Glial cells are non-neuronal cells of the nervous system that create a physical support system for the neurons, which is vital for the proper functioning of both the nervous tissue and the central nervous system of the human brain.
The glial cells are located in both the central nervous system and the peripheral nervous system that do not relay electrical impulses. Their function is to protect the neurons from outside influences and regulate the internal environment of the brain.
What Is The Function of Glial Cells?
While the neurons relay the signals from one neuron cell to another via neuron impulses in order to control different parts of the body, the glial cells support and protect the functions of the neurons. This type of cell in the brain coordinates the actions of the neuron cells by guiding them to their final destination, thus playing an important role in neurotransmission.
Also, glial cells have a protective function, owing to their ability to build myelin sheets around the neuronal axons. In this way, glial cells insulate and hold the neuronal cells in place.
Glial cells are the chief suppliers of nutrition and oxygen to the neurons. By removing the pathogens from the brain, they protect the central nervous system from inflammation and infection.
What Are the Types of Glial Cells?
There are several types of glial cells with specific functions aimed to support the function of the neurons.
The list of glial cell that are present in the central nervous system includes the following types:
- Oligodendrocytes
- Astrocytes
- Ependymal Cells
- Microglial Cells
On the other hand, the list of glial cell that is present in the peripheral nervous system contains two other types:
- Schwann Cells
- Satellite Oligodendrocyte Cells
Oligodendrocytes
Oligodendrocytes are large glial cells in the central nervous system that provide the myelin sheet around the neuronal axons in the central nervous system. They have several dendrites stemming from them. One oligodendrocyte can provide myelination to up to 50 neuronal axons.
These glial cells provide support to the axons by producing myelin. In this way, they insulate the axon which supports the firing of electrical impulses carrying the data of the sensory stimuli. This process is very important for increasing the speed of nerve conduction, i.e. action potential of the cell.
However, the myelin sheet isn’t continuous. There are gaps found between each membrane which aid the efficient spreading of the electrical signals along the nerve cells. These gaps are labeled as The Nodes of Ranvier.
Damage inflicted on this type of glial cells can result in demyelination of the cells, leading to brain tumors, multiple sclerosis, cerebral palsy, and other demyelinating diseases.
Astrocytes
Astrocytes are star-shaped glial cells of the central nervous system. These glial cells are synapses that support the flow of chemical and electrical impulses between the cells and are arranged according to their chemical and electrical communication with the other cells.
Their function is to link the nerve cells to blood vessels, regulate the activity of the surrounding synapses, and form the blood-brain barrier by wrapping the brain capillaries. In this way, astrocyte glial cells prevent toxins from entering the brain.
The blood-brain barrier is a semi-permeable membrane separating the blood from the extracellular fluid of the central nervous system, including the brain and the spinal cord.
Types of Astrocyte Glial cells
There are two major types of astrocytes, according to their structural differences and location:
- Protoplasmic astrocytes – This type of glial cell is located in the gray matter of the brain.
- Fibrous astrocytes – This type of glial cell is located in the white matter of the brain.
Furthermore, they are responsible for providing the ultimate chemical environment for the transmission of neuronal electrical impulses. Astrocyte glial cells also regulate the levels of extracellular neurotransmitters and participate in the body’s immune response.
Ependymal Cells
Ependymal cells (or: ependycites) are a type of glial cells that are found in the fluid-filled ventricles (hollow spaces) of the brain and the central canal of the spinal cord. The membrane of these cells is labeled as an ependyma.
These glial cells are very small in size, so they have to line up together in order to create a membrane. They are responsible for the creation and secretion of the cerebrospinal fluid, which is highly important for the circulation of nutrients from the blood, as well as for the detoxification of the brain.
The surface of these cells is covered with two distinct structures:
- Cilia – A hairlike organelle that projects from the cell body, and supports the immune system with their function.
- Microvilli – A protrusion in the form of small bristles that support the absorption of nutrients and water molecules.
The ependymal cells are also responsible for the process of homeostasis in the body, i.e. an inclination of the body to maintain internal balance (including constant temperature, heartbeat, regular rhythm of breathing, etc.).
Microglial Cells
The microglial cells are the primary immune cells of the CNS. They create the brain’s so-called ‘immune system’. This type of glial cell alerts the brain of a possible disease or an injury.
If a disease occurs, they become activated and remove the dead cells or the pathogen from the brain by activating the process of phagocytosis. This is a process of ‘cell eating’. During phagocytosis, the phagocytes (cells involved in the process), literally digest the other cells or particles.
