{"id":2174,"date":"2014-10-30T20:54:33","date_gmt":"2014-10-30T20:54:33","guid":{"rendered":"https:\/\/courses.candelalearning.com\/apvccs\/?post_type=chapter&p=2174"},"modified":"2019-08-11T20:16:44","modified_gmt":"2019-08-11T20:16:44","slug":"nervous-tissue-2","status":"web-only","type":"chapter","link":"https:\/\/courses.lumenlearning.com\/suny-dutchess-anatomy-physiology\/chapter\/nervous-tissue-2\/","title":{"raw":"Nervous Tissue","rendered":"Nervous Tissue"},"content":{"raw":"
\r\n\r\n \r\n\r\n<\/div>\r\nNervous tissue is composed of two types of cells, neurons and glial cells. Neurons<\/strong> are the primary type of cell that most anyone associates with the nervous system. They are responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to target cells. Glial cells<\/strong>, or glia, are known to play a supporting role for nervous tissue. Ongoing research pursues an expanded role that glial cells might play in signaling, but neurons are still considered the basis of this function. Neurons are important, but without glial support they would not be able to perform their function.\r\n

Neurons<\/h2>\r\nNeurons are the cells considered to be the basis of nervous tissue. They are responsible for the electrical signals that communicate information about sensations, and that produce movements in response to those stimuli, along with inducing thought processes within the brain. An important part of the function of neurons is in their structure, or shape. The three-dimensional shape of these cells makes the immense numbers of connections within the nervous system possible.\r\n

Parts of a Neuron<\/h3>\r\nAs you learned in the first section, the main part of a neuron is the cell body<\/strong>, which is also known as the soma (soma = \u201cbody\u201d). The cell body contains the nucleus and most of the major organelles. But what makes neurons special is that they have many extensions of their cell membranes, which are generally referred to as processes. Neurons are usually described as having one, and only one, axon<\/strong>\u2014an elongated projection that emerges from the cell body and extends to target cells. That single axon can branch repeatedly to communicate with many target cells. It is the axon that propagates the nerve impulse, which is communicated to one or more cells. The other processes of the neuron are dendrites, which receive information from other neurons at specialized areas of contact called\u00a0synapses<\/strong>. The dendrites are usually highly branched processes, providing locations for other neurons to communicate with the cell body. Information flows through a neuron from the dendrites, across the cell body, and down the axon. This gives the neuron a polarity\u2014meaning that information flows in this one direction.\u00a0Figure 1\u00a0shows the relationship of these parts to one another.\r\n\r\nWhere the axon emerges from the cell body, there is a special region referred to as the\u00a0axon hillock<\/strong>. This is a tapering of the cell body toward the axon fiber. Within the axon hillock, the cytoplasm changes to a solution of limited components called\u00a0axoplasm<\/strong>. The beginning of the axon on the axon hillock is referred to as the\u00a0initial segment<\/strong>, which often contains the trigger zone<\/strong>.\u00a0 In the trigger zone, a sufficiently strong voltage stimulus will start an action potential.\r\n\r\n[caption id=\"attachment_4360\" align=\"alignnone\" width=\"1131\"]\"This<\/a> Figure 1. Parts of a Neuron<\/strong> The major parts of the neuron are labeled on a multipolar neuron from the CNS.[\/caption]\r\n\r\nMany axons are wrapped by an insulating substance called myelin, which is actually made from glial cells. Myelin acts as insulation much like the plastic or rubber that is used to insulate electrical wires. A key difference between myelin and the insulation on a wire is that there are gaps in the myelin covering of an axon. Each gap is called a\u00a0node of Ranvier<\/strong>\u00a0and is important to the way that electrical signals travel down the axon.\u00a0 The end of the axon splits into branches called collaterals\u00a0<\/strong>which are branches extending toward the target cell, each of which ends in an enlargement called a\u00a0axon\u00a0<\/strong>terminal<\/b>. The axon terminals frequently form an enlarged bulge at the synapse called the synaptic end bulb<\/strong>, which contains chemical messengers called neurotransmitters <\/strong>that\u00a0are used to contact the target cell.\r\n
Visit this\u00a0site\u00a0to learn about how nervous tissue is composed of neurons and glial cells.<\/a> Neurons are dynamic cells with the ability to make a vast number of connections, to respond incredibly quickly to stimuli, and to initiate movements on the basis of those stimuli. They are the focus of intense research because failures in physiology can lead to devastating illnesses. Why are neurons only found in animals? Based on what this article says about neuron function, why wouldn't they be helpful for plants or microorganisms?<\/div>\r\n

Structural Types of Neurons<\/h3>\r\nThere are many neurons in the nervous system\u2014a number in the trillions. And there are many different types of neurons. They can be classified by many different criteria. The first way to classify them is by the number of processes attached to the cell body. Using the standard model of neurons, one of these processes is the axon, and the rest are dendrites. Because information flows through the neuron from dendrites or cell bodies toward the axon, these names are based on the neuron's polarity (Figure 2).\r\n\r\n[caption id=\"attachment_3675\" align=\"aligncenter\" width=\"1024\"]\"Three Figure 2.\u00a0Neuron Classification by Shape.<\/strong> Unipolar cells have one process that includes both the axon and dendrite. Bipolar cells have two processes, the axon and a dendrite. Multipolar cells have more than two processes, the axon and two or more dendrites.[\/caption]\r\n

