The brain is made up largely of neurons, or nerve cells, blood vessels and glial cells. Glial cells create a supporting structure for the brain. The brain is about 60 percent fat.
There are four kinds of glial cells: oligodendrocytes, astrocytes, ependymal and microglia cells. They protect the brain by keeping out pathogens and toxins.
The neurons and the fibers that surround them are gray matter. The bundles of axon fibers are white matter. Axons are single nerve fibers through which the nerve cells send out impulses. Axons are usually encased in a sheath of fat called myelin. Oligodendrocyte cells are responsible for myelin.
The brain's gray matter is located in the core of the spinal cord and reaches up into the brain stem at the bottom of the brain. White matter wraps around the gray matter in the spinal cord and the brain stem, while gray matter surrounds white matter in the upper parts of the brain. The connections between the cells in the gray matter let the brain interpret signals from the sense organs and plan how to respond to them. White matter serves as communication network between different parts of the brain and between the brain and the spinal cord.