Neuronal activity in nucleus ambiguus during deglutition and vocalization in conscious monkeys

Extracellular recordings were made from the nucleus ambiguus in three conscious Macaca nemestrina monkeys during spontaneous vocalizations and swallows. The temporal relationship of neuronal activity to swallowing was inferred through correlation with the thyroarytenoid electromyographic (EMG) activity. Videofluoroscopic analysis of a fourth monkey during swallows of barium-impregnated fruit juice established the temporal relationship between swallowing and thyroarytenoid EMG activity. Of 691 cells recorded from the nucleus ambiguus and its adjacent area, the neuronal activity of 80 cells showed modulation during swallowing. Sixty-two cells were classified as "active" cells, with increased activity in relation to swallowing, while 18 cells were classified as "suppressed" cells, with tonic activity that reduced with swallowing. A continuum of latency was seen between the onset of modulation of these cells and the onset of swallowing, from "early" before the swallow to "late" after the swallow onset with most of the cells (44 cells) showing modulation near the onset of the swallow. A majority (37) of the 62 active swallowing-related cells also discharged with vocalization, but they demonstrated a lower discharge frequency and a longer burst duration during swallowing. Of the 18 suppressed swallowing-related cells, 11 cells discharged phasically with the respiratory cycle and may be a part of the ventral medullary respiratory center. This chronic awake primate model would preserve the many sensory inputs that may modify the oropharyngeal swallow and may better approximate human physiology.