Unipolar<\/h4>\r\nUnipolar<\/strong> neurons have only one process emerging from the cell. True unipolar neurons are only found in invertebrate animals, so the unipolar neurons in humans are more appropriately called \u201cpseudo-unipolar\u201d neurons. Invertebrate unipolar neurons do not have dendrites. Human unipolar neurons have an axon that emerges from the cell body, but it splits so that the axon can extend along a very long distance. At one end of the axon are dendrites, and at the other end, the axon forms synaptic connections with a target. Unipolar neurons are exclusively sensory neurons and have two unique characteristics. First, their dendrites are receiving sensory information, sometimes directly from the stimulus itself. Secondly, the cell bodies of unipolar neurons are always found in ganglia. Sensory reception is a peripheral function (those dendrites are in the periphery, perhaps in the skin) so the cell body is in the periphery, though closer to the CNS in a ganglion. The axon projects from the dendrite endings, past the cell body in a ganglion, and into the central nervous system.\r\n

Bipolar<\/h4>\r\nBipolar<\/strong> neurons have two processes, which extend from each end of the cell body, opposite to each other. One is the axon and one the dendrite. Bipolar neurons are not very common. They are found mainly in the olfactory epithelium (where smell stimuli are sensed), and as part of the retina.\r\n

Multipolar<\/h4>\r\nMultipolar<\/strong>\u00a0neurons are all of the neurons that are not unipolar or bipolar. They have one axon and two or more dendrites (usually many more). With the exception of the unipolar sensory ganglion cells, and the two specific bipolar cells mentioned above, all other neurons are multipolar. Some cutting edge research suggests that certain neurons in the CNS do not conform to the standard model of \u201cone, and only one\u201d axon. Some sources describe a fourth type of neuron, called an anaxonic neuron. The name suggests that it has no axon (an- = \u201cwithout\u201d), but this is not accurate. Anaxonic neurons are very small, and if you look through a microscope at the standard resolution used in histology (approximately 400X to 1000X total magnification), you will not be able to distinguish any process specifically as an axon or a dendrite. Any of those processes can function as an axon depending on the conditions at any given time. Nevertheless, even if they cannot be easily seen, and one specific process is definitively the axon, these neurons have multiple processes and are therefore multipolar.\r\n

Other Neuron Classifications<\/h3>\r\nNeurons can also be classified on the basis of where they are found, who found them, what they do, or even what chemicals they use to communicate with each other. Some neurons referred to in this section on the nervous system are named on the basis of those sorts of classifications (Figure 3). For example, a multipolar neuron that has a very important role to play in a part of the brain called the cerebellum is known as a Purkinje (commonly pronounced per-KIN-gee) cell. It is named after the anatomist who discovered it (Jan Evangilista Purkinje, 1787\u20131869).\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"650\"]\"This Figure 3.\u00a0Other Neuron Classifications<\/strong>\u00a0Three examples of neurons that are classified on the basis of other criteria. (a) The pyramidal cell is a multipolar cell with a cell body that is shaped something like a pyramid. (b) The Purkinje cell in the cerebellum was named after the scientist who originally described it. (c) Olfactory neurons are named for the functional group with which they belong.[\/caption]\r\n

<\/h3>\r\n

Functional Types of Neurons<\/h3>\r\nNeurons are also classified based on their function in the body.\u00a0 Sensory neurons<\/strong> detect changes in the internal or external environment using receptors on their dendrites, and generate action potentials that are carried along an afferent pathway towards the CNS.\u00a0 They can be either unipolar or bipolar in shape.\u00a0 Motor neurons<\/strong> carry action potentials away from the CNS along an efferent pathway towards a target (effector), such as a muscle or gland, and cause that target to produce a response.\u00a0 These neurons are multipolar shaped.\u00a0 Interneurons<\/strong> are found in the CNS, and depending on their location they can receive signals from sensory neurons, communicate with motor neurons, or send and receive signals from other interneurons in the brain and spinal cord.\u00a0 Interneurons are multipolar.\r\n\r\n[caption id=\"attachment_4386\" align=\"alignnone\" width=\"1239\"]\"Figure<\/a> Figure 4.\u00a0Functional types of neurons<\/strong>\u00a0Sensory neurons use receptors to detect the heat of the candle. Interneurons in the spinal cord receive signals from the sensory neurons. The interneurons can send action potentials to the brain so that the person is aware of the heat when other interneurons in the brain receive the signal. If the heat is high enough, interneurons in the spinal cord can send signals directly to motor neurons in the spinal cord that carry action potentials to the biceps brachii muscle of the arm, causing it to contract.[\/caption]\r\n

