9.2: Anatomy of Nervous Tissue

By the end of this section, you will be able to:

• Describe the basic structure of a neuron
• Identify the different types of neurons based on their structure and function
• List the glial cells of the CNS and describe their function
• List the glial cells of the PNS and describe their function
• Describe the process of myelination and axon regeneration

Nervous tissue is composed of two types of cells: neurons and glia, as shown in Figure \(\PageIndex{1}\). Neurons are the primary type of cell that most anyone associates with the nervous system. They are responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to target cells. Glial cells, or glia, or neuroglia, are known to play a crucial supporting role in nervous tissue. Ongoing research pursues an expanded role that glial cells play in signaling, but neurons are still considered the basis of this function. Neurons are important, but without glial support they would not be able to perform their function.

Figure \(\PageIndex{1}\): Nervous Tissue. A small chunk of tissue from the spinal cord was spread on a microscope slide to show neurons and glial cells. Glial cells are smaller and in larger quantities than neurons. The neuron shown is large and has multiple processes coming out of the soma. (Image credit: "Nervous Tissue Cells" by Jennifer Lange, illustration by Claire McGuire is licensed under CC-BY-NC-SA 4.0. Micrograph provided by Berkshire Community College Bioscience Image Library is in the Public Domain.)

Neurons

Neurons are the basic functional units of the nervous system. They are large cells with a high metabolic rate. They depend on a continuous and abundant supply of oxygen and glucose. Some neurons live an entire lifespan while others are replaced within days. Neurons do not typically undergo mitosis, thus they are not able to divide and produce new neurons. Only certain areas of the brain and the sense organ of smell house mitotic neurons. They are responsible for the electrical signals that communicate information about sensations, that produce movement in response to those stimuli, and induce thought processes within the brain. An important part of the function of neurons is in their structure, or shape. The three-dimensional shape of these cells makes the immense numbers of connections within the nervous system possible.

Parts of a Neuron

As you learned in the previous section, the main part of a neuron is the cell body, which is also known as the soma (soma = “body”). The cell body contains the nucleus and most of the major organelles. What makes neurons special is that they have many extensions of their cell membranes, which are generally referred to as processes. The two types of processes are axons and dendrites. Neurons are usually described as having one, and only one, axon—a fiber that emerges from the cell body and projects to target cells. That single axon can branch repeatedly to communicate with many target cells. It is the axon that propagates the nerve impulse, carrying the electrical signal from the cell body to one or more cells. The region where the axon emerges from the cell body is called the axon hillock. This is a tapering of the cell body toward the axon fiber and it is the location where the outgoing electrical signal is generated. The other processes of the neuron are dendrites which branch off of the cell body, which receive information from other neurons at specialized areas of contact called synapses. The dendrites are usually highly branched processes, providing locations for other neurons to communicate with the cell. Information flows through a neuron from the dendrites, across the cell body, and down the axon. Figure \(\PageIndex{2}\) shows the relationship of these parts to one another.

Many axons are wrapped by an insulating substance called myelin, which is actually made of glial cells. Myelin acts as insulation much like the plastic or rubber that is used to insulate electrical wires. A key difference between myelin and the insulation on a wire is that there are gaps in the myelin covering of an axon. Each gap is called a neurofibril node (or node of Ranvier). These gaps are exchange points for ions that allow electrical signals to travel down the axon. At the end of the axon is the axon (synaptic) terminal, where there are usually several branches extending toward the target cell, each of which ends in an enlargement called a synaptic knob. These knobs are what make the connection with the target cell at the synapse.

Figure \(\PageIndex{2}\) The Neuron: The dendrites that branch off the cell body (soma) receive signals from other neurons. Those signals are transmitted from the dendrites and cell body down the axon to the axon terminals. At the axon terminals, the signal is then propagated to the target cell. (Image credit: "Neuron description" by Dhp1080 is licensed under CC BY-SA 3.0, via Wikimedia Commons.)

