11.1D: Spinal Cord White Matter

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The white matter of the spinal cord is composed of bundles of myelinated axons.

Learning Objectives

Describe the function and compositon of spinal cord white matter

Key Points

White matter is one of the two components of the central nervous system and consists mostly of glial cells and myelinated axons.
The white matter is white because of the fatty substance ( myelin ) that surrounds the nerve fibers. Myelin acts as an electrical insulation. It allows the messages to pass quickly from place to place.
Cerebral and spinal white matter do not contain dendrites, which can only be found in grey matter along with neural cell bodies, and shorter axons.
White matter modulates the distribution of action potentials, acting as a relay and coordinating communication between different brain regions.
White matter in the spinal cord functions as the “wiring”; primarily to carry information.

Key Terms

myelin: A white, fatty, material composed of lipids and lipoproteins, that surrounds the axons of nerves.
white matter: A region of the central nervous system containing myelinated nerve fibers and no dendrites.
cerebral ventricles: Interconnected cavities in the brain where the cerebrospinal fluid is produced.
glial cell: A type of cell, in the nervous system, that provides support for the neurons.

White matter is one of the two components of the central nervous system. It consists mostly of glial cells and myelinated axons and forms the bulk of the deep parts of the brain and the superficial parts of the spinal cord. It is the tissue through which messages pass between different areas of grey matter within the nervous system.

Composition of White Matter

White matter is composed of bundles of myelinated nerve cell processes (or axons). The axons connect various grey matter areas (the locations of nerve cell bodies) of the brain to each other and carry nerve impulses between neurons. The axonal myelin acts as an insulator and increases the speed of transmission of all nerve signals. White matter does not contain dendrites, which are only found in grey matter along with neural cell bodies and shorter axons.

In a freshly cut brain, the tissue of white matter appears pinkish white to the naked eye because myelin is composed largely of lipid tissue that contains capillaries. In nonelderly adults, 1.7-3.6% of the white matter is blood. Myelin is found in almost all long nerve fibers and acts as electrical insulation. This is important because it allows the messages to pass quickly from place to place.

Spinal Cord Columns

The spinal cord white matter is subdivided into columns. The dorsal columns carry sensory information from mechanoreceptors (cells that respond to mechanical pressure or distortion). The axons of the lateral columns ( corticospinal tracts ) travel from the cerebral cortex to contact spinal motor neurons. The ventral columns carry sensory pain and temperature information and some motor information.

Function of White Matter

Long thought to be passive tissue, white matter actively affects how the brain learns and functions. While grey matter is primarily associated with processing and cognition, white matter modulates the distribution of action potentials, acting as a relay and coordinating communication between different brain regions. The brain in general (and especially a child’s brain) can adapt to white-matter damage by finding alternative routes that bypass the damaged white-matter areas; therefore, it can maintain good connections between the various areas of grey matter. Using a computer network as an analogy, the grey matter can be thought of as the actual computers themselves, whereas the white matter represents the network cables connecting the computers together.

Axon Tracts

Within white matter, there are three different kinds of tracts or bundles of axons that connect one part of the brain to another and to the spinal cord:

Projection tracts extend vertically between higher and lower brain and spinal cord centers. They carry information between the cerebrum and the rest of the body. The corticospinal tracts, for example, carry motor signals from the cerebrum to the brainstem and spinal cord.
Commissural tracts cross from one cerebral hemisphere to the other through bridges called commissures. Commissural tracts enable the left and right sides of the cerebrum to communicate with each other.
Association tracts connect different regions within the same hemisphere of the brain. Among their roles, association tracts link perceptual and memory centers of the brain.

White Matter-Grey Matter Interactions

White matter 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 and brain stem nuclei, are spread within the cerebral white matter. The cerebellum is structured in a similar manner as the cerebrum, with a superficial mantle of cerebellar cortex, deep cerebellar white matter (called the “arbor vitae”), 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, and fourth ventricle) are also located deep within the cerebral white matter.

White matter in spinal cord: The spinal cord diagram showing location of the white matter surrounding grey matter.