A Genetic, Anatomical, and Functional Analysis of Nucleus Ambiguus Neurons Controlling Digestive and Cardiorespiratory Functions

The nucleus ambiguus (nAmb), a region found within the medullary reticular formation of the brainstem, controls aspects of swallowing, vocalization, and cardiorespiratory function through its innervation of the pharynx, striated esophageal muscle, larynx, heart, and airways, covering a diverse range of physiological functions. Despite the importance of this region, little was known about what neuronal subtypes exist within the nucleus ambiguus, which organ(s) each subtype innervates, and how they control physiological functions. Thus, identification of these neuronal subtypes within the nucleus ambiguus and their control of respiration, heart function, vocalization, and swallowing were significant knowledge gaps in both the field of neuroscience as well as cardiovascular, respiratory, and digestive medicine. The following studies outlined in this thesis have identified three molecularly distinct neuronal subtypes localized in the nucleus ambiguus. Based on their gene expression and previous studies, we predicted that each molecularly distinct neuron subtype has a different physiological role. We used recombinase-driver mice to access each nucleus ambiguus neuron subtype. Our data shows that one of these nucleus ambiguus subtypes innervates the esophagus; another innervates the pharynx and larynx; and a third innervates the heart. Furthermore, when we optogenetically activated the subtype that innervated the esophagus, we saw esophageal contraction, but no change in heart rate. When we optogenetically activated the subtype that innervated the heart, we saw an immediate and drastic decrease in heart rate. We next focus on elucidating the functional role of the third subtype in pharyngeal and laryngeal control, as well as the necessity of these nucleus ambiguus neuronal subtypes for functional control. Taken together, this data supports the model that the nucleus ambiguus is composed of multiple genetically defined labeled lines that project to individual organs and control those organ related functions. Overall, these studies provide three major advances to understanding the neuronal control of esophageal and cardiorespiratory function: (1) identifies the primary motor and parasympathetic neurons for the esophagus and heart molecularly, anatomically, and functionally, (2) reveals a genetic logic for the functional organization of the nucleus ambiguus; and (3) comprehensively characterizes the gene expression profile of esophageal motor neurons and cardiac vagal preganglionic neurons, which can be mined for potential drug targets to treat swallowing and cardiorespiratory disorders.