Types of Neurons

There are many neurons in the nervous system—a number in the trillions. And there are many different types of neurons. They can be classified by both their structure and their function. Structural classification depends on the number of processes attached to the cell body: one, two, or multiple (Figure \(\PageIndex{3}\)). The functional classification depends on which function the neuron is carrying out: sensation, integration, or motor.

Figure \(\PageIndex{3}\): Neuron Classification by Structure. Pseudounipolar neurons have one process emerging from the soma which contains both the axon and dendrites. Bipolar neurons have two processes emerging from the soma: one is an axon and the other is a dendrite. Multipolar neurons have more than two processes emerging from the soma: one is an axon and two or more other are dendrites. (Image credit: This work by Cenveo is licensed under CC BY 3.0). / Labels modified and cropped from original by Harmony Folse

Unipolar cells have only one process emerging from the cell. True unipolar cells are only found in invertebrate animals, so the unipolar cells in humans are more appropriately called “pseudo-unipolar” cells. Invertebrate unipolar cells do not have dendrites. Human pseudo-unipolar cells have an axon that emerges from the cell body, but it splits so that the axon can extend along a very long distance. At one end of the axon are dendrites, and at the other end, the axon forms synaptic connections with a target. Pseudo-unipolar cells are exclusively sensory neurons and have two unique characteristics. First, their dendrites are receiving sensory information, sometimes directly from the stimulus itself, as is the case for sensory neurons perceiving pressure, temperature change, touch, or pain from the skin. Secondly, the cell bodies of unipolar neurons are always found in ganglia. Sensory reception is a peripheral function (those dendrites are in the periphery, perhaps in the skin) so the cell body is in the periphery, though closer to the CNS in a ganglion. The axon projects from the dendrite endings, past the cell body in a ganglion, and into the central nervous system.

Bipolar cells have two processes, which extend from each end of the cell body, opposite to each other. One is the axon and one the dendrite. Bipolar cells are not very common. They are found mainly in the olfactory epithelium (where smell stimuli are sensed), and as part of the retina, and thus are sensory neurons.

Multipolar neurons are all of the neurons that are not unipolar or bipolar. They have one axon and two or more dendrites (usually many more). Multipolar neurons can have multiple functions. Multipolar neurons can be motor neurons which carry commands from the brain and spinal cord to muscles and glands. Motor neurons are located in the brain, spinal cord or autonomic ganglia. Multipolar neurons can also be classified as interneurons which constitute the majority of neurons. These neurons carry out integrative functions (such as retrieve, process and store information) and facilitate communication between sensory and motor neurons. Interneurons are found exclusively in the central nervous system.

Glial Cells

Glial cells, or neuroglia or simply glia, are the other type of cell found in nervous tissue. They are considered to be supporting cells, and many functions are directed at helping neurons complete their function for communication. The name glia comes from the Greek word that means “glue,". Today, research into nervous tissue has shown that there are many deeper roles that these cells play. And research may find much more about them in the future.

There are six types of glial cells. Four of them are found in the CNS and two are found in the PNS. Table \(\PageIndex{1}\) outlines some common characteristics and functions.

Table \(\PageIndex{1}\): Glial Cell Types by Location and Basic Function.

CNS glia	PNS glia	Basic function
Astrocyte	Satellite cell	Structural and physiological support of the nervous tissue Form the blood-brain barrier (BBB) (Only applies to astrocytes in CNS)
Oligodendrocyte	Schwann Cell (Neurolemmocyte)	Insulation, myelination
Microglia	-	Immune surveillance and phagocytosis
Ependymal cell	-	Creating and circulating CSF

Glial Cells of the CNS

One cell providing support to neurons of the CNS is the astrocyte, so named because it appears to be star-shaped under the microscope (astro- = “star”). Astrocytes have many processes extending from their main cell body (not axons or dendrites like neurons, just cell extensions). Those processes extend to interact with neurons, blood vessels, or the connective tissue covering the CNS that is called the pia mater (Figure \(\PageIndex{6}\)). Generally, they are supporting cells for the neurons in the central nervous system.

