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| − | [[Image:Nervous system diagram.png|thumb|290px|The Human Nervous System]]
| + | #REDIRECT [[Nervous system]] |
| − | The '''nervous system''' is the network of specialized [[cell (biology)|cells]], [[tissue]]s, and [[organ (anatomy)|organ]]s in a multicellular [[animal]] that coordinates the body's interaction with the [[environment]], including sensing internal and external stimuli, monitoring the organs, coordinating the activity of [[muscle]]s, initiating actions, and regulating behavior. At the cellular level, the nervous system is defined by the presence of a special type of excitable cell called a [[neuron]] (or "nerve cell") that transmits impulses. All parts of the nervous system are made of [[nervous tissue]], which contains the two main categories of cells: neurons and supporting glia cells. An example of an organ that is part of the nervous system is the [[brain]], which serves as the center of the nervous system in all [[vertebrate]] and most [[invertebrate]] animals.
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| − | This major coordinating system is found in most [[invertebrate]]s and all [[vertebrate]]s, but is most complex in vertebrate animals. The only multicellular animals that have no nervous system at all are [[sponge]]s, [[placozoa]]ns, and [[mesozoa]]ns, which have very simple body plans. In vertebrates, the nervous system is divided into the [[central nervous system]] (CNS), comprising the brain and [[spinal cord]], and the [[peripheral nervous system]] (PNS), consisting of all the nerves and neurons that reside or extend outside the central nervous system, such as to serve the limbs and organs. The large majority of what are commonly called nerves (which are actually [[axon]]al processes of nerve cells) are considered to be part of the peripheral nervous system.
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| − | ''Cephalization'' is a trend seen in the history of life whereby nervous tissue in more advanced organisms is concentrated toward the anterior of the body. This process culminates in a [[head]] region with sensory organs. The human brain is the most complex known living structure, with 100 billion nerve cells and trillions of neuronal connections; millions of information transfer processes take place in remarkable coordination every second in the human central and peripheral nervous system. There also are more than 1,000 disorders of the human brain and nervous system, with neurological disorders affecting up to one billion people worldwide. [[Neurology]] is the medical specialty dealing with disorders and diseases of the nervous system. [[Neuroscience]] is the field of science that focuses on the study of the nervous system.
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| − | At the most basic level, the function of the nervous system is to send signals from one cell to others, or from one part of the body to others. In order for an individual to grow and develop, it needs to be continuously engaged in reciprocal relationships with its environment. The nervous system is what allows that interaction with the environment. Furthermore, the ubiquity of the nervous system among multicellular organisms reflects the unity in [[nature]].
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| − | ==Overview==
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| − | The nervous system is the part of an [[animal]]'s body that coordinates the voluntary and involuntary actions of the animal and transmits signals between different parts of its body. This coordinating system derives its name from [[nerve]]s, which are cylindrical bundles of fibers that emanate from the brain and central cord, and branch repeatedly to innervate every part of the body (Kandel et al. 2000). Nerves actually consist of a cable-like bundle of [[axon]]s (the long, slender projection of a neuron), along with a variety of membranes that wrap around them and they are capable of transmitting electrical signals called nerve impulses or, more technically, action potentials. Nerves are large enough to have been recognized by the ancient Egyptians, Greeks, and Romans, but their internal structure was not understood until it became possible to examine them using a microscope (Finger 2000).The neurons that give rise to nerves do not lie entirely within the nerves themselves—their cell bodies reside within the brain, central cord, or peripheral ganglia (Kandel et al. 2000).
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| − | ===Cellular components===
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| − | [[File:Chemical synapse schema cropped.jpg|thumb|350px|Major elements in synaptic transmission. An electrochemical wave called an [[action potential]] travels along the [[axon]] of a [[neuron]]. When the wave reaches a [[synapse]], it provokes release of a small amount of [[neurotransmitter]] molecules, which bind to chemical receptor molecules located in the membrane of the target cell.]]
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| − | The nervous system contains two main categories or types of cells: [[neuron]]s and [[glial cell]]s.
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| − | [[Image:Neuron.svg|thumb|350px|The structure of a typical neuron includes four main components (from left to right): dendrites, cell body (or soma), axon, and axon terminal]]
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| − | * '''Neurons''' (also known as '''neurones''' and '''nerve cells''') are electrically excitable [[cell (biology)|cells]] in the [[nervous system]] that process and transmit information from both internal and external environments. Given the diversity of their functions, neurons have a wide variety of structures, sizes, and electrochemical properties. However, most neurons are composed of four main components: A [[Soma (biology)|soma]], or cell body, which contains the [[nucleus]]; one or more [[dendrite|dendritic tree]]s that typically receive input; an [[axon]] that carries an electric impulse; and an [[axon terminal]] that often functions to transmit signals to other cells.
