white matter

Brain tissue is divided into two types: gray matter and white matter. White matter is made up of the axons of neurons -- the long filaments that extend from the cell bodies and carry the electrical signals that carry the messages between neurons. It's the myelin sheathing that makes it look white. There are three major white matter systems, which all connect to form one continuous system: cortical white matter; the corpus callosum; the internal capsule.
White matter tracts are classified according to their pathways:
• projection tracts encompass those that project from the cortex to non-cortical areas, such as the senses and the muscles, and those that project from the thalamus to the cortex.
• commissural tracts cross from one hemisphere to the other, through the corpus callosum and the anterior commissure.
• association tracts connect regions within the same hemisphere (long ones connect different lobes; short ones connect different gyri within the same lobe).

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