The glia , glial cells or simply glia are the whole cell nonneuronal of the nervous system. They were discovered in 1856 by physician Rudolf Virchow while investigating brain tissue.
The functions of neuroglia can be summarized in the maintenance of homeostasis in the nervous system, metabolic support for neurons, myelin formation, destruction of pathogens, removal of dead neurons and structural support for neurons, both in the central nervous system. like the peripheral nervous system.
Although they are not excitable cells, they participate in nerve transmission, assisting neurons to establish synaptic connections between them, recycling neurotransmitters, and in some processes, such as respiration, they appear to have an active regulatory role.
Glial cell types and their functions
Glial cells are usually classified into two large groups, macroglial cells or macroglia , which have an ectodermal (neural) origin, and microglia , which have mesodermal origin. Both types are present in both the central and peripheral nervous systems.
Macroglia of the CNS
In the central nervous system (CNS) several types of glial cells can be found, among the most important we find:
They also regulate the conditions of the external chemical microenvironment of neurons, removing excess potassium, recycling neurotransmitters and regulating blood vasoconstriction and vasodilation. They also appear to have an inhibitory role in neuronal circuits through the detection of changes in the extracellular calcium concentration.
The recycling of neurotransmitters, removing them from the synaptic space, has an important role in the regulation of synaptic function and in preventing the reaching of toxic concentrations of some neurotransmitters, for example glutamate.
Oligodendrocytes are cells with projections that cover the axons of neurons in the central nervous system and form the myelin sheath . The myelin sheath electrically insulates the axons and allows for more efficient nerve transmission.
Radial glial cells
During embryonic development, radial cells act as progenitors of neurons and at the same time guide the migration of new neurons. In adults they remain as specialized glial cells in some areas, such as the retina and cerebellum . In the cerebellum, Bergmann’s glia regulates synaptic plasticity; in the retina, Müller cells are the only macroglia and perform functions similar to astrocytes and oligodendrocytes of the CNS.
- Pituitcytes : specialized glial cells similar to astrocytes that appear in the neurohypophysis or posterior pituitary. Its main function is the storage and release of pituitary hormones . The pituicytes surround the terminal axons and regulate the release of these hormones.
- Tanicytes : are a type of specialized ependymocytes that appear in the third cerebral ventricle and in the lower part of the fourth ventricle. They appear to be involved in the release of GnRH (gonadotropin-releasing hormone) by neurons in the hypothalamus.
Macroglia of the SNP
Fewer glial cells appear in the peripheral nervous system and are usually differentiated cells from the macroglia of the central nervous system. Some of the most important are the following:
- Neurolemocytes : Also called Schwann cells . They have a similar function to oligodendrocytes of the CNS and form the myelin sheath in the axons of the PNS. They have phagocytic activity eliminating cellular debris and also appear to guide the growth of neuronal axons.
- Satellite cells : they are small cells that surround the neuronal bodies in sensory, sympathetic and parasympathetic ganglia . They have an important role in the regulation of the microenvironment in these ganglia and are very sensitive to lesions and inflammatory processes; They seem to be involved in various pathological processes that occur with chronic pain.
- Enteric glial cells : These are glial cells found in the enteric nervous system , the subdivision of the autonomic nervous system that is responsible for directly controlling the digestive system.
Microglial cells are not really considered glial cells, since they do not have the same embryological origin as the rest of the cells of the nervous system . Microglial cells are specialized macrophages that form in the bone marrow and migrate to the nervous system. They are present only in the central nervous system .
Microglial cells have phagocytic activity and are the cells that regulate the immune system response in the central nervous system by acting as antigen-presenting cells . Microglial cell deficiency is seen in several diseases that affect the central nervous system, such as Alzheimer’s, Parkinson’s, and amyotrophic lateral sclerosis.