The blood-brain barrier (BBB) is a layer of specialised cells lining the blood vessels in the brain. Structures called tight junctions seal it together.
Its job is to be selective:
- it lets in what the brain needs (oxygen, glucose, amino acids, certain nutrients)
- and keeps out what could harm it (toxins, pathogens, most of what circulates in the bloodstream at any given moment).
The barrier ensures that swallowing something, digesting it, and absorbing it into your bloodstream doesn’t automatically mean it reaches your brain.
What can cross through the barrier?
Some substances cross easily through passive diffusion because they’re small and fat-soluble enough to pass through the cell membranes.
Most psychiatric medications, including stimulants prescribed for ADHD, are designed to cross this way. Others are actively transported across, and some are actively pumped back out by efflux proteins even after they’ve crossed in. Genetic variants in the genes coding for these pumps can mean that two people taking the same medication at the same dose end up with different amounts reaching their brains.
Blood-brain barrier and neurodivergent brains
Research on the blood-brain barrier in neurodivergent conditions is still in its early stages. The strongest evidence comes from autism, where postmortem tissue shows altered expression of tight junction proteins in both the cortex and cerebellum, alongside increased markers of neuroinflammation.1 For ADHD, preliminary studies hint at blood-brain barrier changes, but the evidence base is still developing.2
For a fuller picture of how this fits into the neurodivergent brain’s anatomy and function, see our neurodivergent brain entry.
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