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| − | The basic function of a neuron is to communicate information, which it does via chemical or electric impulses across a [[synapse]] (the junction between cells). The fundamental process that triggers these impulses is the [[action potential]], an electrical signal that is generated by utilizing the [[membrane potential|electrically excitable membrane]] of the neuron.
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| − | Neurons can be distinguished from other cells in a number of ways, but their most fundamental property is that they communicate with other cells via [[synapse]]s, which are membrane-to-membrane junctions containing molecular machinery that allows rapid transmission of signals, either electrical or chemical.<ref name=KandelCh2/> Many types of neuron possess an [[axon]], a protoplasmic protrusion that can extend to distant parts of the body and make thousands of synaptic contacts.<ref name=KandelCh4/> Axons frequently travel through the body in bundles called nerves.
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| − | Neurons have special structures that allow them to send signals rapidly and precisely to other cells. They send these signals in the form of electrochemical waves traveling along thin fibers called [[axon]]s, which cause chemicals called [[neurotransmitter]]s to be released at junctions called [[synapse]]s. A cell that receives a synaptic signal from a neuron may be excited, inhibited, or otherwise modulated. The connections between neurons form neural circuits that generate an organism's perception of the world and determine its behavior.
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| − | Even in the nervous system of a single species such as humans, hundreds of different types of neurons exist, with a wide variety of morphologies and functions.<ref name=KandelCh4/> These include [[sensory neuron]]s that transmute physical stimuli such as light and sound into neural signals, and [[motor neuron]]s that transmute neural signals into activation of muscles or glands; however in many species the great majority of neurons receive all of their input from other neurons and send their output to other neurons.<ref name=KandelCh2/>
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| − | Along with neurons, the nervous system contains other specialized cells called [[neuroglia|glial cell]]s (or simply glia), which provide structural and metabolic support.
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| − | [[Glial cell]]s (named from the Greek for "glue") are non-neuronal cells that provide support and nutrition, maintain [[homeostasis]], form [[myelin]], and participate in signal transmission in the nervous system.<ref name=Allen2009/> In the [[human brain]], it is estimated that the total number of glia roughly equals the number of neurons, although the proportions vary in different brain areas.<ref name=Azevedo/> Among the most important functions of glial cells are to support neurons and hold them in place; to supply nutrients to neurons; to insulate neurons electrically; to destroy [[pathogen]]s and remove dead neurons; and to provide guidance cues directing the axons of neurons to their targets.<ref name=Allen2009/> A very important type of glial cell ([[oligodendrocyte]]s in the central nervous system, and [[Schwann cell]]s in the peripheral nervous system) generates layers of a fatty substance called [[myelin]] that wraps around axons and provides electrical insulation which allows them to transmit action potentials much more rapidly and efficiently.
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| − | Most neurons send signals via their [[axon]]s, although some types are capable of dendrite-to-dendrite communication. (In fact, the types of neurons called [[amacrine cell]]s have no axons, and communicate only via their dendrites.) Neural signals propagate along an axon in the form of electrochemical waves called [[action potential]]s, which produce cell-to-cell signals at points where [[axon terminal]]s make [[synapse|synaptic]] contact with other cells.<ref name=KandelCh9/>
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| − | Synapses may be electrical or chemical. [[Electrical synapse]]s make direct electrical connections between neurons,<ref name=Hormuzdi/> but [[chemical synapse]]s are much more common, and much more diverse in function.<ref name=KandelCh10/> At a chemical synapse, the cell that sends signals is called presynaptic, and the cell that receives signals is called postsynaptic. Both the presynaptic and postsynaptic areas are full of molecular machinery that carries out the signalling process. The presynaptic area contains large numbers of tiny spherical vessels called [[synaptic vesicle]]s, packed with [[neurotransmitter]] chemicals.<ref name=KandelCh9/> When the presynaptic terminal is electrically stimulated, an array of molecules embedded in the membrane are activated, and cause the contents of the vesicles to be released into the narrow space between the presynaptic and postsynaptic membranes, called the [[synaptic cleft]]. The neurotransmitter then binds to [[neurotransmitter receptor|receptors]] embedded in the postsynaptic membrane, causing them to enter an activated state.<ref name=KandelCh10/> Depending on the type of receptor, the resulting effect on the postsynaptic cell may be excitatory, inhibitory, or modulatory in more complex ways. For example, release of the neurotransmitter [[acetylcholine]] at a synaptic contact between a [[motor neuron]] and a [[muscle cell]] induces rapid contraction of the muscle cell.<ref name=KandelCh11/> The entire synaptic transmission process takes only a fraction of a millisecond, although the effects on the postsynaptic cell may last much longer (even indefinitely, in cases where the synaptic signal leads to the formation of a [[memory trace]]).<ref name=KandelCh4/>
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| − | {{Synapse map}}
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| − | There are literally hundreds of different types of synapses. In fact, there are over a hundred known neurotransmitters, and many of them have multiple types of receptors.<ref name=KandelCh15/>
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| − | * mention article on cocaine/drugs here as well.
