The histological studies of Cajal, Golgi, and a host of successors led to the consensus that the cells of the nervous system can be divided into two broad categories: nerve cells, or neurons, and supporting glial cells (also called neuroglia, or simply glia). Nerve cells are specialized for electrical signaling over long distances. Understanding this process represents one of the more dramatic success stories in modern biology and is the subject of Unit I. In contrast to nerve cells, glial cells support rather than generate electrical signals. They also serve additional functions in the developing and adult brain. Perhaps most important, glia are essential contributors to repair of the damaged nervous system, acting as stem cells in some brain regions, promoting regrowth of damaged neurons in regions where regeneration can usefully occur, and preventing regeneration in other regions where uncontrolled regrowth might do more harm than good.