What occurs when the action potential reaches the axon terminal?

• A. Calcium channels open
• B. Sodium ions flood out
• C. Neurotransmitters are absorbed
• D. Vesicles disintegrate

Answer: (A)

When the action potential reaches the axon terminal, calcium channels open, allowing calcium ions to flow into the neuron. This influx of calcium is crucial because it triggers the release of neurotransmitters stored in vesicles within the axon terminal. The opening of calcium channels is a key step in synaptic transmission, facilitating the communication between neurons by enabling the vesicles to fuse with the presynaptic membrane and release their contents into the synaptic cleft.

The other options do not accurately describe the events occurring at the axon terminal when the action potential arrives. Sodium ions do not flood out; in fact, the movement of sodium ions is more relevant to the depolarization phase of the action potential, which occurs along the length of the axon, rather than at the terminal itself. Neurotransmitters are not absorbed; rather, they are released into the synaptic cleft. Additionally, vesicles do not disintegrate; they merge with the membrane to release their contents. Therefore, the correct answer highlights the critical role of calcium in neurotransmitter release during synaptic transmission.