A brain freeze is an informal term for Sphenopalatine Ganglion Neuralgia. It is when the blood vessels in your brain expand to cause a headache! This is all caused SPG nerves. These nerves are at the back of your mouth and are very sensitive to cold foods like ice cream. SPG nerves or the sphenopalatine ganglion are a bundle of nerves located on the roof of your mouth. These nerves are very sensitive to cold things. This is what causes a brain freeze. The SPG nerves can actually make your blood vessels in your brain expand when the nerves get cold! SPG nerves are important for our survival because they actually regulate our body temperature. They are what makes humans and other mammals warm-blooded.