Glial Cells (Glia): Definition, Function, Types

Nervous tissue is one of four primary kinds of tissue in the human body, with muscle tissue, connective tissue (e.g., bones and ligaments) and epithelial tissue (e.g, skin) completing the set.

Human anatomy and physiology is a marvel of natural engineering, making it difficult to pick which of these tissue types is most striking in diversity and design, but it would be hard to argue against nervous tissue topping this list.

Tissues consist of cells, and the cells of the human nervous system are known as **neurons**, nerve cells or, more colloquially, "nerves."

These can be divided into the nerve cells you may think of when you hear the word "neuron" – that is, functional carriers of electrochemical signals and information – and glial cells or neuroglia, which you may not have heard of at all. "Glia" is Latin for "glue," which, for reasons you'll soon learn, is an ideal term for these supportive cells.

Glial cells appear throughout the body and come in a variety of subtypes, most of which are in the central nervous system or CNS (the brain and the spinal cord) and a small number of which inhabit the peripheral nervous system or PNS (all nervous tissue outside the brain and spinal cord).

These include the astroglia, ependymal cells, oligodendrocytes and microglia of the CNS, and the Schwann cells and satellite cells of the PNS.

Nervous tissue is distinguished from other kinds of tissue in that is is excitable and capable of receiving and transmitting electrochemical impulses in the form of action potentials.

The mechanism for sending signals between neurons, or from neurons to target organs such as skeletal muscle or glands, is the release of neurotransmitter substances across the synapses, or tiny gaps, forming the junctions between the axon terminals of one neuron and the dendrites of the next or a given target tissue.

In addition to dividing the nervous system anatomically into the CNS and the PNS, it can be divided functionally in a number of ways.

For instance, neurons may be classified as motor neurons (also called motoneurons), which are efferent nerves that carry instructions from the CNS and activate skeletal or smooth muscle in the periphery, or sensory neurons, which are afferent nerves that receive input from the outside world or the internal environment and transmit it to the CNS.

Interneurons, as the name suggests, act as relays between these two types of neurons.

Finally, the nervous system includes both voluntary and automatic functions; running a mile is an example of the former, while the associated cardiorespiratory changes that accompany exercise exemplify the latter. The somatic nervous system encompasses voluntary functions, while the autonomic nervous system deals with automatic nervous-system responses.

The human brain alone is home to an estimated 86 billion neurons, so it is not surprising that nerve cells come in a variety of shapes and sizes. About three-fourths of these are glial cells.

While glial cells lack many of the distinctive features of "thinking" nerve cells, it is nevertheless instructive when considering these gluelike cells to consider the anatomy of the functional neurons they support, which have a number of elements in common.

These elements include:

• Dendrites: These are the highly branched structures (the Greek word "dendron" means "tree") radiating outward to receive signals from adjacent neurons that generate action potentials, which are essentially a kind of current flowing down the neuron resulting from the movement of charged sodium and potassium ions across the nerve cell membrane in response to various stimuli. They converge on the cell body.
• **Cell body: **This part of a neuron in isolation looks a lot like a "normal" cell and contains the nucleus and other organelles. Most of the time, it is fed by a wealth of dendrites on one side and gives rise to an axon on the other.
• Axon: This linear structure carries signals away from the nucleus. Most neurons have only one axon, although it may give off a number of axon terminals along its length before it terminates. The zone where the axon meets the cell body is called the axon hillock.
• Axon terminals: These fingerlike projections form the "transmitter" side of synapses. Vesicles, or small sacs, of neurotransmitters are stored here and are released into the synaptic cleft (the actual gap between axon terminals and the target tissue or dendrites on the other side) in response to action potentials zooming down the axon.

Generally, neurons can be divided into four types based on their morphology, or shape: unipolar, bipolar, multipolar and pseudounipolar.

• Unipolar neurons have one structure that projects from the cell body, and it forks into a dendrite and an axon. These are not found in humans or other vertebrates, but are vital in insects. 
• Bipolar neurons have a single axon at one end and a single dendrite at the other, making the cell body a sort of central way station. An example is the photoreceptor cell in the retina at the back of the eye.
• Multipolar neurons, as the name implies, are irregular nerves with a number of dendrites and axons. They are the most common type of neuron and predominate in the CNS, where an unusually high number of synapses are required.
• Pseudounipolar neurons have a single process extending from the cell body, but this very quickly splits into a dendrite and an axon. Most sensory neurons belong to this category.

