The steady state response of brainstem respiratory neuron activity to various levels of PA,CO2 and PA,O2.

Extracellular recordings were made of 52 respiratory neurons in the brainstem of cats, anesthetized (chloralose-urethane), vagotomized and artificially ventilated. Phrenic nerve activity was recorded and quantified as an index of the output of the respiratory neuronal organization in the brainstem. The unit activity was quantified by using the modal spike frequency as a possible indication of the activating effect of one unit on other respiratory neurons (Smolders and Folgering, 1979). Inspiratory neurons showed the strongest reaction to changes in PA,CO2 and/or PA,O2. Expiratory neurons and frequency modulated neurons responded less to changes in chemical drive. Phase spanning neurons did not show any consistent response. Four out of ten continuously firing neurons without any respiratory rhytmicity increased their firing frequency when PA,CO2 was increased. Apart from the increase in modal spike frequency, the respiratory neuronal organization also reacted with an increase in active units (recruitment) when the chemical drive was increased. The relationship between quantified phrenic nerve activity and spike frequency was independent of the stimulus (hypercapnia or hypoxia). A model was developed in which the increase in modal frequency of a unit arouses other units: when the chemical drive increases, progressively more units tend to be recruited into the respiratory neuronal organization in the brainstem.