Glial cells are many times more numerous as compared to neurons. In the brain and spinal cord, neurons exist only in close proximity to glial cells, which comprise about half of the brain’s volume. The attrition rate of mature glial cells (that do divide—unlike neurons) is negligible because of their ability to rapidly proliferate. (Schramm, Derek D., PhD. The Creative Brain. p 4-6. CA: Institute for Natural Resources, Health Update. 2007.)