Spatio-temporal segregation between sensory relay and swallowing pre-motor population activities by optical imaging in the rat nucleus of the solitary tract.

The nucleus tractus solitarius (NTS) contains neurons that relay sensory swallowing commands information from the oropharyngeal cavity and swallowing premotor neurons of the dorsal swallowing group (DSG). However, the spatio-temporal dynamics of the interplay between the sensory relay and the DSG is not well understood. Here, we employed fluorescence imaging after microinjection of the calcium indicator into the NTS in an arterially perfused brainstem preparation of rat (n = 8) to investigate neuronal population activity in the NTS in response to superior laryngeal nerve (SLN) stimulation. Respiratory and swallowing motor activities were determined by simultaneous recordings of phrenic and vagal nerve activity (PNA, VNA). The analysis of SLN stimulation near the threshold triggering a swallowing allowed us to analyze Ca²⁺ signals related to the sensory relay and the DSG. We show that activation of sensory relay neurons triggers spatially confined Ca²⁺ signals exclusively unilateral to the stimulated SLN at short latencies (114.3 ± 94.4 ms). However, SLN-evoked swallowing triggered Ca²⁺ signals bilaterally at longer latencies (200 ± 145.2 ms) and engaged anatomically distributed DSG activity across the dorsal medulla oblongata. The Ca²⁺ signals originating from the DSG preceded evoked VNA swallow motor bursts, thus the swallowing premotor neurons that drive laryngeal motor pools are located outside the DSG. In conclusion, the study illuminates the spatial-temporal features of sensory-motor integration of swallowing in the NTS and further supports the hypothesis that the NTS harbors swallowing pre-motor neurons that may generate the swallowing motor activity, while first-order pre-motor pools are located outside the DSG.