Glial Cells<\/h2>\r\nGlial cells, or neuroglia or simply glia, are the other type of cell found in nervous tissue. They are considered to be supporting cells, and many functions are directed at helping neurons complete their function for communication. The name glia comes from the Greek word that means \u201cglue,\u201d and was coined by the German pathologist Rudolph Virchow, who wrote in 1856: \u201cThis connective substance, which is in the brain, the spinal cord, and the special sense nerves, is a kind of glue (neuroglia) in which the nervous elements are planted.\u201d Today, research into nervous tissue has shown that there are many deeper roles that these cells play. And research may find much more about them in the future.\r\n\r\nThere are six types of glial cells. Four of them are found in the CNS and two are found in the PNS.\u00a0Table\u00a01\u00a0outlines some common characteristics and functions.\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
Table 1. Glial Cell Types by Location and Basic Function<\/th>\r\n<\/tr>\r\n
CNS glia<\/th>\r\nPNS glia<\/th>\r\nBasic function<\/th>\r\n<\/tr>\r\n<\/thead>\r\n
Astrocyte<\/td>\r\nSatellite cell<\/td>\r\nSupport<\/td>\r\n<\/tr>\r\n
Oligodendrocyte<\/td>\r\nSchwann cell<\/td>\r\nInsulation, myelination<\/td>\r\n<\/tr>\r\n
Microglia<\/td>\r\n-<\/td>\r\nImmune surveillance and phagocytosis<\/td>\r\n<\/tr>\r\n
Ependymal cell<\/td>\r\n-<\/td>\r\nCreating CSF<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

Glial Cells of the CNS<\/h3>\r\nOne cell providing support to neurons of the CNS is the\u00a0astrocyte<\/strong>, so named because it appears to be star-shaped under the microscope (astro<\/em>- = \u201cstar\u201d). Astrocytes have many processes extending from their main cell body (not axons or dendrites like neurons, just cell extensions). Those processes extend to interact with neurons, blood vessels, or the connective tissue covering the CNS that is called the pia mater (Figure 4).\r\n\r\n[caption id=\"\" align=\"alignright\" width=\"449\"]\"This Figure 5.\u00a0Glial Cells of the CNS<\/strong> The CNS has astrocytes, oligodendrocytes, microglia, and ependymal cells that support the neurons of the CNS in several ways.[\/caption]\r\n\r\nGenerally, they are supporting cells for the neurons in the central nervous system. Some ways in which they support neurons in the central nervous system are by maintaining the concentration of chemicals in the extracellular space, removing excess signaling molecules, reacting to tissue damage, and contributing to the\u00a0blood-brain barrier (BBB)<\/strong>. The blood-brain barrier is a physiological barrier that keeps many substances that circulate in the rest of the body from getting into the central nervous system, restricting what can cross from circulating blood into the CNS. Nutrient molecules, such as glucose or amino acids, can pass through the BBB, but other molecules cannot. This actually causes problems with drug delivery to the CNS. Pharmaceutical companies are challenged to design drugs that can cross the BBB as well as have an effect on the nervous system.\r\n\r\nLike a few other parts of the body, the brain has a privileged blood supply. Very little can pass through by diffusion. Most substances that cross the wall of a blood vessel into the CNS must do so through an active transport process. Because of this, only specific types of molecules can enter the CNS. Glucose\u2014the primary energy source\u2014is allowed, as are amino acids. Water and some other small particles, like gases and ions, can enter. But most everything else cannot, including white blood cells, which are one of the body\u2019s main lines of defense. While this barrier protects the CNS from exposure to toxic or pathogenic substances, it also keeps out the cells that could protect the brain and spinal cord from disease and damage. The BBB also makes it harder for pharmaceuticals to be developed that can affect the nervous system. Aside from finding efficacious substances, the means of delivery is also crucial.\r\n\r\nAlso found in CNS tissue is the\u00a0oligodendrocyte<\/strong>, sometimes called just \u201coligo,\u201d which is the glial cell type that insulates axons in the CNS. The name means \u201ccell of a few branches\u201d (oligo<\/em>- = \u201cfew\u201d; dendro<\/em>- = \u201cbranches\u201d; -cyte<\/em> = \u201ccell\u201d). There are a few processes that extend from the cell body. Each one reaches out and surrounds an axon to insulate it in myelin. One oligodendrocyte will provide the myelin for multiple axon segments, either for the same axon or for separate axons. The function of myelin will be discussed below.\r\n\r\nMicroglia<\/strong>\u00a0are, as the name implies, smaller than most of the other glial cells. Ongoing research into these cells, although not entirely conclusive, suggests that they may originate as white blood cells, called macrophages, that become part of the CNS during early development. While their origin is not conclusively determined, their function is related to what macrophages do in the rest of the body. When macrophages encounter diseased or damaged cells in the rest of the body, they ingest and digest those cells or the pathogens that cause disease. Microglia are the cells in the CNS that can do this in normal, healthy tissue, and they are therefore also referred to as CNS-resident macrophages.\r\n\r\nThe\u00a0ependymal cell<\/strong>\u00a0is a glial cell that filters blood to make\u00a0cerebrospinal fluid (CSF)<\/strong>, the fluid that circulates through the CNS. Because of the privileged blood supply inherent in the BBB, the extracellular space in nervous tissue does not easily exchange components with the blood. Ependymal cells line each\u00a0ventricle<\/strong>, one of four central cavities that are remnants of the hollow center of the neural tube formed during the embryonic development of the brain. The\u00a0choroid plexus<\/strong>\u00a0is a specialized structure in the ventricles where ependymal cells come in contact with blood vessels and filter and absorb components of the blood to produce cerebrospinal fluid. Because of this, ependymal cells can be considered a component of the BBB, or a place where the BBB breaks down. These glial cells appear similar to epithelial cells, making a single layer of cells with little intracellular space and tight connections between adjacent cells. They also have cilia on their apical surface to help move the CSF through the ventricular space. The relationship of these glial cells to the structure of the CNS is seen in\u00a0Figure 4.\r\n

