Video: Glial cells

Phew! Delivering all of this information is tough work. Fortunately for our hardworking neurons, they're not alone. Meet the glial cells. These diverse groups of cells provide our neurons with the support, nourishment, and protection they need to ensure our neural couriers stay in top form.

So if you're ready to explore the world of glial cells, let's hit the streets and see how these amazing supporters keep our nervous system's delivery team on the move.

Glial cells, also known as neuroglia, are one of the two basic cell types of the nervous system alongside neurons. Glial cells do not propagate action potentials like neurons; instead, they provide support for neurons by maintaining an optimal neuronal microenvironment, myelinating the neurons, and providing support, protection, and nutrition for neurons throughout the nervous system. Now as you can see, there are many different types of glial cells, two of which are found in the peripheral nervous system and four more major types in the central nervous system.

So let's first delve into the glial cells of the peripheral nervous system.

The first of the two major types of peripheral glial cells are called Schwann cells, also known as neurolemmocytes. These cells are responsible for the formation of the myelin sheath, which is the protective layer of lipids and proteins that envelops the axon of a neuron, acting as an insulator and enhancing the speed of impulse transmission. It should be noted, however, that Schwann cells can be differentiated into two types of cells: myelinating Schwann cells and nonmyelinating Schwann cells.

Myelinating Schwann cells form a myelin sheath around a segment of a single neuron. You will also notice these small gaps between bundles of myelin along the axon called myelin sheath gaps, commonly referred to as nodes of Ranvier, which allow electrical impulses to jump from node to node, speeding up the transmission of the electrical impulse.

The axon of the neuron passes through the cytoplasm of the Schwann cell and is held in place by the mesaxon, which is an extension of the Schwann cell's plasma membrane. The outermost nucleated cytoplasmic layer of Schwann cells aids in the regeneration of an axon when it is damaged and is often called the neurolemma, although that term is also alternatively sometimes used for the cell membrane of the neuron.

The nonmyelinating Schwann cells ensheath several small axons forming nonmyelinated axon bundles called Remak bundles. In fact, many axons or parts of axons, also referred to as nerve fibers, in the peripheral nervous system are actually nonmyelinated such as those related to slow conducting functions like pain, temperature, and autonomic functions.

The second major type of peripheral glial cell we're going to cover today are the satellite cells. These are small cuboidal cells found surrounding the cell bodies of neurons located in sensory and autonomic ganglia. They function to support the neuron which they surround, providing them with nourishment, protection, and regulation of their microenvironments.

And that takes care of the different types of glial cells in the peripheral nervous system. Now let's take a look at the glial cells only found in the central nervous system.

Let's take a look at oligodendrocytes first. These cells are analogous to the Schwann cells of the peripheral nervous system and are responsible for the production of a myelin sheath for neurons of the central nervous system. They are characterized by these long arm-like cytoplasmic processes extending from their cell body with which they are able to myelinate multiple axons by spiraling around them to form a myelin sheath. This, of course, is different to a Schwann cell, which can only form a segment of myelin sheath around a single neuron.

The cell body and nucleus of oligodendrocytes remain separate from the myelin sheath which means that no neurolemma is present, unlike in Schwann cells, and this is thought to be one of the reasons that there is little regrowth of damaged axons in the central nervous system. What is similar to myelinated neurons of the peripheral nervous system, however, is the presence of myelin sheath gaps.

The next glial cells of the central nervous system to discuss are the largest and most abundant type: the astrocytes. Astrocytes are the central nervous system's analogue of satellite cells. They present numerous processes, each of which have extensive terminal branching that allow one single astrocyte to be in contact with many neurons and make connections with over a million synapses. This highlights the complexity of the structure and function of astrocytes. Here we're going to highlight some of the major ones.

The astrocyte endfeet serve many different functions. A prominent one is the formation of the glia limitans, a key structural barrier in the central nervous system. The glia limitans consists of two main components: the superficial glia limitans formed by the pial processes of astrocytes which adhere to the basement membrane of the pia mater of the brain and spinal cord, and the vascular glia limitans, also known as the perivascular sheath, formed by the vascular processes of astrocytes, which surround the outer surface of the basement membranes of blood vessels and therefore contributes to the formation and integrity of the blood-brain barrier.

