Exercise 15 Review Sheet Histology of Nervous Tissue

Learning Objectives

Past the cease of this section, you lot volition be able to:

• Describe the bones structure of a neuron
• Identify the different types of neurons on the basis of polarity
• Listing the glial cells of the CNS and describe their function
• Listing the glial cells of the PNS and describe their part

Nervous tissue is composed of two types of cells, neurons and glial cells. Neurons are the primary type of cell that nigh anyone associates with the nervous system. They are responsible for the computation and communication that the nervous organisation provides. They are electrically agile and release chemical signals to target cells. Glial cells, or glia, are known to play a supporting part for nervous tissue. Ongoing inquiry pursues an expanded role that glial cells might play in signaling, simply neurons are withal considered the footing of this role. Neurons are important, simply without glial support they would not be able to perform their function.

Neurons

Neurons are the cells considered to be the ground of nervous tissue. They are responsible for the electric signals that communicate information virtually sensations, and that produce movements in response to those stimuli, along with inducing thought processes within the brain. An important role of the role of neurons is in their structure, or shape. The three-dimensional shape of these cells makes the immense numbers of connections inside the nervous system possible.

Parts of a Neuron

Equally you lot learned in the first section, the main part of a neuron is the jail cell body, which is also known equally the soma (soma = "body"). The prison cell trunk 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 every bit processes. Neurons are usually described as having one, and simply one, axon—a cobweb that emerges from the prison cell torso and projects to target cells. That unmarried axon tin branch repeatedly to communicate with many target cells. It is the axon that propagates the nervus impulse, which is communicated to ane or more cells. The other processes of the neuron are dendrites, which receive information from other neurons at specialized areas of contact called synapses. The dendrites are normally highly branched processes, providing locations for other neurons to communicate with the jail cell trunk. Data flows through a neuron from the dendrites, across the cell body, and down the axon. This gives the neuron a polarity—significant that information flows in this i management. Figure 12.8 shows the relationship of these parts to one another.

Figure 12.viii Parts of a Neuron The major parts of the neuron are labeled on a multipolar neuron from the CNS.

Where the axon emerges from the cell torso, there is a special region referred to equally the axon hillock. This is a tapering of the jail cell body toward the axon fiber. Within the axon hillock, the cytoplasm changes to a solution of limited components called axoplasm. Because the axon hillock represents the commencement of the axon, it is also referred to as the initial segment.

Many axons are wrapped by an insulating substance called myelin, which is actually fabricated from glial cells. Myelin acts every bit insulation much similar the plastic or rubber that is used to insulate electrical wires. A central 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 node of Ranvier and is of import to the style that electrical signals travel downward the axon. The length of the axon between each gap, which is wrapped in myelin, is referred to as an axon segment. At the terminate of the axon is the axon final, where there are unremarkably several branches extending toward the target cell, each of which ends in an enlargement called a synaptic cease bulb. These bulbs are what make the connection with the target cell at the synapse.

Types of Neurons

There are many neurons in the nervous system—a number in the trillions. And there are many different types of neurons. They tin be classified past many dissimilar criteria. The first way to classify them is by the number of processes attached to the cell trunk. Using the standard model of neurons, one of these processes is the axon, and the rest are dendrites. Considering information flows through the neuron from dendrites or cell bodies toward the axon, these names are based on the neuron'due south polarity (Figure 12.ix).

Effigy 12.9 Neuron Classification by Shape Unipolar cells accept one procedure that includes both the axon and dendrite. Bipolar cells have two processes, the axon and a dendrite. Multipolar cells have more than ii processes, the axon and ii or more dendrites.

Unipolar cells take merely one process emerging from the jail cell. Truthful unipolar cells are only found in invertebrate animals, and so the unipolar cells in humans are more appropriately called "pseudo-unipolar" cells. Invertebrate unipolar cells practise non accept dendrites. Human being unipolar cells accept an axon that emerges from the cell body, simply it splits so that the axon tin can extend along a very long distance. At i terminate of the axon are dendrites, and at the other end, the axon forms synaptic connections with a target. Unipolar cells are exclusively sensory neurons and have two unique characteristics. Starting time, their dendrites are receiving sensory information, sometimes directly from the stimulus itself. Secondly, the prison cell bodies of unipolar neurons are always institute in ganglia. Sensory reception is a peripheral function (those dendrites are in the periphery, maybe in the skin) so the jail cell body is in the periphery, though closer to the CNS in a ganglion. The axon projects from the dendrite endings, past the cell torso in a ganglion, and into the key nervous organisation.

Bipolar cells have two processes, which extend from each end of the cell body, opposite to each other. Ane is the axon and one the dendrite. Bipolar cells are not very common. They are establish mainly in the olfactory epithelium (where olfactory property stimuli are sensed), and as part of the retina.

