{"id":4841,"date":"2015-10-28T15:55:52","date_gmt":"2015-10-28T15:55:52","guid":{"rendered":"https:\/\/opentextbc.ca\/biology\/chapter\/16-1-neurons-and-glial-cells\/"},"modified":"2021-03-04T00:07:06","modified_gmt":"2021-03-04T00:07:06","slug":"16-1-neurons-and-glial-cells","status":"publish","type":"chapter","link":"https:\/\/opentextbc.ca\/biology\/chapter\/16-1-neurons-and-glial-cells\/","title":{"raw":"16.1\u00a0Neurons and Glial Cells","rendered":"16.1\u00a0Neurons and Glial Cells"},"content":{"raw":"<div class=\"section module\" title=\"35.1.\u00a0Neurons and Glial Cells\" xml:lang=\"en\">\n<div class=\"titlepage\">\n<div class=\"abstract\">\n<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\nBy the end of this section, you will be able to:\n<div class=\"itemizedlist\">\n<ul class=\"itemizedlist\">\n \t<li class=\"listitem\">List and describe the functions of the structural components of a neuron<\/li>\n \t<li class=\"listitem\">List and describe the four main types of neurons<\/li>\n \t<li class=\"listitem\">Compare the functions of different types of glial cells<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44747-fs-idm129833856\"> <\/span>Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in Figure 16.2. 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 \u201cnerve net.\u201d 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 \u201cbrain\u201d 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\u2014they have neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species.\n\n[caption id=\"attachment_1145\" align=\"aligncenter\" width=\"500\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_01.jpg\"><img class=\"wp-image-4832\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/02\/Figure_35_01_01-1024x963-1.jpg\" alt=\"Figure_35_01_01\" width=\"500\" height=\"470\"><\/a> Figure 16.2.\u00a0 Nervous systems vary in structure and complexity. In (a) cnidarians, nerve cells form a decentralized nerve net. In (b) echinoderms, nerve cells are bundled into fibers called nerves. In animals exhibiting bilateral symmetry such as (c) planarians, neurons cluster into an anterior brain that processes information. In addition to a brain, (d) arthropods have clusters of nerve cell bodies, called peripheral ganglia, located along the ventral nerve cord. Mollusks such as squid and (e) octopi, which must hunt to survive, have complex brains containing millions of neurons. In (f) vertebrates, the brain and spinal cord comprise the central nervous system, while neurons extending into the rest of the body comprise the peripheral nervous system. (credit e: modification of work by Michael Vecchione, Clyde F.E. Roper, and Michael J. Sweeney, NOAA; credit f: modification of work by NIH)[\/caption]\n\n<div id=\"m44747-fig-ch35_01_01\" class=\"figure\" title=\"Figure\u00a035.2.\u00a0\">\n<div class=\"title\"><\/div>\n<span id=\"m44747-fs-idp196332480\"> <\/span>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 \u201cflipped\u201d during the evolution of vertebrates.\n<div id=\"m44747-fs-idm51063712\" class=\"note interactive\">\n<h2 class=\"title\"><span class=\"cnx-gentext-tip-t\">Concept in Action\n<\/span><\/h2>\n<div class=\"body\">\n<div class=\"mediaobject\"><span id=\"m44747-fs-idp31101408\"> <\/span><img src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/vertebrate_evol.png\" alt=\"QR Code representing a URL\" width=\"120\"><\/div>\n<span id=\"m44747-fs-idp57227664\"> <\/span>Watch <a class=\"link\" href=\"http:\/\/openstaxcollege.org\/l\/vertebrate_evol\" target=\"\" rel=\"noopener noreferrer\">this video<\/a> of biologist Mark Kirschner discussing the \u201cflipping\u201d phenomenon of vertebrate evolution.\n\n<\/div>\n<\/div>\n<span id=\"m44747-fs-idm2940400\"> <\/span>The nervous system is made up of <span id=\"m44747-autoid-cnx2dbk-id1382818\"> <\/span><strong>neurons<\/strong>, specialized cells that can receive and transmit chemical or electrical signals, and <span id=\"m44747-autoid-cnx2dbk-id1382822\"> <\/span><strong>glia<\/strong>, 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\u2014it 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.\n\n<span id=\"m44747-fs-idp225761920\"> <\/span>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.\n<div class=\"section\" title=\"Neurons\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44747-fs-idp237666720\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Neurons<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44747-fs-idp248532976\"> <\/span>The nervous system of the common laboratory fly, <span class=\"emphasis\">Drosophila melanogaster<\/span>, 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\u2014from 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.\n\n<span id=\"m44747-fs-idp80925296\"> <\/span>Most neurons share the same cellular components. But neurons are also highly specialized\u2014different types of neurons have different sizes and shapes that relate to their functional roles.\n<div class=\"section\" title=\"Parts of a Neuron\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44747-fs-idp256040352\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Parts of a Neuron<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44747-fs-idp120784864\"> <\/span>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 <a class=\"xref target-figure\" title=\"Figure\u00a035.3.\u00a0\" href=\"#attachment_1147\">Figure 16.3<\/a> for receiving and sending the electrical signals that make neuronal communication possible. <span id=\"m44747-autoid-cnx2dbk-id1453731\"> <\/span><strong>Dendrites<\/strong> are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called <span id=\"m44747-autoid-cnx2dbk-id1453736\"> <\/span><strong>synapses<\/strong>. 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.\n\n<span id=\"m44747-fs-idm14255728\"> <\/span>Once a signal is received by the dendrite, it then travels passively to the cell body. The cell body contains a specialized structure, the <span id=\"m44747-autoid-cnx2dbk-id1482174\"> <\/span><strong>axon hillock<\/strong> that integrates signals from multiple synapses and serves as a junction between the cell body and an <span id=\"m44747-autoid-cnx2dbk-id1482178\"> <\/span><strong>axon<\/strong>. An axon is a tube-like structure that propagates the integrated signal to specialized endings called <span id=\"m44747-autoid-cnx2dbk-id1482183\"> <\/span><strong>axon terminals<\/strong>. 