Neurons in many body regions, and in particular within the brain, are supported by other types of cell that do not themselves transmit electrical signals. Glial cells are a major class of supporting cell. Indeed, the human brain has more glial cells than neurons.

Glial cells have many functions. They surround neurons and provide them with nutrition and physical support. Some glial cells, called astrocytes, support endothelial cells that make up blood vessels in the brain. These endothelial cells are linked by tight junctions (Chapter 10) to form a selective barrier, called the blood–brain barrier, which limits the types of compounds that can diffuse from the blood into the brain. This barrier prevents pathogens and toxic compounds in the blood from entering the brain. Because nerve cells have limited capacity to regenerate after damage, protection of the brain and spinal cord is critically important. Nevertheless, lipid-soluble compounds such as alcohol and certain anesthetics readily diffuse across the blood–brain barrier, and thus they can affect the functioning of the brain and an animal’s mental state. They can also damage and even destroy neurons.

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During development, glial cells help orient neurons as they develop their connections. They also provide electrical insulation to vertebrate neurons that allows nerve signals to be transmitted rapidly, a topic we discuss next.