Final answer:
There is no neurotransmitter release from the axon terminal without extracellular calcium ions because they are essential for the fusion of neurotransmitter vesicles with the presynaptic membrane and subsequent neurotransmitter exocytosis.
Step-by-step explanation:
Neurotransmitter release at a synapse relies on the action potential reaching the axon terminal and causing a depolarization of the presynaptic membrane. This depolarization triggers voltage-gated calcium channels to open, allowing Ca²+ ions to enter the cell. The influx of calcium ions is critical because they bind to proteins on the neurotransmitter vesicles and facilitate their fusion with the presynaptic membrane, leading to exocytosis of the neurotransmitter into the synaptic cleft.
Without calcium ions in the extracellular solution, this process is hindered, preventing neurotransmitter release and thus signal transmission to the postsynaptic neuron. The voltage-gated calcium channels would not open, preventing neurotransmitter release.