Some ways in which glia support neurons in the central nervous system are:

1. forming a structural network that strengthens the nervous tissue
2. maintaining the homeostasis of signaling molecules, water and ions in the extracellular space
3. providing an energy storage
4. defending the nervous tissue from oxidative stress
5. reacting to tissue damage and replacing dead neurons
6. assisting neuronal development in the fetal brain.
7. contributing to the blood-brain barrier (described below).

The blood-brain barrier (BBB) (Figure \(\PageIndex{5}\)) is a physiological barrier that keeps many substances that circulate in the rest of the body from getting into the central nervous system, restricting what can cross from circulating blood into the CNS. Nutrient molecules, such as glucose or amino acids, can pass through the BBB, but other molecules cannot. This actually causes problems with drug delivery to the CNS. Pharmaceutical companies are challenged to design drugs that can cross the BBB as well as have an effect on the nervous system. Like a few other parts of the body, the brain has a privileged blood supply. Very little can pass through by diffusion. Most substances that cross the wall of a blood vessel into the CNS must do so through an active transport process. Because of this, only specific types of molecules can enter the CNS. Glucose—the primary energy source—is allowed, as are amino acids. Water and some other small particles, like gases and ions, can enter. But most everything else cannot, including white blood cells, which are one of the body’s main lines of defense. While this barrier protects the CNS from exposure to toxic or pathogenic substances, it also keeps out the cells that could protect the brain and spinal cord from disease and damage.

Figure \(\PageIndex{5}\): Blood-Brain Barrier. The foot processes of the astrocytes cover the capillaries in the brain. Having this additional layer makes passage from the blood to the brain more difficult, but immune cells such as monocytes can pass through. (Image credits: "Astrocyte with Foot Processes" by Sofia Elizondo is licensed under CC-BY-NC-SA 4.0, modification of image by BruceBlaus)

Also found in CNS tissue is the oligodendrocyte, sometimes called just “oligo,” which is the glial cell type that insulates axons in the CNS (Figure \(\PageIndex{6}\)). The name means “cell of a few branches” (oligo- = “few”; dendro- = “branches”; -cyte = “cell”). There are a few processes that extend from the cell body. Each one reaches out and surrounds an axon to insulate it in myelin. One oligodendrocyte will provide the myelin for multiple axon segments, either for the same axon or for separate axons. The function of myelin will be discussed below.

Figure \(\PageIndex{6}\): Glial Cells of the CNS. Astrocytes form a barrier (blood-brain barrier) between the capillaries and the nervous tissue to protect it. Microglial cells act as defenders of the nervous tissue. Finally, oligodendrocytes wrap around multiple axons to insulate them. (Image credit: "Glia of the CNS" by BruceBlaus, CC BY-SA 4.0, via Wikimedia Commons)

Microglia are, as the name implies, smaller than most of the other glial cells (Figure \(\PageIndex{6}\)). Ongoing research into these cells, although not entirely conclusive, suggests that they may originate as white blood cells, called macrophages, that become part of the CNS during early development. While their origin is not conclusively determined, their function is related to what macrophages do in the rest of the body. When macrophages encounter diseased or damaged cells in the rest of the body, they ingest and digest those cells or the pathogens that cause disease. Microglia are the cells in the CNS that can do this in normal, healthy tissue, and they are therefore also referred to as CNS-resident macrophages.

The ependymal cell is a glial cell that filters blood to make cerebrospinal fluid (CSF), the fluid that circulates through the CNS.

Figure \(\PageIndex{7}\): Ependymal Cells. The ependymal cells cover specialized capillaries in brain's fluid filled spaces. They filter the blood to produce, and then circulate, cerebrospinal fluid (CSF). (Image credit: "Ependymal Cells - Blood-CSF Barrier" by Jennifer Lange is licensed under CC-BY-NC-SA 4.0, modification of image by NIH Medical Arts.)

Glial Cells of the PNS

One of the two types of glial cells found in the PNS is the satellite cell. Satellite cells are found in sensory and autonomic ganglia, where they surround the cell bodies of neurons. They provide support, performing similar functions in the periphery as astrocytes do in the CNS—except, of course, for establishing the BBB.