Astrocytes also provide metabolic support to neurons by modulating the chemical composition of interstitial fluid within the brain through the regulation of the exchange of various ions and molecules between blood and tissue fluid. They provide structural support and plasticity to nervous tissue in the central nervous system through their cytoskeletal network, helping to maintain the organization of neural tissue, supporting changes in neuronal activity, learning, and recovery from injury.

Astrocytes also assist with the replacement of damaged nerve cells by the process of astrocytosis, which involves the proliferation of astrocytes to occupy space left behind by dead neurons. They also assist with the neuronal development of the fetal brain through the release of chemicals that establish and manage connections between neurons.

Astrocytes also play a role in regulating blood flow in the brain by controlling the diameter of vessels they are in contact with through the release of vasoactive substances that result in dilation or constriction of the vessels. This helps ensure that active regions of the brain receive an adequate blood supply.

They also play a role in the central nervous system's immune response to infection or injury by releasing cytokines and chemokines which can modulate the activity of immune cells.

There are two main morphological types of astrocytes: protoplasmic and fibrous astrocytes. Protoplasmic astrocytes have a star-like appearance due to their many radiating processes that wrap around terminal segments of axons, synapses, and dendrites. They are primarily found in the gray matter. Fibrous astrocytes, on the other hand, have relatively few but longer cytoplasmic processes that align with the nerve axons and are mainly found in white matter.

The next group of central nervous system glial cells are ependymal cells. These cells form the ependyma which lines the central canal of the spinal cord and ventricles of the brain. On their surfaces, these cells possess tiny hair-like structures called cilia which beat in a coordinated pattern to influence the direction of flow of cerebrospinal fluid as well as microvilli which assists in the absorption of the cerebrospinal fluid.

This movement and regulation of cerebrospinal fluid is vital for distributing nutrients, removing waste products and maintaining the overall extracellular environment of the central nervous system.

The basal surfaces of ependymal cells contain processes which interdigitate with astrocyte processes, providing structural support by anchoring the ependymal cells in place.

Ependymal cells also form the epithelial layer of the choroid plexus. This layer of ependymal-derived cells, which surrounds the blood vessels in the choroid plexus, is responsible for producing cerebrospinal fluid. It also forms the blood-cerebrospinal fluid barrier through tight junctions that connect the cells, regulating the movement of substances between the blood and the cerebrospinal fluid.

The final group of glial cells in the central nervous system are microglia. Microglia account for only about 5 percent of the glial population in the central nervous system and they are notably smaller and distinct from other glial cell types. They share structural and functional similarities with tissue macrophages, and in fact, are the only nervous system cells which do not originate from the neural tube.

Instead, they show a common ancestry with macrophages outside of the central nervous system and migrate into the central nervous system during early development. This highlights their essential role in immune responses in the central nervous system.

The structure and appearance of microglial cells vary significantly depending on their functional state.

Resting microglia are characterized by a small cell body and branched processes and are involved in ongoing surveillance, maintenance, and supportive neuronal health without active inflammation. Activated microglia, on the other hand, change morphology, increase their immune and phagocytic activities, and produce inflammatory mediators in response to injury or disease.

Overall, microglia play a significant role in immune surveillance within the central nervous system. This includes recognizing and destroying harmful agents such as microbes, toxins, and antigens, removing cellular waste, and terminating dysfunctional or apoptotic neurons, often by means of phagocytosis.

In the developing brain, microglia support nervous tissue development and regeneration as they play a role in removing toxic waste, releasing anti-inflammatory factors, and pruning ineffective synapses, thereby sculpting neuronal circuits.

In response to CNS injury, microglia proliferate and migrate to the damaged area, forming nodules and engaging in phagocytosis of cellular debris, apoptotic cells, or damaged tissue. They also release substances to assist in tissue repair and regeneration.

Additionally, microglia act as antigen-presenting cells which is crucial for central nervous system defense. They process and present antigens on their surface, releasing chemokines to activate lymphocytes across the blood-brain barrier, leading to their activation and subsequent infiltration into the nervous tissue to initiate an immune response.

And that's all for today's tutorial. Make sure to check out our other study units and content on the nervous system.