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| − | ===Function of nervous systems===
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| − | Efficiencies in multicellular organisms are improved through the specialization of collections of cells to perform specific functions, such as [[perception]], [[motion]], [[ingestion]], [[digestion]], and [[reproduction]]—provided the different functions can be coordinated and the product or benefit of each functional group of cells distributed to all the other specialized groups of cells. Coordinating the activity of the specialized groups of cells is the task of the nervous system, whose level of complexity reflects the overall complexity of an organism. Examples are provided here in the [[worm]]s, [[arthropoda]], [[mollusca]], and [[vertebrates]].
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| − | At the most basic level, the function of the nervous system is to send signals from one cell to others, or from one part of the body to others. There are multiple ways that a cell can send signals to other cells. One is by releasing chemicals called [[hormone]]s into the internal circulation, so that they can diffuse to distant sites. In contrast to this "broadcast" mode of signaling, the nervous system provides "point-to-point" signals—neurons project their axons to specific target areas and make synaptic connections with specific target cells.<ref name=Gray170>{{Cite book|title=Psychology |author=Gray PO |edition=5 |publisher=Macmillan |year=2006 |isbn=978-0-7167-7690-1 |page=170}}</ref> Thus, neural signaling is capable of a much higher level of specificity than hormonal signaling. It is also much faster: the fastest nerve signals travel at speeds that exceed 100 meters per second.
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| − | At a more integrative level, the primary function of the nervous system is to control the body.<ref name=KandelCh2/> It does this by extracting information from the environment using sensory receptors, sending signals that encode this information into the central nervous system, processing the information to determine an appropriate response, and sending output signals to muscles or glands to activate the response. The evolution of a complex nervous system has made it possible for various animal species to have advanced perception abilities such as vision, complex social interactions, rapid coordination of organ systems, and integrated processing of concurrent signals. In humans, the sophistication of the nervous system makes it possible to have language, abstract representation of concepts, transmission of culture, and many other features of human society that would not exist without the human brain.
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| − | * The nervous system is susceptible to malfunction in a wide variety of ways, as a result of genetic defects, physical damage due to trauma or poison, infection, or simply aging. The medical specialty of [[neurology]] studies the causes of nervous system malfunction, and looks for interventions that can prevent it or treat it. In the peripheral nervous system, the most commonly occurring type of problem is failure of nerve conduction, which can have a variety of causes including [[diabetic neuropathy]] and demyelinating disorders such as [[multiple sclerosis]] and [[amyotrophic lateral sclerosis]].
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| − | ===Overview of Bilatera, invertebrates, vertebrates===
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| − | All animals more advanced than sponges have nervous systems. However, even sponges, unicellular animals, and non-animals such as slime molds have cell-to-cell signalling mechanisms that are precursors to those of neurons.<ref name=Sakarya/> In radially symmetric animals such as the jellyfish and hydra, the nervous system consists of a diffuse network of isolated cells.<ref name=Ruppert/> In [[bilateria]]n animals, which make up the great majority of existing species, the nervous system has a common structure that originated early in the Cambrian period, over 500 million years ago.<ref name=Balavoine/>
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| − | Nervous systems are found in most multicellular animals, but vary greatly in complexity.<ref name=Columbia/> The only multicellular animals that have no nervous system at all are [[sponge]]s, [[placozoa]]ns and [[mesozoa]]ns, which have very simple body plans. The nervous systems of [[ctenophores]] (comb jellies) and [[cnidarians]] (e.g., anemones, hydras, corals and jellyfishes) consist of a diffuse nerve net. All other types of animals, with the exception of a few types of worms, have a nervous system containing a brain, a central cord (or two cords running in [[parallel (geometry)|parallel]]), and nerves radiating from the brain and central cord. The size of the nervous system ranges from a few hundred cells in the simplest worms, to on the order of 100 billion cells in humans.
