The split-respiratory centre in the cat: responses to hypercapnia.

In 22 cats anaesthetized with halothane, vagotomised, paralyzed with d-tubocurarine and artificially ventilated, the medulla was split in the midline and the response of phrenic motoneurones, efferent vagus nerve and medullary respiratory neurones to hypercapnia was studied. In 18 successful experiments the mid-sagittal incision abolished all electrical activities but an inhalation of a 5% CO2-oxygen mixture promptly restored rhythmic firing in both the medullary units and efferent nerves when PaCO2 reached 55 mm Hg on the average (mean pH = 7.20). Switching the ventilation back to normocapnia was followed by a gradual disappearance of the activities usually when PaCO2 and pH returned to control values. We conclude that splitting the brainstem in cat affects the respiratory rhythmogenesis, presumably by increasing the CO2-threshold of medullary respiratory neurones.