Answer:
4,3,1,5,2
4. Vesicles full of acetylcholine are stored at the axon terminal.
3. Action potential arrives at the neuromuscular junction.
1. Action potential is propagated in the sarcolemma.
5. Acetylcholine is released into the synaptic cleft.
2. Acetylcholine binds to ligand gated sodium channels.
Explanation:
Nerve impulse is generated from somatic motor neurons and reaches neuromuscular junction. The nerve impulse specifically reaches a region named 'synaptic end bulbs' at axon terminal. These synaptic end bulbs are in turn connected to the motor end plate region of a sarcolemma.
As soon as the nerve impulse reaches neuromuscular junction, it triggers the opening of voltage gated ion channels which in turn facilitate the influx of Ca²⁺ from the extracellular fluid to the inside of neuron where synaptic vesicles containing neurotransmitter acetylcholine are already stored.
The Ca²⁺ causes the exocytosis of synaptic vesicles which fuse with the plasma membrane of motor neuron as a result of which acetylcholine is released in the synaptic cleft.
The acetylcholine further binds with the acetylcholine receptor which is present at the motor end plate of a skeletal muscle fibre. These receptors are ligand gated ion channels.
Binding of acetylcholine with its receptor causes the ion channels to open and cations like Na⁺ enter the muscle fibre so as to make the electric potential inside the muscle positive.
The following events of the neuromuscular junction in the order in which they occur - 4,3,5,2,1
4. Vesicles full of acetylcholine are stored at the axon terminal
3. Action potential arrives at the neuromuscular junction
5. Acetylcholine is released into the synaptic cleft
2. Acetylcholine binds to ligand-gated sodium channels
1. Action potential is propagated in the sarcolemma
Before the action potential arrives at the axon terminal, synaptic vesicles containing neurotransmitters are present (stored) in the axon terminal. Once the action potential arrives at the neuromuscular junction, the membrane of the axon terminal is depolarised and causes the release of acetylcholine into the synaptic cleft triggered by calcium influx.
Acetylcholine released into the synaptic cleft binds to ligand-gated sodium channels present on sarcolemma and causes depolarization of sarcolemma resulting in the generation of action potential and then the action potential propagates in the sarcolemma.
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