[Matt---Mrs D link --is a good article.

I will try again to answers your ? but you need to understand more about the brain its to complex-- to teach so you should do more research.

Regarding your first ?--Most of the brains neurons over 90% are located in the cerebral cortex. That part of the brain is divided into --4 lobes --the frontal lobe,parietal lobe,temporal lobe,and the occipital lobe. You can research all there functions. To get to the point Hippocampus is located in the medial temporal lobe inside this lobe is region of the brain known as the --limbic system--. In this system includes hippocampus the amygdala and thlamus,hypothalamus, epithalamus--most have to do with the the processing of memory simply put. To answer your question you need to really understand the brain and its functions. Example --hippocampus is important in memory function short--and long term and behavior. The amgdala prcoccess of memory and emotional reactions and impulses. It effects the medial temporal lobe --but most important hippocampus which is all a part of the limbic system. Which I think is your answer to your ?--- is the Limbic system.
Your 2nd ? ---I already think I have explained this too--- based on what been studied so far---that skringae is likely the effect of long term untreated (chronic depression) rather then the cause. This shrikange has shown to have changed and improved with treatment. This is all that has been understood --more studies are being done.