Information

35.1: Neurons and Glial Cells - Biology

35.1: Neurons and Glial Cells - Biology


We are searching data for your request:

Forums and discussions:
Manuals and reference books:
Data from registers:
Wait the end of the search in all databases.
Upon completion, a link will appear to access the found materials.

Skills to Develop

  • List and describe the functions of the structural components of a neuron
  • List and describe the four main types of neurons
  • Compare the functions of different types of glial cells

Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in Figure (PageIndex{1}). Some organisms, like sea sponges, lack a true nervous system. Others, like jellyfish, lack a true brain and instead have a system of separate but connected nerve cells (neurons) called a “nerve net.” Echinoderms such as sea stars have nerve cells that are bundled into fibers called nerves. Flatworms of the phylum Platyhelminthes have both a central nervous system (CNS), made up of a small “brain” and two nerve cords, and a peripheral nervous system (PNS) containing a system of nerves that extend throughout the body. The insect nervous system is more complex but also fairly decentralized. It contains a brain, ventral nerve cord, and ganglia (clusters of connected neurons). These ganglia can control movements and behaviors without input from the brain. Octopi may have the most complicated of invertebrate nervous systems—they have neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species.

Compared to invertebrates, vertebrate nervous systems are more complex, centralized, and specialized. While there is great diversity among different vertebrate nervous systems, they all share a basic structure: a CNS that contains a brain and spinal cord and a PNS made up of peripheral sensory and motor nerves. One interesting difference between the nervous systems of invertebrates and vertebrates is that the nerve cords of many invertebrates are located ventrally whereas the vertebrate spinal cords are located dorsally. There is debate among evolutionary biologists as to whether these different nervous system plans evolved separately or whether the invertebrate body plan arrangement somehow “flipped” during the evolution of vertebrates.

Link to Learning

Watch this video of biologist Mark Kirschner discussing the “flipping” phenomenon of vertebrate evolution.

The nervous system is made up of neurons, specialized cells that can receive and transmit chemical or electrical signals, and glia, cells that provide support functions for the neurons by playing an information processing role that is complementary to neurons. A neuron can be compared to an electrical wire—it transmits a signal from one place to another. Glia can be compared to the workers at the electric company who make sure wires go to the right places, maintain the wires, and take down wires that are broken. Although glia have been compared to workers, recent evidence suggests that also usurp some of the signaling functions of neurons.

There is great diversity in the types of neurons and glia that are present in different parts of the nervous system. There are four major types of neurons, and they share several important cellular components.

Neurons

The nervous system of the common laboratory fly, Drosophila melanogaster, contains around 100,000 neurons, the same number as a lobster. This number compares to 75 million in the mouse and 300 million in the octopus. A human brain contains around 86 billion neurons. Despite these very different numbers, the nervous systems of these animals control many of the same behaviors—from basic reflexes to more complicated behaviors like finding food and courting mates. The ability of neurons to communicate with each other as well as with other types of cells underlies all of these behaviors.

Most neurons share the same cellular components. But neurons are also highly specialized—different types of neurons have different sizes and shapes that relate to their functional roles.

Parts of a Neuron

Like other cells, each neuron has a cell body (or soma) that contains a nucleus, smooth and rough endoplasmic reticulum, Golgi apparatus, mitochondria, and other cellular components. Neurons also contain unique structures, illustrated in Figure (PageIndex{2}) for receiving and sending the electrical signals that make neuronal communication possible. Dendrites are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called synapses. Although some neurons do not have any dendrites, some types of neurons have multiple dendrites. Dendrites can have small protrusions called dendritic spines, which further increase surface area for possible synaptic connections.

