{"id":3969,"date":"2017-03-27T17:51:37","date_gmt":"2017-03-27T17:51:37","guid":{"rendered":"https:\/\/courses.lumenlearning.com\/wm-biology2\/?post_type=chapter&#038;p=3969"},"modified":"2024-04-26T01:09:02","modified_gmt":"2024-04-26T01:09:02","slug":"neurons","status":"publish","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/wm-biology2\/chapter\/neurons\/","title":{"raw":"Neurons","rendered":"Neurons"},"content":{"raw":"<div class=\"textbox learning-objectives\">\r\n<h3>Learning Outcomes<\/h3>\r\n<ul>\r\n \t<li>List and describe the functions of the structural components of a neuron<\/li>\r\n \t<li>List and describe the four main types of neurons<\/li>\r\n<\/ul>\r\n<\/div>\r\nThe nervous system of the common laboratory fly, <em>Drosophila melanogaster<\/em>, 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.\r\n\r\nMost 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.\r\n<h2>Parts of a Neuron<\/h2>\r\nLike 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 1\u00a0for receiving and sending the electrical signals that make neuronal communication possible. <strong>Dendrites<\/strong> are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called <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.\r\n\r\n[caption id=\"attachment_2653\" align=\"aligncenter\" width=\"1024\"]<img class=\"size-large wp-image-2653\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/07195441\/Figure_35_01_02-1024x687.png\" alt=\"Illustration shows a neuron. The main part of the cell body, called the soma, contains the nucleus. Branch-like dendrites project from three sides of the soma. A long, thin axon projects from the fourth side. The axon branches at the end. The tip of the axon is in close proximity to dendrites of an adjacent nerve cell. The narrow space between the axon and dendrites is called the synapse. Cells called oligodendrocytes are located next to the axon. Projections from the oligodendrocytes wrap around the axon, forming a myelin sheath. The myelin sheath is not continuous, and gaps where the axon is exposed are called nodes of Ranvier.\" width=\"1024\" height=\"687\" \/> Figure 1.\u00a0Neurons contain organelles common to many other cells, such as a nucleus and mitochondria. They also have more specialized structures, including dendrites and axons.[\/caption]\r\n\r\nOnce a signal is received by the dendrite, it then travels passively to the cell body. The cell body contains a specialized structure, the\u00a0<strong>axon hillock<\/strong> that integrates signals from multiple synapses and serves as a junction between the cell body and an <strong>axon<\/strong>. An axon is a tube-like structure that propagates the integrated signal to specialized endings called <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 <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 <strong>nodes of Ranvier<\/strong> and are sites where the signal is \u201crecharged\u201d as it travels along the axon.\r\n\r\nIt 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.\r\n<div class=\"textbox exercises\">\r\n<h3>Practice Question<\/h3>\r\nWhich of the following statements is false?\r\n<ol style=\"list-style-type: lower-alpha;\">\r\n \t<li>The soma is the cell body of a nerve cell.<\/li>\r\n \t<li>Myelin sheath provides an insulating layer to the dendrites.<\/li>\r\n \t<li>Axons carry the signal from the soma to the target.<\/li>\r\n \t<li>Dendrites carry the signal to the soma.<\/li>\r\n<\/ol>\r\n[reveal-answer q=\"962754\"]Show Answer[\/reveal-answer]\r\n[hidden-answer a=\"962754\"]Statement b is false.[\/hidden-answer]\r\n\r\n<\/div>\r\n<h2>Types of Neurons<\/h2>\r\nThere 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 2.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"651\"]<img id=\"7\" class=\"\" src=\"https:\/\/openstax.org\/resources\/36e75d3e77c64e083a6b8cf306e9a9f3c66054c8\" alt=\"Part A shows a pyramidal cell with two long, branched projections on either end of the soma. Dendrites branch from either side. Part B shows a Purkinje cell with highly branched dendrites opposite the axon. Part C shows cells with long, thin axons. The dendrites are less branched than in pyramidal or Purkinje cells.\" width=\"651\" height=\"314\" data-media-type=\"image\/png\" \/> Figure 2. 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]\r\n\r\nWhile there are many defined neuron cell subtypes, neurons are broadly divided into four basic types: unipolar, bipolar, multipolar, and pseudounipolar. Figure 3\u00a0illustrates 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.