White matter - Definition and Overview

White matter is one of two categories of tissue in the nervous system. It forms the deep parts of the brain and the superficial parts of the spinal cord. It is composed of nerve cells processes (axons and dendrites) that connect various parts of the brain to each other and carry nerve impulses to or from the bodies of nerve cells (neuron).

White matter is distinguished in that it is composed of nerve fibers often covered with myelin. This is as opposed to grey matter, which is composed primarally of nerve cell bodies. Generally, white matter can be understood as the parts of the brain responsible for information transmission; whereas, grey matter is responsible for information processing.

The cerebral fiber bundles can be divided into three major groups: Association fibers, commissural fibers and projection fibers. Association fibers form cortical connections within the same hemisphere. An examples is cingulum. Commissural fibers interconnect the two hemispheres (e.g., corpus callosum) while projection fibers are fibers from the cerebral cortex to other parts of the nervous system, e.g, thalamus.