Once a signal is received by the dendrite, it then travels passively to the cell body. The cell body contains a specialized structure, the axon hillock that integrates signals from multiple synapses and serves as a junction between the cell body and an axon. An axon is a tube-like structure that propagates the integrated signal to specialized endings called axon terminals. These terminals in turn synapse on other neurons, muscle, or target organs. Chemicals released at axon terminals allow signals to be communicated to these other cells. Neurons usually have one or two axons, but some neurons, like amacrine cells in the retina, do not contain any axons. Some axons are covered with myelin, which acts as an insulator to minimize dissipation of the electrical signal as it travels down the axon, greatly increasing the speed on conduction. This insulation is important as the axon from a human motor neuron can be as long as a meter—from the base of the spine to the toes. The myelin sheath is not actually part of the neuron. Myelin is produced by glial cells. Along the axon there are periodic gaps in the myelin sheath. These gaps are called nodes of Ranvier and are sites where the signal is “recharged” as it travels along the axon.

It is important to note that a single neuron does not act alone—neuronal communication depends on the connections that neurons make with one another (as well as with other cells, like muscle cells). Dendrites from a single neuron may receive synaptic contact from many other neurons. For example, dendrites from a Purkinje cell in the cerebellum are thought to receive contact from as many as 200,000 other neurons.

Art Connection

Which of the following statements is false?

  1. The soma is the cell body of a nerve cell.
  2. Myelin sheath provides an insulating layer to the dendrites.
  3. Axons carry the signal from the soma to the target.
  4. Dendrites carry the signal to the soma.

Types of Neurons

There are different types of neurons, and the functional role of a given neuron is intimately dependent on its structure. There is an amazing diversity of neuron shapes and sizes found in different parts of the nervous system (and across species), as illustrated by the neurons shown in Figure (PageIndex{3}).

While there are many defined neuron cell subtypes, neurons are broadly divided into four basic types: unipolar, bipolar, multipolar, and pseudounipolar. Figure (PageIndex{4}) illustrates these four basic neuron types. Unipolar neurons have only one structure that extends away from the soma. These neurons are not found in vertebrates but are found in insects where they stimulate muscles or glands. A bipolar neuron has one axon and one dendrite extending from the soma. An example of a bipolar neuron is a retinal bipolar cell, which receives signals from photoreceptor cells that are sensitive to light and transmits these signals to ganglion cells that carry the signal to the brain. Multipolar neurons are the most common type of neuron. Each multipolar neuron contains one axon and multiple dendrites. Multipolar neurons can be found in the central nervous system (brain and spinal cord). An example of a multipolar neuron is a Purkinje cell in the cerebellum, which has many branching dendrites but only one axon. Pseudounipolar cells share characteristics with both unipolar and bipolar cells. A pseudounipolar cell has a single process that extends from the soma, like a unipolar cell, but this process later branches into two distinct structures, like a bipolar cell. Most sensory neurons are pseudounipolar and have an axon that branches into two extensions: one connected to dendrites that receive sensory information and another that transmits this information to the spinal cord.

Everyday Connection: Neurogenesis

At one time, scientists believed that people were born with all the neurons they would ever have. Research performed during the last few decades indicates that neurogenesis, the birth of new neurons, continues into adulthood. Neurogenesis was first discovered in songbirds that produce new neurons while learning songs. For mammals, new neurons also play an important role in learning: about 1000 new neurons develop in the hippocampus (a brain structure involved in learning and memory) each day. While most of the new neurons will die, researchers found that an increase in the number of surviving new neurons in the hippocampus correlated with how well rats learned a new task. Interestingly, both exercise and some antidepressant medications also promote neurogenesis in the hippocampus. Stress has the opposite effect. While neurogenesis is quite limited compared to regeneration in other tissues, research in this area may lead to new treatments for disorders such as Alzheimer’s, stroke, and epilepsy.

How do scientists identify new neurons? A researcher can inject a compound called bromodeoxyuridine (BrdU) into the brain of an animal. While all cells will be exposed to BrdU, BrdU will only be incorporated into the DNA of newly generated cells that are in S phase. A technique called immunohistochemistry can be used to attach a fluorescent label to the incorporated BrdU, and a researcher can use fluorescent microscopy to visualize the presence of BrdU, and thus new neurons, in brain tissue. Figure (PageIndex{5}) is a micrograph which shows fluorescently labeled neurons in the hippocampus of a rat.

