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The nucleus retroambiguus (NRA) is a neuronal cell group in the medullary ventrolateral tegmentum, rostrocaudally between the obex and the first cervical spinal segment. NRA neurons are premotor interneurons with direct projections to the motoneurons of soft palate, pharynx, and larynx in the nucleus ambiguus in the lateral medulla as well as to the motoneurons in the spinal cord innervating diaphragm, abdominal, and pelvic floor muscles and the lumbosacral motoneurons generating sexual posture. These NRA premotor interneurons receive very strong projections from the periaqueductal gray (PAG) in the context of basic survival mechanisms as fight, flight, freezing, sound production, and sexual behavior. In the present study in rat we investigated the physiological motor patterns generated by NRA neurons, as the result of vagal, peripheral chemosensory, and nociceptive stimulation. The results show that the NRA contains phasic respiratory modulated neurons, as well as nonphasic tonically modulated neurons. Stimulation in the various rostrocaudal levels of the NRA generates site-specific laryngeal, respiratory, abdominal, and pelvic floor motor activities. Vagal and peripheral chemosensory stimulation induces both excitatory and inhibitory modulation of phasic NRA-neurons, while peripheral chemosensory and nociceptive stimulation causes excitation and inhibition of nonphasic NRA-neurons. These results are in agreement with the concept that the NRA represents a multifunctional group of neurons involved in the output of the emotional motor system, such as vomiting, vocalization, mating, and changes in respiration. [ABSTRACT FROM AUTHOR]

The midbrain periaqueductal gray (PAG) organizes basic survival behavior, which includes respiration. How the PAG controls respiration is not known. We studied the PAG control of respiration by injecting D,L-homocysteic acid in the PAG in unanesthetized precollicularly decerebrated cats. Injections in different parts of the PAG caused different respiratory effects. Stimulation in the dorsomedial PAG induced slow and deep breathing and dyspnea. Stimulation in the dorsolateralPAGresulted in active breathing and tachypnea consistent with the respiratory changes during fright and flight. Stimulation in the medial part of lateral PAG caused inspiratory apneusis. Stimulation in lateral parts of the lateral and ventrolateral PAG produced respiratory changes associated with vocalization (mews, alternating mews and hisses, or hisses). D,L-Homocysteic acid injections in the caudal ventrolateral PAG induced irregular breathing. These results demonstrate that the PAG exerts a strong influence on respiration, suggesting that it serves as the behavioral modulator of breathing. [ABSTRACT FROM AUTHOR]