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| − | ''Cephalization'' is a trend seen in the history of life whereby nervous tissue in more advanced organisms is concentrated toward the anterior of the body. This process culminates in a [[head]] region with sensory organs. Cephalization is intrinsically connected with a change in [[symmetry (biology)|symmetry]], accompanying the move to [[symmetry (biology)#Bilateral symmetry|bilateral symmetry]] made in [[flatworm]]s, with [[ocelli]] and [[pinna|auricles]] placed in the head region. The cephalization/bilateral symmetry combination allows animals to have sensory organs facing the direction of movement, granting a more focused assessment of the environment into which they are moving.
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| − | ====Bilateria====
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| − | [[File:Bilaterian-plan.svg|thumb|right|alt=A rod-shaped body contains a digestive system running from the mouth at one end to the anus at the other. Alongside the digestive system is a nerve cord with a brain at the end, near to the mouth. |Nervous system of a bilaterian animal, in the form of a nerve cord with segmental enlargements, and a "brain" at the front]]
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| − | The vast majority of existing animals are [[bilateria]]ns, meaning animals with left and right sides that are approximate mirror images of each other. All bilateria are thought to have descended from a common wormlike ancestor that appeared in the [[Cambrian]] period, 550–600 million years ago.<ref name=Balavoine/> The fundamental bilaterian body form is a tube with a hollow gut cavity running from mouth to anus, and a nerve cord with an enlargement (a "ganglion") for each body segment, with an especially large ganglion at the front, called the "brain".
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| − | [[File:Gray797.png|thumb|left|125px|Area of the human body surface innervated by each spinal nerve]]
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| − | Even mammals, including humans, show the segmented bilaterian body plan at the level of the nervous system. The spinal cord contains a series of segmental ganglia, each giving rise to motor and sensory nerves that innervate a portion of the body surface and underlying musculature. On the limbs, the layout of the innervation pattern is complex, but on the trunk it gives rise to a series of narrow bands. The top three segments belong to the brain, giving rise to the forebrain, midbrain, and hindbrain.<ref name=Ghysen>{{Cite journal|author=Ghysen A |title=The origin and evolution of the nervous system |journal=Int. J. Dev. Biol. |volume=47 |issue=7–8 |pages=555–62 |year=2003 |pmid=14756331 |doi= |url=http://www.ijdb.ehu.es/web/paper.php?doi=14756331}}</ref>
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| − | Bilaterians can be divided, based on events that occur very early in embryonic development, into two groups ([[superphylum|superphyla]]) called [[protostomia|protostomes]] and [[deuterostome]]s.<ref name=Erwin/> Deuterostomes include vertebrates as well as [[echinoderm]]s, [[hemichordata|hemichordates]] (mainly acorn worms), and [[Xenoturbellida]]ns.<ref name=Bourlat/> Protostomes, the more diverse group, include [[arthropod]]s, [[mollusc]]s, and numerous types of worms. There is a basic difference between the two groups in the placement of the nervous system within the body: protostomes possess a nerve cord on the ventral (usually bottom) side of the body, whereas in deuterostomes the nerve cord is on the dorsal (usually top) side. In fact, numerous aspects of the body are inverted between the two groups, including the expression patterns of several genes that show dorsal-to-ventral gradients. Most anatomists now consider that the bodies of protostomes and deuterostomes are "flipped over" with respect to each other, a hypothesis that was first proposed by [[Étienne Geoffroy Saint-Hilaire|Geoffroy Saint-Hilaire]] for insects in comparison to vertebrates. Thus insects, for example, have nerve cords that run along the ventral midline of the body, while all vertebrates have spinal cords that run along the dorsal midline.<ref name=Lichtneckert/>
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| − | ==Vertebrate Nervous Systems==
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| − | {|border=1 cellpadding=5 cellspacing=0 style="float:right"
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| − | |+'''Organization of the vertebrate nervous system'''
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| − | |rowspan=4|[[peripheral nervous system|Peripheral]]
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| − | |colspan=2|[[somatic nervous system|Somatic]]
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| − | |rowspan=3|[[autonomic nervous system|Autonomic]]
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| − | |[[sympathetic nervous system|Sympathetic]]
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| − | |[[parasympathetic nervous system|Parasympathetic]]
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| − | |[[enteric nervous system|Enteric]]
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| − | |colspan=3|[[central nervous system|Central]]
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| − | [[File:NSdiagram.svg|thumb|right|450px|Diagram showing the major divisions of the vertebrate nervous system.]]