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 <span id=\"m44747-autoid-cnx2dbk-id1482189\"> <\/span><strong>myelin<\/strong>, 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\u2014from 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 <span id=\"m44747-autoid-cnx2dbk-id1482197\"> <\/span><strong>nodes of Ranvier<\/strong> and are sites where the signal is \u201crecharged\u201d as it travels along the axon.\n\n<span id=\"m44747-fs-idp114206336\"> <\/span>It is important to note that a single neuron does not act alone\u2014neuronal 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.\n<div id=\"m44747-fs-idp48683952\" class=\"note art-connection\">\n<div class=\"title\">\n\n[caption id=\"attachment_1147\" align=\"aligncenter\" width=\"500\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_02.png\"><img class=\"wp-image-4834\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_02-1024x687-1.png\" alt=\"Figure_35_01_02\" width=\"500\" height=\"336\"><\/a> Figure 16.3.\u00a0 Neurons contain organelles common to many other cells, such as a nucleus and mitochondria. They also have more specialized structures, including dendrites and axons.[\/caption]\n\n<\/div>\n<div class=\"body\">\n<div id=\"m44747-fig-ch35_01_02\" class=\"figure\" title=\"Figure\u00a035.3.\u00a0\">\n<div class=\"title\"><\/div>\n<span id=\"m44747-fs-idp61034176\"> <\/span>Which of the following statements is false?\n<div class=\"orderedlist\">\n<ol class=\"orderedlist\">\n \t<li class=\"listitem\">The soma is the cell body of a nerve cell.<\/li>\n \t<li class=\"listitem\">Myelin sheath provides an insulating layer to the dendrites.<\/li>\n \t<li class=\"listitem\">Axons carry the signal from the soma to the target.<\/li>\n \t<li class=\"listitem\">Dendrites carry the signal to the soma.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Types of Neurons\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44747-fs-idp157648880\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Types of Neurons<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44747-fs-idp6233104\"> <\/span>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 16.4.\n\n[caption id=\"attachment_1148\" align=\"aligncenter\" width=\"600\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_03.jpg\"><img class=\"wp-image-4835\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_03-1024x472-1.jpg\" alt=\"Figure_35_01_03\" width=\"600\" height=\"277\"><\/a> Figure 16.4.\u00a0 There is great diversity in the size and shape of neurons throughout the nervous system. Examples include (a) a pyramidal cell from the cerebral cortex, (b) a Purkinje cell from the cerebellar cortex, and (c) olfactory cells from the olfactory epithelium and olfactory bulb.[\/caption]\n\n<span id=\"m44747-fs-idp205596576\"> <\/span>While there are many defined neuron cell subtypes, neurons are broadly divided into four basic types: unipolar, bipolar, multipolar, and pseudounipolar. <a class=\"xref target-figure\" title=\"Figure\u00a035.5.\u00a0\" href=\"#attachment_1149\">Figure 16.5<\/a> 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.\n\n[caption id=\"attachment_1149\" align=\"aligncenter\" width=\"600\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_04.jpg\"><img class=\"wp-image-4836\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_04-1024x550-1.jpg\" alt=\"Figure_35_01_04\" width=\"600\" height=\"322\"><\/a> Figure 16.5.\u00a0 Neurons are broadly divided into four main types based on the number and placement of axons: (1) unipolar, (2) bipolar, (3) multipolar, and (4) pseudounipolar.[\/caption]\n\n<div id=\"m44747-fs-idp128114784\" class=\"note everyday\">\n<div class=\"title\">\n<div class=\"body\">\n<h2 class=\"title\">Neurogenesis<\/h2>\nAt 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\u2019s, stroke, and epilepsy.\n\n<span id=\"m44747-fs-idp144832192\"> <\/span>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 16.6 is a micrograph which shows fluorescently labeled neurons in the hippocampus of a rat.\n\n[caption id=\"attachment_1150\" align=\"aligncenter\" width=\"450\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_05.jpg\"><img class=\"wp-image-4837\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_05.jpg\" alt=\"Figure_35_01_05\" width=\"450\" height=\"357\"><\/a> Figure 16.6.\u00a0 This micrograph shows fluorescently labeled new neurons in a rat hippocampus. Cells that are actively dividing have bromodoxyuridine (BrdU) incorporated into their DNA and are labeled in red. Cells that express glial fibrillary acidic protein (GFAP) are labeled in green. Astrocytes, but not neurons, express GFAP. Thus, cells that are labeled both red and green are actively dividing astrocytes, whereas cells labeled red only are actively dividing neurons. (credit: modification of work by Dr. Maryam Faiz, et. al., University of Barcelona; scale-bar data from Matt Russell)[\/caption]\n\n<div id=\"m44747-fig-ch35_01_05\" class=\"figure\" title=\"Figure\u00a035.6.\u00a0\">\n<div class=\"title\"><\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"m44747-fs-idp195577984\" class=\"note interactive\">\n<h2 class=\"title\"><span class=\"cnx-gentext-tip-t\">Concept in Action\n<\/span><\/h2>\n<div class=\"body\">\n<div class=\"mediaobject\"><span id=\"m44747-fs-idm26856928\"> <\/span><img src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/neurogenesis.png\" alt=\"QR Code representing a URL\" width=\"120\"><\/div>\n<span id=\"m44747-fs-idm104113056\"> <\/span><a class=\"link\" href=\"http:\/\/openstaxcollege.org\/l\/neurogenesis\" target=\"\" rel=\"noopener noreferrer\">This site<\/a> contains more information about neurogenesis, including an interactive laboratory simulation and a video that explains how BrdU labels new cells.