arachnoid mater

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Our Three (Brain) Mothers

Protecting our brain and central nervous system are the meninges, derived from the Greek term for “membrane”. You may have heard of meningitis - this is when the innermost layer of the meninges swells, often due to infection, and can cause nerve or brain damage, and sometimes death.

There are three meningeal layers: the dura mater, arachnoid mater, and pia mater. In Latin, “mater” means “mother”. The term comes from the enveloping nature of these membranes, but we later learned how apt it was, because of how protective and essential the meningeal layers are.

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  • The dura mater is the outermost and toughest membrane. Its name means “tough mother”.

The dura is most important for keeping cerebrospinal fluid where it belongs, and for allowing the safe transport of blood to and from the brain. This layer is also water-tight - if it weren’t, our cerebrospinal fluid (CSF) would leak out, and our central nervous system would have no cushion! Its leathery qualities mean that even when the skull is broken, more often than not, the dura (and the brain it encases) is not punctured.

  • The arachnoid mater is the middle membrane. Its name means “spider-like mother”, because of its web-like nature.

The arachnoid is attached directly to the deep side of the dura, and has small protrusions into the sinuses within the dura, which allows for CSF to return to the bloodstream and not become stagnant. It also has very fine, web-like projections downward, which attach to the pia mater. However, it doesn’t contact the pia mater in the same way as the dura: the CSF flows between the two meningeal layers, in the subarachnoid space. The major superficial blood vessels are on top of the arachnoid, and below the dura.

  • Pia mater is the innermost membrane, which follows the folds (sulci) of the brain and spinal cord most closely. Its name means “tender mother”.

The pia is what makes sure the CSF stays between the meninges, and doesn’t just get absorbed into the brain or spinal cord. It also allows for new CSF from the ventricles to be shunted into the subarachnoid space, and provides pathways for blood vessels to nourish the brain. While the pia mater is very thin, it is water-tight, just like the dura mater. The pia is also the primary blood-brain barrier, making sure that no plasma proteins or organic molecules penetrate into the CSF. 

Because of this barrier, medications which need to reach the brain or meninges must be administered directly into the CSF.

Images:
Anatomy: Practical and Surgical. Henry Gray, 1909.

anonymous asked:

When the brain bleeds, could the blood come out of the person's nose/eyes/ears? Or would the blood only be from burst vessels in the nose/eyes/ears? I'm thinking along the lines of "superpowered character uses their power too much and starts bleeding", which I'm not sure if it breaks your rule of reality (I guess the other question would be Can the brain bleed from overuse/strain?). Thank you so much! The blog has been very helpful :)

Hey there nonny! 

Brain bleeds are an interesting topic for me. I’m going to say up front that “superhero” abilities indeed fall under the Rule of Reality: 

However, we’re talking generally about brains bleeding and where that blood goes, which is absolutely a thing that happens. 

The brain is a lump of neurons wired up all crazy. It’s basically a gray-ish Jell-o. 

That Jell-o sits inside a multi-layered sack, called the meninges. The meningers are a set of three tough, fibrous layers that protect the brain and anchor it to the skull. Observe: 

[Courtesy of WikeMedia Commons

The brain is fed by circulating something called cerebrospinal fluid, or CSF. 

Bleeding can occur in a few places, and where it is determines the kind of bleed. So for example, a subdural hemorrhage is bleeding that occurs underneath the dura mater, the outermost layer of the meninges; an epidural hemorrhage is bleeding between the dura and the skull, and is usually from a tear in the middle meningeal artery, while a subarachnoid hemorrhage is between the arachnoid and pia maters. 

(If you need to remember this for any reason, remember that the brain needs some PADding – the Pia, Arachnoid, and Dura maters are in that order.) 

Now. You asked if blood drains from the mouth, nose, or ears when the brain bleeds. The answer is no, with one noted exception. 

There’s a particular type of skull fractures called a LaForte fracture, in which the facial bones – and the maxilla, the bone that essentially forms the shelf under your eyes and over your hard palate – is fractured. In some types of LaForte fractures, you can see bleeding from the nose or ears. This bleeding will have what’s called a positive halo test: a piece of gauze touched to the blood will show the blood, but also a “halo” of clear CSF around the blood. 

However, without a LaForte fracture, the blood buildup in the skull has nowhere to go, and it starts to squish the brain. (LaForte fractures only occur in significant head trauma, so it’s not exactly a happy finding.) 

The most common “spontaneous” head bleed – that is, non-trauma-related  – is a subarachnoid hemorrhage, and it’s typically due to an aneurysm or an arteriovenous malformation. Blood pressure rises due to stress or exertion, the blood vessel bursts, and there’s bleeding in the skull. Ruh roh! 

….No, seriously, this is a Bad Day. AVM ruptures are often lethal. What happens is that the brain essentially gets squished by all the blood building up in the skull, a process called herniation. When that happens, the character can have severe neurological symptoms and permanent brain damage. Up to 70% of those with subarachnoid hemorrhage die from it. 

In short: you don’t want your character’s actual physical brain to be doing any bleeding. 

In movies and TV, bleeding from the nose is used as a metaphor for psychic strain: 

[Courtesy of GamePadProject.com

Nosebleeds like this come from ruptured capillaries inside the nares, not directly from the brain. However, they’re often also related to stress, exertion, and hypertension. 

Perhaps it’s sentimental of me, but this trope doesn’t actually bother me that much. Maybe it’s that the cause – superpowers – are actually having a negative effect, even if that effect is purely visual. 

What would be more believable to me would be that the character faints after using their powers, or becomes exhausted and dizzy and tired for a few hours after the fact and needs some form of rehydration and sugar in order to keep going. 

Anyway, that’s what’s up with nosebleeds, brain bleeds, and Eleven from Stranger Things! Hope this helped you out. 

xoxo, Aunt Scripty

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