Approximately how many glial cells are there in the human nervous system?

• A. Approximately 100 billion
• B. Approximately 1 trillion
• C. Approximately 10 thousand
• D. Approximately 1 trillion (about 10 times more than neurons)

Answer: (D)

Glial cells greatly outnumber neurons in the human nervous system. They come in several types—astrocytes, oligodendrocytes, and microglia in the CNS, plus Schwann cells and satellite cells in the PNS—that support, insulate, and modulate neuronal function. In many introductory estimates, the total glial population is around a trillion, roughly ten times the number of neurons, which is why the statement about about one trillion glia and tenfold more glia compared to neurons is used. Keep in mind that modern methods have refined these ratios in specific regions, but the trillions-versus-hundreds-of-billions picture is a useful general rule of thumb for understanding glial abundance.