Final answer:
Option D: Voltage-gated Ca²⁺ channels are required for neurotransmitter secretion in response to an action potential, as they permit calcium ion influx which triggers synaptic vesicle fusion and neurotransmitter release.
Step-by-step explanation:
The process of neurotransmitter secretion in response to an action potential requires the presence of voltage-gated Ca²⁺ channels. When an action potential reaches the axon terminal, it triggers the opening of these channels, allowing for an influx of Ca²⁺ ions into the cell. These calcium ions play a critical role in the mechanism that causes synaptic vesicles to fuse with the presynaptic membrane and release their cargo of neurotransmitters into the synaptic cleft. This release mechanism is a vital step in the transmission of signals across a chemical synapse, leading to the continuation or inhibition of the action potential in the postsynaptic cell.
The correct option for the secretion of neurotransmitters in response to an action potential is d. Voltage-gated Ca²⁺ channels. When an action potential reaches the axon terminals, voltage-gated Ca²⁺ channels in the membrane of the synaptic end bulb open. The entry of Ca²⁺ ions into the cell causes synaptic vesicles to fuse with the membrane at the nerve ending, releasing neurotransmitters into the synaptic cleft.