Neuromuscular junction: motor end plate and acetylcholine receptors !
- Topic
The spot where a motor nerve tells a muscle fibre to contract, using acetylcholine. Several diseases and toxins act right here.
Updated 2026-11-01
See it in 3D: Biceps (long head)
What it is
The neuromuscular junction (motor end plate) is the chemical synapse between a motor axon terminal and a skeletal muscle fibre. Acetylcholine released from the terminal binds nicotinic receptors on the folded sarcolemma, and acetylcholinesterase in the cleft ends the signal within milliseconds. Each step is a target: myasthenia gravis attacks the receptors, Lambert–Eaton the presynaptic calcium channels, botulinum toxin the release machinery.
Structure
- The axon loses its myelin and ends in a trough on the fibre surface, covered by a Schwann cell.
- The terminal holds many mitochondria and synaptic vesicles filled with acetylcholine, clustered at active zones.
- Synaptic cleft with a basal lamina that holds acetylcholinesterase.
- Postsynaptic sarcolemma thrown into deep junctional folds; nicotinic receptors pack the crests of the folds, sodium channels the depths.
Step by step
- An action potential reaches the motor axon terminal.
- Voltage-gated P/Q-type calcium channels open and calcium enters the terminal.
- SNARE proteins fuse acetylcholine vesicles with the membrane and acetylcholine floods the cleft.
- Acetylcholine binds nicotinic receptors (ligand-gated cation channels) on the crests of the junctional folds.
- Sodium entry creates the end-plate potential; when it reaches threshold, a muscle action potential spreads into the T tubules.
- Acetylcholinesterase splits acetylcholine; choline is taken back into the terminal to make more.
Where each disorder hits the junction
| Disorder | Target | Typical picture | Repeated stimulation |
|---|---|---|---|
| Myasthenia gravis | Postsynaptic nicotinic ACh receptors (autoantibodies; some anti-MuSK) | Ptosis, diplopia, fatigable weakness; thymoma or thymic hyperplasia | Strength and EMG response fall (decrement) |
| Lambert–Eaton syndrome | Presynaptic P/Q calcium channels (autoantibodies) | Proximal leg weakness, dry mouth, low reflexes; often small cell lung cancer | Strength and response rise (facilitation) |
| Botulism | SNARE proteins inside the terminal (toxin cleaves them) | Descending flaccid paralysis starting with cranial nerves; no fever | Release blocked; may show mild facilitation |
| Guillain–Barré syndrome | Peripheral nerve myelin or axon (not the junction) | Ascending weakness, lost reflexes, after an infection | Nerve conduction slowed or blocked |
Markers
- α-bungarotoxin (labels ACh receptors)
- Acetylcholinesterase histochemistry
- Anti-AChR antibodies
- Anti-VGCC antibodies
Clinical notes
- In myasthenia gravis the attacked folds are internalised and the folds flatten, so fewer receptors remain.
- Acetylcholinesterase inhibitors raise the acetylcholine available in the cleft; this is why they help myasthenia.
- Organophosphate poisoning blocks acetylcholinesterase: too much acetylcholine, first twitching, then paralysis; it is common in farming areas of Colombia.
- Non-depolarising blockers (curare-like) compete for the receptor; succinylcholine depolarises and holds it open.
Test yourself
Think of the answer, then tap the question to see it.
What is the role of the junctional folds at the motor end plate?
They enlarge the postsynaptic surface so more acetylcholine receptors fit; receptors crowd the crests and sodium channels the depths.
Which protein do the antibodies of myasthenia gravis most often target?
The nicotinic acetylcholine receptor on the muscle side; a minority have antibodies to MuSK.
Why does strength improve with repeated effort in Lambert–Eaton syndrome?
Repeated firing lets calcium build up in the terminal, partly overcoming the blocked presynaptic calcium channels, so more acetylcholine is released.
How does botulinum toxin cause paralysis?
It enters the motor terminal and cleaves SNARE proteins, so vesicles cannot fuse and acetylcholine is not released.
See it in 3D
The structures of Durgical's 3D anatomy where it is found.
Related
- Skeletal muscle: sarcomere and contraction
- Peripheral nerve: endoneurium, perineurium, epineurium; myelin
- The neuron and synapse
- Thymus and tonsils under the microscope
Related conditions
Microorganisms
Educational content on normal microscopic structure and its clinical relevance; no substitute for studying slides or for a health professional's advice.
Sources: 10.2 Skeletal Muscle (OpenStax (Rice University), 2022); Myasthenia gravis: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026); Botulism: MedlinePlus Medical Encyclopedia (MedlinePlus, US National Library of Medicine, 2026).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.