The Neuroscience of Grey and White Brain Matter

Neuroscientists primarily study two main components of the central nervous system: grey matter and white matter. These two tissues are distinguished by their composition and function. Understanding their roles is fundamental to comprehending how the brain processes information, controls the body, and changes throughout a person’s life.

Defining Grey Matter

Grey matter, named for its pinkish-gray color, is composed of neuronal cell bodies, dendrites, and axon terminals, along with supportive glial cells. It functions as the brain’s main processing center, handling computation, thought, and memory storage. This tissue is where synapses—the connections between neurons—are located, allowing for the transmission of nerve signals. Different regions of the cerebral cortex are specialized for various tasks, such as language, attention, and problem-solving.

Grey matter forms the outer layer of the cerebrum, known as the cerebral cortex, and is also found in the cerebellum, brain stem, and deep within the cerebrum. In the spinal cord, grey matter is located at the center, forming a butterfly-like shape. This central region is responsible for processing sensory information and controlling motor output.

Understanding White Matter

White matter is located beneath the grey matter of the cortex and is composed of long, bundled axons that connect different grey matter regions. Its white color comes from the myelin sheath, a fatty substance that insulates the axons. Myelin allows for the rapid and efficient conduction of electrical impulses, making it the brain’s primary communication network.

The structure of white matter can be compared to a network of fiber-optic cables. The myelin sheath acts like the coating on an electrical wire, preventing signal loss and increasing transmission speed. These bundles of myelinated axons, known as tracts, form pathways that send nerve signals between brain regions and to the rest of the body. Damage to this tissue can affect the ability to move, use sensory faculties, or react to external stimuli.

The Interplay Between Brain Matter

Grey and white matter work together as an integrated system. Grey matter can be seen as the brain’s processing centers, or “cities,” where information is analyzed. White matter functions as the “highways” that connect these cities, allowing for rapid communication between them.

Consider the process of seeing an object and deciding to pick it up. Sensory information from the eyes is first sent to a grey matter region in the cerebral cortex that processes visual input. Once the object is identified, this information is relayed through white matter tracts to other grey matter areas responsible for decision-making and planning motor actions. Finally, a command is sent via another white matter pathway to the motor cortex, a grey matter region that controls voluntary movement, which then signals the muscles to pick up the object.

This continuous flow of information allows for a seamless integration of perception, thought, and action. The efficiency of the white matter’s communication channels directly impacts the processing speed and power of the grey matter’s hubs. Without these connective pathways, the processing centers in the grey matter would be isolated and unable to function as a cohesive whole.

How Brain Matter Changes Over a Lifetime

The composition and volume of grey and white matter are not static; they change throughout a person’s life in a process known as neuroplasticity. Grey matter volume increases until about the age of eight, after which a process called synaptic pruning begins. This process eliminates weaker synaptic connections while strengthening more frequently used ones, leading to more efficient cognitive processing.

White matter development continues into early adulthood, with the myelination of axons progressing throughout this period. The formation of the myelin sheath may not be complete until after the age of 20. This extended period of myelination contributes to improvements in cognitive functions that rely on rapid communication between brain regions, such as impulse control and long-term planning. The frontal cortex is one of the last brain regions to fully myelinate.

With advancing age, a gradual decline in the volume of both grey and white matter is common and can be associated with changes in cognitive function. These changes include slower processing speed and memory difficulties. Certain diseases also disproportionately affect one type of brain matter. For example, multiple sclerosis damages the myelin in white matter, while Alzheimer’s disease involves a significant loss of grey matter.