Glial Cells of the PNS<\/h3>\r\n[caption id=\"\" align=\"alignright\" width=\"450\"]\"This Figure 6.\u00a0Glial Cells of the PNS<\/strong> The PNS has satellite cells and Schwann cells.[\/caption]\r\n\r\nOne of the two types of glial cells found in the PNS is the\u00a0satellite cell<\/strong>. Satellite cells are found in sensory and autonomic ganglia, where they surround the cell bodies of neurons. This accounts for the name, based on their appearance under the microscope. They provide support, performing similar functions in the periphery as astrocytes do in the CNS\u2014except, of course, for establishing the BBB.\r\n\r\nThe second type of glial cell is the\u00a0Schwann cell<\/strong>, which insulate axons with myelin in the periphery. Schwann cells are different than oligodendrocytes, in that a Schwann cell wraps around a portion of only one axon segment and no others. Oligodendrocytes have processes that reach out to multiple axon segments, whereas the entire Schwann cell surrounds just one axon segment. The nucleus and cytoplasm of the Schwann cell are on the edge of the myelin sheath. The relationship of these two types of glial cells to ganglia and nerves in the PNS is seen in\u00a0Figure 5.\r\n

Myelin<\/h3>\r\nThe insulation for axons in the nervous system is provided by glial cells, oligodendrocytes in the CNS, and Schwann cells in the PNS. Whereas the manner in which either cell is associated with the axon segment, or segments, that it insulates is different, the means of myelinating an axon segment is mostly the same in the two situations. Myelin is a lipid-rich sheath that surrounds the axon and by doing so creates a\u00a0myelin sheath<\/strong>\u00a0that facilitates the transmission of electrical signals along the axon. The lipids are essentially the phospholipids of the glial cell membrane. Myelin, however, is more than just the membrane of the glial cell. It also includes important proteins that are integral to that membrane. Some of the proteins help to hold the layers of the glial cell membrane closely together.\r\n\r\nThe appearance of the myelin sheath can be thought of as similar to the pastry wrapped around a hot dog for \u201cpigs in a blanket\u201d or a similar food. The glial cell is wrapped around the axon several times with little to no cytoplasm between the glial cell layers. For oligodendrocytes, the rest of the cell is separate from the myelin sheath as a cell process extends back toward the cell body. A few other processes provide the same insulation for other axon segments in the area. For Schwann cells, the outermost layer of the cell membrane contains cytoplasm and the nucleus of the cell as a bulge on one side of the myelin sheath. During development, the glial cell is loosely or incompletely wrapped around the axon (Figure\u00a06a). The edges of this loose enclosure extend toward each other, and one end tucks under the other. The inner edge wraps around the axon, creating several layers, and the other edge closes around the outside so that the axon is completely enclosed.\r\n\r\n[caption id=\"attachment_3676\" align=\"aligncenter\" width=\"1024\"]\"This Figure 7.\u00a0The Process of Myelination.<\/strong> Myelinating glia wrap several layers of cell membrane around the cell membrane of an axon segment. A single Schwann cell insulates a segment of a peripheral nerve, whereas in the CNS, an oligodendrocyte may provide insulation for a few separate axon segments. EM \u00d7 1,460,000. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)[\/caption]\r\n\r\n
View the University of Michigan WebScope at\u00a0to see an electron micrograph of a cross-section of a myelinated nerve fiber.<\/a> The axon contains microtubules and neurofilaments that are bounded by a plasma membrane known as the axolemma. Outside the plasma membrane of the axon is the myelin sheath, which is composed of the tightly wrapped plasma membrane of a Schwann cell. What aspects of the cells in this image react with the stain to make them a deep, dark, black color, such as the multiple layers that are the myelin sheath?<\/div>\r\nMyelin sheaths can extend for one or two millimeters, depending on the diameter of the axon. Axon diameters can be as small as 1 to 20 micrometers. Because a micrometer is 1\/1000 of a millimeter, this means that the length of a myelin sheath can be 100\u20131000 times the diameter of the axon.\u00a0Figure\u00a01,\u00a0Figure\u00a04, and\u00a0Figure\u00a05\u00a0show the myelin sheath surrounding an axon segment, but are not to scale. If the myelin sheath were drawn to scale, the neuron would have to be immense\u2014possibly covering an entire wall of the room in which you are sitting.\r\n
\r\n

Disorders of the Nervous Tissue<\/h3>\r\n

Several diseases can result from the demyelination of axons. The causes of these diseases are not the same; some have genetic causes, some are caused by pathogens, and others are the result of autoimmune disorders. Though the causes are varied, the results are largely similar. The myelin insulation of axons is compromised, making electrical signaling slower.<\/p>\r\nMultiple sclerosis (MS) is one such disease. It is an example of an autoimmune disease. The antibodies produced by lymphocytes (a type of white blood cell) mark myelin as something that should not be in the body. This causes inflammation and the destruction of the myelin in the central nervous system. As the insulation around the axons is destroyed by the disease, scarring becomes obvious. This is where the name of the disease comes from; sclerosis means hardening of tissue, which is what a scar is. Multiple scars are found in the white matter of the brain and spinal cord. The symptoms of MS include both somatic and autonomic deficits. Control of the musculature is compromised, as is control of organs such as the bladder.\r\n\r\nGuillain-Barr\u00e9[footnote]pronounced gee-YAN bah-RAY[\/footnote] syndrome is an example of a demyelinating disease of the peripheral nervous system. It is also the result of an autoimmune reaction, but the inflammation is in peripheral nerves. Sensory symptoms or motor deficits are common, and autonomic failures can lead to changes in the heart rhythm or a drop in blood pressure, especially when standing, which causes dizziness.\r\n\r\n<\/div>","rendered":"

