That is a very good question. Then problem with answering is that up until recently MS was thought of as purely a white matter disease. So, ALL of the symptoms that occurred were felt in one way or another to be the results of damage to the white matter. In the last couple years knowledge about the extent of gray matter lesions has grown enormously. We now know that our disability may be more linked to grey matter damage as to white. In fact, things like cerebral atrophy may be more due to the loss of gray cells than of white cells.
In researching the answer to this question I came across a crystal clear description of the functions and differences of grey and white nerve cells. This is it. I rarely take large quotes from other places, but this is definitely worthwhile.
A Quick Lesson in the Difference Between White Matter and Gray Matter"
"Brain cells are called neurons. Neurons are made up of a cell body. In the brain (and spinal cord), the cell bodies are grouped together in organized ways. When looking at the brain with the naked eye, these groups of cell bodies have a grayish color, and so they are called gray matter.
The outer layer of the brain is one area where cell bodies are grouped together; this region of gray matter is called the cerebral cortex. Deeper in the brain are other groups of cell bodies. These are also called gray matter but they are separate from the cerebral cortex and are called nuclei. The two areas of gray matter are connected together in many ways. The connections between these areas allow us to speak and think.
Nerve cells talk to each other using electrical and chemical signals. These signals must be sent very fast. In order for this to occur, cells must be connected by "wires." These wires are like long threads that extend from the nerve cell body. These threads are called axons. Some axons are short and travel from one part of the brain to another. Others are very long, and travel from the brain to the spinal cord. Some nerve cells are several meters long.
Electrical signals travel much faster through wires that are coated or insulated. This prevents them from short-circuiting. Axons are coated with a substance called myelin which allows for fast movement of signals from one area of the brain to another, or from one part of the body to another. Like gray matter, axons are also grouped together. Because myelin appears white to the naked eye, parts of the brain or spinal cord that are made up of axons are called white matter."
The Grey matter is where all of the information that is brought to the brain is put together as higher level thought. It is where we analyze, understand, plan, think, dream, imagine, put together language, be creative, have our personality, really listen, read, remember, grieve, be happy, make judgments, and do all the things that are more than just react.
Damage to the gray matter directly affects our cognitive functions. It affects our memories and our concentration. It affects our attention span, our ability to concentrate and to multitask. It can affect our emotions. But all of those things also require that we be able to connect one part of the gray matter to the others and this is done largely via the white matter (information highways). For instance, to be able to read out loud the gray matter that recognizes the written word must communicate via the white matter with the gray matter that allows us to formulate speech.
So for each of us with MS who have significant cognitive difficulties, we can probably say that we also have some gray matter damage/lesions. I don't think, though, that the science has progressed far enough for our doctors to say with certainty that this or another symptom is specifically from gray matter damage.
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