The tangled part is actually mostly 2mm thickness on the surface[0]. That's why the folds matter so much:

[0] https://28oa9i1t08037ue3m1l0i861-wpengine.netdna-ssl.com/wp-...

Check out this article for a pretty cool overview of the brain: https://waitbutwhy.com/2017/04/neuralink.html