Your nervous system extends over your entire body, the brain doesn't live in a firewalled jar.
replyTo extend the metaphor, the brain may have a robust firewall, but it also transacts with millions of clients over a separate (electric rather than chemical) network.
replySee: vagal neurons and vagus nerve. Serotonin from gut directly impacts those. Your brain still directly interprets that signal from the vagus nerve and uses that to up and down regulate mood and arousal. Impact is still significant
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