Laryngeal and respiratory protective reflexes

Abstract

Swallowing is a complex motor behavior that relies on an interneuronal network of premotor neurons (PMNs) to organize the sequential activity of motor neurons that are active during the buccopharyngeal and esophageal phases. Swallowing PMNs are highly interconnected to multiple areas of the brain stem and the central nervous system and provide a potential anatomic substrate integration of swallowing activity with airway protective reflexes. Because these neurons have synaptic contact with both afferent inputs and motor neurons and exhibit a true central activity, they appear to constitute the swallowing central pattern generator. We studied the viscerotopic organization of the nucleus of the solitary tract (NTS), the nucleus ambiguus (NA), the dorsal motor nucleus (DMN), and the hypoglossal nucleus (XII) using cholera toxin horseradish peroxidase (CT-HRP), a sensitive antegrade and retrograde tracer that effectively labels afferent terminal fields within the NTS as well as swallowing motor neurons and their dendritic fields within the NA, DMN, and XII. We used CT-HRP to provide a comprehensive description of the dendritic architecture of NA motor neurons innervating swallowing muscles. We also conducted studies using pseudorabies virus (PRV), a swine alpha-herpesvirus, to map central neural circuits after injection in the peripheral or central nervous systems. One attenuated vaccine strain, Bartha PRV, has preferential affinity for sites of afferent synaptic contact on the cell body and dendrites and a reactive gliosis that effectively isolates the infected neurons and provides a barrier to the nonspecific spread to adjacent neurons. The findings provide a basis for the central integration of swallowing and respiratory protective reflexes.