On Meninges

In thinking about the Glymphatic system, one cannot forgo consideration of the meninges in our brains. Well, technically, they aren’t inside our brains – they are along the outside of the Blood Brain Barrier. In my opinion though, the location of these three excitement-filled membranous coverings is not nearly as important as their functions. Also, one more thing - we only have three of these.

The first meningeal membrane is known as something called the Dura mater and is the outermost layer (directly under the bones of the skull and vertebrate column). Because of its location, this layer is thick, tough, and inextensible. In most areas, the dura mater folds inwards to constitute dual reflections which partition and divide the cranium into several compartments. This Dural Mater is largely independent in that it maintains its own vasculature (primarily from the middle meningeal artery and vein) but it is also innervated by the trigerminal nerve. This particular layer of the meninges is most notably applicable to subdural hemorrhages, intracranial bleeds, and of course, meningitis (an inflammation and infection of one or more of the meningeal layers).

The second meningeal membrane is medial and known as the Arachnoid Mater. This consists of layers with connective tissue, existing without any particular vasculature or innervation. Essentially, this layer is responsible for protecting a bank of cerebrospinal fluid which in turn acts to provide a cushion for the brain. Below the layer of Arachnoid Mater is a space known as the sub-arachnoid space which is available to allow small projections of matter into the dura and permits cerebrospinal fluid to re-enter the circulatory system via dural venous sinuses. It’s almost like a vehicle by which CSF (cerebrospinal fluid) and ISF (Interstitial Fluid) are renewed or cleansed. The substances are "refreshed" here in that we require a sub – arachnoid space to complete the cycle by which the glia function. CSF and ISF are the media which carry substance deposits from the glia and other neurons/cells.

Lastly, our final meningeal layer is known as the Pia Mater and is located, consequently, below the sub arachnoid space. Due to the presence of the preceding meningeal layers, this section is very thin and tightly adheres to the surface of the brain and spinal cord but does not penetrate the Blood Brain Barrier. This is the only covering which follows the contouring of the brain. Similar to the initial layer discussed (Dura Mater), the Pia Mater is highly vascularized with intricate blood vessels which supply the underlying neural tissue. The vasculature of this particular area seems to get quite confusing as we observe neural tissue, Blood Brain Barrier, and the Pia Mater all interacting across one single interface.

In case you haven’t yet gotten completely bored of all this meningeal talk, allow me to explain exactly what these layers have to do with the Glymphatic System. So as I mentioned earlier, below the Arachnoid Mater is the sub – arachnoid space where CSF and ISF are circulated and cleaned. But the question I immediately arrive to at this point is, what makes the CSF and ISF require this space for renewal? If we refer back to one of the diagrams I came up with last year, we can see that these two types of fluid run through veins and arteries as a medium through which calcium and other substances can be deposited and as a result, regulated. To put it simply, the meningeal layers are almost like a vacation spot for the CSF and ISF to help them refresh and renew their position in the system of neuronal circulation.

Thinking about this concept of the meningeal layers providing the correct and necessary circumstances for the Glymphatic System to function reminded me of Rafi's symposium speech this past June. He spoke about the space available for learning. In a similar way that the three meninges layers, with different characteristics and functions, provide a space for a "recharging" or cleansing of sorts, we all need space, or maybe it's time - I'm not sure which - to recharge and refresh our minds. Not just so that we can learn better academically, but more so that we can truly live better lives. Because, let's face it, everyone needs a vacation at some point. There is no way for the glymphatic system to function for the entirety of someone's life without the sub - arachnoid space. Likewise, there is no way we can function at our prime standard all the time without taking a break. I think we need to find the space to learn how to sit still and relax - to refresh ourselves.