What is the Difference Between White Matter and Grey Matter?

The central nervous system can be divided into white matter and grey matter. These two divisions earned their names because of the difference in their appearance in the gross human anatomy. White matter appears white due to the presence of large amounts of myelin, a fatty protein that serves an insulation function for signal transmission. Grey matter appears grey-brown due to the presence of neuronal cell bodies and capillaries, as well as the relative lack of myelin. White matter and grey matter are different by virtue of their components, functions, and locations in both brain and spinal cord matter.

White matter is sometimes called substantia alba. It contains myelinated axon tracts that connect different areas of grey matter in the brain and spinal cord. These myelinated axons carry nerve impulses from one neuron to another. The presence of myelin allows faster transmission of nerve signals.

Additionally, white matter can be further divided into projection tracts, commissural tracts, and association tracts. Projection tracts carry nerve signals from the cortex to other regions of the brain, or from the brain to other tissues of the body. Commissural tracts allow the left and right cerebral hemispheres to communicate. Association tracts carry nerve signals from one cortical lobe to another.

Grey matter is sometimes called substantia grisea. It mainly contains cell bodies, dendrites, and synapses. Other components of grey matter include neuropil, glial cells, and capillaries. Neuropil is the collective term for dendrites and axons and can be myelinated or unmyelinated, while glial cells refer to astrocytes and oligodendrocytes. In contrast to white matter, which mainly serves to transmit nerve signals, grey matter is where such signals are generated and processed.

White matter and grey matter also differ in terms of distribution. In the brain, grey matter is found on the surface of the cerebral and cerebellar cortices. It is also found in the deeper parts of the cerebrum, particularly the basal ganglia, thalamus, hypothalamus, and different subthalamic nuclei. In the deeper parts of the cerebellum, grey matter is present as the dentate, globose, emboliform, and fastigial nuclei, while in the deeper parts of the brain stem it is present as the substantia nigra as well as the red, olivary, and cranial nerve nuclei. Grey matter is also found in the anterior, lateral, and posterior horns of the spinal cord.

In contrast, white matter forms a large part of the deeper brain matter and the superficial layers of the spinal cord. It connects the cortex with the deeper parts of the brain, and connects the two cerebral hemispheres through a structure called corpus callosum. White matter located in the deeper parts of the cerebellum is sometimes called arbor vitae. In the spinal cord, white matter is more superficial than grey matter, and surrounds the different horns.

White matter and grey matter also differ in terms of their contribution to brain volume and their energy consumption. While white matter constitutes about 60% of the brain volume, grey matter makes up the other 40%. Grey matter, however, is more energy consuming and uses up approximately 94% of the total oxygen that goes to the brain.