Autonomic pharmacology
Introduced| English | Meaning |
|---|---|
| agonist/ˈæɡənɪst/ | A drug that binds a receptor and activates it. |
| blocker | A drug that binds a receptor without activating it and stops the natural transmitter acting: an antagonist. |
| muscarinic receptors | The G-protein-coupled acetylcholine receptors of the parasympathetic targets, such as the heart, gut, glands and eye. |
| acetylcholinesterase | The enzyme in the synaptic cleft that breaks down acetylcholine and ends its signal. |
| anticholinergic | Blocking muscarinic receptors, with effects such as dry mouth, fast heart rate, constipation, urinary retention and confusion. |
Pinpoint pupils on a farm
- A farm worker sprayed with an organophosphate pesticide arrives drooling, sweating, with tiny pupils, cramps and a slow pulse.
- Every sign is one row of the autonomic receptor map, read in excess. Learn the map, and you can predict drugs and poisonings alike.
The adrenergic rows
- α1 constricts blood vessels and dilates the pupil. β1 raises heart rate and force. β2 opens the airways and widens some vessels.
- An agonist 激动剂 pushes a row; a blocker 阻断剂 (antagonist) removes its tone. A β2 agonist inhaler opens airways; a β1 blocker slows the heart.
Match each receptor to its main effect.
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One receptor row, one effect to predict from.
Which drug type relieves an asthma attack?
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β2 stimulation opens the airways.
The muscarinic rows
- Muscarinic receptors 毒蕈碱受体: M3 drives glands (saliva, tears, sweat), constricts the pupil, narrows airways and speeds the gut. M2 slows the heart.
- Sweat glands are the exception: their nerves are sympathetic but release acetylcholine onto muscarinic receptors.
Muscarinic stimulation slows the gut.
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It speeds the gut, causing cramps and diarrhoea.
Cholinergic excess
- Organophosphates block acetylcholinesterase 乙酰胆碱酯酶, so acetylcholine builds up: salivation, tears, sweating, tiny pupils, airway secretions, a slow heart, cramps and diarrhoea.
- Nicotinic sites also over-fire: muscle twitching, then weakness. Atropine blocks the muscarinic effects.
Put the events at a cholinergic synapse in order.
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Organophosphates block the third step, so acetylcholine builds up.
A patient has tiny pupils, drooling, sweating, cramps and a slow pulse after spraying crops. What is the pattern?
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Every sign is muscarinic over-activity from blocked acetylcholinesterase.
Anticholinergic and adrenergic excess
- Anticholinergic 抗胆碱能 excess is the muscarinic map switched off: dry mouth, hot dry skin, large pupils, blurred vision, a fast heart and urinary retention.
- Adrenergic excess (as with amphetamine or cocaine) gives a fast heart, high blood pressure, large pupils and sweating.
Worked example: predict four drugs
- (Fictional teaching set) A β1 blocker lowers heart rate and force. An α1 blocker lowers blood pressure and can cause dizziness on standing.
- A β2 agonist opens airways with little effect on the heart. An M blocker dries secretions and blurs near vision.
Match each drug to a predicted effect.
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Each prediction comes straight off the map.
Worked example: two patients, two maps
- (Fictional) Patient A: tiny pupils, wet skin, cramps and diarrhoea, slow pulse: cholinergic.
- Patient B: dry hot skin, wide pupils, fast pulse, a full bladder: anticholinergic. Both sweating or both dry? The skin separates them.
Two patients both have wide pupils and a fast pulse. One is sweating, the other has hot dry skin. Which is anticholinergic?
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Sweat glands are muscarinic, so blocking them dries the skin; the sweating patient fits adrenergic excess.
Three common slips
- Saying muscarinic stimulation slows the gut. It speeds it: cramps and diarrhoea.
- Calling sweating adrenergic only. Sweat glands use acetylcholine, so anticholinergic patients are hot and dry.
- Mixing muscarinic with nicotinic. Nicotinic receptors sit in ganglia and at the muscle end-plate.
Recap
- α1 vessels and pupil dilation; β1 heart; β2 airways; M3 glands, pupil constriction, gut; M2 slow heart.
- Cholinergic = wet, small pupils, slow heart, cramps; anticholinergic = dry, hot, wide pupils, fast.
- Adrenergic = fast, high pressure, wide pupils, sweating.