3.6: Nervous Tissue

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

• Identify the types of cells that make up nervous tissue
• Explain the general function of neurons and their parts
• Explain the general function of neuroglia

Nervous tissue is characterized as being excitable and capable of sending and receiving electrochemical signals that provide the body with information.

Two main classes of cells make up nervous tissue: the neuron and neuroglia (Figure \(\PageIndex{1}\) and Figure \(\PageIndex{2}\)). Neurons propagate information via electrochemical impulses, called action potentials, which are biochemically linked to the release of chemical signals. Neuroglia play essential roles in structurally and physically supporting neurons, stabilizing synapses, and modulating their information propagation.

A. Cerebellum

B. Cerebral Cortex

Figure \(\PageIndex{1}\): Nervous Tissue. Neurons are large cells with a variety of shapes and sizes, but all have a cell body that contains the nucleus and projections. Neuroglia are tiny cells and show up as dots on the slides. (Image credit: “Nervous Tissue of Cerebellum and Cerebrum" images provided by the Regents of the University of Michigan Medical School under CC-BY-NC-SA 4.0 © 2022)

Figure \(\PageIndex{2}\): Nervous Tissue Cells. In this image tissue from the spinal cord has been smeared on the slide so you can see the neuron structure. (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 display distinctive morphology, well suited to their role as conducting cells, with three main parts:

• The cell body (soma) includes most of the cytoplasm, the organelles, and the nucleus.
• Dendrites branch off the cell body and appear as thin extensions. They transfer the incoming signals to the soma.
• A long “tail,” the axon, extends from the cell body and can be wrapped in an insulating layer, called myelin, formed by neuroglia. It carries the outgoing action potential away to another excitable cell.

Axons split into multiple axon terminals when they are close to their target. At the end of each terminal is a synaptic knob that will pass the electrical signal to the next cell (usually another neuron, muscle fiber, or gland) by stimulating the release of a neurotransmitter. (Figure \(\PageIndex{3}\)) The specific classes of neurons and their functions will be discussed in a later chapter.

Figure \(\PageIndex{3}\): Neural Tissue. Neurons receive, process, and send messages throughout the nervous system and to effectors, such as the heart or stomach. Neuroglia, greatly enlarged here, come in six different types that each have a specific function and a unique shape. This one is a microglia, which clears cellular debris as well as pathogens. (Image credit: "Blausen 0672 - Neurvous tissue - English labels" by Blausen.com staff (2014), license: CC BY. Source: "Medical gallery of Blausen Medical 2014")

Neuroglia

The second class of neural cells comprises the neuroglia or glial cells, which formerly have been characterized as having a simple support role. (Figure \(\PageIndex{4}\)) The word “glia” comes from the Greek word for glue. Recent research is shedding light on the more complex role of neuroglia in the function of the brain and nervous system. These cells were harder to study initially because they did not generate the electrical signals that neurons did, but with more recent techniques we now know that these cells do a lot more than glue the neurons together! Currently known functions include:

• defining synaptic contacts and maintaining the signaling ability of neurons
• maintaining the chemical environment around the neurons
• modulating the rate of signal propagation
• controlling the uptake of neurotransmitters
• providing a scaffold for neurodevelopment
• healing damaged neurons
• regulating fluid transport in the glymphatic pathway
• production and circulation of cerebrospinal fluid

Glial cells outnumber neurons by an approximate 3:1 ratio. The specific types of glial cells and their functions will be discussed in a later chapter.

Figure \(\PageIndex{4}\): Astrocytes. Special imaging techniques are used for visualizing the tiny glial cells. This image uses confocal microscopy to display the characteristic star-like shape of one type of glial cell: the astrocyte. (Image Credit: "Activated Astrocytes" by Pics56, CC BY-SA 4.0, via Wikimedia Commons)

Glia - what are they good for? 

Two neurological mysteries may be partly explained by glial cells: why do we need to sleep? and what makes a person a genius?

Why must we sleep? In terms of survival, sleeping does not make sense. Why would all animals risk not being conscious of their environment for hours every day - what predators might be able to walk up un-noticed?! All animals sleep, even though some sleep with only half their brain at a time, so this brain down time must be crucial to life. Indeed, if deprived of sleep you will die faster than if you are deprived of food or water.

In 2013, a study by Maiken Nedergaard and colleagues at the University of Rochester in New York discovered a system that flushes waste products from the brain. Only while sleeping the cerebrospinal fluid, the liquid surrounding the brain and spinal cord, moves through the brain along a series of channels surrounding the blood vessels flushing out the toxins that accumulated during the waking hours. Since the channels are controlled by the brain’s glial cells, the research team called it the glymphatic system. Nedergaard's team discovered this system in mice, and it has subsequently been shown in humans as well. This raises the interesting possibility that a lack of sleep could lead to neurological diseases such as Alzheimer's, or that sleep disturbances that go along with neurological conditions could lead to further brain damage due to the build up of harmful toxins. For more detailed information about the original study visit https://www.science.org/content/article/sleep-ultimate-brainwasher. To see a video of the pulses of CSF in the human brain visit https://www.bu.edu/articles/2019/cerebrospinal-fluid-washing-in-brain-during-sleep/.

Is genius all in the brain? A long standing question for neuroscientists and philosophers alike is where does genius come from? Some people are just levels above everyone else in terms of their ability to create, discover, and change future knowledge within their field - Galileo Galilei, Leonardo Da Vinci, Johann Wolfgang van Goethe, and, of course, Albert Einstein. Shortly after Einstein's death in 1955, the physician conducting his autopsy, Thomas Harvey, secretly removed and preserved his brain. Photographs were taken and then Harvey supervised the division of the brain into 240 blocks, and created 12 sets of 200 slides containing tissue samples indexed to the blocks. Many of these were sent to eminent neuropathologists of the 1950's, but no studies were published. They did not find anything worth publishing.

In 1978 a young reporter tracked down Thomas Harvey and rediscovered Einstein's brain. Once news got out, Harvey was inundated with requests for samples. The first study to be published based on these samples was by neuroanatomist Marian Diamond of U.C. Berkeley in 1985. She received four sections of the cerebral cortex association regions from both hemispheres and compared the ratio of glial cells in Einstein's brain with that of the preserved brains of 11 other males. Einstein's brain had more glial cells relative to neurons in all areas studied, but in only one area was the difference statistically significant, one that is part of the brain regions responsible for incorporating and synthesizing information from multiple other brain regions. The question remains though - was it the additional glial cells that made Einstein a genius, or was it Einstein's genius that caused him to have more glial cells? To read more about the strange journey of Einstein's brain visit https://www.bbc.com/news/magazine-32354300.

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Concept Review

The most prominent cell of the nervous tissue, the neuron, is characterized mainly by its ability to receive stimuli and respond by generating an electrical signal, known as an action potential, which can travel rapidly over great distances in the body. A typical neuron displays a distinctive morphology: a large cell body branches out into short extensions called dendrites, which receive chemical signals from other neurons, and a long tail called an axon, which relays signals away from the cell to other neurons, muscles, or glands. Other cells in the nervous tissue, the neuroglia, function to keep these neural connections stable and to support signal transmission.

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Review Questions

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Critical Thinking Questions

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Glossary

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Contributors and Attributions

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