Glial Cells

Glial cells, also known as neuroglia or simply glia, are non-neuronal cells in the nervous system that support neurons^[1]. In the image above, the neurons are light brown and orange, and the glial cell is the big red one.

Early estimates suggested a ratio of about 10 glial cells for every neuron, but more recent research has adjusted this ratio significantly. A study published in 2009, using an isotropic fractionator to count cells, indicated that the total number of neurons and glial cells in the average human brain are approximately equal, suggesting a 1:1 ratio^[2]. This ratio can vary depending on the specific region of the brain. For instance, in the cortex, the number of glial cells tends to be greater than the number of neurons, whereas in the cerebellum, the number of neurons is higher. The complexity of this relationship means that it's difficult to provide an exact overall ratio.

Unlike neurons, glial cells do not produce electrical impulses; however, they serve many critical roles in the nervous system:

1. Astrocytes: These are star-shaped glial cells involved in many functions, including the regulation of ion concentration in the extracellular space, the modulation of synaptic transmission, and the maintenance of the blood-brain barrier, which protects the brain from harmful substances^[4]. They also provide neurons with nutrients and have a role in repair and scarring processes after neural injury^[5].

2. Oligodendrocytes and Schwann cells: Oligodendrocytes are present in the central nervous system and Schwann cells in the peripheral nervous system^[6]. Their primary function is to create myelin sheaths around axons, which insulate these extensions of neurons and enhance their transmission of electrical impulses^[7].

3. Microglia: These are the primary immune cells of the nervous system^[8]. They act as the first and main form of active immune defense in the central nervous system. They are responsible for cleaning up the brain of apoptotic cells and debris, and they react quickly to any changes in their environment to protect the nervous system^[9].

4. Ependymal cells: These cells line the ventricles of the brain and the central canal of the spinal cord^[10]. They are involved in the production, circulation, and absorption of cerebrospinal fluid, which protects the brain and spinal cord^[11].

5. Radial glia: These cells function as neural stem cells and also create a scaffold for new neuron migration during development^[12]. They play a crucial role in the development of the nervous system^[13].

Glial cells are critical for the normal functioning and health of the nervous system. They ensure a suitable environment for neurons to function, protect and insulate neurons, provide immune support, and have a crucial role in the development of the nervous system^[14].