Which event most directly triggers the exocytosis of acetylcholine (ACh) from synaptic vesicles at the presynaptic terminal?

• A. Influx of Ca^{2+} ions into the presynaptic neuron
• B. Depolarization of the postsynaptic membrane
• C. Binding of acetylcholine to postsynaptic receptors
• D. Efflux of Na^{+} ions from the presynaptic neuron

Answer: (A)

Verified step by step guidance

1. Understand the context: Exocytosis of acetylcholine (ACh) occurs at the presynaptic terminal of a neuron during synaptic transmission. This process involves the release of neurotransmitters into the synaptic cleft to communicate with the postsynaptic neuron.
2. Identify the key trigger: Exocytosis is typically initiated by an influx of calcium ions (Ca^{2+}) into the presynaptic neuron. This influx occurs when voltage-gated calcium channels open in response to the depolarization of the presynaptic membrane.
3. Clarify the role of Ca^{2+}: Calcium ions bind to specific proteins on the synaptic vesicles, such as synaptotagmin, which facilitates the fusion of the vesicle membrane with the presynaptic membrane, leading to the release of ACh into the synaptic cleft.
4. Eliminate incorrect options: Depolarization of the postsynaptic membrane, binding of acetylcholine to postsynaptic receptors, and efflux of Na^{+} ions from the presynaptic neuron are not directly involved in triggering exocytosis of ACh. These events occur either downstream or are unrelated to the exocytosis process.
5. Conclude: The influx of Ca^{2+} ions into the presynaptic neuron is the event that most directly triggers the exocytosis of acetylcholine from synaptic vesicles.