What comprises the blood-brain barrier and its primary function?

• A. Tight junctions between CNS endothelial cells with astrocyte end-feet; restricts passage of substances from blood to brain while permitting selective transport.
• B. Gap junctions between neurons; main function to synchronize neuronal firing.
• C. Myelin sheath around axons; insulate electrical signals.
• D. Fenestrated capillaries with loose tissue; free exchange between blood and brain.

Answer: (D)

The blood-brain barrier is formed by tight junctions between the endothelial cells of CNS capillaries, with astrocyte end-feet helping to regulate the barrier's properties. This arrangement creates a highly selective barrier that restricts most substances in the blood from entering brain tissue while still allowing essential nutrients, ions, and gases to pass through via specific transport mechanisms. In other words, it acts like a gatekeeper, preventing many harmful substances from crossing into the brain while permitting selective, controlled entry of what the nervous tissue needs.

The other descriptions point to different neural features. Gap junctions between neurons synchronize activity but don’t create a barrier to blood-borne substances. The myelin sheath around axons insulates electrical signals, not vascular permeability. Fenestrated capillaries with loose tissue would allow free exchange between blood and brain, which is the opposite of the selective permeability of the blood-brain barrier.