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  • It goes without saying, but you really don't want toxic chemicals in your brain. Luckily,

  • our brains have a special feature called the blood brain barrier that selectively lets

  • good things in and keeps bad things out. And I find this feature fascinating because it

  • takes advantage of cells and nervous system architecture that don't get a lot of attention

  • in biology classrooms. Neurons usually get the spotlight since they send nervous impulses

  • around the body. But the blood brain barrier features a different type of cell, glial cells.

  • And as we'll see, these underrated cells make up one of the most important aspects

  • of the central nervous system, and have implications for different diseases. Today, we're talking

  • about how the blood brain barrier keeps one of our most important organs safe from harm.

  • A quick circulatory system 101 heremost of our organs receive oxygen and nutrients

  • the same way. The heart pumps oxygenated blood through arteries into systemic circulation,

  • then those arteries branch out into tinier and tinier blood vessels called capillaries

  • that surround different tissues. Capillaries have really thin walls, and oxygen, carbon

  • dioxide, and other compounds can diffuse through those walls to get in or out of the blood

  • stream. And just like any other organ, the central nervous system, which includes the

  • brain and spinal cord, gets oxygen and nutrients from the blood. But these organs are a little

  • more sensitive to changes in their direct environment, so they need a bit more security

  • around their blood supply to keep themselves stable. That's where the Blood Brain Barrier

  • comes in. It's an anatomical separation between the brain's capillaries and the

  • fluid between brain tissue itself. It's made of a few different structures including

  • special endothelial cells, a sheath of connective tissue, and nervous tissue that surrounds

  • those tiny blood vessels in the brain. This thing is purpose built to be a selectively

  • permeable barrier, so let's take a little tour, starting with the inside of the capillaries.

  • Their walls are considerably more thin than the blood vessels around muscles. But while

  • they're thin, the cells that make up the capillary are joined by  tight junctions

  • which lets almost nothing through. The capillaries around some other organs actually have tiny

  • pores in them, while the brain capillaries do not, so already they're sealed tighter

  • by comparison. Surrounding the capillaries are cells called pericytes that wrap around

  • and support the endothelial cells. They can regulate blood flow by squeezing around the

  • vessel and help with maintenance of the whole barrier. They sit under a layer of mostly

  • connective tissue called the basal lamina which surrounds those vessels to give them

  • some structural integrity. Other tissues have lamina too, but what makes this one unique

  • is how it's a single, continuous piece throughout the barrier. That makes this endothelium different

  • from what we see in systemic circulation. As soon as we leave the capillaries, we see

  • more specialized nervous tissue, but not neurons yet. In particular, we bump up against the

  • end-feet of astrocytes, star-shaped cells which are bound to the basal lamina layer.

  • That positioning lets them interact with the blood vessels. For instance, astrocytes can

  • send signals that contract or dilate the blood vessels that feed the brain. Now, the blood

  • brain barrier isn't totally impenetrable, it's just more choosy with what it lets

  • through. And in order to be more selective, the endothelium has a few transporter proteins

  • embedded within it. These transporters let different nutrients in and waste products

  • out. Importantly, one of these transporters allows glucose through the barrier and into

  • the brain. And the brain alone consumes about 20% of our daily glucose, which makes it the

  • hungriest organ by mass in our bodies. To metabolize all that glucose, the blood brain

  • barrier needs to let oxygen in. But oxygen doesn't actually use a transporter. Some

  • things can pass directly through the endothelial cells. Oxygen and Carbon dioxide are lipid

  • soluble. They diffuse directly across the endothelial cells because the cell's membrane

  • is made of lipids. But that's more of a chemical properties thing than strictly a

  • size thing. Like hydrogen is extremely small, but it's hydrophilic, so it can't pass

  • through the barrierThe issue of what can pass through the membrane becomes an even

  • bigger challenge when scientists try to design drugs for diseases that affect the brain like

  • for brain tumors, Alzheimers and multiple sclerosis. So just how do you get something

  • to sneak past the barrier on purpose? Most molecules that can pass through the blood

  • brain barrier are small and dissolve in lipids. But many drugs that we'd want to deliver

  • are comparatively big and insoluble in lipids. So like, the opposite of what we want. One

  • strategy is to include a special protein with the drug that triggers the transporter proteins

  • to let it through. Another strategy is focused on packaging known, studied drugs into smaller

  • packages that can cross the barrier. Same Greek soldiers, different Trojan HorseThese

  • are the things that neurologists and folks in neuroscience have to deal with for anatomy

  • as unique as the human brain. If you're interested in more brain videos, I think you'll

  • like this one on the regions of the brain. As always, make sure you're subscribed and

  • hit the bell so you get notified when we post a new video. Thanks for watching.

It goes without saying, but you really don't want toxic chemicals in your brain. Luckily,

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B2 brain barrier blood oxygen tissue nervous

What Exactly Is the Blood Brain Barrier?

  • 16 2
    林宜悉 posted on 2021/02/11
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