The autonomic nerve terminal: release, reuptake and drug targets
- Mechanism
- Exam priority: medium
Autonomic nerves make, store, release and recycle their transmitters; many drugs act by speeding or blocking one of those steps.
Also called: Varicosities, Norepinephrine reuptake, Ganglionic blockers
See it in 3D: Sympathetic trunk
What it is
Postganglionic autonomic fibers end in chains of swellings (varicosities) that release acetylcholine or norepinephrine onto nearby effector cells. Each transmitter is synthesized in the terminal, packed into vesicles, released by Ca²⁺ entry and then removed: acetylcholine by enzymatic breakdown, norepinephrine mainly by reuptake. Each step is a drug target, and ganglia add another one.
Key facts
- In the adrenal medulla, PNMT converts norepinephrine to epinephrine; cortisol from the cortex flowing through the gland boosts this enzyme.
- Dominant tone decides the result of ganglionic block: vessels (sympathetic) dilate, the heart (parasympathetic) speeds up, gut and bladder become sluggish, pupils dilate, sweating stops.
How it works, step by step
- An action potential reaches a varicosity and opens voltage-gated Ca²⁺ channels.
- Ca²⁺ triggers SNARE-mediated fusion of vesicles with the membrane, releasing transmitter into a wide junction.
- At cholinergic endings, acetylcholinesterase splits acetylcholine within milliseconds; choline returns to the terminal by a high-affinity transporter.
- At adrenergic endings, norepinephrine acts on α and β receptors and on presynaptic α2 receptors that reduce further release (negative feedback).
- NET pumps most norepinephrine back into the terminal; VMAT reloads it into vesicles, and cytoplasmic MAO destroys what is left free.
- Indirect sympathomimetics enter via NET, displace norepinephrine from vesicles and drive it out through the transporter, independent of nerve firing.
- In ganglia, preganglionic acetylcholine acts on nicotinic (NN) receptors; nicotine first stimulates and then, at high levels, blocks by persistent depolarization.
Where drugs act at the terminal
| Step | Effect of blocking or enhancing | Examples |
|---|---|---|
| Vesicular uptake (VMAT) | Stores deplete; less transmitter released | Reserpine |
| Release (adrenergic) | Indirect release raises synaptic norepinephrine | Amphetamine, tyramine, ephedrine |
| Release blocked (adrenergic) | Less norepinephrine released per impulse | Adrenergic neuron blockers (guanethidine type) |
| Reuptake (NET) | Norepinephrine lingers; stronger sympathetic effects | Cocaine, tricyclic antidepressants |
| Breakdown (MAO) | More norepinephrine in stores; tyramine reactions | MAOIs |
| Acetylcholinesterase | Acetylcholine accumulates at muscarinic and nicotinic sites | Neostigmine (reversible), organophosphates (irreversible) |
| Ganglionic nicotinic receptor | Both divisions blocked or stimulated | Nicotine, hexamethonium-type blockers |
Clinical notes
- Organophosphate insecticide poisoning: irreversible cholinesterase block gives muscarinic signs (secretions, bronchospasm, miosis, bradycardia) plus nicotinic weakness and fasciculations.
- A patient taking an MAOI who eats tyramine-rich foods can have a hypertensive crisis, because tyramine releases the enlarged norepinephrine stores.
- Cocaine's vasoconstriction, tachycardia and hypertension come from blocked norepinephrine reuptake.
Test yourself
Think of the answer, then tap the question to see it.
What is the main way norepinephrine is removed from the neuroeffector junction?
Reuptake into the nerve terminal by the norepinephrine transporter (NET); diffusion and breakdown by MAO and COMT remove the rest.
Why does reserpine lower sympathetic activity over days?
It blocks vesicular monoamine uptake (VMAT), so norepinephrine left in the cytoplasm is destroyed by MAO and the stores slowly empty.
A patient receives a ganglionic blocker. Why does the heart rate rise while blood pressure falls?
Resting heart rate is dominated by vagal tone, so blocking it speeds the heart; vessels are dominated by sympathetic tone, so blocking it causes vasodilation and hypotension.
How do amphetamine and tyramine raise synaptic norepinephrine without nerve firing?
They are taken up by NET, displace norepinephrine from vesicles and cause its outward transport into the junction.
Why do organophosphates cause both miosis and muscle fasciculations?
Inhibited acetylcholinesterase lets acetylcholine build up at muscarinic sites (pupil sphincter) and at nicotinic sites (neuromuscular junction).
Connections
See it in 3D1
The structures of Durgical's 3D anatomy where it happens.
In Biochemistry1
Educational physiology content; not a diagnostic or treatment guide and no substitute for a health professional's advice. Values are approximate references. No doses are given.
Sources: Pheochromocytoma (StatPearls (NCBI Bookshelf), 2025); Neuroanatomy, Cranial Nerve 10 (Vagus Nerve) (StatPearls (NCBI Bookshelf), 2025); Insecticide poisoning: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2025).
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Representative model, for educational purposes only.