Neuroglia, commonly called glial cells or simply glia, are the non-neuronal cells of your nervous system. They don’t fire electrical signals the way neurons do, but they perform nearly every support function that keeps neurons alive and working properly: maintaining the chemical environment around nerve cells, speeding up signal transmission, cleaning up used neurotransmitters, guiding brain development, and responding to injury. The human brain contains roughly 40 to 85 billion glial cells, sitting at approximately a 1:1 ratio with neurons. That corrects a longstanding claim, repeated in textbooks for decades, that glia outnumbered neurons 10 to 1.
Where the Name Comes From
The term “neuroglia” was coined by the German physician Rudolf Virchow in 1856. He identified these cells as a distinct population, separate from neurons, in the central nervous system. The word literally translates to “nerve glue,” reflecting the early assumption that glia were passive filler holding the brain together. That view has been thoroughly overturned. Glia are now understood to actively shape how the brain processes information, fights infection, and repairs itself.
Glial Cells in the Brain and Spinal Cord
The central nervous system (your brain and spinal cord) contains four main types of glial cells, each with a distinct job.
Astrocytes
Astrocytes are star-shaped cells that do an enormous amount of housekeeping. They regulate the chemical balance around neurons, mopping up excess ions and neurotransmitters so signals stay clean. They also wrap tightly around blood vessels in the brain, reinforcing what’s known as the blood-brain barrier. This barrier is selective about what gets in and out of brain tissue, and astrocytes strengthen it by tightening the junctions between blood vessel cells and controlling the placement of molecular transporters.
Astrocytes also participate directly in signaling. At a synapse (the gap where one neuron communicates with another), astrocytes have receptors that detect the neurotransmitters being released. When they pick up that signal, they respond by releasing their own chemical messengers, including glutamate, ATP, and D-serine. This three-way exchange between the sending neuron, receiving neuron, and the astrocyte is sometimes called the tripartite synapse. It means astrocytes can fine-tune how strong or weak a neural connection becomes.
Oligodendrocytes
Oligodendrocytes produce myelin, the fatty insulation that wraps around nerve fibers in the brain and spinal cord. Myelin allows electrical signals to travel much faster by forcing them to jump between gaps in the coating, called nodes of Ranvier. A single oligodendrocyte can extend its membranes to insulate segments of up to 60 different nerve fibers at once. The main protein in central nervous system myelin is called proteolipid protein, which differs from the proteins used in the peripheral nervous system’s version of myelin.
Microglia
Microglia are the brain’s resident immune cells. They act as macrophages, patrolling the central nervous system for signs of infection, damage, or cellular debris, then engulfing and digesting whatever they find. Beyond infection defense, microglia play a critical role during brain development and beyond by pruning synapses. They physically engulf dendritic spines (the tiny protrusions where neurons receive signals), eliminating unnecessary connections and refining neural circuits. When this pruning process goes wrong, it has been linked to neurological and psychiatric conditions.
Ependymal Cells
Ependymal cells line the fluid-filled cavities, called ventricles, inside the brain and the central canal of the spinal cord. They are covered in tiny hair-like projections called cilia that beat in coordinated waves. This beating drives the flow of cerebrospinal fluid (CSF), the clear liquid that cushions the brain, clears away toxins, and carries signaling molecules. Ependymal cells at a structure called the choroid plexus are directly involved in producing CSF. Normal CSF flow is essential for brain health, waste clearance, and even guiding the migration of newly born neurons to their correct positions.
Glial Cells in the Rest of the Body
Outside the brain and spinal cord, in your peripheral nervous system, two main types of glia do the heavy lifting.
Schwann Cells
Schwann cells are the peripheral nervous system’s equivalent of oligodendrocytes. They produce the myelin sheath around nerve fibers in your arms, legs, and organs. One key difference: while a single oligodendrocyte insulates segments of many nerve fibers, a single Schwann cell wraps around just one segment of one nerve fiber. To do this, Schwann cells expand their surface area roughly 2,000-fold, creating the multilayered myelin sheath needed for fast signal conduction.
Not all Schwann cells make myelin. A subtype called Remak Schwann cells surrounds smaller nerve fibers that don’t need insulation, bundling them together and providing metabolic support. When a peripheral nerve is injured, another subtype, repair Schwann cells, activates. These cells help break down damaged myelin and axon debris, recruit immune cells to clean the area, and then elongate to two or three times their normal length. They line up in columns called Büngner bands, forming tracks that guide regrowing nerve fibers back to their targets. This is a major reason peripheral nerves can regenerate after injury, while central nervous system damage is much harder to reverse.
Satellite Cells
Satellite cells surround the cell bodies of neurons in clusters called ganglia, located just outside the brain and spinal cord. They provide structural support and help regulate the chemical environment around these neurons, functioning somewhat like the astrocytes of the peripheral nervous system.
How Glia Differ From Neurons
The core distinction is that glial cells do not participate directly in the electrical signaling and synaptic communication that neurons use to transmit information. Neurons generate action potentials (rapid voltage changes that travel along nerve fibers) and release neurotransmitters at synapses. Glia support every step of that process without doing the signaling themselves. They maintain the right chemical conditions for neurons to fire, they insulate nerve fibers so signals move faster, they clear away used neurotransmitters after a signal has been sent, and they provide the physical scaffolding that helps wire the nervous system during development.
Glia also differ in their ability to divide. Most mature neurons cannot reproduce, but several types of glial cells retain this ability throughout life. This is relevant both for normal brain maintenance and for disease, since glial cell division that goes unchecked can lead to tumors.
Diseases Linked to Glial Dysfunction
Because glia are involved in so many critical functions, their malfunction underlies a range of serious neurological conditions.
Multiple sclerosis (MS) is the most well-known demyelinating disease. In MS, the immune system attacks the myelin produced by oligodendrocytes in the brain and spinal cord. Without intact myelin, nerve signals slow down or fail entirely, causing symptoms that can include vision problems, numbness, weakness, and difficulty with coordination. All major types of glial cells play roles in MS progression. Microglia can drive inflammation and influence whether damaged myelin gets repaired. Astrocytes can become direct targets of immune attack in related conditions like neuromyelitis optica, where antibodies target a water channel protein on astrocyte membranes.
Gliomas are tumors that arise from glial cells. They are the most common type of primary brain tumor, and their behavior ranges from slow-growing and manageable to highly aggressive. The specific type of glial cell involved determines the tumor’s classification: astrocytomas originate from astrocytes, oligodendrogliomas from oligodendrocytes, and ependymomas from ependymal cells.
Glial cells have also been implicated in neurodegenerative diseases like Alzheimer’s, where microglia and astrocytes contribute to the inflammatory environment that damages neurons over time. However, MS pathology is distinct from purely degenerative diseases because it features recurring waves of inflammatory demyelinating lesions that can strike anywhere in the central nervous system.