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| − | [[Image:Gray839.png|right|thumb|250px|Autonomic nervous system innervation, showing the sympathetic and parasympathetic (craniosacral) systems, in red and blue, respectively]]
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| − | The sympathetic nervous system is a main subdivision of the [[autonomic nervous system]] (ANS), which in turn is part of the [[peripheral nervous system]] (PNS). It is sometimes referenced as the '''sympathetic division''' of the autonomic nervous system (Marieb and Hoehn, 2010).
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| − | The [[vertebrate]] [[nervous system]] is divided into the [[central nervous system]] (CNS), comprising the [[brain]] and [[spinal cord]], and the peripheral nervous system (PNS), consisting of all the [[nerve]]s and [[neuron]]s that reside or extend outside the central nervous system, such as to serve the limbs and [[organ (anatomy)|organs]]. The large majority of what are commonly called nerves (which are actually axonal processes of nerve cells) are considered to be part of the peripheral nervous system.
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| − | The '''peripheral nervous system''', in turn, is commonly divided into two subsystems, the [[somatic nervous system]] and the autonomic nervous system. The somatic nervous system (or sensory-somatic nervous system) involves nerves just under the [[skin]] and serves as the sensory connection between the outside environment and the CNS. These nerves are under conscious control, but most have an automatic component, as is seen in the fact that they function even in the case of a coma (Anissimov 2007). In humans, the somatic nervous system consists of 12 pairs of cranial nerves and 31 pairs of spinal nerves (Chamberlin and Narins 2005).
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| − | The '''autonomic nervous system''' is typically presented as that portion of the peripheral nervous system that is independent of conscious control, acting involuntarily and subconsciously (reflexively), and innervating heart muscle, [[endocrine system|endocrine glands]], exocrine glands, and smooth muscle (Chamberlin and Narins 2005). In sending fibers to three tissues—cardiac muscle, smooth muscle, or glandular tissue—the autonomic nervous system provides stimulation, sympathetic or parasympathetic, to control smooth muscle contraction, regulate cardiac muscle, or stimulate or inhibit glandular secretion. (In contrast, the somatic nervous system innervates skeletal muscle tissue, rather than smooth, cardiac, or glandular tissue.) Unlike the somatic nervous system, which always excites [[muscle]] tissue, the autonomic nervous system can either excite or inhibit innervated tissue (Chamberlin and Narins 2005).
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| − | The autonomic nervous system is subdivided into the sympathetic nervous system, the parasympathetic nervous system, and the enteric nervous system. In general, the sympathetic nervous system increases activity and [[metabolism|metabolic]] rate (the "fight or flight response"), while the parasympathetic slows activity and metabolic rate, returning the body to normal levels of function (the "rest and digest state") after heightened activity from sympathetic stimulation (Chamberlin and Narins 2005). The enteric nervous system innervates areas around the intestines, [[pancreas]], and [[gall bladder]], dealing with digestion, and so forth.
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| − | ** In most types of animals it consists of two main parts, the [[central nervous system]] (CNS) and the [[peripheral nervous system]] (PNS). The CNS contains the [[brain]] and [[spinal cord]]. The PNS consists mainly of [[nerve]]s, which are long fibers that connect the CNS to every other part of the body. The PNS includes [[motor neuron]]s, mediating voluntary movement, the [[autonomic nervous system]], comprising the [[sympathetic nervous system]] and the [[parasympathetic nervous system]] and regulating involuntary functions, and the [[enteric nervous system]], a semi-independent part of the nervous system whose function is to control the gastrointestinal system.
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| − | Most associated tissues and organs have nerves of both the sympathetic and the parasympathetic nervous systems. The two system can stimulate the target organs and tissues in opposite ways, such as sympathetic stimulation to increase heart rate and parasympathetic to decrease heart rate, or the sympathetic stimulation resulting in pupil dilation, and the parasympathetic in pupil constriction or narrowing (Chamberlin and Narins 2005). Or, they can both stimulate activity in concert, but in different ways, such as both increasing [[saliva]] production by [[salivary gland]]s, but with sympathetic stimulation yielding viscous or thick saliva and parasympathetic yielding watery saliva. Likewise, in human reproduction, they work in concert with the parasympathetic promoting erection of genitals and the sympathetic promoting ejaculation and vaginal contractions (Campbell et al. 2008).
