"White Matter Matters"

The brain has always been regarded by scientists as two distinct domains – gray matter and white matter. Gray matter consists of neurons while white matter contains axon bodies wrapped in sheets of myelin. For decades scientists have concluded that gray matter is responsible for learning, memory, mental illness, and more, but now researchers are turning their attention to the fatty substance surrounding the axon: myelin.

Oligodendrocytes are glial cells that lay down myelin in the brain, and they do so differentially at various stages in the human lifespan. A newborn, for example, has virtually no myelin coating his/her axons. Myelination occurs in different areas of the brain until about age 25, at which point it basically halts. Interestingly, the degree of myelination depends on the activities that the subject partakes in; professional pianists have a very dense coating of myelin in the region of the brain that connects finger movement with making music. These and other studies were performed using diffusion tensor imaging (DTI), which is similar in concept to an MRI only more detailed. Thus it is likely that experience does influence the formation and location of myelin.

A question that arises when studying myelin formation is – why do some neurons contain myelin and others do not? By one interpretation, it takes a great amount of precision for neuronal signals to arrive in one location at the correct time. Some must travel faster than others, and the speed is regulated by the degree of myelination and the number of nodes of Ranvier. The speed of a signal can vary from 30 milliseconds all the way to 300 milliseconds. Thus coordination is key when considering the timing of all of the impulses it takes to formulate a thought.

In addition to myelin’s role in learning and cognition, lack of myelination can also play a vital role in disease. Diseases that arise from missing or incomplete myelination include multiple sclerosis, cerebral palsy, Alexander disease, and possibly schizophrenia, autism, bipolar disorder and dyslexia. Patients with schizophrenia, for example, generally show signs of the disorder during late adolescence; this also happens to be when the forebrain undergoes myelination.

The effects and extent of influence of myelination are not yet fully understood, and of course more research is necessary. The study of the brain is an ever-evolving process that unfolds a bit more each day, revealing mysterious secrets and surprises.

Fields, Douglas R. "White Matter Matters." Scientific American March 2008. p54-61. 03-27-08.