Glial cells
Despite the complexity and cellular density of the nervous tissue, it mainly consists of two large categories of cells [52, 93] : neurons [41, 52, 64, 94, 100] , which are the major and ultimate players in the transmission of nerve impulses, and glial cells [50, 57, 114] - also called gliocytes or neuroglia [116] - which act as "baby-sitters" for the nerve cells, among other roles.
While the nervous system contains about 100 billion neurons [1, 4], gliocytes are nearly 10 times more numerous [114, 115]; this proves the undoubtedly important role these cells play (Note: The 10:1 glia-to-neuron ratio is a historical myth; recent research indicates a ratio of roughly 1:1 in the human brain).
Upon their discovery in the second half of the 19th century [100, 116, 117] , it was thought that glial cells (from "glia," meaning "glue") primarily served as a glue to bind nerve cells together (hence their name) [40, 118]. Over time, many fundamental functions crucial to the operation of neurons have been attributed to them [40, 57].
Unlike neurons, which are mostly amitotic, glial cells can and do reproduce [36, 119].
There are two categories of gliocytes [120]:
Neuroglia: including the four types of gliocytes in the central nervous system: astrocytes, oligodendrocytes, ependymal cells, and microglia.
Gliocytes of the peripheral nervous system: including satellite cells and Schwann cells.
Astrocytes :
These are the most abundant and largest gliocytes [1,35, 94,121]; they are star-shaped with several processes. A distinction is made between: Type I astrocytes [35, 70] , which are in contact with blood capillaries, and Type II astrocytes [35, 70] , which surround neurons and synaptic clefts, thereby preventing the dispersion of neurotransmitters.
Astrocytes have many functions [104] , several of which are still under investigation. For instance:
They play a vital role in the formation of the blood-brain barrier [39].
They ensure the supply of oxygen and nutrients to neurons [40].
They help maintain an appropriate chemical environment for the production of action potentials by neurons [40, 116].
They capture excess neurotransmitters in the synaptic cleft and participate in their metabolism [57, 116].
They provide structural support by forming a network that maintains the architecture of the nervous tissue [2].
They form glial scars in damaged areas of the brain [50].
They guide the migration of neurons to their final locations during development [120], and perform many other functions…
Oligodendrocytes :
Oligodendrocytes [1, 49, 81, 100, 104, 122] are smaller and have fewer processes than astrocytes. They also serve as a support network for CNS neurons but primarily provide their myelination [100, 123].
Each oligodendrocyte gives off several processes that wrap around axons, which then become surrounded by a large number of concentric layers (30 to 100) [40, 100]. These layers form the myelin sheath [104, 113, 124, 125] : a substance composed mainly of lipids that insulates and protects axons, similar to the plastic around electrical wires. Most importantly, this myelin sheath serves to accelerate the speed of nerve impulses [125]. An oligodendrocyte can myelinate up to 30 or even 50 adjacent axons [4, 100].
Ependymal cells :
Ependymal cells [126] are cuboidal (cube-like) or columnar (tall); they are often ciliated and form a simple epithelium that lines the internal cavities of the CNS. They ensure the secretion of cerebrospinal fluid and facilitate its circulation.
Microglia :
Microglia [104] are small, star-shaped cells with few processes; they share the same embryonic origins as monocytes and macrophages.
Microglia protect the cells of the central nervous system against infectious and toxic attacks. They can migrate to injured regions and eliminate debris from dead cells. Their protective role is of great importance because the cells of the immune system do not have access to the central nervous system.
Satellite cells :
These flattened cells surround neuronal cell bodies within the ganglia, performing regulatory functions analogous to those of astrocytes [40,128,129].
Schwann cells :
Schwann cells [39,124,127], also known as neurolemmocytes [128, 129], are flattened cells that envelop axons in the peripheral nervous system to form the myelin sheath.
Each Schwann cell myelinates a portion of a single axon [40, 124, 126, 130]. Occasionally, a neurolemmocyte may surround axons without forming a myelin sheath; these axons are then said to be unmyelinated [131].
Several neurolemmocytes are organized in a chain around a single axon; the constriction defined by each Schwann cell and its neighbor is called a node of Ranvier [81, 94]. It is at this level that collaterals can emerge from the axon [99]. The zone between two nodes of Ranvier is called the internodal segment.
Schwann cells play a trophic and nourishing role for peripheral axons. They can accelerate the speed of nerve impulses by up to 100 times [132] , and they also play a very important role in the regeneration of peripheral nervous system axons in the event of injury [100]...