\n

 <\/p>\n<\/div>\n

Nervous tissue is composed of two types of cells, neurons and glial cells. Neurons<\/strong> are the primary type of cell that most anyone associates with the nervous system. They are responsible for the computation and communication that the nervous system provides. They are electrically active and release chemical signals to target cells. Glial cells<\/strong>, or glia, are known to play a supporting role for nervous tissue. Ongoing research pursues an expanded role that glial cells might play in signaling, but neurons are still considered the basis of this function. Neurons are important, but without glial support they would not be able to perform their function.<\/p>\n

Neurons<\/h2>\n

Neurons are the cells considered to be the basis of nervous tissue. They are responsible for the electrical signals that communicate information about sensations, and that produce movements in response to those stimuli, along with inducing thought processes within the brain. An important part of the function of neurons is in their structure, or shape. The three-dimensional shape of these cells makes the immense numbers of connections within the nervous system possible.<\/p>\n

Parts of a Neuron<\/h3>\n

As you learned in the first section, the main part of a neuron is the cell body<\/strong>, which is also known as the soma (soma = \u201cbody\u201d). The cell body contains the nucleus and most of the major organelles. But what makes neurons special is that they have many extensions of their cell membranes, which are generally referred to as processes. Neurons are usually described as having one, and only one, axon<\/strong>\u2014an elongated projection that emerges from the cell body and extends to target cells. That single axon can branch repeatedly to communicate with many target cells. It is the axon that propagates the nerve impulse, which is communicated to one or more cells. The other processes of the neuron are dendrites, which receive information from other neurons at specialized areas of contact called\u00a0synapses<\/strong>. The dendrites are usually highly branched processes, providing locations for other neurons to communicate with the cell body. Information flows through a neuron from the dendrites, across the cell body, and down the axon. This gives the neuron a polarity\u2014meaning that information flows in this one direction.\u00a0Figure 1\u00a0shows the relationship of these parts to one another.<\/p>\n

Where the axon emerges from the cell body, there is a special region referred to as the\u00a0axon hillock<\/strong>. This is a tapering of the cell body toward the axon fiber. Within the axon hillock, the cytoplasm changes to a solution of limited components called\u00a0axoplasm<\/strong>. The beginning of the axon on the axon hillock is referred to as the\u00a0initial segment<\/strong>, which often contains the trigger zone<\/strong>.\u00a0 In the trigger zone, a sufficiently strong voltage stimulus will start an action potential.<\/p>\n

\"This<\/a><\/p>\n

Figure 1. Parts of a Neuron<\/strong> The major parts of the neuron are labeled on a multipolar neuron from the CNS.<\/p>\n<\/div>\n

Many axons are wrapped by an insulating substance called myelin, which is actually made from glial cells. Myelin acts as insulation much like the plastic or rubber that is used to insulate electrical wires. A key difference between myelin and the insulation on a wire is that there are gaps in the myelin covering of an axon. Each gap is called a\u00a0node of Ranvier<\/strong>\u00a0and is important to the way that electrical signals travel down the axon.\u00a0 The end of the axon splits into branches called collaterals\u00a0<\/strong>which are branches extending toward the target cell, each of which ends in an enlargement called a\u00a0axon\u00a0<\/strong>terminal<\/b>. The axon terminals frequently form an enlarged bulge at the synapse called the synaptic end bulb<\/strong>, which contains chemical messengers called neurotransmitters <\/strong>that\u00a0are used to contact the target cell.<\/p>\n

Visit this\u00a0site\u00a0to learn about how nervous tissue is composed of neurons and glial cells.<\/a> Neurons are dynamic cells with the ability to make a vast number of connections, to respond incredibly quickly to stimuli, and to initiate movements on the basis of those stimuli. They are the focus of intense research because failures in physiology can lead to devastating illnesses. Why are neurons only found in animals? Based on what this article says about neuron function, why wouldn’t they be helpful for plants or microorganisms?<\/div>\n

Structural Types of Neurons<\/h3>\n

There are many neurons in the nervous system\u2014a number in the trillions. And there are many different types of neurons. They can be classified by many different criteria. The first way to classify them is by the number of processes attached to the cell body. Using the standard model of neurons, one of these processes is the axon, and the rest are dendrites. Because information flows through the neuron from dendrites or cell bodies toward the axon, these names are based on the neuron’s polarity (Figure 2).<\/p>\n

\"Three<\/p>\n

Figure 2.\u00a0Neuron Classification by Shape.<\/strong> Unipolar cells have one process that includes both the axon and dendrite. Bipolar cells have two processes, the axon and a dendrite. Multipolar cells have more than two processes, the axon and two or more dendrites.<\/p>\n<\/div>\n

