Question

question 2
the entrance of ____ into the presynaptic neuron triggers the release of a neurotransmitter.
enter answer

Explanation:

When an action - potential reaches the presynaptic terminal, voltage - gated calcium channels open. The influx of calcium ($Ca^{2+}$) ions into the presynaptic neuron causes synaptic vesicles to fuse with the presynaptic membrane and release neurotransmitters into the synaptic cleft.

Answer:

Calcium ions ($Ca^{2+}$)