What triggers the release of neurotransmitters into the synaptic cleft?

The release of neurotransmitters into the synaptic cleft is primarily triggered by the influx of calcium ions (Ca²⁺) into the presynaptic terminal. When an action potential reaches the presynaptic terminal, voltage-gated calcium channels open, allowing Ca²⁺ to flow into the neuron. This influx of calcium ions is crucial because it initiates a series of intracellular events that culminate in the fusion of neurotransmitter-containing vesicles with the presynaptic membrane. This fusion process releases neurotransmitters into the synaptic cleft, leading to signal transmission to the postsynaptic neuron.

Calcium ions play a critical role in this process because they act as a signaling molecule. The elevated intracellular calcium concentration prompts various proteins to facilitate the docking and release of vesicles. Therefore, without the influx of Ca²⁺, neurotransmitter release would not occur effectively, underscoring the significance of this ion in synaptic transmission.