What Are Glial Cells and What Do They Do?

The Other Brain Cells

You've likely heard of the "gray matter" of the brain, which is made up of cells called neurons, but a lesser-known type of brain cell is what makes up the "white matter." These are called glial cells.

What Are Glial Cells?

Originally, glial cells—also called glia or neuroglia—were believed to just provide structural support. The word "glia" literally means "neural glue." Relatively recent discoveries, though, have revealed that they perform all kinds of functions in the brain and the nerves that run throughout your body. As a result, research has exploded and we've learned volumes about them. Still, much more is left to learn.

Types of Glial Cells

Primarily, glial cells provide support for the neurons. Think of them as a secretarial pool for your nervous system, plus the janitorial and maintenance staff. They may not do the big jobs, but without them, those big jobs would never get done.

Glial cells come in multiple forms, each of which performs certain specific functions that keeps your brain operating correctly—or not, if you have a disease that impacts these important cells.

Your central nervous system (CNS) is made up of your brain and the nerves of your spinal column. Five types that are present in your CNS are:

  • Astrocytes
  • Oligodendrocytes
  • Microglia
  • Ependymal cells
  • Radial glia

You also have glial cells in your peripheral nervous system (PNS), which comprises the nerves in your extremities, away from the spine. Two types of glial cells there are:

  • Schwann cells
  • Satellite cells
1

Astrocytes

NANCY KEDERSHA/UCLA/SCIENCE PHOTO LIBRARY/Getty Images

The most common type of glial cell in the central nervous system is the astrocyte, which is also called astroglia. The "astro" part of the name because refers to the fact that they look like stars, with projections going out all over the place.

Some, called protoplasmic astrocytes, have thick projections with lots of branches. Others, called fibrous astrocytes have long, slender arms that branch less frequently. The protoplasmic type is generally found among neurons in the gray matter while the fibrous ones are typically found in white matter. In spite of these differences, they perform similar functions.

Astrocytes have several important jobs, including:

  • Forming the blood-brain barrier (BBB). The BBB is like a strict security system, only letting in substances that are supposed to be in your brain while keeping out things that could be harmful. This filtering system is essential for keeping your brain healthy.
  • Regulating the chemicals around neurons. The way neurons communicate is via chemical messengers called neurotransmitters. Once a chemical has delivered its message to a cell, it basically sits there cluttering things up until an astrocyte recycles it through a process called reuptake. The reuptake process is the target of numerous medications, including anti-depressants. Astrocytes also clean up what's left behind when a neuron dies, as well as excess potassium ions, which are chemicals that play an important role in nerve function.
  • Regulating blood flow to the brain. For your brain to process information properly, it needs a certain amount of blood going to all of its different regions. An active region gets more than an inactive one.
  • Synchronizing the activity of axons. Axons are long, thread-like parts of neurons and nerve cells that conduct electricity to send messages from one cell to another.

Astrocyte dysfunction has been potentially linked to numerous neurodegenerative diseases, including:

Animal models of astrocyte-related disease are helping researchers learn more about them with the hope of discovering new treatment possibilities.

2

Oligodendrocytes

Oligodendrocytes come from neural stem cells. The word is composed of Greek terms that, all together, mean "cells with several branches." Their main purpose is to help information move faster along axons.

Oligodendrocytes look like spikey balls. On the tips of their spikes are white, shiny membranes that wrap around the axons on nerve cells. Their purpose is to form a protective layer, like the plastic insulation on electrical wires. This protective layer is called the myelin sheath.

The sheath isn't continuous, though. There's a gap between each membrane that's called the "node of Ranvier," and it's the node that helps electrical signals spread efficiently along nerve cells. The signal actually hops from one node to the next, which increases the velocity of the nerve conduction while also reducing how much energy it takes to transmit it. Signals along myelinated nerves can travel as fast as 200 miles per second.

At birth, you only have a few myelinated axons, and the amount of them keeps growing until you're about 25- to 30-years-old. Myelination is believed to play an important role in intelligence.

Oligodendrocytes also provide stability and carry energy from blood cells to the axons.

The term "myelin sheath" may be familiar to you because of its association with multiple sclerosis. In that disease, it's believed that the body's immune system attacks the myelin sheaths, which leads to dysfunction of those neurons and impaired brain function. Spinal cord injuries may also cause damage to myelin sheaths.

Other diseases believed to be associated with oligodendrocyte dysfunction include:

Some research suggests that oligodendrocytes may be damaged by the neurotransmitter glutamate, which, among other functions, stimulates areas of your brain so you can focus and learn new information. However, in high levels, glutamate is considered an "excitotoxin," which means that it can overstimulate cells until they die.