\n\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Glia\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44747-fs-idm1127504\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Glia<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44747-fs-idp1149712\"> <\/span>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\u2014most brain tumors are caused by mutations in glia.\n<div class=\"section\" title=\"Types of Glia\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44747-fs-idp181018656\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Types of Glia<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<span id=\"m44747-fs-idm2931648\"> <\/span>There are several different types of glia with different functions, two of which are shown in <a class=\"xref target-figure\" title=\"Figure\u00a035.7.\u00a0\" href=\"#attachment_1152\">Figure 16.7<\/a>. <span id=\"m44747-autoid-cnx2dbk-id1236579\"> <\/span><strong>Astrocytes<\/strong>, shown in <a class=\"xref target-figure\" title=\"Figure\u00a035.8.\u00a0\" href=\"#attachment_1153\">Figure 16.8<\/a><span class=\"bold\"><strong>a<\/strong><\/span> 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\u2014a 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. <span id=\"m44747-autoid-cnx2dbk-id1236599\"> <\/span><strong>Satellite glia <\/strong>provide nutrients and structural support for neurons in the PNS. <span id=\"m44747-autoid-cnx2dbk-id1236602\"> <\/span><strong>Microglia<\/strong> scavenge and degrade dead cells and protect the brain from invading microorganisms. <span id=\"m44747-autoid-cnx2dbk-id1236606\"> <\/span><strong>Oligodendrocytes<\/strong>, shown in <a class=\"xref target-figure\" title=\"Figure\u00a035.8.\u00a0\" href=\"#attachment_1153\">Figure 16.8<\/a><span class=\"bold\"><strong>b<\/strong><\/span> 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 <span id=\"m44747-autoid-cnx2dbk-id1236624\"> <\/span><strong>Schwann cell <\/strong>provides myelin for only one axon as the entire Schwann cell surrounds the axon. <span id=\"m44747-autoid-cnx2dbk-id1236628\"> <\/span><strong>Radial glia <\/strong>serve as scaffolds for developing neurons as they migrate to their end destinations. <span id=\"m44747-autoid-cnx2dbk-id1236632\"> <\/span><strong>Ependymal <\/strong>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.\n\n[caption id=\"attachment_1152\" align=\"aligncenter\" width=\"600\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_06.jpg\"><img class=\"wp-image-4839\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_06-1024x499-1.jpg\" alt=\"Figure\u00a035.7.\u00a0 Glial cells support neurons and maintain their environment. Glial cells of the (a) central nervous system include oligodendrocytes, astrocytes, ependymal cells, and microglial cells. Oligodendrocytes form the myelin sheath around axons. Astrocytes provide nutrients to neurons, maintain their extracellular environment, and provide structural support. Microglia scavenge pathogens and dead cells. Ependymal cells produce cerebrospinal fluid that cushions the neurons. Glial cells of the (b) peripheral nervous system include Schwann cells, which form the myelin sheath, and satellite cells, which provide nutrients and structural support to neurons.\" width=\"600\" height=\"292\"><\/a> Figure 16.7.\u00a0<br>Glial cells support neurons and maintain their environment. Glial cells of the (a) central nervous system include oligodendrocytes, astrocytes, ependymal cells, and microglial cells. Oligodendrocytes form the myelin sheath around axons. Astrocytes provide nutrients to neurons, maintain their extracellular environment, and provide structural support. Microglia scavenge pathogens and dead cells. Ependymal cells produce cerebrospinal fluid that cushions the neurons. Glial cells of the (b) peripheral nervous system include Schwann cells, which form the myelin sheath, and satellite cells, which provide nutrients and structural support to neurons.[\/caption]\n\n<div id=\"m44747-fig-ch35_01_06\" class=\"figure\" title=\"Figure\u00a035.7.\u00a0\">\n<div class=\"body\">\n<div class=\"mediaobject\">\n<div class=\"mediaobject\">\n\n[caption id=\"attachment_1153\" align=\"aligncenter\" width=\"600\"]<a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_07.jpg\"><img class=\"wp-image-4840\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_07-1024x479-1.jpg\" alt=\"Figure\u00a035.8.\u00a0 (a) Astrocytes and (b) oligodendrocytes are glial cells of the central nervous system. (credit a: modification of work by Uniformed Services University; credit b: modification of work by Jurjen Broeke; scale-bar data from Matt Russell)\" width=\"600\" height=\"280\"><\/a> Figure 16.8.\u00a0<br>(a) Astrocytes and (b) oligodendrocytes are glial cells of the central nervous system. (credit a: modification of work by Uniformed Services University; credit b: modification of work by Jurjen Broeke; scale-bar data from Matt Russell)[\/caption]\n\n<div class=\"section module\" title=\"35.5.\u00a0Nervous System Disorders\" xml:lang=\"en\">\n<div class=\"section\" title=\"Other Neurological Disorders\">\n<h2 class=\"section\" title=\"Stroke\">Summary<\/h2>\n<div class=\"section\" title=\"Stroke\">\n<div class=\"section\">\n<div class=\"body\">\n\nThe 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.\n<div class=\"cnx-eoc summary\">\n<div class=\"section\">\n\n&nbsp;\n<div class=\"body\">\n<div id=\"m44747-fs-idp120487536\" class=\"exercise\">\n<div class=\"title\">\n<div class=\"bcc-box bcc-info\">\n<h3>Exercises<\/h3>\n<div class=\"chapter\" title=\"Chapter\u00a035.\u00a0The Nervous System\">\n<div class=\"cnx-gentext-exercise cnx-gentext-autogenerated\">\n<ol>\n \t<li>Which of the following statements is false?\n<ol>\n \t<li>The soma is the cell body of a nerve cell.<\/li>\n \t<li>Myelin sheath provides an insulating layer to the dendrites.<\/li>\n \t<li>Axons carry the signal from the soma to the target.<\/li>\n \t<li>Dendrites carry the signal to the soma.<\/li>\n<\/ol>\n<\/li>\n \t<li>Neurons contain ________, which can receive signals from other neurons.\n<ol>\n \t<li>axons<\/li>\n \t<li>mitochondria<\/li>\n \t<li>dendrites<\/li>\n \t<li>Golgi bodies<\/li>\n<\/ol>\n<\/li>\n \t<li>A(n) ________ neuron has one axon and one dendrite extending directly from the cell body.