Marguerite, a first-year doctoral student, scrutinizes the neural tissue that she prepared for study under her microscope. she knows from experience that the areas that look gray have a high density of cell bodies, while the areas that look white consist of large bundles of myelinated

Axons. Myelin is a fatty substance surrounding axons of some nerve cells insulating them. This insulation allows for faster transfer of the action potential (up to 120 m/s instead of 2 m/s). In the peripheral nervous system myelin is made by Schwann cells. In the central nervous system it is made by oligodendrocytes.