Link to Learning

This site contains more information about neurogenesis, including an interactive laboratory simulation and a video that explains how BrdU labels new cells.

While glia are often thought of as the supporting cast of the nervous system, the number of glial cells in the brain actually outnumbers the number of neurons by a factor of ten. Neurons would be unable to function without the vital roles that are fulfilled by these glial cells. Glia guide developing neurons to their destinations, buffer ions and chemicals that would otherwise harm neurons, and provide myelin sheaths around axons. Scientists have recently discovered that they also play a role in responding to nerve activity and modulating communication between nerve cells. When glia do not function properly, the result can be disastrous—most brain tumors are caused by mutations in glia.

Types of Glia

There are several different types of glia with different functions, two of which are shown in Figure (PageIndex{6}). Astrocytes, shown in Figure (PageIndex{7}) make contact with both capillaries and neurons in the CNS. They provide nutrients and other substances to neurons, regulate the concentrations of ions and chemicals in the extracellular fluid, and provide structural support for synapses. Astrocytes also form the blood-brain barrier—a structure that blocks entrance of toxic substances into the brain. Astrocytes, in particular, have been shown through calcium imaging experiments to become active in response to nerve activity, transmit calcium waves between astrocytes, and modulate the activity of surrounding synapses.

Satellite glia provide nutrients and structural support for neurons in the PNS. Microglia scavenge and degrade dead cells and protect the brain from invading microorganisms. Oligodendrocytes, shown in Figure (PageIndex{7}) form myelin sheaths around axons in the CNS. One axon can be myelinated by several oligodendrocytes, and one oligodendrocyte can provide myelin for multiple neurons. This is distinctive from the PNS where a single Schwann cell provides myelin for only one axon as the entire Schwann cell surrounds the axon. Radial glia serve as scaffolds for developing neurons as they migrate to their end destinations. Ependymal cells line fluid-filled ventricles of the brain and the central canal of the spinal cord. They are involved in the production of cerebrospinal fluid, which serves as a cushion for the brain, moves the fluid between the spinal cord and the brain, and is a component for the choroid plexus.

Summary

The nervous system is made up of neurons and glia. Neurons are specialized cells that are capable of sending electrical as well as chemical signals. Most neurons contain dendrites, which receive these signals, and axons that send signals to other neurons or tissues. There are four main types of neurons: unipolar, bipolar, multipolar, and pseudounipolar neurons. Glia are non-neuronal cells in the nervous system that support neuronal development and signaling. There are several types of glia that serve different functions.

Art Connections

[link] Which of the following statements is false?

  1. The soma is the cell body of a nerve cell.
  2. Myelin sheath provides an insulating layer to the dendrites.
  3. Axons carry the signal from the soma to the target.
  4. Dendrites carry the signal to the soma.

[link] B

Glossary

astrocyte
glial cell in the central nervous system that provide nutrients, extracellular buffering, and structural support for neurons; also makes up the blood-brain barrier
axon
tube-like structure that propagates a signal from a neuron’s cell body to axon terminals
axon hillock
electrically sensitive structure on the cell body of a neuron that integrates signals from multiple neuronal connections
axon terminal
structure on the end of an axon that can form a synapse with another neuron
dendrite
structure that extends away from the cell body to receive messages from other neurons
ependymal
cell that lines fluid-filled ventricles of the brain and the central canal of the spinal cord; involved in production of cerebrospinal fluid
glia
(also, glial cells) cells that provide support functions for neurons
microglia
glia that scavenge and degrade dead cells and protect the brain from invading microorganisms
myelin
fatty substance produced by glia that insulates axons
neuron
specialized cell that can receive and transmit electrical and chemical signals
nodes of Ranvier
gaps in the myelin sheath where the signal is recharged
oligodendrocyte
glial cell that myelinates central nervous system neuron axons
radial glia
glia that serve as scaffolds for developing neurons as they migrate to their final destinations
satellite glia
glial cell that provides nutrients and structural support for neurons in the peripheral nervous system
Schwann cell
glial cell that creates myelin sheath around a peripheral nervous system neuron axon
synapse
junction between two neurons where neuronal signals are communicated

