What Are Glial Cells and What Do They Do?

Astrocytes

The most common type of glial cell in the CNS is the astrocyte or astroglia. The "astro" part of the name is because the cells have projections that make them look star-shaped.

There are different kinds of astrocytes. For example, protoplasmic astrocytes have thick projections with lots of branches. Fibrous astrocytes have long, slender arms with few branches.

Protoplasmic astrocytes are generally found among neurons in the gray matter of the brain while the fibrous ones are typically found in white matter.

While they're found in different places, they do similar jobs, including:

• Forming the blood-brain barrier (BBB): The BBB is like a strict security system for the brain. It only lets 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 neurotransmitters: Neurons communicate using chemical messengers called neurotransmitters. Once the message is delivered, neurotransmitters hang around until an astrocyte recycles them. This reuptake process is the target of many medications, including antidepressants.
• Cleaning up: Astrocytes also clean up what's left behind when a neuron dies, as well as excess potassium ions (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 blood 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 between cells.
• Brain energy metabolism and homeostasis: One of the most important roles of astrocytes is to regulate metabolism in the brain by storing sugar (glucose) from the blood and providing it as fuel for neurons.

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

Oligodendrocytes

Oligodendrocytes come from neural stem cells. The word is made up of a few Greek terms that mean "cells with several branches."

The main purpose of oligodendrocytes is to help information move faster along axons in the brain.

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

The sheath is not continuous, though. There's a gap between each membrane that's called the "node of Ranvier." This node helps electrical signals spread efficiently along nerve cells.

The signal actually hops from one node to the next and increases the velocity of the nerve conduction while also reducing how much energy it takes to transmit it.

At birth, you only have a few myelinated axons, but the number 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" is often used when talking about multiple sclerosis (MS) because this part gets damaged in the disease.

In people with MS, it's thought that the body's immune system attacks the myelin sheaths, which leads to dysfunction of the neurons and impaired brain function. Spinal cord injuries can also damage myelin sheaths.

Other diseases associated with oligodendrocyte dysfunction include:

• Leukodystrophies
• Tumors called oligodendrogliomas
• Schizophrenia
• Bipolar disorder

Microglia

Microglia are tiny glial cells ("micro" means small). They act as the brain's own dedicated immune system. The brain needs its own immune system because the blood-brain barrier isolates the brain from the rest of your body.

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

When microglia respond to an injury, it causes inflammation as part of the healing process.

Sometimes, the response causes problems. For example, in Alzheimer's disease, microglia are hyperactivated and cause too much inflammation. The response may lead to amyloid plaques and other brain changes related to Alzheimer's.

Along with Alzheimer's, other conditions linked to microglial dysfunction include:

• Fibromyalgia
• Chronic neuropathic pain
• Autism spectrum disorders
• Schizophrenia

Microglia are believed to have many jobs, including playing a "housekeeping" role in learning-associated brain plasticity and guiding the development of the brain.

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 is not very efficient.

Microglia detect unnecessary synapses and "prune" them, just as a gardener prunes a rose bush to keep it healthy.

Ependymal Cells

Ependymal cells make up the thin membrane lining the central canal of the spinal cord and the passageways (ventricles) of the brain (ependyma). They also make cerebrospinal fluid and have an important role in the blood-brain barrier.

Ependymal cells are very small and line up tightly to form the membrane. Inside the ventricles, they have little hairlike projections (cilia) that wave back and forth to keep 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.

The fluid is also necessary to maintain homeostasis of your brain, which means regulating its temperature and other features that keep it operating as well as possible.

Radial Glia

Radial glia is believed to be a type of stem cell. This type of cell can create other cells. In the developing brain, stem cells are the "parents" of neurons, astrocytes, and oligodendrocytes.

When you were an embryo, these cells also provided the "scaffolding" for developing neurons. They provide the long fibers that guide young brain cells into place as your brain forms.

Since they have an important role as stem cells, especially as creators of neurons, researchers have looked at radial glia to learn more about how to repair brain damage from illness or injury.

Later in life, these cells contribute to your brain's ability to change and adapt (neuroplasticity).       

Schwann Cells

Schwann cells are named for Theodor Schwann, the physiologist who discovered them.

They function a lot like oligodendrocytes by providing myelin sheaths for axons. However, Schwann cells are found in the peripheral nervous system (PNS) rather than the CNS.

Instead of being a central cell with membrane-tipped arms, Schwann cells form spirals directly around the axon. The nodes of Ranvier sit between them, just as they do with oligodendrocytes, and 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, it can "eat" the nerve's axons and provide a protected path for a new axon to form.

There are a few diseases that involve the Schwann cells, such as:

• Guillain-Barre' syndrome
• Charcot-Marie-Tooth disease
• Schwannomatosis
• Chronic inflammatory demyelinating polyneuropathy
• Leprosy

Schwann cells might also be involved in some forms of chronic pain. The activation of the cells after nerve damage might contribute to dysfunction in a type of nerve fibers called nociceptors, which sense environmental factors such as heat and cold.

There has been exciting research on transplanting Schwann cells for spinal cord injury and other types of peripheral nerve damage.

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 satellite cells but many researchers believe they're similar to astrocytes. However, they're found in the PNS, not the CNS.

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

The neurons with satellite cells make up clusters of nerve cells in the autonomic nervous system and the sensory system called ganglia.

The autonomic nervous system regulates your internal organs, while your sensory system is what allows you to see, hear, smell, touch, feel, 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.

Like microglia, satellite cells detect and respond to injury and inflammation, but their role in repairing cell damage is not yet understood.

It's also thought that satellite cells help transport several neurotransmitters and other substances, including:

• Glutamate
• GABA
• Norepinephrine
• Adenosine triphosphate
• Substance P
• Capsaicin
• Acetylcholine

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.

Summary

There are several kinds of glial cells in your brain and the nerves throughout your body. Each type has a special—and important—job in keeping your brain working at its best.

If these cells get damaged or are affected by a disease, it can cause problems in your nervous system.

We have a sense of what glial cells do in the body, but still have a lot left to learn.