The nervous systems consists of two main types of cells: neurons and glial cells. The basic structure and function of the neuron have been covered here, so today we will turn our attention towards their previously ignored partner: glial cells or glia. The name “glia” is derived from Greek, meaning “glue”, and suggests the initial impression that glial cells acted as the glue of the nervous system, holding neurons together.
More recent studies, however, have proven that the tasks fulfilled by these cells go beyond mere structural support. Research continues to unravel the importance of these cells not only in providing a scaffold for neurons, but also in synaptic transmission, the fight against brain pathogens, and breathing.
There are several types of glia: astrocytes, oligodendrocytes, microglia, and ependymal cells are found in the central nervous system, while Schwann cells and satellite cells are part of the peripheral nervous system. In the next paragraphs, we will describe the basic function of each type of glial cell mentioned above.
Astrocytes (Figure 1) are star-shaped glia, with numerous functions in the nervous system. Among these functions, some of the most important ones are:
- regulation of the ion concentration in the extracellular space
- metabolic support for neurons
- modulation of the synaptic transmission
- reuptake and release of neurotransmitters
While astrocytes act mainly in the central nervous system, similar functions in the peripheral nervous system are accomplished by satellite cells.
Oligodendrocytes (Figure 2) function primarily as myelin producers in the central nervous system. They accomplish this task by sending out projections which wrap around axons. A secondary role of these glial cells is the production of neurotrophic and growth factors.
In contrast to oligodendrocytes, Schwann cells are responsible for myelinating axons in the peripheral nervous system. They do this not by sending out projections, as oligodendrocytes do, but by wrapping themselves around the axons. Therefore, while an oligodendrocyte can myelinate multiple axons, a Schwann cell can only myelinate a single axon. And since a single Schwann cell is much smaller compared to an axon, several of them need to wrap around one axon in order to myelinate it. During this process, the Schwann cells do not fuse, but leave some gaps between them, called the nodes of Ranvier. Furthermore, these cells are responsible for axonal regeneration in the peripheral nervous system.
Every system in the body needs defensive mechanisms across pathogens. This role is usually fulfilled by the various types of cells of the immune system. In the brain, however, there are some specialized glial cells which aid in this task, i.e. microglia (Figure 3). In addition to acting as macrophages, microglia also play a role in extracellular signaling.
Finally, perhaps the least studied glia are ependymal cells. So far, we know that they are involved in forming the blood-brain barrier and in producing the cerebrospinal fluid.
As you can see from this short introduction, glia are diverse and fascinating cells of the nervous system, without which it couldn’t properly function. Nevertheless, more research is necessary in order to explore the true depth of the roles fulfilled by these previously ignored carers of the nervous system.
Did you enjoy this article? Do you have any questions for us? Let us know in the comments what you thought and if you would like to learn more about these amazing supporters of the nervous system!