\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"599\"]<img id=\"8\" class=\"\" src=\"https:\/\/openstax.org\/resources\/61b4d087adb9b7a2c1298f5f2e7b6263fb852901\" alt=\"The unipolar cell has a single, long axon extending from the cell body. The bipolar neuron has two axons projecting from opposite sides of the cell body. The multipolar neuron has one long axon and several short, highly branched axons extending in all directions. The pseudounipolar neuron has one axon that forms two branches a short distance from the cell body, each of which extends in a different direction.\" width=\"599\" height=\"335\" data-media-type=\"image\/png\" \/> Figure 3. 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]\r\n\r\n<div class=\"textbox tryit\">\r\n<h3>Try It<\/h3>\r\nhttps:\/\/assess.lumenlearning.com\/practice\/98db62af-c7a4-4005-8859-ab3d70001b5a\r\nhttps:\/\/assess.lumenlearning.com\/practice\/7941520d-5911-4af5-83ac-5dfb799aabab\r\n<\/div>","rendered":"<div class=\"textbox learning-objectives\">\n<h3>Learning Outcomes<\/h3>\n<ul>\n<li>List and describe the functions of the structural components of a neuron<\/li>\n<li>List and describe the four main types of neurons<\/li>\n<\/ul>\n<\/div>\n<p>The nervous system of the common laboratory fly, <em>Drosophila melanogaster<\/em>, 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>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<h2>Parts of a Neuron<\/h2>\n<p>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 1\u00a0for receiving and sending the electrical signals that make neuronal communication possible. <strong>Dendrites<\/strong> are tree-like structures that extend away from the cell body to receive messages from other neurons at specialized junctions called <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<div id=\"attachment_2653\" style=\"width: 1034px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" aria-describedby=\"caption-attachment-2653\" class=\"size-large wp-image-2653\" src=\"https:\/\/s3-us-west-2.amazonaws.com\/courses-images\/wp-content\/uploads\/sites\/1223\/2017\/02\/07195441\/Figure_35_01_02-1024x687.png\" alt=\"Illustration shows a neuron. The main part of the cell body, called the soma, contains the nucleus. Branch-like dendrites project from three sides of the soma. A long, thin axon projects from the fourth side. The axon branches at the end. The tip of the axon is in close proximity to dendrites of an adjacent nerve cell. The narrow space between the axon and dendrites is called the synapse. Cells called oligodendrocytes are located next to the axon. Projections from the oligodendrocytes wrap around the axon, forming a myelin sheath. The myelin sheath is not continuous, and gaps where the axon is exposed are called nodes of Ranvier.\" width=\"1024\" height=\"687\" \/><\/p>\n<p id=\"caption-attachment-2653\" class=\"wp-caption-text\">Figure 1.\u00a0Neurons contain organelles common to many other cells, such as a nucleus and mitochondria. They also have more specialized structures, including dendrites and axons.<\/p>\n<\/div>\n<p>Once a signal is received by the dendrite, it then travels passively to the cell body. The cell body contains a specialized structure, the\u00a0<strong>axon hillock<\/strong> that integrates signals from multiple synapses and serves as a junction between the cell body and an <strong>axon<\/strong>. An axon is a tube-like structure that propagates the integrated signal to specialized endings called <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 <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 <strong>nodes of Ranvier<\/strong> and are sites where the signal is \u201crecharged\u201d as it travels along the axon.<\/p>\n<p>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 class=\"textbox exercises\">\n<h3>Practice Question<\/h3>\n<p>Which of the following statements is false?<\/p>\n<ol style=\"list-style-type: lower-alpha;\">\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<div class=\"qa-wrapper\" style=\"display: block\"><span class=\"show-answer collapsed\" style=\"cursor: pointer\" data-target=\"q962754\">Show Answer<\/span><\/p>\n<div id=\"q962754\" class=\"hidden-answer\" style=\"display: none\">Statement b is false.<\/div>\n<\/div>\n<\/div>\n<h2>Types of Neurons<\/h2>\n<p>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 2.<\/p>\n<div style=\"width: 661px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"7\" class=\"\" src=\"https:\/\/openstax.org\/resources\/36e75d3e77c64e083a6b8cf306e9a9f3c66054c8\" alt=\"Part A shows a pyramidal cell with two long, branched projections on either end of the soma. Dendrites branch from either side. Part B shows a Purkinje cell with highly branched dendrites opposite the axon. Part C shows cells with long, thin axons. The dendrites are less branched than in pyramidal or Purkinje cells.\" width=\"651\" height=\"314\" data-media-type=\"image\/png\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 2. 