White matter

White matter (a.k.a. substantia alba) is one of the main tissue types in the central nervous system, alongside gray matter. It consists of nerve fibers (axons) supported by neuroglia. The nerve fibers are protected by a fatty covering called the myelin sheath, which gives white matter its white color. These fibers in the white matter allow for communication and information exchange between different areas of gray matter.

In the spinal cord, the white matter surrounds the centrally placed gray matter. It can be divided into distinct regions, including the anterior, lateral, and posterior white columns or funiculi. The anterior white columns are located between the anterior motor roots, the posterior column is between the posterior sensory roots, and the lateral white columns exist between the anterior and posterior nerve roots on both sides. Functionally, the nerve fibers can be grouped into bundles or tracts known as ascending and descending tracts. Ascending tracts are involved in sensory functions, such as transmitting information about vibration, proprioceptive, and two-point discrimination through the fasciculus gracilis and cuneatus, as well as carrying pain, temperature, touch, and pressure sensations via the spinothalamic tracts. Descending tracts are responsible for motor functions, specifically controlling fine motor control of distal extremities through the corticospinal tract.

All white matter tracts in the spinal cord pass through different parts of the brainstem and connect with the thalamus, cerebellum, and various regions of the brain both sub-cortically and cortically.

Within the cerebral hemispheres, white matter fibers can be classified into three groups based on their connections: commissural fibers, association fibers, and projection fibers. Commissural fibers connect corresponding regions between the two hemispheres, including the corpus callosum, fornix, anterior and posterior commissures, and the habenular commissure. Association fibers connect different cortical regions within the same hemisphere; they can be short association fibers, connecting nearby regions, or long association fibers, connecting distant regions of the cerebral cortex. Projection fibers refer to the sensory and motor tracts that radiate from the thalamus and internal capsule, projecting to different parts of the cerebral cortex.

When observing the radiological images, we find that the white matter appears whitish or lighter in appearance on T1 weighted MRI scans of the brain, as compared to gray matter which appears darker. This presentation is flipped over in T2 weighted MRI and FLAIR images of the brain, where the substantia alba (white matter) tissue appears grayish or darker, while the gray matter appears whitish (or lighter) in color.