Central chemoreceptors are stimulated directly by which substance?

Central chemoreceptors, located in the medulla oblongata, primarily respond to changes in the concentration of hydrogen ions (H+) in the cerebrospinal fluid. While carbon dioxide (CO2) does not directly stimulate these receptors, it plays a crucial role in the regulation of hydrogen ion concentration. When CO2 levels in the blood increase, it diffuses into the cerebrospinal fluid, where it is converted to carbonic acid. This process results in an increase in hydrogen ions, which then stimulates the central chemoreceptors. Therefore, the correct answer highlights that hydrogen ions are the direct stimulants for the central chemoreceptors, making it the most accurate choice in this context.