MS Questions

Actually, the best part of the last MS checkup was getting some questions that have puzzled me since day one cleared up.

MS is always described as a disease in which the insulating myelin covers of nerve cells are damaged. Yet, the main diagnostic tool used is an MRI of the brain. I asked Dr. Kipp the following questions:

Are nerve cells and brain cells the same?
Do all nerve cells have myelin sheaths?
Do you get sclerosis on nerve cells throughout the body?

His answer was a qualified no, nerve cells and brain cells are not the same. I my words, it would seem there is a sliding scale of change. Most [but not all] nerve cells outside the brain and spinal cord do not have myelin sheaths, and consequently do not get lesions.

The name multiple sclerosis refers to scars (sclerae—better known as plaques or lesions) in particular in the white matter of the brain and spinal cord. [from wikipedia]

So I asked, is describing MS as a nerve cell de-myelining disease a misnomer? Should it not be described as a brain cell de-myelining disease?
His answer was again qualified; a qualified yes. Yes, it is a more a brain cell de-myelining disease, except it also affects cells in the brain stem and spinal cord, [where the brain attaches??].

Because MS affects cells in the brain, the blood brain barrier [BBB] becomes significant. [my question]Is MS a result of a failure or compromise of the BBB?