- White matter is one of the two principal components of the central nervous system (CNS), the other being gray matter. It is named for its pale appearance, which is primarily due to the high concentration of myelin, a lipid-rich substance that surrounds many axons.
- Myelin acts as an insulator, increasing the speed and efficiency of electrical signal transmission between neurons. As such, white matter is fundamentally responsible for communication within the brain and spinal cord, acting as a high-speed information highway that connects different brain regions and links the brain with the rest of the body.
- Structurally, white matter is composed predominantly of myelinated axons, along with glial cells such as oligodendrocytes, which produce and maintain the myelin sheath, and astrocytes, which provide metabolic and structural support. Unlike gray matter, which contains most of the brain’s neuronal cell bodies and is involved in processing and integration of information, white matter specializes in signal transmission, carrying nerve impulses between neurons located in different parts of the CNS.
- In the brain, white matter lies beneath the gray matter of the cerebral cortex and includes major fiber tracts that interconnect various regions of the brain. These tracts are typically classified into three types:
- Association fibers connect different parts of the same hemisphere, allowing intra-hemispheric communication (e.g., the superior longitudinal fasciculus).
- Commissural fibers connect the two hemispheres, with the most prominent being the corpus callosum, which facilitates communication between the left and right sides of the brain.
- Projection fibers connect the cerebral cortex with lower brain structures and the spinal cord (e.g., corticospinal tracts), transmitting signals that control motor and sensory functions.
- In the spinal cord, white matter is located on the outside, surrounding the central gray matter. It contains ascending tracts that carry sensory information from the body to the brain and descending tracts that carry motor commands from the brain to muscles and glands. These tracts ensure the bidirectional flow of information, essential for coordinated movement, reflexes, and perception.
- Functionally, the integrity of white matter is essential for efficient brain function, particularly for tasks involving coordination, memory, learning, attention, and processing speed. Disruption or damage to white matter can significantly impair cognitive and motor abilities. For example, multiple sclerosis (MS) is a demyelinating disease where the immune system attacks the myelin sheath, leading to slowed or blocked nerve signals. Symptoms of MS can include muscle weakness, loss of coordination, vision problems, and cognitive difficulties.
- White matter continues to develop and mature well into early adulthood, especially in areas of the brain associated with executive function, decision-making, and emotional regulation. This prolonged development helps explain why cognitive and emotional maturity continues to improve into the late teens and early twenties. White matter is also capable of plasticity, adapting to changes through learning and experience, although to a lesser extent than gray matter.
- Age-related changes in white matter are a natural part of the aging process. Over time, white matter may exhibit degeneration or reduced volume, especially in regions like the frontal lobe. Such changes have been linked to declines in processing speed, memory, and executive function. Moreover, white matter abnormalities have been implicated in a variety of psychiatric and neurodevelopmental conditions, including schizophrenia, autism spectrum disorders, bipolar disorder, and Alzheimer’s disease.
