Many medications introduced into the bloodstream cannot directly affect the neurons of the cns because

The answer to the question above is this: the endothelium of CNS capillaries forms a blood-brain barrier. The blood-brain barrier is actually a mechanism that the capillaries that carry blood to the central nervous system can do. What this barrier does is that it filters and only allows certain substances (and not all), that's why this cannot directly affect the neurons in the central nervous system.