3

Microglia

As their name suggests, microglia are tiny glial cells. They act as the brain's own dedicated immune system, which is necessary since the BBB isolates the brain from the rest of your body.

Microglia are alert to signs of injury and disease. When they detect it, they charge in and take care of the problem—whether it means clearing away dead cells or getting rid of a toxin or pathogen.

When they respond to an injury, microglia cause inflammation as part of the healing process. In some cases, such as Alzheimer's disease, they may become hyper-activated and cause too much inflammation. That's believed to lead to the amyloid plaques and other problems associated with the disease.

Along with Alzheimer's, illnesses that may be linked to microglial dysfunction include:

Microglia are believed to have many jobs beyond that, including roles in learning-associated plasticity and guiding the development of the brain, in which they have an important housekeeping function.

Our brains create a lot of connections between neurons that allow them to pass information back and forth. In fact, the brain creates a lot more of them than we need, which isn't efficient. Microglia detect unnecessary synapses and "prune" them, just as a gardener prunes a rose bush to keep it healthy.

Microglial research has really taken off in recent years, leading to an ever-increasing understanding of their roles in both health and disease in the central nervous system.

4

Ependymal Cells

Ependymal cells are primarily known for making up a membrane called the ependyma, which is a thin membrane lining the central canal of the spinal cord and the ventricles (passageways) of the brain. They also create cerebrospinal fluid.

Ependymal cells are extremely small and line up tightly together to form the membrane. Inside the ventricles, they have cilia, which look like little hairs, that wave back and forth to get the cerebrospinal fluid circulating.

Cerebrospinal fluid delivers nutrients to and eliminates waste products from the brain and spinal column. It also serves as a cushion and shock absorber between your brain and skull. It's also important for homeostasis of your brain, which means regulating its temperature and other features that keep it operating as well as possible.

Ependymal cells also are involved in the BBB.

5

Radial Glia

Radial glia are believed to be a type of stem cell, meaning that they create other cells. In the developing brain, they're the "parents" of neurons, astrocytes, and oligodendrocytes. When you were an embryo, they also provided a scaffolding for developing neurons, thanks to long fibers that guide young brain cells into place as your brain forms.

Their role as stem cells, especially as creators of neurons, makes them the focus of research on how to repair brain damage from illness or injury.

Later in life, they play roles in neuroplasticity as well.                   

6

Schwann Cells

Schwann cells are named for physiologist Theodor Schwann, who discovered them. They function a lot like oligodendrocytes in that they provide myelin sheaths for axons, but they exist in the peripheral nervous system (PNS) rather than the CNS.

However, instead of being a central cell with membrane-tipped arms, Schwann cells form spirals directly around the axon. The nodes of Ranvier lie between them, just as they do between the membranes of oligodendrocytes, and they assist in nerve transmission in the same way.

Schwann cells are also part of the PNS's immune system. When a nerve cell is damaged, they have the ability to, essentially, eat the nerve's axons and provide a protected path for a new axon to form.

Diseases involving Schwann cells include:

We've had some promising research on transplanting Schwann cells for spinal cord injury and other types of peripheral nerve damage.

Schwann cells are also implicated in some forms of chronic pain. Their activation after nerve damage may contribute to dysfunction in a type of nerve fibers called nociceptors, which sense environmental factors such as heat and cold.

7

Satellite Cells

Satellite cells get their name from the way they surround certain neurons, with several satellites forming a sheath around the cellular surface. We're just beginning to learn about these cells but many researchers believe they're similar to astrocytes.

Satellite cells' main purpose appears to be regulation the environment around the neurons, keeping chemicals in balance.

The neurons that have satellite cells make up something called gangila, which are clusters of nerve cells in the autonomic nervous system and sensory system. The autonomic nervous system regulates your internal organs, while your sensory system is what allows you to see, hear, smell, touch, and taste.

Satellite cells deliver nutrition to the neuron and absorb heavy metal toxins, such as mercury and lead, to keep them from damaging the neurons.

They're also believed to help transport several neurotransmitters and other substances, including:

Like microglia, satellite cells detect and respond to injury and inflammation. However, their role in repairing cell damage isn't yet well understood.

Satellite cells are linked to chronic pain involving peripheral tissue injury, nerve damage, and a systemic heightening of pain (hyperalgesia) that can result from chemotherapy.

A Word From Verywell

Much of what we know, believe, or suspect about glial cells is new knowledge. These cells are helping us understand how the brain works and what's going on when things don't work like they're supposed to.

It's certain that we have much more to learn about glia, and we're likely to gain new treatments for myriad diseases as our pool of knowledge grows.

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