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.<\/p>\n<\/div>\n<p>While there are many defined neuron cell subtypes, neurons are broadly divided into four basic types: unipolar, bipolar, multipolar, and pseudounipolar. Figure 3\u00a0illustrates 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<div style=\"width: 609px\" class=\"wp-caption aligncenter\"><img loading=\"lazy\" decoding=\"async\" id=\"8\" class=\"\" src=\"https:\/\/openstax.org\/resources\/61b4d087adb9b7a2c1298f5f2e7b6263fb852901\" alt=\"The unipolar cell has a single, long axon extending from the cell body. The bipolar neuron has two axons projecting from opposite sides of the cell body. The multipolar neuron has one long axon and several short, highly branched axons extending in all directions. The pseudounipolar neuron has one axon that forms two branches a short distance from the cell body, each of which extends in a different direction.\" width=\"599\" height=\"335\" data-media-type=\"image\/png\" \/><\/p>\n<p class=\"wp-caption-text\">Figure 3. 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.<\/p>\n<\/div>\n<div class=\"textbox tryit\">\n<h3>Try It<\/h3>\n<p>\t<iframe id=\"assessment_practice_98db62af-c7a4-4005-8859-ab3d70001b5a\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/98db62af-c7a4-4005-8859-ab3d70001b5a?iframe_resize_id=assessment_practice_id_98db62af-c7a4-4005-8859-ab3d70001b5a\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe><br \/>\n\t<iframe id=\"assessment_practice_7941520d-5911-4af5-83ac-5dfb799aabab\" class=\"resizable\" src=\"https:\/\/assess.lumenlearning.com\/practice\/7941520d-5911-4af5-83ac-5dfb799aabab?iframe_resize_id=assessment_practice_id_7941520d-5911-4af5-83ac-5dfb799aabab\" frameborder=\"0\" style=\"border:none;width:100%;height:100%;min-height:300px;\"><br \/>\n\t<\/iframe>\n<\/div>\n\n\t\t\t <section class=\"citations-section\" role=\"contentinfo\">\n\t\t\t <h3>Candela Citations<\/h3>\n\t\t\t\t\t <div>\n\t\t\t\t\t\t <div id=\"citation-list-3969\">\n\t\t\t\t\t\t\t <div class=\"licensing\"><div class=\"license-attribution-dropdown-subheading\">CC licensed content, Shared previously<\/div><ul class=\"citation-list\"><li>Biology 2e. <strong>Provided by<\/strong>: OpenStax. <strong>Located at<\/strong>: <a target=\"_blank\" href=\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\">http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8<\/a>. <strong>License<\/strong>: <em><a target=\"_blank\" rel=\"license\" href=\"https:\/\/creativecommons.org\/licenses\/by\/4.0\/\">CC BY: Attribution<\/a><\/em>. <strong>License Terms<\/strong>: Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction<\/li><\/ul><\/div>\n\t\t\t\t\t\t <\/div>\n\t\t\t\t\t <\/div>\n\t\t\t <\/section>","protected":false},"author":17,"menu_order":5,"template":"","meta":{"_candela_citation":"[{\"type\":\"cc\",\"description\":\"Biology 2e\",\"author\":\"\",\"organization\":\"OpenStax\",\"url\":\"http:\/\/cnx.org\/contents\/185cbf87-c72e-48f5-b51e-f14f21b5eabd@10.8\",\"project\":\"\",\"license\":\"cc-by\",\"license_terms\":\"Access for free at https:\/\/openstax.org\/books\/biology-2e\/pages\/1-introduction\"}]","CANDELA_OUTCOMES_GUID":"c562b4f3-047a-4686-8cf8-7fe086f3b1ac, e4512597-6c62-497f-91ff-4e8c7b83ccbc, ebda304a-f503-4867-acaf-c3587769f8cb","pb_show_title":"on","pb_short_title":"","pb_subtitle":"","pb_authors":[],"pb_section_license":""},"chapter-type":[],"contributor":[],"license":[],"class_list":["post-3969","chapter","type-chapter","status-publish","hentry"],"part":3966,"_links":{"self":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/3969","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters"}],"about":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/types\/chapter"}],"author":[{"embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/users\/17"}],"version-history":[{"count":10,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/3969\/revisions"}],"predecessor-version":[{"id":8504,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/3969\/revisions\/8504"}],"part":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/parts\/3966"}],"metadata":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapters\/3969\/metadata\/"}],"wp:attachment":[{"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/media?parent=3969"}],"wp:term":[{"taxonomy":"chapter-type","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/pressbooks\/v2\/chapter-type?post=3969"},{"taxonomy":"contributor","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/contributor?post=3969"},{"taxonomy":"license","embeddable":true,"href":"https:\/\/courses.lumenlearning.com\/wm-biology2\/wp-json\/wp\/v2\/license?post=3969"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}