Glial Cells (neuroglia) are the non-excitable supporting cells of the nervous system. All glial cells are much smaller but far more numerous than the nerve cells. They form a major component of the nervous tissue and include the the following.
- Neuroglial cells, found in the parenchyma of brain and spinal cord.
- Ependymal cells lining the internal cavities or ventricles;
- Capsular or satellite cells, surrounding neurons of the sensory and autonomic ganglia.
- Schwann cells, forming sheaths for axons of peripheral nerves.
- Several types of supporting cells, ensheathing the motor and sensory nerve terminals, and supporting the sensory epithelia.
Classification of glial cells:
The neuroglial cells, found in the parenchyma of brain and spinal cord, are broadly classified as:
- Macroglia, of ectodermal (neural) origin, comprising astrocytes, oligodendrocytes, and glioblasts.
- Microglia, of mesodermal origin.
As the name suggests, these cells are star-shaped because of their numerous processes radiating in all directions. Astrocytes are of two types.
- Protoplasmic astrocytes, with thick and symmetrical processes are found in the grey matter.
- Fibrous astrocytes, with thin and asymmetrical processes, are found in the white matter.
The processes of astrocytes often end in plate-like expansions on the blood vessels, ependyma, and pial surface of the CNS.
As the name suggests these cells have fewer cell processes. According to their distribution, the oligodendrocytes may be intrafascicular, or perineuronal.
- The intrafascicular cells are found in the myelinated tracts.
- The perineuronal cells are seen on the surface of the somata of neurons.
These are stem cells which can differentiate into macroglial cells. They are particularly numerous beneath the ependyma.
These are the smallest of the glial cells which have a flattened cell body with a few short, fine processes. They are often related to capillaries, and are said to be phagocytic in nature. Microglial cells are possibly derived from the circulating monocytes which migrate into the CNS during the late fetal and early postnatal life.
Functions of Glial cells:
- They provide mechanical support to neurons.
- Because of their non-conducting nature, the glial cells act as insulators between the neurons and prevent neuronal impulses from spreading in unwanted directions.
- They can remove the foreign material and cell debris by phagocytosis.
- They can repair the damaged areas of nervous tissue by proliferation (gliosis) they form glial scar tissue, and fill the gaps left by degenerated neurons.
- Glial cells can take up and store neurotransmitters released by the neighboring synapses. These can-either be metabolized or released again from the glial cells.
- They help in neuronal functions by maintaining a suitable metabolic and ionic environment for the neurons.
- Oligodendrocytes myelinate tracts.
- Ependymal cells are concerned with exchanges of materials between brain and Cerebrospinal Fluid.