The following are events in the transmission of a signal at a chemical synapse.

Neurotransmitter binds with receptors associated with the postsynaptic membrane.
Calcium ions rush into neuron's cytoplasm.
An action potential depolarizes the membrane of the presynaptic axon terminal.
The ligand-gated ion channels open.
The synaptic vesicles release neurotransmitter into the synaptic cleft.

Which sequence of events is correct?

Question

The following are events in the transmission of a signal at a chemical synapse.

Neurotransmitter binds with receptors associated with the postsynaptic membrane.
Calcium ions rush into neuron's cytoplasm.
An action potential depolarizes the membrane of the presynaptic axon terminal.
The ligand-gated ion channels open.
The synaptic vesicles release neurotransmitter into the synaptic cleft.

Which sequence of events is correct?

BlackTom AI · Accepted Answer

To determine the correct sequence, I’ll map each event to its place in the classic chemical synapse transmission sequence and then compare the options.
Option A: 2 → 3 → 5 → 4 → 1
This suggests calcium influx occurs before the action potential depolarizes the presynaptic membrane. Biophysically, calcium entry is triggered by depolarization via voltage-gated calcium channels, so this ordering is incorrect.
Option B: 4 → 3 → 1 → 2 → 5
Here, ligand-gated channels opening is placed before the action potential and before vesicle release. In reality, postsynaptic receptor activation (ligand-gated channels opening) depends on neurotransmitter release, which requires vesicle fusion after Ca2+ influx; so this sequence misplaces the trigger for postsynaptic effects.
Option C: 3 → 2 → 5 → 1 → 4
This sequence starts with the presynaptic membrane depolarization (action potential, 3), followed by Ca2+ influx (2), then vesicle fusion and neurotransmitter release (5), then neurotransmitter binding to postsynaptic receptors (1), and finally ligand-gated channels opening (4). This aligns with the established order: depolarization → Ca2+ entry → vesicle exocytosis → transmitter-receptor binding → postsynaptic channel opening.
Option D: 1 → 2 → 3 → 4 → 5
This places neurotransmitter binding to receptors before depolarization and Ca2+ entry, and even before vesicle release, which cannot occur without the presynaptic action potential and Ca2+ influx. It contradictions the causal sequence of synaptic transmission.
In summary, the only option that preserves the causal dependencies—depolarization triggering Ca2+ influx, which triggers vesicle release, which leads to transmitter binding and subsequent receptor channel opening—is 3 → 2 → 5 → 1 → 4. Thus, option C is the correct sequence. The other options fail because they invert or misplace key causal steps such as Ca2+ entry, vesicle fusion, or transmitter-receptor interaction relative to the triggering action potential and vesicle release.

Bio 93 Section CDE 2025 Final Exam Practice, Part 1

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Bio 93 Section CDE 2025 Final Exam Practice, Part 1

View Explanation

Log in for full answers

Similar Questions

Which of the following explains why one-way synaptic transmission occurs?

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