Glial cells, also called glial cells or neuroglia, are cell which are non-neuronal and are located within the central nervous system and the peripheral nervous system
that provides physical and metabolic support to neurons, including
neuronal insulation and communication, and nutrient and waste transport.
Glial cells are a general term for many types of glial cell, for example microglial, astrocytes, and Schwann cells, each having their own functions within the body. Each type of glial cell performs specific jobs that keep the brain functioning.
Primarily, glial cells provide support and protection to the neurons ( nerve cells ), maintain homeostasis, cleaning up debris, and forming myelin. They essentially work to care for the neurons and the environment they are in.
It is believed that German biologist Rudolf Virchow was the first to discover glial cells in 1856. Whilst looking for connective tissue in the brain, Virchow identified substance connected to the neurons.
This material was given the name nervenkitt in German, and neuroglial in Greek, which both translate to nerve glue. It was understood that glial cells only functioned as glue for the neurons, having a passive role to the neurons’ active role in the brain.
Some later researchers proposed that glial cells fed the neurons, whilst others believed they might be insulators for the electrical activity of the neurons.
Now, we have a more thorough understanding of glia’s role in the brain, that they serve active and important functions to the overall maintenance of the brain and peripheral areas.
Glial cells differ to neurons in terms of structure. Neurons will have an axon and dendrites which are used to transfer electrical signals between other nerve cells. Glial cells, however, do not have axons or dendrites.
This means that glial cells do not participate directly in synaptic interactions and electrical signaling, although they are supportive in helping the neurons maintain these functions.
Also, although glial cells have complex extensions from their cell bodies, since they do not have axons or dendrites, this makes them typically smaller than neurons. Astrocytes, which are the largest type of glial cell, has a diameter of 40-50 microns.
Despite being smaller in size, glial cells are more numerous than neurons. Depending on the mammal, glial cells can make up between 33% and 66% of the total brain mass, outnumbering neurons by a ratio of around ten to one.
In This Article
Types of glial in the central nervous system (CNS)
Astrocytes are the most numerous types of glial cells and account for about half of all the cells in the brain.
Astrocytes are star-shaped cells, restricted to the brain and spinal cord which makes up the CNS, their main function being to maintain the environment for neuronal signaling.
They do this through controlling the levels of neurotransmitters surrounding the synapses. These cells have the ability to sense the levels of neurotransmitter in synapses and can then respond by releasing molecules that directly influence the neuronal activity.
Because of this, astrocytes are important for modifying synapses and moreover how neurons communicate.
This type of glial cell is also responsible for cleaning up what is left behind after synaptic transmission.
Once a message has been received and transmitted to the next neuron, the astrocytes will recycle any of the left-over neurotransmitters.
Similarly, once a neuron has died, the astrocytes will clean this up as well as any excess potassium ions there may be in the environment. Astrocytes are also important for forming the blood brain barrier. This barrier is important as it will only allow in substances that are supposed to be in the brain, therefore keeping out anything harmful.
With the help of astrocytes, this filtering of substances is essential for maintaining a healthy brain. Also, astrocytes store glucose from the blood and utilise this to fuel the neurons, thus astrocytes are important for regulating metabolism as well as homeostasis.
Another important type of glial cell which are restricted to the CNS are oligodendrocytes. These cells have the appearance of balls with spikes all around them. On the tips of their spikes are white, shiny membranes.
The purpose of this structure, especially the white membranes, is to wrap around the axons of neurons. When these oligodendrocytes wrap around the axons, they form a protective layer on the axon which is called the myelin sheath.
Myelin sheath is a substance that is rich in fat and is laminated, which provides thorough insulation to the neurons. Myelin sheath functions in the same way that wire cables have insulation surrounding them.
Myelin is important to allow electrical signals to travel faster down the axon, therefore influencing the speed of action potential conduction.
Without myelin, the electrical impulses going down the axon would be a lot slower, resulting in delayed and disrupted signals. As such, oligodendrocytes are essential for providing support to neurons for quicker signaling.
Microglial are small cells with an oval-shaped cell body and many small branches projecting out of it to enable them to move about. The main function of these cells is to respond to any injuries or diseases in the CNS.
When injury and disease are detected, the microglial are alerted and respond by moving to the injury site in order to either clear away any dead cells or to remove any harmful toxins or pathogens that may be present.
The cells are therefore especially important for maintaining the health of the CNS and are known as immune cells. Microglia also play a role in the development of the brain.
Typically, far more synapses are created than are needed, when only the strongest and most important ones need to survive.
Microglia directly contribute to removing synapses that are deemed as unnecessary, a process known as synaptic pruning.
Ependymal cells are located in the CNS that are column shaped and typically line up together to form a membrane.
This membrane is called the ependyma, which is a thin membrane lining the spinal cord and ventricles of the brain . In the ventricles, these cells have tiny hairlike structures on them called cilia, which face the open space of the cavities they line.
