Carbon dioxide sensitivity of the central chemosensitive mechanisms. An exploration by direct stimulation in rats.

In urethane-anaesthetised adult albino rats ventral surface of the brainstem was stimulated chemically by increasing the local CO2 concentration and electrically. Two areas were demarcated on the ventral surface of the brainstem, one which showed an increase in pulmonary ventilation on chemical and electrical stimulation, and another which showed a decrease in pulmonary ventilation and sometimes even respiratory arrest. EEG activity recorded from the area from where increased pulmonary ventilation was obtained showed a synchronous slow wave activity during chemical stimulation and inhalation of a CO2-air mixture. This area is situated 0.5--1 mm lateral to the mid-line extending up to the rootlets of the VIIth to IXth cranial nerves. The response increased proportionately on increasing the strength of the chemical stimulus, till it reached a plateau. In carotid body denervated and chronic hypoxic animals, the magnitude of the responses was shown to be increased, probably due to increased sensitivity of the central chemosensitive mechanisms.