In the central nervous system (CNS) and the peripheral nervous system (PNS), we find non-neuronal cells known as glia or glial cells. The term „glia“ was first coined by the German anatomist Rudolf Virchow in 1856, who believed these cells acted as „glue“ to hold the nervous system together. However, it wasn’t until the 20th century that the true complexity and diverse functions of glial cells were revealed. Today, we know that glia play essential roles beyond structural support, including modulating neuronal activity, maintaining homeostasis, facilitating information processing in the brain, and protecting neurons from injury.

Glial cells are smaller than neurons but are equal in number to neurons in the human brain (approximately 85 billion glial cells). The glia-to-neuron ratio varies depending on the brain region. In the CNS, glial cells include astrocytes, oligodendrocytes, microglia, ependymal cells, tanycytes, and radial glia (including Bergmann glia in the cerebellum and radial Müller cells in the retina). In the PNS, glial cells include Schwann cells, satellite cells, and enteric glial cells. (full summary)