We know empirically that different neurotransmitters are associated with different emotional states and behaviors. If you don’t believe me, try taking cocaine (dopamine agonist) and seeing if you don’t notice an obvious change in your mood.
As for why this works the way it does? To answer that question you’d need to fundamentally understand how consciousness and subjective experiences arise from physical matter. I don’t think anyone actually knows that – it’s one of the biggest mysteries of the universe.
Any ELI5 for this is going to be a huge simplification. Neurotransmission helps you understand the basics of how one neuron communicates to the next. Unfortunately, in terms of complexity, the human brain has perhaps 86 billions neurons, and trillions to quadrillions of synapses. The gap between understanding one neuron firing or not and understanding mood and behavior is one we’ve been working on since the late 1800s with only partial success. If your question was “How do we go from binary being 0s and 1s to a computer that can beat humans at chess or play movies,” that would be a lot more straightforward. A computer is the closest metaphor for the human brain, and still not a perfect one.
Let’s start with behavior very simply. For a simple worm, it all boils down to moving toward food and mates, but away from threats, and not moving so much that it wastes energy. For this, you need a basic network of neurons. Some will take input from smell receptors to sense food nearby, and send excitatory signals to neurons which in turn signal to make certain cells contract so the worm can move. If the worm has just eaten, input from the full stomach will signal to other neurons which send inhibitory signals to the same cells, discouraging movement as it’s not needed. The sum of these signals will determine the balance between “go” and “no go.”
Mood and behavior are properties that emerge from incredibly complex systems of neurons that we’re still a long way from accurately modeling or understanding.
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