HFCS is (depending on the grade) generall 55% fructose / 42(ish)% glucose, and some small amounts of other sugars. Corn is the donor plant.
In comparison, Apple sugars are 90% fructose and Pears are 70% fructose, way above HF corn syrup sugars. Honey is about 53% (very, very close). None of those appear to be in the "oh my God we're gonna die" list for worrying that you're brain doesn't realize it's full yet.
It's worth noting that corn starch is coverted to HFCS using hydrochloric acid (the same acid found in the human stomach), followed by a water washing, followed by an enzymatic conversion of dextrose (similar to the same process which is used in baking bread, where enzymes convert starch to simple sugars like fructose to be used by the yeast in the fermentation process), followed by water washing and distillation. It's the same process used for countless "all natural" products which we have consumed for thousands of years, but done more precisely and on a much larger scale.
It will still make you fat if you eat too much of it, but it's not magical and doesn't have some insurmountable effect on the brain. No more than a good steak, a loaf of awesome fresh bread, or a decadent wedge cheese. You'll eat way more of all of those than you need before your brain tells you you're full.