The brain is the internal organ which controls all the bodily functions of a body, either concioussly or unconcioussly. Goa'uld symbiotes will burrow into the brain, taking control of the host. (SG1: "Children of the Gods", "The Enemy Within")
Humans use only five to ten percent of their brain power. If a human should download the Ancient's repository of knowledge, their brain activity will increase to about ninety percent. However, humans have not evolved to the point where their brains can handle that much information. (SG1: "The Fifth Race")
The Asgard brain is significantly more massive than humans. (SG1: "Fragile Balance")