Final answer:
Calcium ions (Ca²⁺) entering the presynaptic neuron trigger synaptic vesicles to merge with the membrane and release Acetylcholine (ACh), which then binds with receptors on the postsynaptic neuron or muscle cell, facilitating depolarization and signal transmission.
Step-by-step explanation:
The migration of synaptic vesicles and subsequent exocytosis of Acetylcholine (ACh) are primarily caused by an influx of calcium ions (Ca²⁺) into the presynaptic neuron. When an action potential reaches the axon terminal, it triggers the opening of voltage-gated Na⁺ channels followed by the opening of voltage-gated Ca²⁺ channels. This causes Ca²⁺ to enter the cell and bind to proteins on the synaptic vesicles, which then facilitates the vesicles' fusion with the presynaptic membrane. Through the process of exocytosis, ACh is released into the synaptic cleft and diffuses across to bind with ACh receptors on the postsynaptic neuron or muscle cell, leading to depolarization and signal transmission.