How does the brain initiate the process of releasing the neurotransmitters?

Just reading up on the basics of neuroscience. Had a basic question on the signal generation. I understand the concepts of sodium, potassium pumps and how the action potential travels through the axon and how the calcium channel signals the release of the neurotransmitters from the vesicles. From this question I also understand what dictates the neurotransmitters released. Understanding the brain: how are neurotransmitters released in the brain?

my question – how does this process begin at the brain? For example, how does the hypothalamus decide to to send the initial signal to the set of neurons to release norepinephrine? Is this via a similar sodium/potassium mechanism?