Multipolar neurons are all of the neurons that are not unipolar or bipolar. They take 1 axon and 2 or more dendrites (usually many more than). With the exception of the unipolar sensory ganglion cells, and the two specific bipolar cells mentioned above, all other neurons are multipolar. Some cut edge research suggests that certain neurons in the CNS practice not adjust to the standard model of "1, and only one" axon. Some sources describe a quaternary type of neuron, called an anaxonic neuron. The name suggests that information technology has no axon (an- = "without"), merely this is not authentic. 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 lot will non exist able to distinguish any process specifically as an axon or a dendrite. Whatsoever of those processes can function equally an axon depending on the weather at any given time. Nevertheless, even if they cannot be hands seen, and one specific process is definitively the axon, these neurons take multiple processes and are therefore multipolar.

Neurons tin can also be classified on the footing of where they are institute, 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 organisation are named on the basis of those sorts of classifications (Figure 12.ten). 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. Information technology is named later on the anatomist who discovered information technology (January Evangelista Purkinje, 1787–1869).

Figure 12.ten Other Neuron Classifications Three 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 information technology. (c) Olfactory neurons are named for the functional group with which they belong.

Glial Cells

Glial cells, or neuroglia or simply glia, are the other type of jail cell found in nervous tissue. They are considered to exist supporting cells, and many functions are directed at helping neurons complete their part for communication. The proper name glia comes from the Greek word that ways "glue," and was coined by the German pathologist Rudolph Virchow, who wrote in 1856: "This connective substance, which is in the brain, the spinal string, and the special sense nerves, is a kind of glue (neuroglia) in which the nervous elements are planted." Today, inquiry into nervous tissue has shown that in that location are many deeper roles that these cells play. And research may find much more about them in the hereafter.

There are six types of glial cells. Four of them are found in the CNS and two are found in the PNS. Table 12.ii outlines some common characteristics and functions.

Glial Jail cell Types by Location and Basic Function

CNS glia	PNS glia	Basic function
Astrocyte	Satellite cell	Support
Oligodendrocyte	Schwann jail cell	Insulation, myelination
Microglia	-	Immune surveillance and phagocytosis
Ependymal cell	-	Creating CSF

Table 12.two

Glial Cells of the CNS

1 cell providing support to neurons of the CNS is the astrocyte, and then named considering information technology appears to exist star-shaped under the microscope (astro- = "star"). Astrocytes have many processes extending from their main cell trunk (not axons or dendrites like neurons, just cell extensions). Those processes extend to collaborate with neurons, blood vessels, or the connective tissue covering the CNS that is chosen the pia mater (Effigy 12.xi). More often than not, they are supporting cells for the neurons in the central nervous arrangement. Some ways in which they back up neurons in the primal 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 blood-brain barrier (BBB). The blood-brain bulwark is a physiological barrier that keeps many substances that circulate in the residual of the body from getting into the cardinal nervous organization, restricting what can cantankerous from circulating claret into the CNS. Nutrient molecules, such as glucose or amino acids, can laissez passer through the BBB, but other molecules cannot. This really causes issues with drug delivery to the CNS. Pharmaceutical companies are challenged to design drugs that can cross the BBB every bit well every bit have an effect on the nervous organization.

Effigy 12.11 Glial Cells of the CNS The CNS has astrocytes, oligodendrocytes, microglia, and ependymal cells that back up the neurons of the CNS in several ways.

Like a few other parts of the trunk, the brain has a privileged blood supply. Very fiddling 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—the primary free energy source—is immune, as are amino acids. H2o and some other small-scale particles, like gases and ions, can enter. Just near everything else cannot, including white blood cells, which are 1 of the torso's chief lines of defence. While this barrier protects the CNS from exposure to toxic or pathogenic substances, information technology also keeps out the cells that could protect the brain and spinal cord from disease and damage. The BBB as well makes it harder for pharmaceuticals to exist adult that tin can affect the nervous organization. Aside from finding efficacious substances, the means of delivery is too crucial.

Besides establish in CNS tissue is the oligodendrocyte, sometimes chosen merely "oligo," which is the glial cell type that insulates axons in the CNS. The name means "cell of a few branches" (oligo- = "few"; dendro- = "branches"; -cyte = "prison cell"). 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 aforementioned axon or for separate axons. The function of myelin will be discussed below.

Microglia are, equally 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 on development. While their origin is not conclusively adamant, their role is related to what macrophages practice in the remainder of the body. When macrophages run across 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 tin can do this in normal, good for you tissue, and they are therefore also referred to as CNS-resident macrophages.

