03. Glial cells

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Despite the complexity and the cell density of nerve tissue, it consists mainly of two major types of cells [ 52, 93 ]: The neurons [ 41 , 52 , 64 , 94 , 100 ] and ultimate key players regarding the transmission of nerve impulses, and glial cells [ 50 , 57 , 114 ] also called gliocytes or glia [ 116 ], which act as babysitters among other nerve cells.

If the nervous system contains about 100 billion neurons [ 1 , 4 ], the gliocytes are almost 10 times more likely [ 114 , 115 ], this proves certainly important that these cells may play.

At the beginning of their discovery in the second half of the 19th century [ 100 , 116 , 117 ], it was thought that glial cells (glia of glue in English which means glue) used essentially glue to attach the nerve cells with each other (hence their appointment) [ 40 , 118 ]. Over time, we are constantly assign several fundamental and crucial to the functioning of neurons [functions 40 , 57 ].

Unlike neurons that are mostly amitotic, glial cells can reproduce [ 36 , 119 ].

There are two categories of gliocytes [ 120 ]:

• Glia: gliocytes includes the central nervous system which are 4 in number: astrocytes, oligodendrocytes, microglia and épendymocytes.
• Gliocytes the peripheral nervous system including satellite cells and Schwann cells.

1. Astrocytes [ 1 , 35 , 94 , 121 ]:

These are the most abundant and most voluminous gliocytes, they have a star shape with several extensions. One distinguishes: type I astrocytes [ 35 , 70 ] which are in contact with the blood capillaries, and the type II astrocytes [ 35 , 70 ] that surround neurons and synaptic clefts thereby preventing dispersion of neurotransmitters.

Astrocytes have many functions [ 104 ] many of which are still under study. As follows:

• They play a crucial role in the formation of the blood-brain barrier [ 39 ].
• They supply of oxygen and nutrients to neurons [ 40 ].
• They help maintain proper production of action potentials by neurons [chemical medium 40 , 116 ].
• They capture the excess neurotransmitters at the synaptic cleft and participate in their metabolism [ 57 , 116 ].
• They play a supporting role in forming a network that maintains the structure and architecture of the nervous tissue [ 2 ].
• They realize glial scars in areas of altered brain [ 50 ].
• They also help to direct the migration of neurons to their final locations during development [ 120 ], and provide many other functions.

2. Oligodendrocytes [ 1 , 49 , 81 , 100 , 104 , 122 ]:

Oligodendrocytes are smaller and have fewer extensions than astrocytes. They also play a role of support network for CNS neurons but mainly provide their myelination [ 100 , 123 ].

Each oligodendrocyte sends several extensions which wrap around axons, they are then surrounded by a large number of concentric layers (30 to 100) [ 40 , 100 ]. These layers are called the myelin sheath [ 104 , 113 , 124 , 125 ]: A substance composed mainly of lipids used to insulate and protect axons as does the plastic around electrical son. But more importantly, this myelin sheath serves to accelerate the speed of nerve impulses [ 125 ]. An oligodendrocyte can myelinate up to 30 or 50 adjacent axons [ 4 , 100 ].

3. The épendymocytes [ 126 ]:

The épendymocytes have a cubic or cylindrical shape, and they often form a single ciliated epithelium that lines the cavities of the central nervous system. They ensure the secretion of Cerebrospinal Fluid and promote circulation.

4. Microglia [ 104 ]:

Microglia are small stellate cells with few extensions, they have the same embryonic origin as monocytes and macrophages.

Microglia protect the cells of the central nervous system against infectious and toxic insults. They can migrate to injured areas and remove the debris of dead cells. Their protective role is important because the cells of the immune system do not have access to the central nervous system.

5. The satellite cells [ 40 , 128 , 129 ]:

This flattened cells are arranged around the cell bodies of the neurons in the lymph. They have similar functions to astrocytes.

6. Schwann cells [ 39 , 124 , 127 ]:

These cells are also called (neurolemmocytes [ 128 , 129 ]), they are flat cells that form the myelin sheaths surrounding the axons in the peripheral nervous system.

Each myelinated Schwann cell part of a single axon [ 40 , 124 , 126 , 130 ]. Sometimes a neurolemmocyte surrounds axons without forming myelin sheath, then we say that these axons are (or non-myelinated amyélinisés [ 131 ]).
 
Several neurolemmocytes organize rosary around a single axon, strangulation that delimits each Schwann cell with its neighbor is called (node ​​of Ranvier [ 81 , 94 ]). It is at this level that collateral can emerge from the axon [ 99 ]. The area bounded by two nodes of Ranvier area is called (internode space).

Schwann cells play a trophic role of nurturing and axons at the peripheral level, they help to accelerate the speed of nerve impulses up to 100 times [ 132 ] and they also play a very important role in the regeneration of axons device in case of any injuries [nervous system 100 ] ...