The primary direct stimulus for excitation of central chemoreceptors is

The central chemoreceptors monitor the H+ concentration of cerebrospinal fluid (CSF), including the brain interstitial fluid.CO2 readily penetrates membranes, including the blood-brain barrier, whereas H+ and HCO3- penetrate slowly. The CO2 that enters the brain and CSF is promptly hydrated. The H2CO3 dissociates so that the local H+ concentration rises. The H+ concentration in brain interstitial fluid parallels the aerial PCO2. Experimentally produced changes in the PCO2 of CSF have minor, variable effects on respiration as long as the H+ concentration is held constant, but any increase in spinal fluid H+ concentration stimulates chemoreceptorsRef: Ganong's Review of Medical Physiology Twenty-Third Edition, Page No: 629