Neurobiology Faculty Candidate Talk: Chen Ran: The Coding of Visceral Senses in the Brain

The discoveries of the coding principles of vision, somatosensation, olfaction, gustation, and audition are all landmark achievements. By contrast, little is known about how the brain encodes information from the internal organs to generate our visceral senses. This critical missing piece of the puzzle prevents a complete understanding of how the brain encodes sensory information to control behavior.
Most internal senses are signaled to the brain by the vagus nerve, which directly connects the visceral organs to the brain. The vagus nerve then relays internal signals to the NTS of the brainstem, the first information-processing hub of bodily cues in the brain. Here we developed a novel in vivo two-photon mouse brainstem calcium imaging platform. This system allowed us, for the first time, to record the activities of thousands of NTS neurons with single-cell resolution and to visualize the sensory map directly. This innovative approach opened a new chapter of viscerosensory biology and enabled us to make a series of discoveries: (1) Diverse chemo- and mechanosensory signals within the same organ are represented by overlapping NTS neuronal populations; (2) by contrast, different internal organs are encoded by distinct neuronal populations that form dedicated "labeled lines", each of which comprises heterogeneous cell types; (3) the brain creates a point-to-point map to represent internal organs, forming a "visceral homunculus" in the brainstem that arises from the salt-and-pepper representation in the vagal ganglia; (4) the spatial map of organs depends on local inhibition. When inhibition is blocked, neurons representing specific organs become responsive to multiple organs, and the brainstem map is blurred. Our study provides the first systematic analysis of the coding logic of visceral senses, laying the foundation for future research to understand interoceptive codes throughout the brain.