Organic systems, which are often referred to as unified systems, play a significant role in ensuring the harmonious, balanced, and coordinated functioning of individual parts of the human organism and the organism as a whole.
In addition, we don’t imply only the nervous system, but also the immune system, and the endocrine system. It provides reactions of the organism in relation to any change in the environment.
The external environment implies not only the environment in which the organism as a whole is located but also numerous environments within the organism that are external to the components of the nervous system.
These components detect changes of a mechanical or chemical nature but also require appropriate parts of the organism to respond appropriately to these changes.
The nerve tissue, which forms the morphological and functional basis of the nervous system, is formed by two groups of cells. One group consists of functional, basic cells – nerve cells or neurons, while the other group comprises the accompanying nerve tissue cells called the glia cells.
The glia cells are the topic of this article. We will talk about the types of glia cells, their functions, as well as correlations with other brain elements and systems.
From the histoanatomical and physiological point of view, it is common to distinguish the central and peripheral nervous systems. The central nervous system (marked as CNS) consists of the brain and the spinal cord while the peripheral nervous system (PNS) includes nerves that extend between the CNS and other organs and tissues, as well as the ganglia.
The nerve and glia cells that form the CNS and PNS differ in their embryonic origin. The neuroectoderm is, for the most part, the source of CNS nerve and glial cells. The proliferation of ancestral cells called ventricular cells forms the so-called postmitotic neurons, on the one hand, and glioblasts, on the other.
In most cases, postmitotic neurons do not exhibit proliferative properties but are distinguished by their ability of migration during which they go through the process of selection and then through the process of differentiation.
Glioblasts also have the ability to migrate, and their differentiation produces the largest number of CNS glial cell types. Neurons and supporting cells forming PNS originate from nerve crest material or ectodermic placode material.
Types, Characteristics, and Functions of the Glia Cells
Most glia cells, accompanying the nerve tissue cells, are compared by many authors, considering their function in nerve tissue, with connective tissue cells and those authors call them “supporting cells” (1). Glia cells differ from each other both in their involvement in the construction of the central or peripheral nervous system and in the role they play.
They differ from neurons in the following ways:
- They do not have axon but only have dendrites;
- They do not have channels for the transfer of sodium ions, but only those for potassium ions;
- They do not generate action potentials and thus do not have the ability to transmit stimuli;
- They keep their ability to divide throughout the lifetime.
Glia cells differ from each other both according to the part of the nervous system they are located in, whether they are found in the central or peripheral CNS, and according to their embryonic origin.
In the central nervous system, the supporting cells are collectively referred to as neuroglia. This group of cells includes, on the one hand, ependymal cells delimiting the brain cells, ventricles and the central, ependymal duct in the spinal cord as well as choroidal epithelial cells that form choroid plexuses located within the brain chambers.
On the other hand, astrocytes, oligodendrocytes and microglial cells are also classified as neuroglia. Astrocytes and oligodendrocytes have their embryonic origin in common – as well as the CNS nerve cells, they originate from neuroectoderms. The embryonic origin of microglial cells differs in most researchers.
One category of astrocytes make contact with nerve cell bodies and the outer surface of the capillaries, and forms the outer border, glia limitans in the brain. Oligodendrocytes around the axonal nerve cell extensions form a sheath called the myelin sheath (2).
The supporting cells of the peripheral nervous system are Schwann cells and amphitic, satellite, or capsular cells that are present in ganglia. Schwann cells, such as oligodendrocytes in the central nervous system, form a myelin sheath around the axons of the nerve cells or neurons (3).
These are the only nerve tissue cells that have a lamina on their surface. Accompanying cells are classified as radial cells that excel in the embryonic nerve tissue. These cells, in fact, provide physical support to the nerve cells during their migration to the corresponding CNS regions.
The Myelin sheath and its role
We have already been said that morphological differences between nerve cells can be observed in the level of their axons – some are myelinated, while the others do not have the myelin sheath.
This sheath, which is scientifically called “myelin sheath”, in physiological terms, is very important for the nerve cell function. Moreover, it is found in all vertebrates but is relatively rare in invertebrates.
