Explain why a nerve has to hit depolarization in order for it to send a signal.

Answer 1

When a neuron accumulates enough depolarizations at the axon hillock to reach the threshold potential, an action potential will be triggered. The action potential will be sent down the axon, all the way to the axon terminal. Once it reaches the axon terminal, it will cause synaptic vesicles (usually containing AcH or any other neurotransmitter) to be exocytosed.

There is a lot of tiny details that I left out since I'm not too sure on how detailed of an answer you wanted. Feel free to send me a message or schedule a lesson if you want a more detailed explanation!