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The two mechanisms I've seen mentioned are the cerebrospinal fluid (CSF) acting as a hydraulic cushion or shock absorber, and CSF creating a micro-gravity environment through buoyancy.

I have very little understanding of the former, but I do have a vague idea about the latter: because the brain is submerged in a fluid (the densities of the brain and CSF are also similar I think), it experiences a buoyant force upwards that counteracts the force of gravity on the brain, hence decreasing its "effective weight" from around 1400g to about 50g.

How does this help precisely? The explanation I've seen that I could understand is that it makes it so that the brain isn't pressed up against the base of the skull due to gravity, and so that it doesn't get deformed under its own weight. This buoyancy also helps the various structures attached to the brain, like nerve roots and blood vessels, hold it up and support it. Wikipedia also says that it allows the brain to maintain its density without being impaired by its own weight, which would cut off blood supply and kill neurons in the lower sections without CSF. This explanation is plausible but I'm not sure if it's the entire story.

There's also this bit from "Neuroscience in Medicine" that says the decrease in effective weight decreases injury by reducing brain momentum in response to stresses/strains inflicted on the head. How does that work? I'm not super familiar with the physics of buoyancy, I just know the basics. I don't understand how it reduces momentum during movement.

I couldn't find anything that went into the physics of the whole hydraulic cushion thing, and I’d appreciate an explanation.

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    $\begingroup$ This is a cool question! Here's another way to understand the "momentum" part: A function of CSF that comes from its physical properties as fluid is to protect the brain from the damage that would result from a sudden movement of the skull. Any rapid acceleration/deceleration of the head has the potential to injure the delicate contents contained within. The CSF helps reduce the potential damage in such an event by acting as a cushion and a shock absorber.(ncbi.nlm.nih.gov/books/NBK470578) $\endgroup$
    – Bipasha
    Commented Aug 28, 2020 at 5:09
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    $\begingroup$ @Bipasha thanks! So essentially, “reducing momentum” is more or less the just saying the same thing as “it acts as a hydraulic cushion”. That makes things simpler. $\endgroup$
    – Dahen
    Commented Aug 28, 2020 at 6:26

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In terms of buoyancy, you are correct in your understanding that CSF creates a micro-gravity environment around the brain. This buoyant force essentially counteracts the downward pull of gravity on the brain, reducing its effective weight as you described. This decrease in effective weight plays a crucial role in maintaining proper positioning and preventing deformation of neural structures within the skull. As for how CSF acts as a hydraulic cushion specifically, think of it as providing support and distributing pressure evenly throughout different regions surrounding the brain and spinal cord. The fluid-filled spaces allow for dynamic adjustments in response to changes such as changes in posture or intracranial pressure fluctuations.
1. CSF compression: When the brain is subjected to a sudden force, the CSF is compressed, which increases its pressure.
2. Pressure transmission: The increased pressure is transmitted through the CSF to the surrounding skull and meninges (the protective membranes covering the brain).
3. Force distribution: The pressure is then distributed evenly throughout the skull and meninges, reducing the force applied to any single point on the brain.
4. Shock absorption: The CSF acts as a shock absorber, reducing the amplitude of the force transmitted to the brain.

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