The neuromuscular junction !
- Mechanism
- Exam priority: high
Where a motor nerve meets a muscle fiber, it releases acetylcholine; each nerve impulse reliably fires one muscle impulse.
Updated 2026-11-01
Also called: Motor end plate, Myoneural junction
See it in 3D: Vastus lateralis · Diaphragm
What it is
The neuromuscular junction is the chemical synapse between a motor neuron terminal and a skeletal muscle fiber. A nerve impulse opens Ca²⁺ channels in the terminal, acetylcholine (ACh) is released, binds nicotinic receptors on the folded end plate and produces an end-plate potential large enough to trigger a muscle action potential. Acetylcholinesterase ends the signal within milliseconds.
How it works, step by step
- The motor neuron action potential invades the axon terminal.
- Depolarization opens voltage-gated Ca²⁺ channels at the active zones; Ca²⁺ rushes in.
- Ca²⁺ binds synaptotagmin and SNARE proteins fuse ACh vesicles with the membrane; each vesicle holds roughly 10,000 ACh molecules.
- ACh diffuses across the cleft and binds nicotinic receptors packed at the crests of the junctional folds.
- The channels let mostly Na⁺ in: the end plate depolarizes (end-plate potential).
- The end-plate potential opens voltage-gated Na⁺ channels in the folds' troughs and launches a muscle action potential in both directions.
- Acetylcholinesterase splits ACh into acetate and choline within milliseconds, so one nerve impulse gives one muscle impulse.
- Choline is recycled into the terminal and resynthesized into ACh by choline acetyltransferase.
Action potential
Rest −90 mVThreshold −60 mVPeak +30 mVDuration ≈ 2–4 ms
Tap a phase (the pills above or its band) or a refractory period (ARP, RRP) at the base of the plot.
- Mainly Na⁺
- Mainly K⁺
- Rest
- Threshold
- ARP: absolute refractory period
- RRP: relative refractory period
Reference values
| Parameter | Value | Note |
|---|---|---|
| End-plate potential amplitude | ≈ 50–75 mV of depolarization | well above the ≈ 20 mV needed |
| ACh per vesicle (quantum) | ≈ 10,000 molecules | |
| Muscle fiber resting potential | ≈ −80 to −90 mV |
Approximate values for a healthy adult at rest; each laboratory sets its own ranges.
What can go wrong at the junction
| Agent or disease | Site | Effect |
|---|---|---|
| Botulinum toxin | Presynaptic SNARE proteins | No ACh release: descending flaccid paralysis |
| Lambert-Eaton antibodies | Presynaptic P/Q Ca²⁺ channels | Less ACh release; proximal weakness, low reflexes |
| Myasthenia gravis antibodies | Postsynaptic nicotinic receptor | Fewer working receptors; weakness that worsens with use (ptosis, diplopia) |
| Organophosphates and carbamates | Acetylcholinesterase | ACh builds up: muscarinic excess, fasciculations, then depolarizing paralysis |
| Coral snake (Micrurus) venom | Postsynaptic receptor (mainly) | Neurotoxic paralysis with little local swelling |
Clinical notes
- Myasthenia gravis: fatigable weakness of eye, bulbar and limb muscles; acetylcholinesterase inhibitors help by letting ACh act longer, and respiratory muscles (diaphragm) can fail in a crisis.
- Organophosphate pesticide poisoning is a frequent intoxication in rural Colombia: salivation, miosis, bronchorrhea, bradycardia and muscle weakness from too much ACh.
- Coral snake bites (Micrurus species, present in Colombia) can paralyze breathing muscles by blocking neuromuscular transmission.
Test yourself
Think of the answer, then tap the question to see it.
Why does a myasthenia gravis patient's ptosis get worse late in the day while Lambert-Eaton weakness improves with a few seconds of effort?
In myasthenia the receptors are reduced, so falling ACh release with repetition drops the end-plate potential below threshold. In Lambert-Eaton, repeated firing builds up presynaptic Ca²⁺ and transiently boosts ACh release.
How can an acetylcholinesterase inhibitor overdose itself cause muscle paralysis?
ACh stays in the cleft and keeps the end plate depolarized. Nearby Na⁺ channels stay inactivated, so no new muscle action potentials fire: a depolarizing block after initial fasciculations.
What is the safety factor at the neuromuscular junction and why does it matter clinically?
The end-plate potential is several times larger than the depolarization needed to reach threshold. Diseases that lower ACh release or receptor number only cause weakness once this margin is used up.
Why does removing extracellular Ca²⁺ stop neuromuscular transmission but not the nerve action potential?
The nerve spike depends on Na⁺ and K⁺. Vesicle fusion, however, is triggered by Ca²⁺ entering the terminal; without it, no ACh is released.
Why does botulinum toxin cause a flaccid rather than a spastic paralysis?
It cleaves SNARE proteins in the motor terminal, so ACh is never released and the muscle receives no excitation at all. Tone and reflexes are lost.
See it in 3D
The structures of Durgical's 3D anatomy where it happens.
Related topics
- Skeletal muscle excitation–contraction coupling
- Synaptic transmission
- The nerve action potential
- Motor units, recruitment and summation
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: 10.3 Muscle Fiber Contraction and Relaxation (OpenStax (Rice University), 2022); 12.5 Communication Between Neurons (OpenStax (Rice University), 2022); Hall JE. Guyton and Hall Textbook of Medical Physiology, 13th ed. (ch. 7). Reference book used to check facts; its text and figures are not reproduced (Elsevier, 2016).
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Representative model, for educational purposes only.