quote:

The cause is a dilation of blood vessels in the head. The dilation may be caused by a nerve center located above the roof of your mouth -- when this nerve center gets cold, it seems to over-react and tries to heat your brain.

Therefore, the easy way to avoid "brain freeze" is to keep cold things away from the roof of your mouth!

quote:

The reaction can be sometimes triggered within a few seconds after a very cold substance consumed comes into contact with the stomach. The body's response to cold environments is to vasoconstrict the peripheral vasculature (to reduce the diameter of blood vessels). This vasoconstriction is in place to reduce blood flow to the area, and thus minimize heat loss to keep warmth in the body. After vasoconstriction, they return to normal status and artery size results in massive dilation (vasodilation) of the arteries that supply the palate (descending palatine arteries). The nerves in the region of the palate (greater and lesser palatine nerves) sense this pain and transmit the sensation of this pain back to the trigeminal ganglia. This results in pain that is referred to the forehead and below the orbit, other regions from which the trigeminal nerve receives sensation (This phenomenon is partially similar to the referred pain that is present in the left arm when someone is having a myocardial infarction).