Many medications introduced into the bloodstream cannot directly affect the neurons of the CNS because the neurilemma is impermeable to most molecules forming a blood-brain barrier. oligodendrocytes form a continuous myelin sheath around the axons. ependymal cells restrict the flow of interstitial fluid between the capillaries and the neurons. Schwann cells form a capsule around neurons. C the astrocytes isolate the CNS by forming a blood-brain barrier