The enteric nervous system contains ~500 million neurons — more than the spinal cord — earning the gut its nickname as the "second brain." The vagus nerve serves as a bidirectional highway: 80% of its fibers carry sensory signals from gut to brain (afferent), while 20% carry motor commands downward (efferent). Gut bacteria produce ~90% of the body's serotonin, short-chain fatty acids (SCFAs) like butyrate that cross the blood-brain barrier, GABA precursors, and tryptophan metabolites affecting mood. Dysbiosis — microbial imbalance — is linked to depression, anxiety, and neurodegenerative disease via inflammatory cytokine cascades and altered vagal tone.