The gut-brain connection is well established, but the mechanisms have been murky. Researchers now report a previously unknown fast neural pathway: specific gut bacteria produce metabolites that directly activate a population of sensory neurons in the intestinal wall, which project via the vagal nerve to the nucleus tractus solitarius in the brainstem in under 100 milliseconds — faster than the time it takes for a signal to reach the prefrontal cortex. This pathway is distinct from the hormonal and immune channels previously identified, and it's active. Disrupting it in mice causes measurable anxiety-like behavior within 48 hours.