Effect of pharmacological inhibition of the pontine respiratory group on swallowing interneurons in the dorsal medulla oblongata.

OBJECTIVES: To examine the role of neurons of the pontine respiratory group (PRG) overlapping with the Kölliker-Fuse nucleus in the regulation of swallowing, we compared the activity of swallowing motor activities and interneuron discharge in the dorsal swallowing group in the medulla before and after pharmacological inhibition of the PRG. METHODS: In 23 in situ perfused brainstem preparation of rats, we recorded the activities of the vagus (VNA), hypoglossal (HNA), and phrenic nerves (PNA), and swallowing interneurons of the dorsal medulla during fictive swallowing elicited by electrical stimulation of the superior laryngeal nerve or oral water injection. Subsequently, respiratory- and swallow-related motor activities and single unit cell discharge were assessed before and after local microinjection of the GABA-receptor agonist muscimol into the area of PRG ipsilateral to the recording sites of swallowing interneurons. RESULTS: After muscimol injection, the amplitude and duration of swallow-related VNA bursts decreased to 71.3 ± 2.84 and 68.1 ± 2.76 % during electrically induced swallowing and VNA interburst intervals during repetitive swallowing decreased. Similar effects were observed for swallowing-related HNA. The swallowing motor activity was similarly qualitatively altered during physiologically induced swallowing. All 23 neurons were changed in either discharge duration or frequency after PRG inhibition, however, the general discharge patterns in relation to the motor output remained unchanged. CONCLUSION: Descending synaptic inputs from PRG provide control of the primary laryngeal sensory gate and synaptic activity of the PRG partially determine medullary cell and cranial motor nerve activities that govern the pharyngeal stage of swallowing.

Firing characteristics of swallowing interneurons in the dorsal medulla during physiologically induced swallowing in perfused brainstem preparation in rats.

Oropharyngeal swallowing is centrally mediated by a swallowing central pattern generator (Sw-CPG) in the medulla oblongata. The activity of the Sw-CPG depends on the sensory inputs determined by physical and chemical bolus properties. Here we investigate the sensory-motor integration during swallowing arising from different sensory sources. To do so we electrically stimulated the superior laryngeal nerve and we triggered swallowing with oral injections of distilled water or capsaicin solution and extracellularly recorded from swallowing interneurons in arterially perfused brainstem preparations of rats. We recorded the activities of 40 neurons, while monitoring the motor activities of the phrenic, vagal and hypoglossal nerves. Eighteen neurons responded to electrical stimulation of the ipsilateral superior laryngeal nerve, and 6 neurons were excited by oral fluid injection, while 16 non-respiratory neurons did not receive afferent inputs to either electrical or physiological stimuli. The cellular activities displayed by swallowing interneurons during electrical and physiological stimulation of pharyngeal and laryngeal afferent input reveal complex adaptations of the timing of firing patterns and frequencies. The modulation of neuronal activity is likely to contribute to the coordination of efficient bolus transfer during the pharyngeal stage of swallowing.