I recently purchased the book “The Magnesium Miracle” from Carolyn Dean, M.D., N.D. (available at Amazon.com). I will quote some parts of the book.
About 60-65 percent of all our magnesium is housed in our bones and teeth. The remaining 35-40 percent is found in the rest of the body, including muscle and tissue cells and body fluids. The highest concentrations are in the heart and brain cells, so it is no wonder that the major symptoms of magnesium deficiency affect the heart and the brain.