The neuromuscular junction

A neuromuscular junction is where a motor neurone meets a skeletal muscle fibre. When an impulse arrives, the neurone releases the neurotransmitter acetylcholine (ACh), which diffuses across a tiny gap and binds to receptors on the muscle's membrane, the motor end plate. That depolarises the muscle fibre and starts the chain of events that makes it contract. The enzyme acetylcholinesterase then breaks the ACh down so the signal stops.

Structure of the neuromuscular junction

The definitions follow OpenStax. AQA's A-level specification asks for the detailed structure of both a synapse and a neuromuscular junction, and a comparison of how transmission works across each.

How the neuromuscular junction works

1. An action potential travels down the motor neurone to its ending.
2. This causes calcium ions to flow into the neurone ending.
3. The calcium makes the vesicles release acetylcholine into the synaptic cleft.
4. Acetylcholine crosses the cleft by diffusion and attaches to receptor molecules in the motor end plate.
5. These receptors are sodium channels. They open, sodium ions flow in, and the membrane depolarises: an end plate potential.
6. An action potential spreads across the muscle fibre and down its T-tubules, causing the sarcoplasmic reticulum to release calcium ions.
7. Calcium ions move tropomyosin out of the way, so myosin can bind to actin and the fibre contracts.
8. Acetylcholinesterase breaks acetylcholine down into acetyl and choline, so the receptors close and the muscle can relax.

These steps follow OpenStax's account of excitation-contraction coupling. Step 8 matters: OpenStax notes that without acetylcholinesterase, acetylcholine would stay bound to its receptors and the muscle contraction would go on for longer than wanted. What happens inside the muscle fibre in step 7 is covered in sliding filament theory.

Neuromuscular junction vs synapse

A neuromuscular junction works like a cholinergic synapse, one that uses acetylcholine, but there are important differences:

AQA's specification includes temporal and spatial summation, and inhibition by inhibitory synapses, for synapses between neurones. The reflex arc guide shows synapses in a nervous pathway, and saltatory conduction covers how the impulse travels down the motor neurone to the junction.

Predicting the effect of drugs

AQA says students should be able to use information provided to predict and explain the effects of specific drugs on a synapse, without recalling the names of drugs. The method is the same every time: find the step the drug changes, then follow the chain forward.

Worked example: a calcium channel blocker

A drug stops calcium ions entering the motor neurone ending. Predict the effect on the muscle.

1. Step affected: calcium can't enter the neurone ending when the action potential arrives.
2. So the vesicles don't release acetylcholine into the cleft.
3. No acetylcholine binds to the receptors, so the sodium channels on the motor end plate stay closed.
4. There is no depolarisation and no action potential in the muscle fibre.
5. So the muscle fibre doesn't contract, even though the motor neurone is still carrying impulses.

The same approach works for any drug in a question: one that copies acetylcholine, one that blocks its receptors, or one that changes how fast it is broken down. Each changes a different step, so write out the chain from that step onwards.

Common mistakes

• Saying the impulse jumps across the gap. Acetylcholine diffuses across the synaptic cleft; the electrical signal does not cross.
• Forgetting calcium at the neurone ending. Calcium entering the neurone is what triggers acetylcholine release.
• Mixing up the two lots of calcium. Calcium enters the neurone ending to release ACh; separately, the sarcoplasmic reticulum releases calcium inside the muscle fibre for contraction.
• Leaving out acetylcholinesterase. Without it the receptors would stay activated and the muscle wouldn't relax properly.
• Saying the neuromuscular junction joins two neurones. It joins a motor neurone to a muscle fibre.

Synapses and muscles are A-level only in AQA's specification, so practise them on today's A-level Biology: Year 2 short timed paper. The human physiology course covers neurones and reflexes.

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