Cilia move in a coordinated pattern to encourage the directional flow of cerebrospinal fluid, which they also produce. Cerebrospinal fluid works by allowing nutrients and other substances to reach the neurons as well as filtering out any harmful molecules.
It also works as a cushion and shock absorber between the brain and the skull, as well as maintaining homeostasis of the brain such as regulating temperature.
A final type of glial in the CNS to discuss are radial glial. Radial glia is believed to be a type of stem cell, meaning they can generate other cells.
These cells are able to make neurons as well as other types of glial such as astrocytes and oligodendrocytes.
Their role as stem cells, especially as being creators of neurons, makes them a target of interest for researchers who are looking into how to repair brain damage from injury and illness, or their role as the brain ages.
Types of glial in the peripheral nervous system (PNS)
Schwann cells work in a similar fashion to oligodendrocytes as they also produce myelin sheath for the axons of neurons, however, they are located in the PNS.
The plasma membrane of these Schwann cells spirals around the axons of neurons to form the fatty insulation that is required for faster transmission of electrical signals.
Schwann cells can be either myelinating or non-myelinating. Whilst myelinating Schwann cells wrap around the axons of neurons, non-myelinating Schwann cells do not wrap around the axons, but they still provide support and cushioning to them.
Also, each Schwann cell form a single myelin sheath around an axon, whereas oligodendrocytes form myelin sheaths for multiple surrounding axons.
In addition to insulating axons, Schwann cells are critical in response to axon damage within the PNS as they can help in regenerating these damaged axons.
When any type of injury occurs, the Schwann cells are sent to the injury site to remove the dead cells. The Schwann cells also have the capability to occupy the original space of the neurons and regenerate the fibers in such a way that they are able to return to their original target sites.
The precentral gyrus is a part of the brain’s cortex responsible for executing voluntary movements, located in the most posterior position of the frontal lobe, outlining the temporal lobes.
The precentral gyrus is the anatomical location of the primary motor cortex, which is what this gyrus is commonly known as. This gyrus works by creating and organizing a map of the body, known as the homunculus or ‘little man’.
The precentral gyrus is believed to contain the motor control for the torso, arms, hands, fingers, and head. This gyrus also works by controlling the motor movements on the body’s contralateral side, meaning the opposite side to which it is located within the brain.
Satellite cells small glia in the PNS that works by surrounding neurons in the sensory, sympathetic, and parasympathetic ganglia. Ganglia are clusters of nerve selves within the autonomic nervous system as well as the sensory system.
The autonomic nervous system regulates the internal organs, whilst the sensory system is important for our senses to work. These cells are thought to be similar to astrocytes in the CNS as they work in similar ways.
Satellite cells’ main purpose is in the regulation of the environment surrounding the neurons and they are thought to provide nutrient support and protection to these neurons.
These cells also absorb harmful toxins so that they do not damage the neurons, as well as detecting and responding to injury and disease in the same way that microglia do.
As previously discussed, glia cells are especially important for the overall functioning and support of neurons. Therefore, if these cells are damaged in any way, can result in many complications, depending on the cells that have been damaged.
Neurodegenerative disorders are particularly involved in glial damage. There is evidence that microglia may become hyperactivated, promoting neuroinflammation which can result to the characteristic toxic protein deposits seen in Alzheimer’s disease.
As microglia in particular is related to the immune system, other conditions which are linked to damaged microglia include chronic neuropathic pain and fibromyalgia.
If microglia are prevented from responding to injury and disease, this can result in chronic pain for individuals. Glial cells in general tend to degenerate in several neurodegenerative diseases, therefore loss of glial cells may contribute to the impairment of learning and memory.
Abnormalities in the process of forming myelin sheath in the CNS (through oligodendrocytes) has been associated with behavioral and cognitive dysfunctions because of signaling of the neurons being weakened.
These dysfunctions have the potential to result in various mental health conditions such as schizophrenia and bipolar disorder.
Dysfunction in forming myelin sheath in the PNS (through Schwann cells) can result in weakened reflexes, weakness, sensory loss, and sometimes paralysis. Guillain-Barre Syndrome is a peripheral demyelinating disease affecting the PNS whereby the immune system attacks healthy neurons in the PNS.
This can result in symptoms such as numbness, weakness, and sometimes even death if it affects the muscles involved in respiration. Although this condition targets the axons of neurons, this also damages the Schwann cells as a result and makes them redundant.
Guillain-Barre Syndrome can be treated through intravenous immunoglobulin which is a treatment comprised of blood donation that contain healthy antibodies, in order to prevent harmful antibodies damaging the axons of neurons.
Although there are not currently any known cures for neurodegenerative diseases that can affect glial cells, it has been suggested that some lifestyle changes can increase the number of new neurons and glial cells being produced.
Exercising, eating healthy foods, and completing exercises for the mind have some support behind them for increasing the number of new cells in certain areas of the brain.
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