Our brain is roughly 50% white matter and 50% grey matter. Anecdotally, people believe grey matter is the “seat of intelligence”. In the scientific community, grey matter was traditionally thought of as the hub of computation while white matter for information transfer. This belief was based on several studies that showed changes in grey matter volume with regular cognitive activity. However, it seems wasteful to have half of the brain devoted solely to information transfer. Thus, more researchers have started looking at white matter part of the brain to understand their role better. Recently, I have been to two talks that look into this topic – one form neuroimaging point of view and the other from cellular point of view.
Perhaps, the most striking discovery on the importance of white matter was shown by Dr. Heidi Johansen-Berg of Oxford University, one of the speakers whose talk I attended. In a study published in Nature Neuroscience, her lab showed for the first time that white matter structure can also be changed with training. In this elegant paper, they got 24 volunteers to learn juggling balls (kudos to the volunteers!). The brain scans before and after showed differences in white matter structure in their brain. This study also refuted the claim that once you are an adult your brain cannot change anymore.
However, as mentioned in a previous post, these neuroimaging data is unable to show why or how things happen and merely shows us the change from one point to another. To understand white matter better, we have to go to a cellular level and the first question we ask is why is white matter white? White matter is made up of a bundle of axon fibers (thread like structures coming out of the cell body of a neuron). These axon fibers are covered in “myelin” – an insulating material that speeds up nerve signals. Since the myelin are white in color the myelin covered axon bundles are collectively called white matter. As an analogy, think of your home electrical system – the wires covered in insulating material are the white matter while your appliances are the grey matter.
So how does myelination take place in the first place and how does the amount of myelination change while juggling? That was the topic for the next lecture I went to given by Dr. Dr Ragnhildur Thora Karadottir of Cambridge University. When our brain is still maturing, some cells called the oligodendrocyte progenitor cells (OPC) are present in the brain that seek out large diameter neurons and myelinate them. These large neurons are important for synchronization of basic functions like suckling. Recently, it has been found that some of these OPCs hang around even when we are older. It is now thought that these OPCs that loiter around in the brain are responsive to some chemicals that are secreted during a training (like juggling). So when these chemicals are secreted, the OPCs become active and start myelinating more axons. Dr. Khadottir’s lab found that the mystery chemical might be Glutamate. Glutamate is the major excitatory neurotransmitter in the central nervous system which means that this chemical increase the activity in our neurons. When juggling and release of glutamate occurs at the same time, the OPCs sense it and goes on to myelinate new axons or remyelinate old ones.
P.S. I have added “LEARN TO JUGGLE” in my PhD checklist.
Reference to the juggling paper:
Scholz, Jan et al. “Training Induces Changes in White Matter Architecture.” Nature neuroscience 12.11 (2009): 1370–1371. PMC. Web. 1 Nov. 2016.