\n<ol>\n \t<li>unipolar<\/li>\n \t<li>bipolar<\/li>\n \t<li>multipolar<\/li>\n \t<li>pseudounipolar<\/li>\n<\/ol>\n<\/li>\n \t<li>Glia that provide myelin for neurons in the brain are called ________.\n<ol>\n \t<li>Schwann cells<\/li>\n \t<li>oligodendrocytes<\/li>\n \t<li>microglia<\/li>\n \t<li>astrocytes<\/li>\n<\/ol>\n<\/li>\n \t<li>How are neurons similar to other cells? How are they unique?<\/li>\n \t<li>Multiple sclerosis causes demyelination of axons in the brain and spinal cord. Why is this problematic?<\/li>\n<\/ol>\n<strong>Answers<\/strong>\n<ol>\n \t<li>B<\/li>\n \t<li>C<\/li>\n \t<li>B<\/li>\n \t<li>B<\/li>\n \t<li>Neurons contain organelles common to all cells, such as a nucleus and mitochondria. They are unique because they contain dendrites, which can receive signals from other neurons, and axons that can send these signals to other cells.<\/li>\n \t<li>Myelin provides insulation for signals traveling along axons. Without myelin, signal transmission can slow down and degrade over time. This would slow down neuronal communication across the nervous system and affect all downstream functions.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Glossary<\/h3>\n<strong>astrocyte: <\/strong>glial cell in the central nervous system that provide nutrients, extracellular buffering, and structural support for neurons; also makes up the blood-brain barrier\n<strong>axon hillock: <\/strong>electrically sensitive structure on the cell body of a neuron that integrates signals from multiple neuronal connections\n<strong>axon terminal: <\/strong>structure on the end of an axon that can form a synapse with another neuron\n<strong>axon: <\/strong>tube-like structure that propagates a signal from a neuron\u2019s cell body to axon terminals\n<strong>dendrite: <\/strong>structure that extends away from the cell body to receive messages from other neurons\n<strong>ependymal: <\/strong>cell that lines fluid-filled ventricles of the brain and the central canal of the spinal cord; involved in production of\n<strong>glia: <\/strong>(also, glial cells) cells that provide support functions for neurons\n<strong>microglia: <\/strong>glia that scavenge and degrade dead cells and protect the brain from invading microorganisms\n<strong>myelin: <\/strong>fatty substance produced by glia that insulates axons\n<strong>neuron: <\/strong>specialized cell that can receive and transmit electrical and chemical signals\n<strong>nodes of Ranvier: <\/strong>gaps in the myelin sheath where the signal is recharged\n<strong>oligodendrocyte: <\/strong>glial cell that myelinates central nervous system neuron axons\n<strong>radial glia: <\/strong>glia that serve as scaffolds for developing neurons as they migrate to their final destinations\n<strong>Schwann cell: <\/strong>glial cell that creates myelin sheath around a peripheral nervous system neuron axon\n<strong>satellite glia: <\/strong>glial cell that provides nutrients and structural support for neurons in the peripheral nervous system\n<strong>synapse: <\/strong>junction between two neurons where neuronal signals are communicated\n\n<\/div>\n&nbsp;\n<h3><strong>\u00a0<\/strong><\/h3>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>","rendered":"<div class=\"section module\" title=\"35.1.\u00a0Neurons and Glial Cells\" xml:lang=\"en\">\n<div class=\"titlepage\">\n<div class=\"abstract\">\n<div class=\"bcc-box bcc-highlight\">\n<h3>Learning Objectives<\/h3>\n<p>By the end of this section, you will be able to:<\/p>\n<div class=\"itemizedlist\">\n<ul class=\"itemizedlist\">\n<li class=\"listitem\">List and describe the functions of the structural components of a neuron<\/li>\n<li class=\"listitem\">List and describe the four main types of neurons<\/li>\n<li class=\"listitem\">Compare the functions of different types of glial cells<\/li>\n<\/ul>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44747-fs-idm129833856\"> <\/span>Nervous systems throughout the animal kingdom vary in structure and complexity, as illustrated by the variety of animals shown in Figure 16.2. 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 \u201cnerve net.\u201d 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 \u201cbrain\u201d 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\u2014they have neurons that are organized in specialized lobes and eyes that are structurally similar to vertebrate species.<\/p>\n<figure id=\"attachment_1145\" aria-describedby=\"caption-attachment-1145\" style=\"width: 500px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_01.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4832\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/02\/Figure_35_01_01-1024x963-1.jpg\" alt=\"Figure_35_01_01\" width=\"500\" height=\"470\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/02\/Figure_35_01_01-1024x963-1.jpg 1024w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/02\/Figure_35_01_01-1024x963-1-300x282.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/02\/Figure_35_01_01-1024x963-1-768x722.jpg 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/02\/Figure_35_01_01-1024x963-1-65x61.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/02\/Figure_35_01_01-1024x963-1-225x212.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/02\/Figure_35_01_01-1024x963-1-350x329.jpg 350w\" sizes=\"auto, (max-width: 500px) 100vw, 500px\" \/><\/a><figcaption id=\"caption-attachment-1145\" class=\"wp-caption-text\">Figure 16.2.\u00a0 Nervous systems vary in structure and complexity. In (a) cnidarians, nerve cells form a decentralized nerve net. In (b) echinoderms, nerve cells are bundled into fibers called nerves. In animals exhibiting bilateral symmetry such as (c) planarians, neurons cluster into an anterior brain that processes information. In addition to a brain, (d) arthropods have clusters of nerve cell bodies, called peripheral ganglia, located along the ventral nerve cord. Mollusks such as squid and (e) octopi, which must hunt to survive, have complex brains containing millions of neurons. In (f) vertebrates, the brain and spinal cord comprise the central nervous system, while neurons extending into the rest of the body comprise the peripheral nervous system. (credit e: modification of work by Michael Vecchione, Clyde F.E. Roper, and Michael J. Sweeney, NOAA; credit f: modification of work by NIH)<\/figcaption><\/figure>\n<div id=\"m44747-fig-ch35_01_01\" class=\"figure\" title=\"Figure\u00a035.2.