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It is known that some pathologies cause brain shrinkage, for example, it is said that depression causes shrinkage in the hippocampus.

My problem to understand the statement, is that I don't undestand exactly how a brain region shrinks, or what they mean when they say that some pathology causes a brain region to shrink. Does the neurons and everything in them become smaller? Do the cellular membranes become shrink somehow and make the intracellular space smaller? or is it just that a large part of neurons stop firing and they say the brain has "shrinked" because the activity region has become smaller.

Basically, I want to know what happens or what is required in order to have a brain region shrink.

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    $\begingroup$ You are trying to get a specific mechanical answer for an observational descriptor that can have multiple mechanisms. Imagine asking the same question about the phrase " tissue damage" There is no single mechanical cause. the brain or an area of the brain is smaller than it was prior, that can have many causes, especially given many contextual uses. $\endgroup$ – John Nov 24 '18 at 15:18
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Brain shrinkage = brain atrophy, which means a decrease in volume of the brain tissue, which involves different types of degeneration on the cellular level.

Here is a Wikipedia article about cerebral atrophy with a list of ~30 causes, such as Alzheimer's disease, stroke, cerebral palsy, malnutrition, AIDS, etc. Mechanisms can involve: loss of cytoplasmatic proteins, loss of complete neurons or connections between them, etc.

Alcohol consumption and frontal lobe shrinkage: study of 1432 non-alcoholic subjects (BMJ):

The brain tends to shrink physiologically with age. Heavy alcohol consumption seems to exaggerate this shrinkage in social drinkers. Moderate alcohol consumption does not seem to affect brain volume.

Vitamin B12, cognition, and brain MRI measures (Neurology):

Methylmalonate, a specific marker of B12 deficiency, may affect cognition by reducing total brain volume...

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