Ambiguous motoneurons discharging synchronously with ultrasonic vocalization in rats

Unit discharges associated with ultrasonic vocalization were recorded from the rat medullary region including the nucleus ambiguous (NA). Three types of units were distinguished: ultrasound-related (US), expiration (EXP) and inspiration (INSP) units. Both US and EXP units discharged with tonic bursts mostly prior to ultrasounds. US units were silent during respiration while EXP units discharged synchronously with expiration. During ultrasonic vocalization INSP units suppressed their discharges which were otherwise synchronized with inspiration. Histological examination revealed that US and EXP units were recorded from the ventral division of the NA and the adjacent medullary region where the NA contained large cells and the adjacent region consisted of small cells. These results suggest that both US and EXP units are NA motoneurons responsible for ultrasound emission. EXP units were always supplied with respiratory afferents while US units were not.