A variety of analogies help describe the relationship between bona fide nerves and the more numerous glia in their midst.

For example, if you regard nervous tissue as an underground subway system, the tracks and tunnels themselves might be seen as neurons, and the various concrete walking passages for maintenance workers and the beams around the tracks and tunnels can be seen as glia.

Alone, the tunnels would be nonfunctional and would likely collapse; similarly, without the subway tunnels, the substance preserving the integrity of the system would be no more than purposeless piles of concrete and metal.

The key difference between glia and nerve cells is that glia do not transmit electrochemical impulses. In addition, where glia meet neurons or other glia, these are ordinary junctions – glia do not form synapses. If they did, they would be incapable of doing their job properly; "glue," after all, only works when it can adhere to something.

In addition, glia have only one type of process connected to the cell body, and unlike full-fledged neurons, they retain the ability to divide. This is necessary given their function as support cells, which subjects them to more wear and tear than nerve cells and does not require them to be as exquisitely specialized as electrochemically active neurons.

Astrocytes are star-shaped cells that help maintain the blood-brain barrier. The brain does not simply permit all molecules to flow into it unchecked into it through the cerebral arteries, but instead filters out most chemicals it does not need and perceives as potential threats.

These neuroglia communicate with other astrocytes via gliotransmitters, which are the glial cells' version of neurotransmitters.

Astrocytes, which can be further divided into protoplasmic and fibrous types, can sense the level of glucose and ions such as potassium in the brain and thereby regulate the flux of these molecules across the blood-brain barrier. The sheer abundance of these cells makes them a major source of basic structural support for the brain functions.

Ependymal cells line the brain's ventricles, which are internal reservoirs, as well as the spinal cord. They produce cerebrospinal fluid (CSF), which serves to cushion the brain and spinal cord in the event of trauma by offering a watery buffer between the bony exterior of the CNS (the skull and the bones of the vertebral column) and the nervous tissue underneath.

Ependymal cells, which also play an important role in nerve regeneration and repair, are arranged in some parts of the ventricles into cube shapes, forming the choroid plexus, a mover of molecules such as white blood cells into and out of the CSF.

"Oligodendrocyte" means "cell with a few dendrites" in Greek, an appellation that stems from their relatively delicate appearance compared to astrocytes, which appear as they do thanks to the robust number of processes radiating in all directions from the cell body. They are found in both the grey matter and the white matter of the brain.

The main job of oligodendrocytes is to manufacture myelin, the waxy substance that coats the axons of "thinking" neurons. This so-called myelin sheath, which is discontinuous and marked by naked portions of the axon called nodes of Ranvier, is what allows neurons to transmit action potentials at high speeds.

The three aforementioned CNS neuroglia are considered macroglia, owing to their comparatively large size. Microglia, on the other hand, serve as the immune system and the clean-up crew of the brain. They both sense threats and actively combat them, and they clear away dead and damaged neurons.

Microglia are believed to play a role in neurological development by eliminating some of the "extra" synapses the maturing brain usually creates in its "better safe than sorry" approach to establishing connections between neurons in the grey and white matter.

They have also been implicated in the pathogenesis of Alzheimer's disease, where excessive microglial activity may contribute to the inflammation and excessive protein deposits that are characteristic of the condition.

Satellite cells, found only in the PNS, wrap themselves around neurons in collections of nerve bodies called ganglia, which are not unlike the substations of an electrical power grid, almost like miniature brains in their own right. Like the astrocytes of the brain and the spinal cord, the participate in the regulation of the chemical environment in which they are found.

Located mainly in the ganglia of the autonomic nervous system and sensory neurons, satellite cells are believed to contribute to chronic pain through an unknown mechanism. They provide nourishing molecules as well as structural support to the nerve cells they serve.

Schwann cells are the PNS analog of oligodendrocytes in that they provide the myelin that encases the neurons in this division of the nervous system. There are differences in how this is done, however; whereas oligodendrocytes can myelinate multiple parts of the same neuron, a single Schawnn cell's reach is limited to a lone segment of an axon between nodes of Ranvier.

They operate by releasing their cytoplasmic material into the areas of the axon where myelin is needed.

Related article: Where are Stem Cells Found?