atropine is the drug of choice for organophosphate poisoning to reverse muscarinic effects, while pralidoxime reactivates the phosphorylated enzyme before ageing.
- Pilocarpine is a directly acting muscarinic agonist used to lower intraocular pressure in acute angle-closure glaucoma and to treat xerostomia.
- Neostigmine and pyridostigmine are reversible anticholinesterases used in myasthenia gravis; edrophonium is short-acting and was used diagnostically.
- Atropine causes a dry mouth, blurred vision, mydriasis, tachycardia, urinary retention and hot flushed skin, remembered as dry as a bone and mad as a hatter.
- Adrenaline is the drug of choice for anaphylaxis and acts on alpha and beta receptors, raising blood pressure and relieving bronchospasm.
- Beta-1 receptors act mainly on the heart to raise rate and force, while beta-2 receptors relax bronchial and uterine smooth muscle.
- Alpha-1 stimulation causes vasoconstriction and mydriasis; alpha-2 stimulation on presynaptic terminals reduces noradrenaline release.
- Phenoxybenzamine is an irreversible alpha blocker used before surgery for phaeochromocytoma, given before beta blockade to avoid a hypertensive crisis.
- Prazosin, a selective alpha-1 blocker, causes first-dose hypotension and is used in hypertension and benign prostatic hyperplasia.
- Propranolol is non-selective and can worsen asthma and mask the warning signs of hypoglycaemia in diabetics.
- Salbutamol is a selective beta-2 agonist for acute asthma; its adverse effects include tremor, tachycardia and hypokalaemia.
- Cocaine and tricyclic antidepressants block noradrenaline reuptake, potentiating sympathetic effects.
- Tyramine in fermented foods can trigger a hypertensive crisis in patients taking monoamine oxidase inhibitors.
Autonomic receptors and key drugs
- **Muscarinic agonist:** Pilocarpine for glaucoma and dry mouth; bethanechol for post-operative urinary retention.
- **Anticholinesterase:** Neostigmine and pyridostigmine for myasthenia; physostigmine crosses the blood-brain barrier.
- **Alpha blocker:** Prazosin for hypertension and prostate; phenoxybenzamine before phaeochromocytoma surgery.
- **Beta blocker:** Propranolol non-selective, avoid in asthma; atenolol and metoprolol are cardioselective.
NMC competencies in this chapter
- **PH1.13:** Cholinergic and anticholinergic drugs and their clinical uses
- **PH1.14:** Adrenergic and antiadrenergic drugs and their clinical uses
Frequently Asked Questions
What is the antidote for organophosphate poisoning?
Atropine reverses the muscarinic effects and is titrated to secretions, while pralidoxime reactivates acetylcholinesterase if given before the enzyme ages.
Why is adrenaline the first drug in anaphylaxis?
It acts on alpha and beta receptors to raise blood pressure, relieve bronchospasm and reduce mucosal oedema, addressing every life-threatening feature at once.
Why avoid propranolol in asthma?
Blocking beta-2 receptors in the airways can trigger bronchospasm, so a non-selective beta blocker is hazardous in patients with reactive airway disease.
Why is phenoxybenzamine given before a beta blocker in phaeochromocytoma?
Alpha blockade must come first so that unopposed alpha vasoconstriction does not cause a hypertensive crisis when beta receptors are blocked.
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