The human brain has about as many neurons as glial cells. These are divided into four major groups: the microglia, the astrocytes, the NG2 glial cells, and the oligodendrocytes. Oligodendrocytes function primarily as a type of cellular insulating tape: They form long tendrils, which consist largely of fat-like substances and do not conduct electricity. These wrap around the axons, which are the extensions through which the nerve cells send their electrical impulses. This prevents short circuits and accelerates signal forwarding.