The primary direct stimulus for excitation of central chemo-receptors is:
High-Yield Explanation
The primary direct stimulus for excitation of central chemo receptors is |H+. Increased in blood pCO2 causes CO2 to diffuse the blood brain barrier with ease where it reacts with water to form carbonic acid acid. This carbonic acid in turn dissociates into H+ and bicarbonate ions. This H+ ions then have a potent stimulatory effect on the central chemo receptors. Ref: Review of Medical Physiology, 22nd Edition By William F Ganong, Page 675;Textbook of Medical Physiology By Khurana, Page 458