okay to answer your question changes and sympathetic tone have a limited effect on cerebral blood flow at normal paco2 levels however the sympathetic nervous system seems to attenuate the CO2 induced increase in cerebral blood flow this phenomenon May indicate a moderate direct effect of the sympathetic nervous system on the cerebral vascular tour for the more sympathetic activation tends to increase ventilation and dust can indirectly increase cerebral vascular tone. I don't take biology but I'm just saying what i know.
hope this helps