[Durgical](https://durgical.com/en/) › [DurgiPedia](https://durgical.com/en/durgipedia/) › [Physiology](https://durgical.com/en/physiology/) › [Nerve and muscle](https://durgical.com/en/physiology/#s-nerve_muscle) › The neuromuscular junction

[Physiology](https://durgical.com/en/physiology/) · [Nerve and muscle](https://durgical.com/en/physiology/#s-nerve_muscle)

# 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](https://durgical.com/en/anatomy/vastus-lateralis-muscle/) · [Diaphragm](https://durgical.com/en/anatomy/diaphragm/)

## Key points

- Sequence: nerve AP → voltage-gated Ca²⁺ entry (P/Q type) → vesicle fusion (SNARE proteins) → ACh → nicotinic N_M receptor → end-plate potential → muscle AP.
- The nicotinic receptor is a ligand-gated cation channel (Na⁺ in, K⁺ out); it needs two ACh molecules to open.
- Safety factor: the end-plate potential is normally several times larger than needed, so every nerve spike triggers a muscle spike; diseases eat up this margin.
- Acetylcholinesterase in the synaptic cleft hydrolyzes ACh; choline is taken back up to make new ACh.
- Myasthenia gravis = antibodies against the ACh receptor (postsynaptic, worsens with use); Lambert-Eaton = antibodies against presynaptic Ca²⁺ channels (improves briefly with repeated effort).
- Botulinum toxin cleaves SNARE proteins and stops ACh release: flaccid paralysis.

## 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 mV **Threshold** −60 mV **Peak** +30 mV **Duration** ≈ 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

The muscle action potential that the end-plate potential launches; tap a phase.

## 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.

## Memory aid

Calcium calls, ACh answers, the esterase hangs up.

## 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.

[Add to my cards (5)](https://durgical.app/estudiar/?add=/en/physiology/the-neuromuscular-junction/)

## See it in 3D

The structures of Durgical's 3D anatomy where it happens.

- [Vastus lateralis](https://durgical.com/en/anatomy/vastus-lateralis-muscle/)
- [Diaphragm](https://durgical.com/en/anatomy/diaphragm/)

## Related topics

- [Skeletal muscle excitation–contraction coupling](https://durgical.com/en/physiology/skeletal-muscle-excitation-contraction-coupling/)
- [Synaptic transmission](https://durgical.com/en/physiology/synaptic-transmission/)
- [The nerve action potential](https://durgical.com/en/physiology/the-nerve-action-potential/)
- [Motor units, recruitment and summation](https://durgical.com/en/physiology/motor-units-recruitment-and-summation/)

[← All of physiology](https://durgical.com/en/physiology/)

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](https://openstax.org/books/anatomy-and-physiology-2e/pages/10-3-muscle-fiber-contraction-and-relaxation) (OpenStax (Rice University), 2022); [12.5 Communication Between Neurons](https://openstax.org/books/anatomy-and-physiology-2e/pages/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.

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Source: https://durgical.com/en/physiology/the-neuromuscular-junction/
License: CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). Cite as: DurgiPedia (durgical.com), https://durgical.com/en/physiology/the-neuromuscular-junction/