The ions responsible for the release of acetylcholine (ACh) from neurons are calcium ions (Ca²⁺).

Here's a breakdown of the process:

1. Action Potential Arrival: When an action potential arrives at the axon terminal of a neuron, it depolarizes the membrane.
2. Calcium Channels Open: This depolarization opens voltage-gated calcium channels located in the membrane of the axon terminal.
3. Calcium Influx: Calcium ions (Ca²⁺) flow into the axon terminal from the extracellular space, down their electrochemical gradient.
4. Vesicle Fusion: The increase in intracellular calcium concentration triggers the fusion of synaptic vesicles (which contain ACh) with the presynaptic membrane. Proteins like synaptotagmin act as calcium sensors to trigger vesicle fusion.
5. ACh Release: The fusion of vesicles with the presynaptic membrane results in the release of ACh into the synaptic cleft via exocytosis.

Essentially, the influx of calcium ions is the critical trigger for the exocytosis of neurotransmitters like ACh.

For further reading, you might find these resources helpful:

• "Physiology, Acetylcholine" - StatPearls
• "The molecular mechanism of neurotransmitter release" - Science