Morphologically, on the surfaces of the myelinated axons, there is a sheath formed by a smaller or larger number of closely adjacent glial cell membranes – Schwann cells in the peripheral and oligodendroglial cells in the central nervous system.
The myelin sheath is not continuous but is formed from segments. A Schwan cell, which is one type of the glia cells, forms a single segment while one oligodendrocyte can form seven to 70 myelin segments.
The beginning of the process of myelination of the nerve cells of the peripheral nervous system can be observed after the Schwann cell attachment to the axon and after the formation of cytoplasmic passages that begin to encompass it (3).
The edges of these passages approach each other. When they meet, the Schwann cell completely protects the axon and the place at which the contact between the extracellular surfaces of the glial cell membrane occurs is called mesaxone (3).
However, the process of axon wrapping is not yet complete. One part of the mesaxon continues its helical progression around the axon surface, primarily by movement of the cell membrane. It is certain that cytoplasm also participates in this movement because it is present in the region which, by analogy with the region in which the cells move, can be called the front of progression.
Nevertheless, the majority of the cytoplasm of the Schwann cell as well as the nucleus occupies a peripheral position with respect to the axon and the helical threads that gradually form.
When the myelination process is complete, one can notice numerous cell membrane windings of Schwann cells around the axons. On the preparations observed under the light microscope, these segments are observed as homogeneous sheaths, while the preparations observed under the transmission electron microscope in the area of the myelin sheath exhibit a kind of periodicity – the shift of contrasted lines and non-contrasted “stripes”.
Within the contrasted lines, however, it is possible to distinguish between the main lines and the intermediate lines by their thickness and degree of contrast. The major lines, which are more intensely contrasted and exhibit greater thickness, reflect the encounter of the cytoplasmic surfaces of the plasma membrane.
Intermediates represent the place of mutual support of extracellular surfaces of the glial cells. The site of their contact closest to the surface of the axon is called the inner mesaxon, and the one farthest from the axon is called the outer mesaxon.
Plasma membranes of glial cells forming myelin sheath differ in the lipid and protein composition from the membranes of other supporting cells – it contains 70% lipid and 30% protein.
According to some studies, sphingomyelin is quantitatively significant among the Schwann cell membrane lipids, but it also contains a lot of cholesterol and phosphatidylethanolamine. Peripheral and transmembrane proteins are also distinguished from proteins specific to these membranes.
Undoubtedly, both categories of membrane proteins play a large role in the formation of the myelin sheath and its subsequent maintenance.
Glia cells or glial cells are supporting cells of the nerve tissue that nourish, protect, and support the neurons and form an insulating, myelin sheath around them. Most of these cells are compared to connective tissue cells thanks to their function and are called nerve tissue supporting cells.
In addition to the undoubted supporting role, glial cells have many other functions, including the role in building the myelin sheath around the axon in the CNS oligodendrocytes and in the PNS Schwann cells, participating in healing processes after brain injury, maintaining ion homeostasis (especially K + ions) and pH of extracellular fluid, synthesizing the precursors of some neurotransmitters, such as glutamine (glutamate chemical mediator precursor), and the role of being brain macrophages because they turn into phagocytes during any inflammation or injury.
- Jäkel S, Dimou L. Glial Cells and Their Function in the Adult Brain: A Journey through the History of Their Ablation. Front Cell Neurosci. 2017 Feb 13;11:24. doi: 10.3389/fncel.2017.00024. PMID: 28243193; PMCID: PMC5303749. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5303749/
- Araque A, Navarrete M. Glial cells in neuronal network function. Philos Trans R Soc Lond B Biol Sci. 2010 Aug 12;365(1551):2375-81. doi: 10.1098/rstb.2009.0313. PMID: 20603358; PMCID: PMC2894949. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2894949/
- Salzer JL. Schwann cell myelination. Cold Spring Harb Perspect Biol. 2015 Jun 8;7(8):a020529. doi: 10.1101/cshperspect.a020529. PMID: 26054742; PMCID: PMC4526746. Found online at: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC4526746/