The second type of glial cell is the Schwann cell (neurolemmocyte), which insulate axons with myelin in the periphery. Schwann cells are different than oligodendrocytes, in that a Schwann cell wraps around a portion of only one axon segment and no others. Oligodendrocytes have processes that reach out to multiple axon segments, whereas the entire Schwann cell surrounds just one axon segment. The nucleus and cytoplasm of the Schwann cell are on the edge of the myelin sheath (Figure \(\PageIndex{8}\)).

Figure \(\PageIndex{8}\): Glial Cells of the PNS. Satellite cells are wrapped around a unipolar neuron of a peripheral ganglion to support and protect it. Schwann cells are wrapped onto the neuron's axon to insulate it with their myelin sheath. (Image credit: "Glia of the PNS" by BruceBlaus, CC BY-SA 4.0, via Wikimedia Commons.)

DISORDERS OF THE...

Glial cells: Gliomas

Glioma is a type of tumor that occurs in the glial cells of the brain or spinal cord (Figure \(\PageIndex{9}\)). Remember that tumor begins when the cell loses control over the checkpoints of mitosis and start proliferating excessively. Since neurons do not go under mitosis and can't produce new neurons, the only cells undergoing mitosis in the nervous system are the glial cells. Three types of glial cells can produce tumors. Gliomas are classified according to the type of glial cell involved in the tumor, as well as the tumor's genetic features, which can help predict how the tumor will behave over time and the treatments most likely to work. Types of glioma include:

• Astrocytomas, involving astrocytes
• Ependymomas, involving ependymal cells
• Oligodendrogliomas, involving oligodendrocytes

A glioma can affect the brain function and be life-threatening depending on its location and rate of growth. Gliomas are one of the most common types of primary brain tumors.

Figure \(\PageIndex{9}\): MRI of a 4-year old boy with a glioma. (Image credit: "Glioma" by Manoj Kumar Raghu H Ramakrishnaiah Rohan Samant is licensed under CC BY-SA 4.0, via Wikimedia Commons.)

Myelin

The insulation for axons in the nervous system is provided by oligodendrocytes in the CNS and Schwann cells in the PNS. These cells use the same mechanism to insulate axon segments. Myelin is a lipid-rich sheath that surrounds the axon and by doing so creates a myelin sheath that facilitates the transmission of electrical signals along the axon. The lipids are essentially the phospholipids of the glial cell membrane. The axon has parts that are not covered by myelin called nodes of Ranvier which ensure the propagation of the electrical signal. Axons can be completely enclosed by myelin (myelinated) (Figure \(\PageIndex{10}\) and Figure \(\PageIndex{11}\)) or not fully enclosed by it (unmyelinated). In the PNS, unmyelinated axons generally are less than 2 μm in diameter and myelinated axons are more than 1–2 μm in diameter. In the CNS, almost all axons with diameters greater than 0.2 μm are myelinated. In unmyelinated axons, their speed of electrical transmission is slower than myelinated nerves. Unmyelinated axons are primarily sensory fibers (transmitting sensations like dull pain, temperature changes, and itch) and autonomic fibers.

In myelinated axons, the appearance of the myelin sheath can be thought of as similar to the pastry wrapped around a hot dog for “pigs in a blanket” or a similar food. The glial cell is wrapped around the axon several times with little to no cytoplasm between the glial cell layers. For oligodendrocytes in the CNS, the rest of the cell is separate from the myelin sheath as a cell process extends back toward the cell body. A few other processes provide the same insulation for other axon segments in the area. For Schwann cells in the PNS, the outermost layer of the cell membrane contains cytoplasm and the nucleus of the cell as a bulge on one side of the myelin sheath. During development, the glial cell is loosely or incompletely wrapped around the axon. During myelination, the edges of this loose enclosure extend toward each other, and one end tucks under the other. The inner edge wraps around the axon, creating several layers, and the other edge closes around the outside so that the axon is completely enclosed. At the end of myelination, the myelin sheath is a thick layer that covers the axon. (Figure \(\PageIndex{10}\)) demonstrates the different myelination patterns of oligodendrocytes and Schwann cells.

