A pyramidal neuron from the hippocampus, stained for green fluorescent protein.

The human brain is a grayish-pink, jellylike ball with many ridges and grooves on its surface. A newborn baby’s brain weighs less than 1 pound (0.5 kilogram). By the time a person is 6 years old, the brain has reached its full weight of about 3 pounds (1.4 kilograms). Most of the brain’s nerve cells are present at birth. The increase in weight comes from growth of nerve cells, development and growth of supporting cells, and development of connections among cells. During this six- year period, a person learns and acquires new behavior patterns at the fastest rate in life.

A network of blood vessels supplies the brain with the vast quantities of oxygen and food that it requires. The human brain makes up only about 2 percent of the total body weight, but it uses about 20 percent of the oxygen used by the entire body when at rest. The brain can go without oxygen for only three to five minutes before serious damage results.

The brain is at the upper end of the spinal cord. The spinal cord is a cable of nerve cells that extends from the neck about two-thirds of the way down the backbone. The spinal cord carries messages between the brain and other parts of the body. In addition, 12 pairs of nerves connect the brain directly with certain parts of the body.

The brain works somewhat like both a computer and a chemical factory. Brain cells produce electrical signals and send them from cell to cell along pathways called circuits. As in a computer, these circuits receive, process, store, and retrieve information. Unlike a computer, however, the brain creates its electrical signals by chemical means. The proper functioning of the brain depends on many complicated chemical substances produced by brain cells.


The main anatomical regions of the vertebrate brain, shown for shark and human. The same parts are present, but they differ greatly in size and shape.

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