Unipolar<\/h4>\n

Unipolar<\/strong> neurons have only one process emerging from the cell. True unipolar neurons are only found in invertebrate animals, so the unipolar neurons in humans are more appropriately called \u201cpseudo-unipolar\u201d neurons. Invertebrate unipolar neurons do not have dendrites. Human unipolar neurons have an axon that emerges from the cell body, but it splits so that the axon can extend along a very long distance. At one end of the axon are dendrites, and at the other end, the axon forms synaptic connections with a target. Unipolar neurons are exclusively sensory neurons and have two unique characteristics. First, their dendrites are receiving sensory information, sometimes directly from the stimulus itself. Secondly, the cell bodies of unipolar neurons are always found in ganglia. Sensory reception is a peripheral function (those dendrites are in the periphery, perhaps in the skin) so the cell body is in the periphery, though closer to the CNS in a ganglion. The axon projects from the dendrite endings, past the cell body in a ganglion, and into the central nervous system.<\/p>\n

Bipolar<\/h4>\n

Bipolar<\/strong> neurons have two processes, which extend from each end of the cell body, opposite to each other. One is the axon and one the dendrite. Bipolar neurons are not very common. They are found mainly in the olfactory epithelium (where smell stimuli are sensed), and as part of the retina.<\/p>\n

Multipolar<\/h4>\n

Multipolar<\/strong>\u00a0neurons are all of the neurons that are not unipolar or bipolar. They have one axon and two or more dendrites (usually many more). With the exception of the unipolar sensory ganglion cells, and the two specific bipolar cells mentioned above, all other neurons are multipolar. Some cutting edge research suggests that certain neurons in the CNS do not conform to the standard model of \u201cone, and only one\u201d axon. Some sources describe a fourth type of neuron, called an anaxonic neuron. The name suggests that it has no axon (an- = \u201cwithout\u201d), but this is not accurate. Anaxonic neurons are very small, and if you look through a microscope at the standard resolution used in histology (approximately 400X to 1000X total magnification), you will not be able to distinguish any process specifically as an axon or a dendrite. Any of those processes can function as an axon depending on the conditions at any given time. Nevertheless, even if they cannot be easily seen, and one specific process is definitively the axon, these neurons have multiple processes and are therefore multipolar.<\/p>\n

Other Neuron Classifications<\/h3>\n

Neurons can also be classified on the basis of where they are found, who found them, what they do, or even what chemicals they use to communicate with each other. Some neurons referred to in this section on the nervous system are named on the basis of those sorts of classifications (Figure 3). For example, a multipolar neuron that has a very important role to play in a part of the brain called the cerebellum is known as a Purkinje (commonly pronounced per-KIN-gee) cell. It is named after the anatomist who discovered it (Jan Evangilista Purkinje, 1787\u20131869).<\/p>\n

\"This<\/p>\n

Figure 3.\u00a0Other Neuron Classifications<\/strong>\u00a0Three examples of neurons that are classified on the basis of other criteria. (a) The pyramidal cell is a multipolar cell with a cell body that is shaped something like a pyramid. (b) The Purkinje cell in the cerebellum was named after the scientist who originally described it. (c) Olfactory neurons are named for the functional group with which they belong.<\/p>\n<\/div>\n

<\/h3>\n

Functional Types of Neurons<\/h3>\n

Neurons are also classified based on their function in the body.\u00a0 Sensory neurons<\/strong> detect changes in the internal or external environment using receptors on their dendrites, and generate action potentials that are carried along an afferent pathway towards the CNS.\u00a0 They can be either unipolar or bipolar in shape.\u00a0 Motor neurons<\/strong> carry action potentials away from the CNS along an efferent pathway towards a target (effector), such as a muscle or gland, and cause that target to produce a response.\u00a0 These neurons are multipolar shaped.\u00a0 Interneurons<\/strong> are found in the CNS, and depending on their location they can receive signals from sensory neurons, communicate with motor neurons, or send and receive signals from other interneurons in the brain and spinal cord.\u00a0 Interneurons are multipolar.<\/p>\n

\"Figure<\/a><\/p>\n

Figure 4.\u00a0Functional types of neurons<\/strong>\u00a0Sensory neurons use receptors to detect the heat of the candle. Interneurons in the spinal cord receive signals from the sensory neurons. The interneurons can send action potentials to the brain so that the person is aware of the heat when other interneurons in the brain receive the signal. If the heat is high enough, interneurons in the spinal cord can send signals directly to motor neurons in the spinal cord that carry action potentials to the biceps brachii muscle of the arm, causing it to contract.<\/p>\n<\/div>\n

Glial Cells<\/h2>\n

Glial cells, or neuroglia or simply glia, are the other type of cell found in nervous tissue. They are considered to be supporting cells, and many functions are directed at helping neurons complete their function for communication. The name glia comes from the Greek word that means \u201cglue,\u201d and was coined by the German pathologist Rudolph Virchow, who wrote in 1856: \u201cThis connective substance, which is in the brain, the spinal cord, and the special sense nerves, is a kind of glue (neuroglia) in which the nervous elements are planted.\u201d Today, research into nervous tissue has shown that there are many deeper roles that these cells play. And research may find much more about them in the future.<\/p>\n