\u00a0\">\n<div class=\"title\"><\/div>\n<p><span id=\"m44747-fs-idp196332480\"> <\/span>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 \u201cflipped\u201d during the evolution of vertebrates.<\/p>\n<div id=\"m44747-fs-idm51063712\" class=\"note interactive\">\n<h2 class=\"title\"><span class=\"cnx-gentext-tip-t\">Concept in Action<br \/>\n<\/span><\/h2>\n<div class=\"body\">\n<div class=\"mediaobject\"><span id=\"m44747-fs-idp31101408\"> <\/span><img decoding=\"async\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/vertebrate_evol.png\" alt=\"QR Code representing a URL\" width=\"120\" \/><\/div>\n<p><span id=\"m44747-fs-idp57227664\"> <\/span>Watch <a class=\"link\" href=\"http:\/\/openstaxcollege.org\/l\/vertebrate_evol\" target=\"\" rel=\"noopener noreferrer\">this video<\/a> of biologist Mark Kirschner discussing the \u201cflipping\u201d phenomenon of vertebrate evolution.<\/p>\n<\/div>\n<\/div>\n<p><span id=\"m44747-fs-idm2940400\"> <\/span>The nervous system is made up of <span id=\"m44747-autoid-cnx2dbk-id1382818\"> <\/span><strong>neurons<\/strong>, specialized cells that can receive and transmit chemical or electrical signals, and <span id=\"m44747-autoid-cnx2dbk-id1382822\"> <\/span><strong>glia<\/strong>, 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\u2014it 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.<\/p>\n<p><span id=\"m44747-fs-idp225761920\"> <\/span>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.<\/p>\n<div class=\"section\" title=\"Neurons\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44747-fs-idp237666720\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Neurons<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44747-fs-idp248532976\"> <\/span>The nervous system of the common laboratory fly, <span class=\"emphasis\">Drosophila melanogaster<\/span>, 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\u2014from 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.<\/p>\n<p><span id=\"m44747-fs-idp80925296\"> <\/span>Most neurons share the same cellular components. But neurons are also highly specialized\u2014different types of neurons have different sizes and shapes that relate to their functional roles.<\/p>\n<div class=\"section\" title=\"Parts of a Neuron\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44747-fs-idp256040352\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Parts of a Neuron<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44747-fs-idp120784864\"> <\/span>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 <a class=\"xref target-figure\" title=\"Figure\u00a035.3.\u00a0\" href=\"#attachment_1147\">Figure 16.3<\/a> for receiving and sending the electrical signals that make neuronal communication possible. <span id=\"m44747-autoid-cnx2dbk-id1453731\"> <\/span><strong>Dendrites<\/strong> are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called <span id=\"m44747-autoid-cnx2dbk-id1453736\"> <\/span><strong>synapses<\/strong>. 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.<\/p>\n<p><span id=\"m44747-fs-idm14255728\"> <\/span>Once a signal is received by the dendrite, it then travels passively to the cell body. The cell body contains a specialized structure, the <span id=\"m44747-autoid-cnx2dbk-id1482174\"> <\/span><strong>axon hillock<\/strong> that integrates signals from multiple synapses and serves as a junction between the cell body and an <span id=\"m44747-autoid-cnx2dbk-id1482178\"> <\/span><strong>axon<\/strong>. An axon is a tube-like structure that propagates the integrated signal to specialized endings called <span id=\"m44747-autoid-cnx2dbk-id1482183\"> <\/span><strong>axon terminals<\/strong>. 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 <span id=\"m44747-autoid-cnx2dbk-id1482189\"> <\/span><strong>myelin<\/strong>, 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\u2014from 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 <span id=\"m44747-autoid-cnx2dbk-id1482197\"> <\/span><strong>nodes of Ranvier<\/strong> and are sites where the signal is \u201crecharged\u201d as it travels along the axon.<\/p>\n<p><span id=\"m44747-fs-idp114206336\"> <\/span>It is important to note that a single neuron does not act alone\u2014neuronal 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.<\/p>\n<div id=\"m44747-fs-idp48683952\" class=\"note art-connection\">\n<div class=\"title\">\n<figure id=\"attachment_1147\" aria-describedby=\"caption-attachment-1147\" style=\"width: 500px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_02.png\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4834\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_02-1024x687-1.png\" alt=\"Figure_35_01_02\" width=\"500\" height=\"336\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_02-1024x687-1.png 1024w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_02-1024x687-1-300x201.png 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_02-1024x687-1-768x515.png 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_02-1024x687-1-65x44.png 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_02-1024x687-1-225x151.png 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_02-1024x687-1-350x235.png 350w\" sizes=\"auto, (max-width: 500px) 100vw, 500px\" \/><\/a><figcaption id=\"caption-attachment-1147\" class=\"wp-caption-text\">Figure 16.3.\u00a0 Neurons contain organelles common to many other cells, such as a nucleus and mitochondria. They also have more specialized structures, including dendrites and axons.<\/figcaption><\/figure>\n<\/div>\n<div class=\"body\">\n<div id=\"m44747-fig-ch35_01_02\" class=\"figure\" title=\"Figure\u00a035.3.\u00a0\">\n<div class=\"title\"><\/div>\n<p><span id=\"m44747-fs-idp61034176\"> <\/span>Which of the following statements is false?<\/p>\n<div class=\"orderedlist\">\n<ol class=\"orderedlist\">\n<li class=\"listitem\">The soma is the cell body of a nerve cell.