HUMAN BRAIN
The human brain is the command center for the human nervous system. It receives input from the sensory organs and sends output to the muscles. The human brain has the same basic structure as other mammal brains, but is larger in relation to body size than any other brains. *FACTS ABOUT THE HUMAN BRAIN* The human brain is the largest brain of all vertebrates relative to body size It weighs about 3.3 lbs. (1.5 kilograms) The brain makes up about 2 percent of a human's body weight The cerebrum makes up 85 percent of the brain's weight It contains about 86 billion nerve cells (neurons) — the "gray matter" It contains billions of nerve fibers (axons and dendrites) — the "white matter" These neurons are connected by trillions of connections, or synapses.