This may be a dumb question. I'm not a Neuroscientist. I'm just trying to learn more about the brain to improve my AI learning algorithms…

I understand that there are different kinds of neurotransmitters that either increase or decrease the possibility of firing an electrical impulse through the axon. I also understand that these Neurotransmitters do not actually pass through the whole cell (Correct me if I'm wrong…), so they only travel from the Cell's terminal to the next cell's body. 


So my question is: How do these Neurotransmitters get into the cell's terminal? Blood? And the more important question: How are these Neurotransmitters distributed among the different neurons? It cannot be at random, or equally distributed, because we have different "reactions" to different Neurotransmitters. For example if Dopamine is produced it gets sent to these different cells in our brain which makes the activation of its cell more likely (if I understood correctly). Dopamine is also known to increase our feelings of lust and makes us more focussed (and more). But if Dopamine got sent to ALL of the Neurons in our brain, thus make the activation of EVERY cell more likely, it would bring total chaos to our brain. (Eg. Make our fear-part of the brain activate more easily, but this is not the case.). So how are these Neurotransmitters distributed?



Thanks in advance