There are six types of glial cells. Four of them are found in the CNS and two are found in the PNS.\u00a0Table\u00a01\u00a0outlines some common characteristics and functions.<\/p>\n\n\n\n\n\n\n\n\n\n
Table 1. Glial Cell Types by Location and Basic Function<\/th>\n<\/tr>\n
CNS glia<\/th>\nPNS glia<\/th>\nBasic function<\/th>\n<\/tr>\n<\/thead>\n
Astrocyte<\/td>\nSatellite cell<\/td>\nSupport<\/td>\n<\/tr>\n
Oligodendrocyte<\/td>\nSchwann cell<\/td>\nInsulation, myelination<\/td>\n<\/tr>\n
Microglia<\/td>\n–<\/td>\nImmune surveillance and phagocytosis<\/td>\n<\/tr>\n
Ependymal cell<\/td>\n–<\/td>\nCreating CSF<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n

Glial Cells of the CNS<\/h3>\n

One cell providing support to neurons of the CNS is the\u00a0astrocyte<\/strong>, so named because it appears to be star-shaped under the microscope (astro<\/em>– = \u201cstar\u201d). Astrocytes have many processes extending from their main cell body (not axons or dendrites like neurons, just cell extensions). Those processes extend to interact with neurons, blood vessels, or the connective tissue covering the CNS that is called the pia mater (Figure 4).<\/p>\n

\"This<\/p>\n

Figure 5.\u00a0Glial Cells of the CNS<\/strong> The CNS has astrocytes, oligodendrocytes, microglia, and ependymal cells that support the neurons of the CNS in several ways.<\/p>\n<\/div>\n

Generally, they are supporting cells for the neurons in the central nervous system. Some ways in which they support neurons in the central nervous system are by maintaining the concentration of chemicals in the extracellular space, removing excess signaling molecules, reacting to tissue damage, and contributing to the\u00a0blood-brain barrier (BBB)<\/strong>. The blood-brain barrier is a physiological barrier that keeps many substances that circulate in the rest of the body from getting into the central nervous system, restricting what can cross from circulating blood into the CNS. Nutrient molecules, such as glucose or amino acids, can pass through the BBB, but other molecules cannot. This actually causes problems with drug delivery to the CNS. Pharmaceutical companies are challenged to design drugs that can cross the BBB as well as have an effect on the nervous system.<\/p>\n

Like a few other parts of the body, the brain has a privileged blood supply. Very little can pass through by diffusion. Most substances that cross the wall of a blood vessel into the CNS must do so through an active transport process. Because of this, only specific types of molecules can enter the CNS. Glucose\u2014the primary energy source\u2014is allowed, as are amino acids. Water and some other small particles, like gases and ions, can enter. But most everything else cannot, including white blood cells, which are one of the body\u2019s main lines of defense. While this barrier protects the CNS from exposure to toxic or pathogenic substances, it also keeps out the cells that could protect the brain and spinal cord from disease and damage. The BBB also makes it harder for pharmaceuticals to be developed that can affect the nervous system. Aside from finding efficacious substances, the means of delivery is also crucial.<\/p>\n

Also found in CNS tissue is the\u00a0oligodendrocyte<\/strong>, sometimes called just \u201coligo,\u201d which is the glial cell type that insulates axons in the CNS. The name means \u201ccell of a few branches\u201d (oligo<\/em>– = \u201cfew\u201d; dendro<\/em>– = \u201cbranches\u201d; –cyte<\/em> = \u201ccell\u201d). There are a few processes that extend from the cell body. Each one reaches out and surrounds an axon to insulate it in myelin. One oligodendrocyte will provide the myelin for multiple axon segments, either for the same axon or for separate axons. The function of myelin will be discussed below.<\/p>\n

Microglia<\/strong>\u00a0are, as the name implies, smaller than most of the other glial cells. Ongoing research into these cells, although not entirely conclusive, suggests that they may originate as white blood cells, called macrophages, that become part of the CNS during early development. While their origin is not conclusively determined, their function is related to what macrophages do in the rest of the body. When macrophages encounter diseased or damaged cells in the rest of the body, they ingest and digest those cells or the pathogens that cause disease. Microglia are the cells in the CNS that can do this in normal, healthy tissue, and they are therefore also referred to as CNS-resident macrophages.<\/p>\n

The\u00a0ependymal cell<\/strong>\u00a0is a glial cell that filters blood to make\u00a0cerebrospinal fluid (CSF)<\/strong>, the fluid that circulates through the CNS. Because of the privileged blood supply inherent in the BBB, the extracellular space in nervous tissue does not easily exchange components with the blood. Ependymal cells line each\u00a0ventricle<\/strong>, one of four central cavities that are remnants of the hollow center of the neural tube formed during the embryonic development of the brain. The\u00a0choroid plexus<\/strong>\u00a0is a specialized structure in the ventricles where ependymal cells come in contact with blood vessels and filter and absorb components of the blood to produce cerebrospinal fluid. Because of this, ependymal cells can be considered a component of the BBB, or a place where the BBB breaks down. These glial cells appear similar to epithelial cells, making a single layer of cells with little intracellular space and tight connections between adjacent cells. They also have cilia on their apical surface to help move the CSF through the ventricular space. The relationship of these glial cells to the structure of the CNS is seen in\u00a0Figure 4.<\/p>\n

Glial Cells of the PNS<\/h3>\n
\"This<\/p>\n

Figure 6.\u00a0Glial Cells of the PNS<\/strong> The PNS has satellite cells and Schwann cells.<\/p>\n<\/div>\n

