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I understand the origin of cranial nerves are within the blood brain barrier. I'm assuming as they extend distally there's a point at which their vasculature is not encapsulated by the blood brain barrier. Is this true? Any idea where the blood brain barrier stops?

Thank you.

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    $\begingroup$ Did you google (e.g.) Blood Brain Barrier? The Blood Brain Barrier isn't a membrane or sac, like the dura mater. It exists because of capillary structure within the central nervous system. The origin of all nerves (anything coming off the brain or spinal cord) is "within the BBB". I'm unsure of what you're asking. $\endgroup$ – anongoodnurse Oct 19 '17 at 7:54
  • $\begingroup$ Thanks anongoodnurse. I've researched it quite a bit. Although I haven't found anything to confirm or deny, I'm assuming the cranial nerves as they emerge from the Pons, etc. would be encapsulated in the BBB. I'm also assuming that the BBB doesn't extend all the way along the cranial nerves until they end, because that would mean you have a Blood Brain Barrier outside of the CNS. Somewhere along the way from the Pons to the end of the cranial nerve, it stops. Is this right? $\endgroup$ – Hairgami_Master Oct 19 '17 at 20:21
  • $\begingroup$ I will ask you a question which can answer yours, though I think you still don't quite understand the BBB. How can you administer a drug which can bypass the BBB? Think cancer treatment. I'm sorry to use the Socratic method, but that's largely how doctors are taught. Teach a man to fish and all that. $\endgroup$ – anongoodnurse Oct 19 '17 at 20:41
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I can't find a great answer to this either, but there is a known concept of a CNS-PNS transition zone, specifically as it pertains to myelination of cranial nerves. With the exception of CN 1 & 2 (which are effectively wholely CNS) the other cranial nerves are anatomically similar to spinal nerves, i.e. they're myelinated by Schwann cells and have an endoneurial blood supply. But they are also myelinated by oligodendrocytes for a short distance (varies by nerve) after taking of from their CNS nuclei before reaching that transition zone. According to this paper, that transition zone should mark the end of the astrocyte coverage which is necessary for BBB function. It's not a dead-ringer of an answer but this explanation makes as good physiologic sense as anything I've seen.

https://www.ncbi.nlm.nih.gov/pubmed/10697296

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