This is a side view of a human brain, facing left. The lights are showing you a few of the areas that use the neurotransmitter acetylcholine. Let's look at one neuron close up. Here's a neuron that starts in the septal nucleus of the hypothalamus and sends its axon to the hippocampus. Let's follow the action potential down the axon to the terminal to see how acetylcholine is synthesized.