<\/li>\n<li class=\"listitem\">Myelin sheath provides an insulating layer to the dendrites.<\/li>\n<li class=\"listitem\">Axons carry the signal from the soma to the target.<\/li>\n<li class=\"listitem\">Dendrites carry the signal to the soma.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Types of Neurons\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44747-fs-idp157648880\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Types of Neurons<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44747-fs-idp6233104\"> <\/span>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 16.4.<\/p>\n<figure id=\"attachment_1148\" aria-describedby=\"caption-attachment-1148\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_03.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4835\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_03-1024x472-1.jpg\" alt=\"Figure_35_01_03\" width=\"600\" height=\"277\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_03-1024x472-1.jpg 1024w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_03-1024x472-1-300x138.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_03-1024x472-1-768x354.jpg 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_03-1024x472-1-65x30.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_03-1024x472-1-225x104.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_03-1024x472-1-350x161.jpg 350w\" sizes=\"auto, (max-width: 600px) 100vw, 600px\" \/><\/a><figcaption id=\"caption-attachment-1148\" class=\"wp-caption-text\">Figure 16.4.\u00a0 There is great diversity in the size and shape of neurons throughout the nervous system. Examples include (a) a pyramidal cell from the cerebral cortex, (b) a Purkinje cell from the cerebellar cortex, and (c) olfactory cells from the olfactory epithelium and olfactory bulb.<\/figcaption><\/figure>\n<p><span id=\"m44747-fs-idp205596576\"> <\/span>While there are many defined neuron cell subtypes, neurons are broadly divided into four basic types: unipolar, bipolar, multipolar, and pseudounipolar. <a class=\"xref target-figure\" title=\"Figure\u00a035.5.\u00a0\" href=\"#attachment_1149\">Figure 16.5<\/a> 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.<\/p>\n<figure id=\"attachment_1149\" aria-describedby=\"caption-attachment-1149\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_04.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4836\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_04-1024x550-1.jpg\" alt=\"Figure_35_01_04\" width=\"600\" height=\"322\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_04-1024x550-1.jpg 1024w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_04-1024x550-1-300x161.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_04-1024x550-1-768x413.jpg 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_04-1024x550-1-65x35.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_04-1024x550-1-225x121.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_04-1024x550-1-350x188.jpg 350w\" sizes=\"auto, (max-width: 600px) 100vw, 600px\" \/><\/a><figcaption id=\"caption-attachment-1149\" class=\"wp-caption-text\">Figure 16.5.\u00a0 Neurons are broadly divided into four main types based on the number and placement of axons: (1) unipolar, (2) bipolar, (3) multipolar, and (4) pseudounipolar.<\/figcaption><\/figure>\n<div id=\"m44747-fs-idp128114784\" class=\"note everyday\">\n<div class=\"title\">\n<div class=\"body\">\n<h2 class=\"title\">Neurogenesis<\/h2>\n<p>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\u2019s, stroke, and epilepsy.<\/p>\n<p><span id=\"m44747-fs-idp144832192\"> <\/span>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 16.6 is a micrograph which shows fluorescently labeled neurons in the hippocampus of a rat.<\/p>\n<figure id=\"attachment_1150\" aria-describedby=\"caption-attachment-1150\" style=\"width: 450px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_05.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4837\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_05.jpg\" alt=\"Figure_35_01_05\" width=\"450\" height=\"357\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_05.jpg 544w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_05-300x238.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_05-65x51.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_05-225x178.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_05-350x277.jpg 350w\" sizes=\"auto, (max-width: 450px) 100vw, 450px\" \/><\/a><figcaption id=\"caption-attachment-1150\" class=\"wp-caption-text\">Figure 16.6.\u00a0 This micrograph shows fluorescently labeled new neurons in a rat hippocampus. Cells that are actively dividing have bromodoxyuridine (BrdU) incorporated into their DNA and are labeled in red. Cells that express glial fibrillary acidic protein (GFAP) are labeled in green. Astrocytes, but not neurons, express GFAP. Thus, cells that are labeled both red and green are actively dividing astrocytes, whereas cells labeled red only are actively dividing neurons. (credit: modification of work by Dr. Maryam Faiz, et. al., University of Barcelona; scale-bar data from Matt Russell)<\/figcaption><\/figure>\n<div id=\"m44747-fig-ch35_01_05\" class=\"figure\" title=\"Figure\u00a035.6.\u00a0\">\n<div class=\"title\"><\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div id=\"m44747-fs-idp195577984\" class=\"note interactive\">\n<h2 class=\"title\"><span class=\"cnx-gentext-tip-t\">Concept in Action<br \/>\n<\/span><\/h2>\n<div class=\"body\">\n<div class=\"mediaobject\"><span id=\"m44747-fs-idm26856928\"> <\/span><img decoding=\"async\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/neurogenesis.png\" alt=\"QR Code representing a URL\" width=\"120\" \/><\/div>\n<p><span id=\"m44747-fs-idm104113056\"> <\/span><a class=\"link\" href=\"http:\/\/openstaxcollege.org\/l\/neurogenesis\" target=\"\" rel=\"noopener noreferrer\">This site<\/a> contains more information about neurogenesis, including an interactive laboratory simulation and a video that explains how BrdU labels new cells.