Medullary neuronal activities in gasping induced by pharyngeal stimulation and hypoxia

Abstract

We examined the hypothesis that medullary respiratory-related and non-respiratory-related neuronal activities are similarly altered with the 'aspiration reflex', induced by mechanical stimulation of the epipharyngeal mucosa, and gasping, induced by severe hypoxia. Extracellular neuronal activities were recorded in decerebrate, paralyzed and ventilated cats. Phrenic activity and neuronal activities were monitored in eupnea and gasping. Seventy-one unit activities were recorded in the lateral medulla including the nucleus tractus solitorii (NTS), lateral tegmental field (LTF) and the nucleus ambiguus (NA). The respiratory modulation of a neuronal activity was quantified by a η 2 statistic (Orem, J. and Dick, T., 1983, J. Neurophysiol. 50: 1098-1107). The η 2 values of the units ranged from 0.02 to 0.93. Inspiratory-related activities with relative high η 2 values (n = 16) were recorded in the region closed to the NTS. Phase-spanning (n = 7) and expiratory related activities (n = 10) were recorded in the ventral medullary region. Units with low η 2 values (n = 29) and with no spontaneous activity (n = 9) in eupnea were recorded in the region of the LTF. In both 'aspiration reflex and gasping, inspiratory-related activities were augmented and expiratory-related activities were suppressed. Tonic units were activated and additional activities were recruited. The modulation of the neuronal activities to gasping induced by anoxia was identical to that induced by pharyngeal stimulation in either hyperoxia or severe hypoxia. We concluded that medullary gasping mechanism is recruited by pharyngeal stimulation. In addition, the present findings are compatible with the idea that different brainstem mechanisms are responsible for the control of eupnea and gasping.