Neuromuscular Junctions (AQA A Level Biology): Revision Note
Exam code: 7402
Neuromuscular junctions
Neuromuscular junctions are specialised synapses located at the junctions between neurones and muscles
The presynaptic cell is a motor neurone
The postsynaptic cell is a muscle cell
Transmission at a neuromuscular junction
When an impulse travelling along the axon of a motor neurone arrives at a neuromuscular junction, the signal is transmitted at the synapse as follows:
calcium ion channels open and calcium ions diffuse into the neurone
vesicles containing the neurotransmitter acetylcholine (ACh) fuse with the presynaptic membrane
ACh is released and diffuses across the cleft where it binds to receptor proteins on the sarcolemma
sodium ion channels in the sarcolemma open, allowing sodium ions to diffuse into the muscle cell
the sarcolemma is depolarised, and if a threshold is reached then an action potential is generated
the action potential is transmitted into the interior of the muscle cell via deeply infolded regions of the sarcolemma known as T-tubules
voltage-gated calcium ion channels in the membranes of the sarcoplasmic reticulum open
calcium ions diffuse out of the sarcoplasmic reticulum and into the sarcoplasm surrounding the myofibrils
the protein filaments inside muscle cells move, and muscle contraction occurs
When stimulation ends the enzyme acetylcholinesterase (AChE) breaks down ACh and the products are reabsorbed by the presynaptic cell
Comparing cholinergic synapses and neuromuscular junctions
As neuromuscular junctions are a types of synapse, they share many features with cholinergic synapses, e.g. in both:
calcium ions trigger the release of neurotransmitter
acetylcholine is the neurotransmitter
acetylcholine moves across the cleft by diffusion
acetylcholine binds to receptors on the postsynaptic membrane to open sodium ion channels
acetylcholine is broken down by AChE
There are also many differences between the two types of synapse:
Cholinergic synapses | Neuromuscular junctions |
|---|---|
Between two neurones | Between a motor neurone and a muscle cell |
Postsynaptic membrane has no folds | Postsynaptic membrane has folds in which AChE can be stored |
No T-tubules | T-tubules are present |
Can be excitatory or inhibitory | Always excitatory |
ACh binds to receptors on postsynaptic cell | ACh binds to receptors on muscle cell |
Action potential may be initiated and transmitted onwards | The end of a nerve pathway |
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