Blood Brain Barrier (BBB)

In this lesson we’re going to talk about the blood brain barrier. We’ve mentioned it before, but we want to clear up a few things about what it isn’t, as well as what it is and why it exists.

First off, we want to make it clear that the blood brain barrier is not a physical structure, it’s not something that blocks things at the base of the brain before they come in. What it actually is is a set of characteristics within the vessels and cells in the brain that prevent certain substances from entering. It’s like a bouncer in a club. It is very selective of who or what is allowed in.

So if you remember from the first lesson, the blood brain barrier is controlled by the astrocytes. You can see here, this is a blood vessel and the astrocytes are surrounding it completely. They’re right up against each other and they form what are called tight junctions. Those tight junctions are what help to prevent certain substances from entering into the actual nerve tissue. It’s very selective and protective of the brain. Things it keeps out are things like chemicals and neurotoxins, certain medications – which we’ll see in a second, and many pathogens. The super small pathogens like bacteria can sometimes sneak past if they find a weakness somewhere in these tight junctions. And there are also ways we can manipulate medications to help carry them across the blood brain barrier. It’s like getting into the club with a VIP.

So sometimes it’s hard to look at these anatomy and physiology topics and see how it affects our patients, so we’re gonna show you two practical examples of where this comes into play with our patients. There are many more than this, but hopefully it helps you see why this is an important topic to understand.

The first practical application is bacterial meningitis. These microscopic bacteria, as I’ve said, can find their way into the meninges of the brain and cause an infection. The problem is that most of our common antibiotics actually can’t cross the blood brain barrier. That’s why you’ll hear that bacterial meningitis is so much harder to treat than viral or other causes. The medications we have can’t actually get to the infection. So a lot of times we can just support the patient and give anti-inflammatories, but there’s also an option for the intrathecal route, which is administering medications directly into the CSF, usually through an epidural. This isn’t common, but it is an option. You can learn more about meningitis in the meningitis lesson later in the course.

The second practical application is in Parkinson’s disease. The primary issue in Parkinson’s is an imbalance of dopamine in the brain. The drug levodopa helps to increase dopamine levels. The problem is that it is metabolized into dopamine before it can cross the blood brain barrier. That dopamine can’t actually cross the blood brain barrier. SO…what we do to trick the system and get the levodopa into the brain is we attach carbidopa to it. This prevents it from metabolizing and helps carry the levodopa across the blood brain barrier. So I remember it this way: Carbidopa Carries and Levodopa Levels. We’ll talk more about Parkinson’s in the Parkinson’s lesson later in this course.

So let’s recap. The blood brain barrier acts like a bouncer to prevent certain substances from entering the nerve tissues. It is controlled by astrocytes that form tight junctions around the capillaries in the brain. Practically, this becomes an issue when we need to get medications across the blood brain barrier. Sometimes there are solutions and other times we have to find other ways to support the patient.

Okay, that’s it for our refresher on neuro anatomy. Be sure to refer back to this module as we begin talking about disease processes if you need a better understanding of something. Now, go out and be your best selves today! And, as always, Happy nursing!