79 8.3 Neurons and Neuroglia

Created by CK-12 Foundation/Adapted by Christine Miller

Figure 8.3.1 A micrograph of human nervous tissue or modern art?

Life as Art

This colourful picture (Figure 8.3.1) could be an abstract work of modern art. You might imagine it hanging in an art museum or art gallery. In fact, the picture illustrates real life — not artistic creation. It is a micrograph of human . The neon green structures in the picture are . The neuron is one of two basic types of cells in the nervous system. The other type is the cell.

Neurons

 — also called nerve cells — are electrically excitable cells that are the main functional units of the . Their function is to transmit , and they are the only type of human cells that can carry out this function.

Neuron Structure

Figure 8.3.2 shows the structure of a typical neuron. Click on each of the main parts to learn about their functions.

Figure 8.3.2 The structure of a typical neuron. 

Neurogenesis

Fully differentiated neurons, with all their special structures, cannot divide and form new daughter neurons. Until recently, scientists thought that new neurons could no longer be formed after the brain developed prenatally. In other words, they thought that people were born with all the brain neurons they would ever have, and as neurons died, they would not be replaced. However, new evidence shows that additional neurons can form in the brain, even in adults, from the division of undifferentiated neural stem cells found throughout the brain. The production of new neurons is called . The extent to which it can occur is not known, but it is not likely to be very great in humans.

Neurons in Nervous Tissues

The in the and consists of gray matter and white matter.  contains mainly non-myelinated structures, including the cell bodies and dendrites of neurons. It is gray only in cadavers. Living gray matter is actually more pink than gray (see Figure 8.3.3) consists mainly of axons covered with a , which gives them their white colour. White matter also makes up the of the . Nerves consist of long bundles of myelinated that extend to muscles, organs, or glands throughout the body. The axons in each nerve are bundled together like wires in a cable. Axons in nerves may be more than a metre long in an adult. The longest nerve runs from the base of the spine to the toes.

White and Gray Matter
Figure 8.3.3 You can see the layers of (pinkish) gray matter and white matter in this photo of a brain from a recently deceased human patient.

Types of Neurons

There are hundreds of different types of neurons in the human nervous system that exhibit a variety of structures and functions. Nonetheless, many neurons can be classified functionally based on the direction in which they carry nerve impulses.

  •  carry nerve impulses from  in  and  to the . They change physical stimuli (such as touch, light, and sound) into nerve impulses.
  • , like the one in the diagram below (Figure 8.3.4), carry nerve impulses from the central nervous system to muscles and glands. They change nerve signals into the activation of these structures.
  • Within the spinal cord or brain,  carry nerve impulses back and forth, often between sensory and motor neurons.

 

Neuromuscular Junction
Figure 8.3.4 The axon in this diagram is part of a motor neuron. It transmits nerve impulses from the central nervous system to a skeletal muscle, causing it to contract.

 

Neuroglia

In addition to neurons, nervous tissues also consist of , also called . The root of the word glial comes from a Greek word meaning “glue,” which reflects earlier ideas about the role of neuroglia in nervous tissues. Neuroglia were thought to be little more than “glue” holding together the all-important neurons, but this is no longer the case. They are now known to play many vital roles in the nervous system. There are several different types of neuroglia, each with a different function. You can see six types of neuroglia in Figure 8.3.5.

Types of Neuroglia
Figure 8.3.5 Different types of glial cells (neuroglia) are found in the central nervous system and peripheral nervous system.

In general, neuroglia provide support for neurons and help them carry out the basic function of nervous tissues, which is to transmit nerve impulses. For example, s in the central nervous system and s in the peripheral nervous system generate the lipids that make up , which increase the speed of nerve impulses’ transmission. Functions of other neuroglia cells include holding neurons in place, supplying neurons with nutrients, regulating the repair of neurons, destroying pathogens, removing dead neurons, and directing axons to their targets. Neuroglia may also play a role in the transmission of nerve impulses, but this is still under study. Unlike mature neurons, mature glial cells retain the ability to divide by undergoing mitosis.

In the human brain, there are generally roughly equal numbers of neurons and neuroglia. If you think intelligence depends on how many neurons you have, think again. Having a relatively high number of neuroglia is actually associated with higher intelligence. When Einstein’s brain was analyzed, researchers discovered a significantly higher-than-normal ratio of neuroglia to neurons in areas of the brain associated with mathematical processing and language. On an evolutionary scale, as well, an increase in the ratio of neuroglia to neurons is associated with greater intelligence in species.

Feature: My Human Body

Would you like your brain to make new neurons that could help you become a better learner? When it comes to learning new things, what college student wouldn’t want a little more brain power? If research about rats applies to humans, then sustained aerobic exercise (such as running) can increase neurogenesis in the adult brain, and specifically in the hippocampus, a brain structure important for learning temporally and/or spatially complex tasks, as well as memory. Although the research is still at the beginning stages, it suggests that exercise may actually lead to a “smarter” brain. Even if the research results are not ultimately confirmed for humans, though, it can’t hurt to get more aerobic exercise. It is certainly beneficial for your body, if not your brain!