<\/p>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"section\" title=\"Glia\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44747-fs-idm1127504\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Glia<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44747-fs-idp1149712\"> <\/span>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\u2014most brain tumors are caused by mutations in glia.<\/p>\n<div class=\"section\" title=\"Types of Glia\">\n<div class=\"titlepage\">\n<div>\n<div>\n<h2 id=\"m44747-fs-idp181018656\"><span class=\"cnx-gentext-section cnx-gentext-autogenerated\"><span class=\"cnx-gentext-section cnx-gentext-t\">Types of Glia<\/span><\/span><\/h2>\n<\/div>\n<\/div>\n<\/div>\n<p><span id=\"m44747-fs-idm2931648\"> <\/span>There are several different types of glia with different functions, two of which are shown in <a class=\"xref target-figure\" title=\"Figure\u00a035.7.\u00a0\" href=\"#attachment_1152\">Figure 16.7<\/a>. <span id=\"m44747-autoid-cnx2dbk-id1236579\"> <\/span><strong>Astrocytes<\/strong>, shown in <a class=\"xref target-figure\" title=\"Figure\u00a035.8.\u00a0\" href=\"#attachment_1153\">Figure 16.8<\/a><span class=\"bold\"><strong>a<\/strong><\/span> 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\u2014a 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. <span id=\"m44747-autoid-cnx2dbk-id1236599\"> <\/span><strong>Satellite glia <\/strong>provide nutrients and structural support for neurons in the PNS. <span id=\"m44747-autoid-cnx2dbk-id1236602\"> <\/span><strong>Microglia<\/strong> scavenge and degrade dead cells and protect the brain from invading microorganisms. <span id=\"m44747-autoid-cnx2dbk-id1236606\"> <\/span><strong>Oligodendrocytes<\/strong>, shown in <a class=\"xref target-figure\" title=\"Figure\u00a035.8.\u00a0\" href=\"#attachment_1153\">Figure 16.8<\/a><span class=\"bold\"><strong>b<\/strong><\/span> 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 <span id=\"m44747-autoid-cnx2dbk-id1236624\"> <\/span><strong>Schwann cell <\/strong>provides myelin for only one axon as the entire Schwann cell surrounds the axon. <span id=\"m44747-autoid-cnx2dbk-id1236628\"> <\/span><strong>Radial glia <\/strong>serve as scaffolds for developing neurons as they migrate to their end destinations. <span id=\"m44747-autoid-cnx2dbk-id1236632\"> <\/span><strong>Ependymal <\/strong>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.<\/p>\n<figure id=\"attachment_1152\" aria-describedby=\"caption-attachment-1152\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_06.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4839\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_06-1024x499-1.jpg\" alt=\"Figure\u00a035.7.\u00a0 Glial cells support neurons and maintain their environment. Glial cells of the (a) central nervous system include oligodendrocytes, astrocytes, ependymal cells, and microglial cells. Oligodendrocytes form the myelin sheath around axons. Astrocytes provide nutrients to neurons, maintain their extracellular environment, and provide structural support. Microglia scavenge pathogens and dead cells. Ependymal cells produce cerebrospinal fluid that cushions the neurons. Glial cells of the (b) peripheral nervous system include Schwann cells, which form the myelin sheath, and satellite cells, which provide nutrients and structural support to neurons.\" width=\"600\" height=\"292\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_06-1024x499-1.jpg 1024w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_06-1024x499-1-300x146.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_06-1024x499-1-768x374.jpg 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_06-1024x499-1-65x32.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_06-1024x499-1-225x110.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_06-1024x499-1-350x171.jpg 350w\" sizes=\"auto, (max-width: 600px) 100vw, 600px\" \/><\/a><figcaption id=\"caption-attachment-1152\" class=\"wp-caption-text\">Figure 16.7.\u00a0<br \/>Glial cells support neurons and maintain their environment. Glial cells of the (a) central nervous system include oligodendrocytes, astrocytes, ependymal cells, and microglial cells. Oligodendrocytes form the myelin sheath around axons. Astrocytes provide nutrients to neurons, maintain their extracellular environment, and provide structural support. Microglia scavenge pathogens and dead cells. Ependymal cells produce cerebrospinal fluid that cushions the neurons. Glial cells of the (b) peripheral nervous system include Schwann cells, which form the myelin sheath, and satellite cells, which provide nutrients and structural support to neurons.<\/figcaption><\/figure>\n<div id=\"m44747-fig-ch35_01_06\" class=\"figure\" title=\"Figure\u00a035.7.\u00a0\">\n<div class=\"body\">\n<div class=\"mediaobject\">\n<div class=\"mediaobject\">\n<figure id=\"attachment_1153\" aria-describedby=\"caption-attachment-1153\" style=\"width: 600px\" class=\"wp-caption aligncenter\"><a href=\"http:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2015\/03\/Figure_35_01_07.jpg\"><img loading=\"lazy\" decoding=\"async\" class=\"wp-image-4840\" src=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_07-1024x479-1.jpg\" alt=\"Figure\u00a035.8.\u00a0 (a) Astrocytes and (b) oligodendrocytes are glial cells of the central nervous system. (credit a: modification of work by Uniformed Services University; credit b: modification of work by Jurjen Broeke; scale-bar data from Matt Russell)\" width=\"600\" height=\"280\" srcset=\"https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_07-1024x479-1.jpg 1024w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_07-1024x479-1-300x140.jpg 300w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_07-1024x479-1-768x359.jpg 768w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_07-1024x479-1-65x30.jpg 65w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_07-1024x479-1-225x105.jpg 225w, https:\/\/opentextbc.ca\/biology\/wp-content\/uploads\/sites\/96\/2021\/03\/Figure_35_01_07-1024x479-1-350x164.jpg 350w\" sizes=\"auto, (max-width: 600px) 100vw, 600px\" \/><\/a><figcaption id=\"caption-attachment-1153\" class=\"wp-caption-text\">Figure 16.8.