Along with neurons, the nervous system contains other specialized cells called glial cells—or simply, glia. Meaning "glue" in Greek, glia are non-neuronal cells that play an essential role in the function of the nervous system. In the human brain, it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas.
Glia perform a number of critical tasks that enable neuron function, including providing physical support and nutrients, destroying pathogens and clearing dead neurons, and directing the growing axons of neurons in the developing brain. One of the most notable functions of glia is to insulate neurons electrically. This is done by generating layers of a fatty substance called myelin that wraps around axons, thus providing electrical insulation for neurons and allowing them to transmit action potentials much more rapidly and efficiently. In the central nervous system (CNS), these particular glial cell subtypes are called oligodendrocytes (Figure 3), and in the peripheral nervous system, Schwann cells.
Neuroglia of the CNS are grouped broadly into microglia and macroglia (Figure 1). Microglia are a type of glial cell that are the resident macrophages of the brain and spinal cord, and thus act as the first and main form of active immune defense in the CNS.
There are a number of macroglia subtypes in the CNS, each with a distinct role. Astrocytes are the most abundant type of macroglial cell, and are also referred to as astroglia (Figure 2). Astrocytes have numerous projections that anchor neurons to their blood supply. They also regulate the external chemical environment of neurons by removing excess ions and recycling neurotransmitters released during synaptic transmission.
Other macroglia include oligodendrocytes, mentioned above, which are the cells responsible for myelinating the axons of the CNS. Ependymal cells are another glial subtype that line the ventricles of the CNS. These cells create and secrete cerebrospinal fluid (CSF) and beat their cilia to help circulate the CSF. Finally, radial glia play a critical role in the developing nervous system where they function both as neuronal progenitors and as scaffolding upon which newborn neurons migrate.