One of the two types of glial cells found in the PNS is the\u00a0satellite cell<\/strong>. Satellite cells are found in sensory and autonomic ganglia, where they surround the cell bodies of neurons. This accounts for the name, based on their appearance under the microscope. They provide support, performing similar functions in the periphery as astrocytes do in the CNS\u2014except, of course, for establishing the BBB.<\/p>\n

The second type of glial cell is the\u00a0Schwann cell<\/strong>, which insulate axons with myelin in the periphery. Schwann cells are different than oligodendrocytes, in that a Schwann cell wraps around a portion of only one axon segment and no others. Oligodendrocytes have processes that reach out to multiple axon segments, whereas the entire Schwann cell surrounds just one axon segment. The nucleus and cytoplasm of the Schwann cell are on the edge of the myelin sheath. The relationship of these two types of glial cells to ganglia and nerves in the PNS is seen in\u00a0Figure 5.<\/p>\n

Myelin<\/h3>\n

The insulation for axons in the nervous system is provided by glial cells, oligodendrocytes in the CNS, and Schwann cells in the PNS. Whereas the manner in which either cell is associated with the axon segment, or segments, that it insulates is different, the means of myelinating an axon segment is mostly the same in the two situations. Myelin is a lipid-rich sheath that surrounds the axon and by doing so creates a\u00a0myelin sheath<\/strong>\u00a0that facilitates the transmission of electrical signals along the axon. The lipids are essentially the phospholipids of the glial cell membrane. Myelin, however, is more than just the membrane of the glial cell. It also includes important proteins that are integral to that membrane. Some of the proteins help to hold the layers of the glial cell membrane closely together.<\/p>\n

The appearance of the myelin sheath can be thought of as similar to the pastry wrapped around a hot dog for \u201cpigs in a blanket\u201d or a similar food. The glial cell is wrapped around the axon several times with little to no cytoplasm between the glial cell layers. For oligodendrocytes, the rest of the cell is separate from the myelin sheath as a cell process extends back toward the cell body. A few other processes provide the same insulation for other axon segments in the area. For Schwann cells, the outermost layer of the cell membrane contains cytoplasm and the nucleus of the cell as a bulge on one side of the myelin sheath. During development, the glial cell is loosely or incompletely wrapped around the axon (Figure\u00a06a). The edges of this loose enclosure extend toward each other, and one end tucks under the other. The inner edge wraps around the axon, creating several layers, and the other edge closes around the outside so that the axon is completely enclosed.<\/p>\n

\"This<\/p>\n

Figure 7.\u00a0The Process of Myelination.<\/strong> Myelinating glia wrap several layers of cell membrane around the cell membrane of an axon segment. A single Schwann cell insulates a segment of a peripheral nerve, whereas in the CNS, an oligodendrocyte may provide insulation for a few separate axon segments. EM \u00d7 1,460,000. (Micrograph provided by the Regents of University of Michigan Medical School \u00a9 2012)<\/p>\n<\/div>\n

View the University of Michigan WebScope at\u00a0to see an electron micrograph of a cross-section of a myelinated nerve fiber.<\/a> The axon contains microtubules and neurofilaments that are bounded by a plasma membrane known as the axolemma. Outside the plasma membrane of the axon is the myelin sheath, which is composed of the tightly wrapped plasma membrane of a Schwann cell. What aspects of the cells in this image react with the stain to make them a deep, dark, black color, such as the multiple layers that are the myelin sheath?<\/div>\n

Myelin sheaths can extend for one or two millimeters, depending on the diameter of the axon. Axon diameters can be as small as 1 to 20 micrometers. Because a micrometer is 1\/1000 of a millimeter, this means that the length of a myelin sheath can be 100\u20131000 times the diameter of the axon.\u00a0Figure\u00a01,\u00a0Figure\u00a04, and\u00a0Figure\u00a05\u00a0show the myelin sheath surrounding an axon segment, but are not to scale. If the myelin sheath were drawn to scale, the neuron would have to be immense\u2014possibly covering an entire wall of the room in which you are sitting.<\/p>\n

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Disorders of the Nervous Tissue<\/h3>\n

Several diseases can result from the demyelination of axons. The causes of these diseases are not the same; some have genetic causes, some are caused by pathogens, and others are the result of autoimmune disorders. Though the causes are varied, the results are largely similar. The myelin insulation of axons is compromised, making electrical signaling slower.<\/p>\n

Multiple sclerosis (MS) is one such disease. It is an example of an autoimmune disease. The antibodies produced by lymphocytes (a type of white blood cell) mark myelin as something that should not be in the body. This causes inflammation and the destruction of the myelin in the central nervous system. As the insulation around the axons is destroyed by the disease, scarring becomes obvious. This is where the name of the disease comes from; sclerosis means hardening of tissue, which is what a scar is. Multiple scars are found in the white matter of the brain and spinal cord. The symptoms of MS include both somatic and autonomic deficits. Control of the musculature is compromised, as is control of organs such as the bladder.<\/p>\n

Guillain-Barr\u00e9[1]<\/sup><\/a> syndrome is an example of a demyelinating disease of the peripheral nervous system. It is also the result of an autoimmune reaction, but the inflammation is in peripheral nerves. Sensory symptoms or motor deficits are common, and autonomic failures can lead to changes in the heart rhythm or a drop in blood pressure, especially when standing, which causes dizziness.<\/p>\n<\/div>\n\n\t\t\t

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Candela Citations<\/h3>\n\t\t\t\t\t
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