- The brain’s white matter is situated under the surface gray matter or cerebral cortex of the brain.
- White matter is composed of nerve cell axons, which extend from the neuron cell bodies of gray matter. These axon fibers form connections between nerve cells.
- White matter nerve fibers serve to connect the cerebrum with different areas of the brain and spinal cord.
White matter contains nerve fibers that are wrapped with nervous tissue cells known as neuroglia. Neuroglia called oligodendrocytes form an insulating coat or myelin sheath that wraps around neuronal axons. The myelin sheath is composed of lipids and proteins and functions to speed up nerve impulses. White brain matter appears white due to its high composition of myelinated nerve fibers. It is the lack of myelin in the neuronal cell bodies of the cerebral cortex that makes this tissue appear gray.
Most of the subcortical region of the brain is composed of white matter with masses of gray matter dispersed throughout. Conglomerates of gray matter that are located below the cortex include the basal ganglia, cranial nerve nuclei, and midbrain structures such as the red nucleus and substantia nigra.
White Matter Fiber Tracts
The primary function of the brain’s white matter is to provide a pathway for connecting the different areas of the brain. Should this brain matter become damaged, the brain can rewire itself and establish new nerve connections between gray and white matter. White matter axon bundles of the cerebrum are composed of three main types of nerve fiber tracts: commissural fibers, association fibers, and projection fibers.
Commissural fibers connect corresponding regions of the left and right brain hemispheres.
- Corpus Callosum – thick bundle of fibers located within the medial longitudinal fissure (separates the brain hemispheres). The corpus callosum connects the left and right frontal lobes, temporal lobes, and occipital lobes.
- Anterior Commissure – small fiber bundles that make connections between the temporal lobes, olfactory bulbs, and amygdalae. The anterior commissure forms the anterior wall of the third ventricle and is thought to be involved in pain sensation.
- Posterior Commissure – white matter fibers that cross the upper region of the cerebral aqueduct and interconnect the pretectal nuclei. These nuclei are involved in pupillary light reflex and control the diameter of the pupils in response to intense changes in light.
- Fornix – an arching band of nerve fibers that connect the hippocampus in each brain hemisphere. The fornix also connects the hippocampus to the mamillary body of the hypothalamus and projects to the anterior nuclei of thalamus. It is a structure of the limbic system and is important to the transfer of information between the hemispheres of the brain.
- Habenular Commissure – band of nerve fibers located in the diencephalon that are positioned in front of the pineal gland and connect the habenular nucleus of each brain hemisphere. Habenular nuclei are nerve cells of the epithalamus and a component of the limbic system.
Association fibers connect cortex regions within the same hemisphere. There are two types of association fibers: short and long fibers. Short association fibers can be found just below the cortex and deep within white matter. These fibers connect brain gyri. Long association fibers connect cerebral lobes within brain regions.
- Cingulum – band of fibers located within the cingulate gyrus that connect the cingulate gyrus and frontal lobes with the gyri of the hippocampus (also called parahippocampal gyri).
- Arcuate Fasciculus – long association fiber tracts that connect frontal lobe gyri with the temporal lobe.
- Dorsal Longitudinal Fasciculus – thin fiber tracts that connect the hypothalamus with portions of the midbrain.
- Medial Longitudinal Fasciculus – fiber tracts that connect areas of the mesencephalon with cranial nerves that control eye muscles (oculomotor, trochlear, and abducent cranial nerves) and with spinal cord nuclei in the neck.
- Superior Longitudinal Fasciculus – long association fiber tracts that connect the temporal, frontal, and occipital lobes.
- Inferior Longitudinal Fasciculus – long association fiber tracts that connect the occipital and temporal lobes.
- Occipitofrontal Fasciculus – association fibers that branch into superior and inferior tracts that connect the occipital and frontal lobes.
- Uncinate Fasciculus – long association fibers that connect the frontal and temporal lobes of the cortex.
Projection fibers connect the cerebral cortex to the brainstem and spinal cord. These fiber tracts help to relay motor and sensory signals between the central nervous system and peripheral nervous system.
White Matter Disorders
White matter brain disorders typically result from abnormalities related to the myelin sheath. A lack or loss of myelin disrupts nerve transmissions and causes neurological problems. A number of diseases can affect white matter including multiple sclerosis, dementia, and leukodystrophies (genetic disorders that result in abnormal development or destruction of white matter). Destruction of myelin or demyelination can also result from inflammation, blood vessel problems, immune disorders, nutritional deficiencies, stroke, poisons, and certain drugs.