8.3 Summary

  •  are one of two major types of nervous system cells. They are electrically excitable cells that transmit nerve impulses.
  • are the other major type of nervous system cells. There are many types of neuroglia and they have many specific functions. In general, neuroglia function to support, protect, and nourish neurons.
  • The main parts of a neuron include the , , and . The cell body contains the . Dendrites receive nerve impulses from other cells, and the axon transmits nerve impulses to other cells at axon terminals. A  is a complex membrane junction at the end of an axon terminal that transmits signals to another cell.
  • Axons are often wrapped in an electrically-insulating , which is produced by neuroglia. Electrical signals occur at gaps in the myelin sheath, called , which speeds the conduction of nerve impulses down the axon.
  • , or the formation of new neurons by cell division, may occur in a mature human brain, but only to a limited extent.
  • The nervous tissue in the brain and spinal cord consists of (which contains unmyelinated cell bodies and dendrites of neurons) and (which contains mainly myelinated axons of neurons). Nerves of the peripheral nervous system consist of long bundles of myelinated axons that extend throughout the body.
  • There are hundreds of types of neurons in the human nervous system, but many can be classified on the basis of the direction in which they carry nerve impulses.  carry nerve impulses away from the body and toward the central nervous system,  carry them away from the central nervous system and toward the body, and  often carry them between sensory and motor neurons.

8.3 Review Questions

  1. Describe the myelin sheath and nodes of Ranvier. How does their arrangement allow nerve impulses to travel very rapidly along axons?
  2. Define neurogenesis. What is the potential for neurogenesis in the human brain?
  3. Relate neurons to different types of nervous tissues.
  4. Compare and contrast sensory and motor neurons.
  5. Identify the role of interneurons.
  6. Identify four specific functions of neuroglia.
  7. What is the relationship between the proportion of neuroglia to neurons and intelligence?

8.3 Explore More

Thriving in the Face of Adversity | Stephanie Buxhoeveden | TEDxHerndon, TEDx Talks, 2015.

You can grow new brain cells. Here’s how | Sandrine Thuret,
TED, 2015.

 

Attributions

Figure 8.3.1

Nervous Tissue Confocal Microscopy/ Mouse brain, confocal microscopy by ZEISS Microscopy on Flickr is used under a CC BY-NC-ND 2.0 (https://creativecommons.org/licenses/by-nc-nd/2.0/) license.

Figure 8.3.2

Parts of a Neuron by Open Stax on Wikimedia Commons is used and adapted by Christine Miller under the CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 8.3.3

White_and_Gray_Matter by OpenStax  on Wikimedia Commons is used under a CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/) license.

Figure 8.3.4

Neuromuscular Junction by CK-12 Foundation is used under a CC BY-NC 3.0 (https://creativecommons.org/licenses/by-nc/3.0/) license.

©CK-12 Foundation Licensed under CK-12 Foundation is licensed under Creative Commons AttributionNonCommercial 3.0 Unported (CC BY-NC 3.0) • Terms of Use • Attribution

Figure 8.3.5

TypesofNeuroglia by BruceBlaus on Wikimedia Commons is used under a CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en) license.

References

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2016, May 18). Figure 12.3 Gray matter and white matter [digital image].  In Anatomy and Physiology (Section 12.1). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/12-1-basic-structure-and-function-of-the-nervous-system

Betts, J. G., Young, K.A., Wise, J.A., Johnson, E., Poe, B., Kruse, D.H., Korol, O., Johnson, J.E., Womble, M., DeSaix, P. (2016, May 18). Figure 12.8 Parts of a neuron [digital image].  In Anatomy and Physiology (Section 12.2). OpenStax. https://openstax.org/books/anatomy-and-physiology/pages/12-2-nervous-tissue

Blausen.com staff. (2014). Types of neuroglia cells [digital image]. Medical gallery of Blausen Medical 2014. WikiJournal of Medicine, 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436. Wikiversity.org. https://en.wikiversity.org/wiki/WikiJournal_of_Medicine/Medical_gallery_of_Blausen_Medical_2014

Brainard, J/ CK-12 Foundation. (2016). Figure 3 The axon in this diagram is part of a motor neuron.  [digital image]. In CK-12 College Human Biology (Section 10.3) [online Flexbook]. CK12.org. https://www.ck12.org/book/ck-12-college-human-biology/section/10.3/

TED. (2015, October 30). You can grow new brain cells. Here’s how | Sandrine Thuret. YouTube. https://www.youtube.com/watch?v=B_tjKYvEziI&feature=youtu.be

TEDx Talks. (2015, April 3). Thriving in the face of adversity | Stephanie Buxhoeveden | TEDxHerndon. YouTube. https://www.youtube.com/watch?v=zuLOT6GsAxw&feature=youtu.be

License

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Human Biology by Christine Miller is licensed under a Creative Commons Attribution-NonCommercial 4.0 International License, except where otherwise noted.

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