Mitochondrial dysfunction in glial cells: Implications for neuronal homeostasis and survival

Mitochondrial dysfunction is central to the pathogenesis of neurological disorders. Neurons rely on oxidative phosphorylation to meet their energy requirements and thus alterations in mitochondrial function are linked to energy failure and neuronal cell death. Furthermore, in neurons, dysfunctional mitochondria are reported to increase the steady-state levels of reactive oxygen species derived from the leakage of electrons from the electron transport chain. Research aimed at understanding mitochondrial dysfunction and its role in neurological disorders has been primarily geared towards neurons. In contrast, the effects of mitochondrial dysfunction in glial cells' function and its implication for neuronal homeostasis and brain function has been largely understudied. Unlike neurons and oligodendrocytes, astrocytes and microglia do not degenerate upon the impairment of mitochondrial function, as they rely primarily on glycolysis to produce energy and have a higher antioxidant capacity than neurons. However, recent evidence highlights the role of mitochondrial metabolism and signaling in glial cell function. In this work, we review the functional role of mitochondria in glial cells and the evidence regarding its potential role regulating neuronal homeostasis and disease progression.

Keywords: Astrocytes Calcium Free fatty acid oxidation Glycolysis Inflammation Microglia Mitochondria Oligodendrocytes Redox.

Copyright © 2017 Elsevier B.V. All rights reserved.

Figures

Neuronal metabolism, redox homeostasis and…

Neuronal metabolism, redox homeostasis and signaling are supported by neighboring glial cells. 1.1:…

Mitochondrial metabolism and signaling in…

Mitochondrial metabolism and signaling in astrocytes. 2.1: Glucose in astrocytes is used for…


More Than Just ‘Glue’

Glia take many forms to perform their specialized functions: Some are sheathlike, while others are spindly, bushy or star-shaped. Many tangle around neurons and form a network so dense that individual cells are hard to distinguish. To some early observers, they didn’t even look like cells — they were considered a supportive matrix within the skull. This prompted the 19th-century researcher Rudolph Virchow to dub this non-neuronal material “neuroglia,” drawing on the Greek word for glue.

In this magnified image of brain tissue, neurons (blue) are surrounded by large numbers of glial cells, including astrocytes (red) and oligodendrocytes (green). Jonathan Cohen/NIH

One reason glia were given such short shrift was that when researchers first began staining nervous system tissue, their methods revealed the convoluted shapes of neurons but rendered only select glia visible. Santiago Ramón y Cajal, who is credited with the discovery of neurons and widely regarded as the founder of neuroscience, illustrated one subtype of glia but lumped the rest together as “the third element.” His focus on neurons set the stage for the burgeoning field of neuroscience but shoved the glia behind the curtains.

In addition, some glia are challenging to study because their fates are so entwined with those of neurons that it’s hard to learn about them separately. If researchers try to learn about the glia’s functions by knocking them out and observing the effects, the neurons they support will die along with them.

But the revolution in cell biology techniques in recent decades has generated an arsenal of tools offering greater access to glia, Shaham said. Advances in live imaging, fluorescent labeling and genetic manipulation are revealing the breadth of glia’s forms and functions.


Watch the video: Εισαγωγή στο κύτταρο (July 2022).


Comments:

  1. Zolotaxe

    According to mine, at someone the letter alexie :)

  2. Egidius

    I suggest you try looking at google.com

  3. Kerr

    Bravo, great phrase and timely

  4. Faelar

    Thanks immense for the help in this matter, now I will not admit such error.

  5. Claudio

    Let will be your way. Do, as want.



Write a message