In this note, we are going to discuss the detailed structure of neuroglia.
But first, let’s recall what the main function of the nervous system is.
It provides interaction of all other organ systems and also monitors all changes in the internal and external environment, producing the corresponding response.
For ease of understanding, the nervous system is structurally subdivided into the central nervous system (or CNS), which includes the brain and spinal cord, and the peripheral nervous system (or PNS), which includes nerves and ganglia outside the CNS.
Both the central and peripheral parts of the nervous system consist of nervous tissue, which in turn is formed by neurons (or nerve cells) and neuroglia.
The structure of a neuron is discussed in a separate topic.
The cellular components of neuroglia differ in the central and peripheral nervous systems.
Neuroglia in the CNS
Depending on the size of the cells, in the CNS, we can distinguish macroglia and microglia.
Astrocytes
Astrocytes are star-shaped cells with numerous processes.
At the distal points of these processes, there are enlargements in the form of feet, which form connections with neurons and blood vessels. These enlargements are called end-feet processes.
A collection or cluster of astrocytes is called astroglia.
Astrocytes contain GFAP (glial fibrillary acidic protein).
Based on its concentration, two types of astrocytes are distinguished:
- protoplasmic
- fibrous
Astroglia performs several functions.
Firstly, it is part of the blood-brain barrier (BBB).
This is a formation that allows some substances to easily enter from the blood into neurons, while completely blocking others.
In other words, it acts as a kind of filter or barrier.
The BBB consists of several elements:
- Endothelial cells of capillaries, which are very closely attached to each other
- Basal membrane of the endothelium
- Astrocytes
- Pericytes – these are small cells located at contact points between endotheliocytes
Secondly, astrocytes perform a supporting function, meaning they fill the space between neurons.
Thirdly, astrocytes have a unique “capture” system that removes neurotransmitters from the synaptic cleft.
And fourthly, astrocytes are capable of division.
In case of CNS damage, they begin to proliferate, filling the intercellular space, and form what is called a glial scar (or astrocytic scar).
Oligodendrocytes
The long processes of some neurons, called axons, are covered with a myelin sheath.
This is a unique lipid-protein barrier that limits the nerve impulse, preventing it from spreading beyond the boundaries of the axon.
The myelin sheath in the central nervous system is formed by oligodendrocytes, and in the peripheral nervous system — by Schwann cells.
The difference lies in the fact that in the CNS, one oligodendrocyte forms the myelin sheath for several axon segments (and sometimes for several neurons at once).
In the PNS, a single Schwann cell forms just one myelin segment.
The myelin-free sections are called nodes of Ranvier.
They contain many ion channels.
This feature allows the impulse to “jump” from one myelinated segment to another, significantly increasing the speed of signal transmission.
Moreover, in the PNS, some axons are not covered with a myelin sheath.
However, Schwann cells still provide some protection for them: several axons are dipped inside the cell.
Ependymocytes
Three types of ependymal glia are distinguished:
- Ependymocytes that line the spinal canal and brain ventricles. They have multiple cilia on their surface.
- Tanycytes, which have a process at the basal end that adjoins neurons and blood vessels.
It is used for the evaluation of cerebrospinal fluid composition and facilitating its transport between vessels and brain ventricles.
- And the third type are choroidal ependymocytes.
They line the surface of the vascular plexuses and take part in the synthesis of cerebrospinal fluid.
Microglia
Microglia are the smallest glial cells in the CNS.
Unlike the other listed cells, which develop from neuroectoderm, microglia originate from bone marrow monocytes and enter the CNS after birth.
These cells are capable of phagocytosis and essentially act as macrophages, playing a connecting role between the CNS and the immune system.
In case of CNS damage, they capture remnants of dead neurons, viruses, bacteria, and tumor cells.
Neuroglia in PNS
Neuroglia in the peripheral nervous system consists of two types of cells:
- Schwann cells, whose myelinating function we have previously discussed.
Additionally, they participate in the regeneration of damaged axons.
- And satellite cells, which surround the neuron bodies in peripheral ganglia.
It is believed that they perform the same function as astrocytes in the CNS.
Neuroglia: Structure & Function
- nervous system
- systema nervosum
- central nervous system
- systema nervosum centrale
- brain
- encephalon
- spinal cord
- medulla spinalis
- peripheral nervous system
- systema nervosum periphericum
- nerves
- nervi
- ganglia
- ganglia
- nucleus
- nucleus
- ganglion
- ganglion
