Why is there no neurotransmitter release from the axon terminal when there are no calcium ions in the extracellular solution? 1) Neurotransmitter channels in the nerve terminal are Ca²⁺ dependent 2) Action potential propagation in the axon is Ca²⁺ dependent 3) Exocytosis of the synaptic vesicles is Ca²⁺ dependent 4) Our visualization of neurotransmitter release is Ca²⁺ dependent