Final answer:
The conversion of an action potential to a chemical signal occurs when calcium ions enter the axon terminal as a result of the depolarization of the presynaptic membrane at the axon terminal. The correct answer is option (a).
Step-by-step explanation:
The electrical signal known as the action potential is converted to a chemical signal within a process involving neurons and synapses. This transition occurs when an action potential that travels along the axon reaches the very end of the axon, referred to as the axon terminal. At the axon terminal, the influx of calcium ions is a critical step in this conversion from electrical to chemical signaling.
As the action potential depolarizes the membrane of the axon terminal, voltage-gated sodium (Na) channels open, allowing sodium ions to enter the neuron, further depolarizing the presynaptic membrane. Subsequently, this encourages the opening of voltage-gated calcium (Ca²) channels. It's the entry of calcium ions through these channels that signals synaptic vesicles containing neurotransmitters to fuse with the presynaptic membrane, thereby releasing the neurotransmitters into the synaptic cleft. Finally, these neurotransmitters can bind to receptors on the postsynaptic cell, continuing the communication process between neurons.
Therefore, the correct option which describes when the electrical signal of an action potential is first converted to a chemical signal is option a) Calcium ions enter the axon terminal.