The following steps refer to various stages in transmission at a chemical synapse.
1. Neurotransmitter binds with receptors associated with the postsynaptic membrane.
2. Calcium ions rush into the neuron's cytoplasm.
3. An action potential depolarizes the membrane of the presynaptic axon terminal.
4. The ligand-gated ion channels open.
5. The synaptic vesicles release a supply of neurotransmitters into the synaptic cleft.

Which sequence of events is correct?

a) 1-2-3-4-5
b) 1-3-5-4-2
c) 3-2-5-1-4
d) 3-4-1-2-5

The correct option is;

c) 3-2-5-1-4

Explanation:

1. Upon reaching the axon terminals, the action potential depolarizes the membrane.

The voltage-gated calcium channels present in the cell membrane are activated when the action potential reaches the axon terminal.

2. Due to the higher concentration gradient, the calcium ion, Ca²⁺, which are present at a higher concentration outside than inside the neuron rush into the cytoplasm (the cell) of the neuron.

3. The synaptic vesicles fuses with the axon terminal membrane due to the presence of the Ca²⁺ ions and neurotransmitters are released into the synaptic cleft.

4. By diffusing across the synaptic cleft, the neurotransmitters bind to the proteins of the receptor on the postsynaptic cell.

5. The opening or closing of the ligand-gated ion channels in the cell membrane then results from the activation of the postsynaptic receptors leading to depolarization (increasing the positivity of the inside of the cell)  or hyper-polarization (increasing the negativity of the inside of the cell) subject to the present.

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