White matter can be understood as the counterpart of gray matter in the brain. It consists of conduction pathways (nerve fibers) whose white coloration comes from their medullary structure. The white matter is part of the central nervous system and is also called substantia alba or medulla or medullary substance. In the spinal cord, it is located next to the gray matter. There it is divided into anterior, lateral and posterior cord. In the brain, the white nerve fibers are located in the inner regions and are surrounded by the gray matter. The myelinated conduction pathways, i.e. medullary extensions of nerve cells, also have accumulations of gray nerve cell bodies. These are the so-called nuclear areas in the spinal cord and brain.
What is white matter?
Those myelin sheaths responsible for the white coloration of the substance are formed by so-called glial cells in the central nervous system. These also belong to the white matter. On the other hand, the bodies of the nerve cells are almost not located in this area, except for the development before birth. Mainly on the surface, the white matter is located in the areas of the spinal cord and brain stem. Nerve fibers from a coinciding point of origin and with the same destination are grouped into bundles, strands or tracts. In the cerebrum, the white matter is located in the central area and is also arranged in strands. Further, the course of the nerve cords continues through the brainstem area and the so-called cerebellar peduncles into the medulla of the cerebellum.
Anatomy and structure
In terms of volume, the white matter fills almost half of the human brain. Overall, it can be thought of as a complicated system of several million connecting cables. Each of these strands features an outgrowth of nerve cells that detects, relays, and transmits signals. Science refers to this as an axon. It is usually wrapped with the fatty myelin that provides its white coloration. The bundles, strands, and tracts of nerves divide again and may reconnect, allowing areas of the brain that are far apart to be linked. Thus, white matter is very significant for all processes in the brain related to learning. If the nerve cords show disturbances, this can have an extremely negative effect on the mental performance of the person. The imaging techniques available today can clearly visualize the white matter and point to its causative effect with regard to possible mental and psychological disorders. Likewise, they show the influence of the white matter on intelligence and thinking ability. Thus, it can be proven that the nerve fibers determine the flow of information between the individual brain areas to a much greater extent than suspected. An active brain, challenged to lively activity, will increase its white matter under certain circumstances. When a person learns something new or acquires many new skills on a musical instrument, for example, the white matter of the brain increases quantitatively. It is therefore trainable, which was originally thought to be impossible. Conversely, however, this also reveals the extent to which white matter contributes to the decline in versatile thinking ability in old age.
Function and tasks
New insights have likewise been gained in recent years about myelin, that fatty whitish sheath surrounding the conduction pathways. Initially, it was assumed that this so-called myelin sheath served solely to insulate the nerve fibers. Later, however, the question arose as to why some fibers have no sheath, while others have a thin or thick one. For a long time, it could not be fully explained why the myelin sheath has microscopic gaps (Ranvier’s lacing rings) at millimeter intervals. Now it has become clear that nerve impulses travel about a hundred times faster along a wrapped (myelinated) conduction path than along an exposed one. Thanks to the “insulating tape,” the electrical signals hop across the cord rings, so to speak. This is noticeable in the central nervous system as well as in the various extremities.
Diseases
The lifelong development of human white matter is characterized by ups and downs. During childhood and adolescence, its volume increases at a relatively steady rate. It continues to increase until the age of 40 to 50. Then, however, the white matter more or less slowly reduces again.Accordingly, mental performance gradually declines. The flow of information between the individual brain regions comes to a standstill because the number of nerve fibers coated with myelin decreases. Research indicates that the total length of myelinated fibers in a person at age 20 is about 149,000 kilometers, but then declines to about 82,000 kilometers by age 80. However, this does not necessarily mean that older people lose their acquired knowledge. It is usually well preserved into old age. The brain has the ability to compensate for certain deficits on its own. A meaningful experiment with younger and older subjects showed that reactions in the motor area slow down with age. However, behind this increased reaction threshold, the researchers suspected a strategy of the brain to avoid hasty and thus possibly erroneous reactions. In fact, the older subjects reacted more slowly than the younger ones, but also achieved a lower error rate. In addition, older people were found to be better at activating certain brain areas compared with younger people, despite their white matter deficits.
