White matter
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White matter is one of the two main solid components of the central nervous system. It forms the bulk of the deep parts of the brain and the superficial parts of the spinal cord. Aggregates of grey matter, such as the basal ganglia (thalamus, caudate nucleus, putamen, globus pallidus, subthalamic nucleus, accumbens nucleus), brain stem nuclei (e.g. red nucleus, substantia nigra, nuclei of the cranial nerves) are spread within the cerebral white matter. The cerebellum is structured in a similar manner, with a superficial mantle of cerebellar cortex, deep cerebellar white matter and aggregates of grey matter surrounded by deep cerebellar white matter (dentate nucleus, globose nucleus, emboliform nucleus, and fastigial nucleus). The fluid-filled cerebral ventricles (lateral ventricles, third ventricle, cerebral aqueduct, fourth ventricle) are also located deep within the cerebral white matter. White matter is composed of nerve cell processes, i.e. extensions (axons), which connect various grey matter areas of the brain to each other and carry nerve impulses to and from the nerve cell bodies within the central nervous system (neurons). Cerebral and spinal white matter does not contain dendrites, which can only be found in gray matter, along with neural cell bodies and shorter axons.
White matter is distinguished in that it is composed of axonal nerve fibers, covered by a myelin sheath. The role of myelin is to facilitate fast nerve impulse (action potential) conduction at a low metabolic cost. Myelin in the central nervous system is formed by oligodengrocytes, a specialized type of glia (neuroglia) cells. A single oligodendrocyte provides the myelin sheath for multiple axons. In the peripheral nervous system, the myelin sheath is provided by Schwann cells. Multiple Schwann cells contribute to the myelin sheath of a single axon in a cranial or spinal/peripheral nerve. This distinction is clinically relevant, since damage to a single oligodendrocyte leads to demyelination of multiple axons, as opposed to damage to a Schwann cell, which affects only one peripheral axon. Grey matter is composed primarily of nerve cell bodies. Generally, white matter can be understood as the parts of the brain and spinal cord responsible for information transmission; whereas, gray matter is mainly responsible for information processing. White matter injuries may be reversible, while gray matter regeneration is less likely.
The axonal fiber bundles (tracts) can be divided into three major groups: association fibers, commissural fibers and projection fibers. Association fibers connect different areas of the brain within the same hemisphere. Examples include the cingulum, superior longitudinal fasciculus, inferior longitudinal fasciculus, uncinate fasciculus. Commissural fibers interconnect the two cerebral hemispheres (e.g., corpus callosum, anterior commissure, posterior commissure), while projection fibers connect different cortical areas with subcortical structures (e.g. thalamo-cortical projection fibers, optic radiation), fornix), the brain stem (e.g. cortico-mesencephalic, cortico-pontine and cortico-bulbar tracts), the cerebellum (superior, middle and inferior cerebellar peduncle), and the spinal cord (e.g. cortico-spinal tract, also known as pyramidal tract or motor pathway).fr:Substance blanche pl:Substancja biała ro: Substanta alba de: Weisse Substanz