Damage inflicted to this type of glial cells can result in:
- Parkinson’s disease
- Frontal Temporal dementias
- ALS
- Traumatic Brain Injuries
- Ischaemic Strokes
- Epilepsy
Schwann Cells
Schwan cells are a type of glial cell that comprises the main structure of the peripheral nervous system. They share the function with the oligodendrocytes from the central nervous system.
Namely, they also create myelin sheets for the neuron axons, but they perform their function in the peripheral nervous system. This fatty layer increases the conduction of the neuron, thus speeding the transmission of information.
Unlike the oligodendrocytes that form membranes, the Schwann cells form spirals around the axon. The Nodes of Ranvier are located between these structures, assisting the transmission of the sensory signals by the neurons.
When Schwann cells are damaged, that influences the immune system of the body which can lead to the following syndromes and diseases:
- Guillain-Barre’ syndrome – A rare neurological auto-immune disorder in which the body’s immune system attacks parts of the peripheral nervous system by mistake. The attack is triggered by an infection, but instead of fighting the pathogen, the body’s immune system attacks the healthy nerves in the brain.
The affected individual can experience symptoms such as severe pain, difficulty in swallowing, speaking, chewing, and vision; abnormal heart rate, and digestive problems.
- Charcot-Marie-Tooth disease – An inherited disorder that causes damage to the peripheral nerves. As a result of this genetic illness, the motor and sensory nerves are damaged, which leads to muscular atrophy, i.e. smaller and weaker muscles.
The affected individual can experience symptoms such as loss of sensation, muscle contractions, and difficulty walking.
- Schwannomatosis – This is also a rare genetic disorder that results in the formation of mostly benign tumors due to mutation of the Schwann cells. The affected individual can experience the following symptoms: painless or painful growth or swelling on the face, loss of hearing and coordination, paralysis of the face, etc.
- Chronic inflammatory demyelinating polyneuropathy (CIDP) – A rare neurological disorder characterized by inflammation of both the peripheral nerves and the nerve roots, as well as degradation of the myelin sheath covering the nerves.
The affected individual can experience the following symptoms: numbness, weakness, fatigue, pain, etc.
- Leprosy – This is a contagious disease that affects the skin, nerves, eyes, and mucous membranes. It’s caused by the Mycobacterium leprae bacteria, which is transmitted through contact with nasal fluids on surfaces, coughing, or sneezing.
There are two forms of the disease: tuberculoid and lepromatous. The affected individual can experience symptoms, such as enlarged nerves, large bumps and lumps on the skin, nosebleed, discoloration of the skin, as well as paralysis.
Satellite Oligodendrocyte Cells
The satellite oligodendrocyte cells surround the cells in the peripheral nervous system. Their main function is to regulate the chemical balance around the neurons. In addition, they deliver nutrition and protect the neurons from toxic substances that enter the brain.
These type of glial cells also support the transport of several neurotransmitters, including:
- Glutamate
- GABA
- Norepinephrine
- Adenosine triphosphate
- Capsaicin
- Acetylcholine
To Sum Up
Without the glial cells, the neurons would not be able to perform their function in the brain. They guide and support the communication between the neurons, as well as protect both them and the brain from toxic substances.
To be more specific, glial cells shield the brain from the harmful ions and chemicals in the extracellular fluid, thus supporting the removal of toxins from the brain. Also, they assist the synapses between the neurons, aid the repair of injured nerves, and protect the brain from neurodegeneration.
If the glial cells are dysfunctional, they can trigger cell mutation, which creates the basis for the growth of most brain tumors.
Fun Facts
Did you know?
- Microglia is the smallest glial cell. It’s derived from the mesoderm and is the less abundant glial cell in the brain.
- Satellite glial cells react to chronic pain as a result of nerve damage.
- Since the brain is the most voracious organ in the human body (it consumes 10 times more nutrients and oxygen), it would starve without the glial cells.
- Pathogens are infectious agents or microorganisms that can trigger a disease of infection, such as germs, bacteria, virus, or fungus.
- Glial cells communicate via intracellular waves of calcium molecules.
- Spanish neuroscientist Pío del Río Hortega is the person responsible for the discovery of the oligodendrocytes.
- Brain tumors that occur as a result of the mutation of the oligodendrocytes are labeled as oligodendrogliomas.
- The nerve impulses travel at speeds between 3–400 ft/s (1–120 m/s), depending on the type of nerve. Movement is faster in myelin-coated axons.
- Astrocytes are found in the connective tissue of the central nervous system.
- Bergmann glia is the former name of the glial cells, according to the scientist who discovered them, Karl Bergmann.