The ependymal prison cell is a glial jail cell that filters blood to make cerebrospinal fluid (CSF), the fluid that circulates through the CNS. Considering of the privileged blood supply inherent in the BBB, the extracellular space in nervous tissue does not easily substitution components with the blood. Ependymal cells line each ventricle, 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 choroid plexus is a specialized structure in the ventricles where ependymal cells come in contact with claret vessels and filter and absorb components of the blood to produce cerebrospinal fluid. Because of this, ependymal cells can exist considered a component of the BBB, or a place where the BBB breaks down. These glial cells announced similar to epithelial cells, making a single layer of cells with little intracellular space and tight connections betwixt adjacent cells. They likewise take 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 Effigy 12.11.

Glial Cells of the PNS

One of the two types of glial cells found in the PNS is the satellite jail cell. Satellite cells are found in sensory and autonomic ganglia, where they environs the cell bodies of neurons. This accounts for the proper name, based on their appearance nether the microscope. They provide support, performing like functions in the periphery every bit astrocytes exercise in the CNS—except, of course, for establishing the BBB.

The second type of glial jail cell is the Schwann prison cell, which insulate axons with myelin in the periphery. Schwann cells are dissimilar than oligodendrocytes, in that a Schwann cell wraps around a portion of only one axon segment and no others. Oligodendrocytes have processes that achieve out to multiple axon segments, whereas the unabridged Schwann jail cell surrounds just one axon segment. The nucleus and cytoplasm of the Schwann jail 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 Figure 12.12.

Figure 12.12 Glial Cells of the PNS The PNS has satellite cells and Schwann cells.

Myelin

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 way in which either prison cell is associated with the axon segment, or segments, that it insulates is different, the means of myelinating an axon segment is more often than not the same in the two situations. Myelin is a lipid-rich sheath that surrounds the axon and by doing so creates a myelin sheath that facilitates the manual of electrical signals forth the axon. The lipids are substantially the phospholipids of the glial cell membrane. Myelin, withal, is more than simply the membrane of the glial cell. It also includes of import proteins that are integral to that membrane. Some of the proteins help to hold the layers of the glial cell membrane closely together.

The appearance of the myelin sheath can exist thought of equally like to the pastry wrapped around a hot domestic dog for "pigs in a coating" or a like food. The glial cell is wrapped around the axon several times with little to no cytoplasm between the glial cell layers. For oligodendrocytes, the balance 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 jail cell membrane contains cytoplasm and the nucleus of the jail cell as a burl on i side of the myelin sheath. During development, the glial jail cell is loosely or incompletely wrapped around the axon (Figure 12.13a). The edges of this loose enclosure extend toward each other, and i end tucks under the other. The inner edge wraps effectually the axon, creating several layers, and the other edge closes around the outside so that the axon is completely enclosed.

Myelin sheaths can extend for one or two millimeters, depending on the bore of the axon. Axon diameters tin can be as pocket-sized equally 1 to 20 micrometers. Because a micrometer is ane/thou of a millimeter, this ways that the length of a myelin sheath can be 100–thou times the diameter of the axon. Effigy 12.8, Figure 12.eleven, and Figure 12.12 evidence the myelin sheath surrounding an axon segment, just are not to scale. If the myelin sheath were drawn to scale, the neuron would have to be immense—mayhap roofing an entire wall of the room in which you are sitting.

Figure 12.xiii The Process of Myelination Myelinating glia wrap several layers of cell membrane around the cell membrane of an axon segment. A single Schwann jail cell insulates a segment of a peripheral nerve, whereas in the CNS, an oligodendrocyte may provide insulation for a few carve up axon segments. EM × 1,460,000. (Micrograph provided by the Regents of University of Michigan Medical School © 2012)

Disorders of the...

Nervous Tissue

Several diseases tin can result from the demyelination of axons. The causes of these diseases are not the aforementioned; some have genetic causes, some are acquired past pathogens, and others are the result of autoimmune disorders. Though the causes are varied, the results are largely like. The myelin insulation of axons is compromised, making electric signaling slower.

Multiple sclerosis (MS) is one such disease. It is an example of an autoimmune affliction. The antibodies produced by lymphocytes (a blazon 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. Every bit the insulation around the axons is destroyed by the affliction, scarring becomes obvious. This is where the name of the disease comes from; sclerosis ways hardening of tissue, which is what a scar is. Multiple scars are establish in the white matter of the encephalon and spinal string. The symptoms of MS include both somatic and autonomic deficits. Command of the musculature is compromised, equally is command of organs such as the float.

Guillain-Barré (pronounced gee-YAN bah-RAY) syndrome is an instance of a demyelinating disease of the peripheral nervous organization. It is besides the result of an autoimmune reaction, merely the inflammation is in peripheral fretfulness. Sensory symptoms or motor deficits are common, and autonomic failures can atomic number 82 to changes in the middle rhythm or a drop in claret pressure, especially when standing, which causes dizziness.

obrienfulot1991.blogspot.com

Source: https://openstax.org/books/anatomy-and-physiology/pages/12-2-nervous-tissue

Share this post