\u00a0<br \/>(a) Astrocytes and (b) oligodendrocytes are glial cells of the central nervous system. (credit a: modification of work by Uniformed Services University; credit b: modification of work by Jurjen Broeke; scale-bar data from Matt Russell)<\/figcaption><\/figure>\n<div class=\"section module\" title=\"35.5.\u00a0Nervous System Disorders\" xml:lang=\"en\">\n<div class=\"section\" title=\"Other Neurological Disorders\">\n<h2 class=\"section\" title=\"Stroke\">Summary<\/h2>\n<div class=\"section\" title=\"Stroke\">\n<div class=\"section\">\n<div class=\"body\">\n<p>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.<\/p>\n<div class=\"cnx-eoc summary\">\n<div class=\"section\">\n<p>&nbsp;<\/p>\n<div class=\"body\">\n<div id=\"m44747-fs-idp120487536\" class=\"exercise\">\n<div class=\"title\">\n<div class=\"bcc-box bcc-info\">\n<h3>Exercises<\/h3>\n<div class=\"chapter\" title=\"Chapter\u00a035.\u00a0The Nervous System\">\n<div class=\"cnx-gentext-exercise cnx-gentext-autogenerated\">\n<ol>\n<li>Which of the following statements is false?\n<ol>\n<li>The soma is the cell body of a nerve cell.<\/li>\n<li>Myelin sheath provides an insulating layer to the dendrites.<\/li>\n<li>Axons carry the signal from the soma to the target.<\/li>\n<li>Dendrites carry the signal to the soma.<\/li>\n<\/ol>\n<\/li>\n<li>Neurons contain ________, which can receive signals from other neurons.\n<ol>\n<li>axons<\/li>\n<li>mitochondria<\/li>\n<li>dendrites<\/li>\n<li>Golgi bodies<\/li>\n<\/ol>\n<\/li>\n<li>A(n) ________ neuron has one axon and one dendrite extending directly from the cell body.\n<ol>\n<li>unipolar<\/li>\n<li>bipolar<\/li>\n<li>multipolar<\/li>\n<li>pseudounipolar<\/li>\n<\/ol>\n<\/li>\n<li>Glia that provide myelin for neurons in the brain are called ________.\n<ol>\n<li>Schwann cells<\/li>\n<li>oligodendrocytes<\/li>\n<li>microglia<\/li>\n<li>astrocytes<\/li>\n<\/ol>\n<\/li>\n<li>How are neurons similar to other cells? How are they unique?<\/li>\n<li>Multiple sclerosis causes demyelination of axons in the brain and spinal cord. Why is this problematic?<\/li>\n<\/ol>\n<p><strong>Answers<\/strong><\/p>\n<ol>\n<li>B<\/li>\n<li>C<\/li>\n<li>B<\/li>\n<li>B<\/li>\n<li>Neurons contain organelles common to all cells, such as a nucleus and mitochondria. They are unique because they contain dendrites, which can receive signals from other neurons, and axons that can send these signals to other cells.<\/li>\n<li>Myelin provides insulation for signals traveling along axons. Without myelin, signal transmission can slow down and degrade over time. This would slow down neuronal communication across the nervous system and affect all downstream functions.<\/li>\n<\/ol>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<div class=\"textbox key-takeaways\">\n<h3>Glossary<\/h3>\n<p><strong>astrocyte: <\/strong>glial cell in the central nervous system that provide nutrients, extracellular buffering, and structural support for neurons; also makes up the blood-brain barrier<br \/>\n<strong>axon hillock: <\/strong>electrically sensitive structure on the cell body of a neuron that integrates signals from multiple neuronal connections<br \/>\n<strong>axon terminal: <\/strong>structure on the end of an axon that can form a synapse with another neuron<br \/>\n<strong>axon: <\/strong>tube-like structure that propagates a signal from a neuron\u2019s cell body to axon terminals<br \/>\n<strong>dendrite: <\/strong>structure that extends away from the cell body to receive messages from other neurons<br \/>\n<strong>ependymal: <\/strong>cell that lines fluid-filled ventricles of the brain and the central canal of the spinal cord; involved in production of<br \/>\n<strong>glia: <\/strong>(also, glial cells) cells that provide support functions for neurons<br \/>\n<strong>microglia: <\/strong>glia that scavenge and degrade dead cells and protect the brain from invading microorganisms<br \/>\n<strong>myelin: <\/strong>fatty substance produced by glia that insulates axons<br \/>\n<strong>neuron: <\/strong>specialized cell that can receive and transmit electrical and chemical signals<br \/>\n<strong>nodes of Ranvier: <\/strong>gaps in the myelin sheath where the signal is recharged<br \/>\n<strong>oligodendrocyte: <\/strong>glial cell that myelinates central nervous system neuron axons<br \/>\n<strong>radial glia: <\/strong>glia that serve as scaffolds for developing neurons as they migrate to their final destinations<br \/>\n<strong>Schwann cell: <\/strong>glial cell that creates myelin sheath around a peripheral nervous system neuron axon<br \/>\n<strong>satellite glia: <\/strong>glial cell that provides nutrients and structural support for neurons in the peripheral nervous system<br \/>\n<strong>synapse: <\/strong>junction between two neurons where neuronal signals are communicated<\/p>\n<\/div>\n<p>&nbsp;<\/p>\n<h3><strong>\u00a0<\/strong><\/h3>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n<\/div>\n","protected":false},"author":90,"menu_order":27,"template":"","meta":{"pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":"cc-by"},"chapter-type":[],"contributor":[],"license":[57],"class_list":["post-4841","chapter","type-chapter","status-publish","hentry","license-cc-by"],"part":4830,"_links":{"self":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters\/4841","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/users\/90"}],"version-history":[{"count":1,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters\/4841\/revisions"}],"predecessor-version":[{"id":4842,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters\/4841\/revisions\/4842"}],"part":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/parts\/4830"}],"metadata":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapters\/4841\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/media?parent=4841"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/pressbooks\/v2\/chapter-type?post=4841"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/contributor?post=4841"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/opentextbc.ca\/biology\/wp-json\/wp\/v2\/license?post=4841"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}