A

B

Figure \(\PageIndex{10}\) Myelination in the CNS and PNS: A) One oligodendrocyte in the CNS is able to myelinate multiple axons. It's multiple processes branch out and wrap themselves around multiple axons. B) One Schwann cell in the PNS forms myelin by wrapping itself multiple times over one segment of the axon. One Schwann cell is only able to myelinate one segment of the axon. (A: Image credit: "Oligodendrocyte illustration" by Holly Fischer, CC BY 3.0, via Wikimedia Commons) (B: Image credit: "Dorsal Ventral Body Cavities" by OpenStax is licensed under CC BY 4.0) / Labels modified by Harmony Folse

Figure \(\PageIndex{11}\): Myelinated Axons. This false colored scanning electron micrograph image shows the myelinated axons in a rat spinal cord. In the CNS axons are myelinated by oligodentrocytes. (Image credit: "Myelinated Axons" by Tom Deerinck and Mark Ellisman, National Center for Microscopy and Imaging Research (NCMIR) is licensed under CC BY NC 2.0.)

Regeneration of Axons

Traumatic injuries can cause damage to nerves. When nerves are damaged within the PNS as in the case of a laceration, the axons distal to the injury degenerate. The Schwann cells that were wrapping the injured axons proliferate to form a new tube of myelin that guides the growing axons through physical and chemical support (Figure \(\PageIndex{12}\)). Most of the times, axons can cross the injury site and can successfully grow back to the original targets. Sometimes the injury can affect also the surrounding tissues and form a scar that axons cannot cross. In that case, the axons are not able to grow back, unless surgery is performed to eliminate the scar tissue. The situation is different when injuries affect the CNS as in the case of spinal cord injuries. The oligodendrocytes respond to injury by undergoing apoptosis or becoming inactive. This contributes to the formation of scar tissue. Moreover, injury-activated astrocytes release molecules that inhibit axonal growth. Consequently, injured axons in the CNS are not able to grow back to their original target and nerve injuries in the CNS are permanent.

Figure \(\PageIndex{12}\): Axon Regeneration in the PNS. A neuron of the PNS innervates its target with its myelinated axon covered by Schwann cells. Following an injury, the part of the axon distal to the injury degenerates, leaving an axonal stump. The Schwann cells proliferate and produce new Schwann cells that form a tube to guide the axon back to its original target. Macrophages help clear up the debris from the axonal degeneration. The axonal stump grows following the tube of Schwann cells, which wrap it with myelin. The axon reaches the original target and the axon regeneration is complete. (Image credit: ”Axon Regeneration in the PNS" by Chiara Mazzasette is a derivative from the original work of Daniel Donnelly and is licensed by CC BY 4.0.)

DISORDERS OF THE...

Nervous Tissue: Demyelinating Diseases

Several diseases can result from the demyelination of axons. The causes of these diseases are not the same; some have genetic causes, some are caused by pathogens, and others are the result of autoimmune disorders. Though the causes are varied, the results are largely similar. The myelin insulation of axons is compromised, making electrical signaling slower.

Multiple sclerosis (MS) is one such disease. It is an example of an autoimmune disease. The antibodies produced by lymphocytes (a type of white blood cell) mark myelin as something that should not be in the body. This causes inflammation and the destruction of the myelin in the central nervous system. As the insulation around the axons is destroyed by the disease, scarring becomes obvious. This is where the name of the disease comes from; sclerosis means hardening of tissue, which is what a scar is. Multiple scars are found in the white matter of the brain and spinal cord. The symptoms of MS include both somatic and autonomic deficits. Control of the musculature is compromised, as is control of organs such as the bladder.

Guillain-Barré (pronounced gee-YAN bah-RAY) syndrome is an example of a demyelinating disease of the peripheral nervous system. It is also the result of an autoimmune reaction, but the inflammation is in peripheral nerves. Sensory symptoms or motor deficits are common, and autonomic failures can lead to changes in the heart rhythm or a drop in blood pressure, especially when standing, which causes dizziness.

Contributors and Attributions

• OpenStax Anatomy & Physiology (CC BY 4.0). Access for free at https://openstax.org/books/anatomy-and-physiology