Scientists and doctors find two major types of tissue when they examine samples from the brain and the spinal cord--white matter and gray matter. These two tissue types are located in different areas of the central nervous system, contain different types of cells and perform different functions. They are also differentiated by disease; the different symptoms produced by neurological symptoms partially depend on whether those diseases attack white matter or gray matter. To keep people healthy and functioning, both the gray matter and the white matter need to be intact.
The most important cells in your brain and spinal cord are neurons, which are the cells that allow you to think, feel, move and speak. There are two basic parts to a neuron. The cell body, which is roughly spherical, receives inputs from lots of other neurons. The axon, a long and thin appendage, is connected to the cell body, and it helps the neuron send messages to other neurons. This anatomy of the neuron, with one "listening" portion and one "talking" portion, allows neurons to talk to each other, which lets you sense or see something, decide what to do and respond to it.
Cellular Properties of Gray Matter
Gray matter is primarily composed of neuronal cell bodies, which are all clustered together into specific locations in the brain and spinal cord. All of the gray matter in your outer brain forms the cerebral cortex, which performs almost all of your conscious thoughts. There is also gray matter deeper in your brain; structures like the thalamus and the basal ganglia are also gray matter, and they help you to sense the environment and coordinate your movements.
Cellular Properties of White Matter
White matter is primarily composed of the axons that are connected to neuronal cell bodies. It gets its white color because most of these axons are coated in a sheath called myelin. Myelin helps to speed nerve conduction, so that messages can travel faster through your brain. White matter also contains supporting cells called glia, which help neurons get nutrition and protect the brain from infection.
Differences in Location
In the brain, gray matter makes up the outermost surface and some deeper structures, whereas white matter tends to make up parts of the brain closer to the middle. Therefore, an injury to the brain that does not penetrate very far is more likely to injure gray matter than white matter. This arrangement is reversed in the spinal cord; gray matter makes up the center of the spinal cord, while white matter makes up the outer portion.
Different diseases of the nervous system affect different parts of the brain, and therefore cause different symptoms. Many diseases that affect cognition and higher thinking involve the gray matter of the cerebral cortex; for example, scientists have found that patients with Alzheimer's Disease lose cell bodies in the gray matter, according to a 2003 article, "Dynamics of Gray Matter Loss in Alzheimer's Disease," in "The Journal of Neuroscience." Diseases that affect white matter may affect signal transmission, but may not affect cognition; multiple sclerosis is an example of such a disease.