Neuromuscular Junctions and Comparisons to Synapse
Laura Armstrong
Teacher
Contents
Recall Questions
This topic requires prior knowledge of the cholinergic synapse. You can test your knowledge on this below.
What is a synapse?
A junction between two neurones where neurotransmitters transmit signals across a synaptic cleft.
What is the function of acetylcholine in synaptic transmission?
It binds to receptors on the postsynaptic membrane to initiate depolarisation.
What is the name of the enzyme that breaks down acetylcholine?
Acetylcholinesterase.
Topic Explainer Video
What Is a Neuromuscular Junction?
A neuromuscular junction (NMJ) is a synapse between a motor neurone and a skeletal muscle fibre. It allows the nervous system to control muscle contraction.
It is where a motor neurone communicates with a muscle fibre to initiate contraction.
Key Structures at the NMJ
| Structure | Function |
| Motor neurone end bulb | Contains vesicles of acetylcholine (ACh). |
| Synaptic cleft | Gap between neurone and sarcolemma of muscle. |
| Sarcolemma (muscle membrane) | Contains ACh receptors and voltage-gated ion channels. |
| T-tubules | Folds of sarcolemma that carry the depolarisation into the muscle fibre. |
| Sarcoplasmic reticulum | Releases calcium ions (Ca²⁺) into the cytoplasm. |
Step-by-Step: Stimulation of a Muscle Fibre
- Action potential arrives at the axon terminal of a motor neurone.
- Voltage-gated Ca²⁺ channels open, and calcium ions diffuse into the neurone.
- Synaptic vesicles move towards and fuse with the presynaptic membrane, releasing ACh into synaptic cleft via exocytosis.
- ACh diffuses across the synaptic cleft and binds to ACh receptors on the sarcolemma.
- This causes Na⁺ channels to open, leading to depolarisation of the sarcolemma.
- The action potential (wave of depolarisation) travels along the T-tubules, reaching the sarcoplasmic reticulum (SR).
- SR releases Ca²⁺ ions into the sarcoplasm.
- Ca²⁺ initiates contraction via the sliding filament mechanism (covered in prior study note).
- Acetylcholinesterase breaks down ACh in the cleft → choline and ethanoic acid are reabsorbed into the presynaptic terminal.
Comparison: NMJ vs Cholinergic Synapse
| Feature | Neuromuscular Junction | Cholinergic Synapse |
| Postsynaptic cell | Muscle fibre. | Another neurone. |
| Response | Always excitatory (triggers contraction). | Can be excitatory or inhibitory. |
| Neurotransmitter | Acetylcholine. | Acetylcholine. |
| Enzyme present | Acetylcholinesterase. | Acetylcholinesterase. |
| Effect | Triggers muscle action potential. | Triggers or inhibits next neurones action potential. |
| Summation required? | No – one AP usually sufficient. | Yes – temporal or spatial summation often needed. |
| End result | Muscle contraction. | Nerve impulse propagation (or inhibition). |
Key Terms
- Acetylcholine (ACh): Neurotransmitter that binds to receptors on the sarcolemma.
- Acetylcholinesterase: Enzyme that breaks down ACh to stop signal.
- Sarcolemma: Muscle cell membrane.
- T-tubules: Extensions of sarcolemma that carry action potentials into the fibre.
- Sarcoplasmic reticulum: Stores and releases calcium ions.
No answer provided.
Exam Tip
Use precise terminology: "depolarises the sarcolemma" NOT “stimulates the muscle”.
No answer provided.
Describe how an action potential at a neuromuscular junction leads to muscle contraction. (6 marks)
- Action potential arrives at motor neurone terminal → voltage-gated Ca²⁺ channels open.
- Ca²⁺ enters the neurone by diffusion → ACh vesicles fuse with presynaptic membrane and release ACh.
- ACh diffuses across the synaptic cleft and binds to receptors on the sarcolemma.
- Na⁺ channels open and Na⁺ diffuses in → sarcolemma depolarises
- Action potential generated in muscle.
- Action potential spreads via T-tubules → stimulates sarcoplasmic reticulum to release Ca²⁺.
- Ca²⁺ initiates sliding filament mechanism → muscle contracts.
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!