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| − | One way to envision the difference between the systems in humans is to call the sympathetic nervous system the E division (exercise, excitement, emergency, embarrassment) and the parasympathetic nervous system the D division (digestion, defecation, diuresis) (Marieb and Hoehn, 2010). A rarely used (but useful) acronym used to summarize the functions of the parasympathetic nervous system in human beings is SLUDD ([[saliva|salivation]], [[tears|lacrimation]], [[urination]], [[digestion]], and [[defecation]]).
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| − | The nervous system of [[vertebrate]] animals is often divided into the [[central nervous system]] and the [[peripheral nervous system]]. The CNS comprises the [[brain]] and [[spinal cord]]. The PNS comprises all other nerves and neurons that do not lie within the central nervous system. The large majority of what are commonly called nerves (which are actually axonal processes of nerve cells) are considered to be part of the peripheral nervous system.
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| − | The peripheral nervous system is divided further into the [[somatic nervous system]] and the [[autonomic nervous system]].
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| − | The [[somatic nervous system]] is responsible for coordinating the body's movements, and also for receiving external stimuli. It is the system that regulates activities that are under conscious control.
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| − | The [[autonomic nervous system]], which usually acts apart from conscious control, comprises the [[sympathetic nervous system|sympathetic division]], [[parasympathetic nervous system|parasympathetic division]], and [[enteric nervous system|enteric division]]. The sympathetic nervous system responds to impending danger or [[stress (medicine)|stress]], and is responsible for the increase of one's heartbeat and blood pressure, among other physiological changes, along with the sense of excitement one feels due to the increase of [[adrenaline]] in the system. The parasympathetic nervous system, on the other hand, is evident when a person is resting and feels relaxed, and is responsible for such things as the constriction of the pupil, the slowing of the heart, the dilation of the blood vessels, and the stimulation of the digestive and [[genitourinary]] systems. The role of the enteric nervous system is to manage every aspect of digestion, from the esophagus to the stomach, small intestine, and colon.
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| − | [[File:NSdiagram.svg|thumb|right|450px|Diagram showing the major divisions of the vertebrate nervous system.]]
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| − | The nervous system of [[vertebrate|vertebrate animals]] (including humans) is divided into the central nervous system (CNS) and peripheral nervous system (PNS).<ref name=KandelCh17/>
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| − | The [[central nervous system]] (CNS) is the largest part, and includes the [[brain]] and [[spinal cord]].<ref name=KandelCh17/> The [[spinal cavity]] contains the spinal cord, while the [[head]] contains the brain. The CNS is enclosed and protected by [[meninges]], a three-layered system of membranes, including a tough, leathery outer layer called the [[dura mater]]. The brain is also protected by the skull, and the spinal cord by the [[vertebra]]e.
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| − | The [[peripheral nervous system]] (PNS) is a collective term for the nervous system structures that do not lie within the CNS.<ref name=Gray233/> The large majority of the axon bundles called nerves are considered to belong to the PNS, even when the cell bodies of the neurons to which they belong reside within the brain or spinal cord. The PNS is divided into [[somatic nervous system|somatic]] and [[viscera]]l parts. The somatic part consists of the nerves that innervate the skin, joints, and muscles. The cell bodies of somatic sensory neurons lie in [[dorsal root ganglion|dorsal root ganglia]] of the spinal cord. The visceral part, also known as the [[autonomic nervous system]], contains neurons that innervate the internal organs, blood vessels, and glands. The autonomic nervous system itself consists of two parts: the [[sympathetic nervous system]] and the [[parasympathetic nervous system]]. Some authors also include sensory neurons whose cell bodies lie in the periphery (for senses such as hearing) as part of the PNS; others, however, omit them.<ref name=Hubbardvii>{{Cite book|title=The peripheral nervous system |author=Hubbard JI |publisher=Plenum Press |year=1974 |page=vii |isbn=978-0-306-30764-5}}</ref>
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| − | [[File:Visible Human head slice.jpg|thumb|200px|left|Horizontal section of the head of an adult female, showing skin, skull, and brain with grey matter (brown in this image) and underlying white matter]]
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| − | The vertebrate nervous system can also be divided into areas called [[grey matter]] ("gray matter" in American spelling) and [[white matter]].<ref name=Purves15/> Grey matter (which is only grey in preserved tissue, and is better described as pink or light brown in living tissue) contains a high proportion of cell bodies of neurons. White matter is composed mainly of [[myelin]]ated axons, and takes its color from the myelin. White matter includes all of the nerves, and much of the interior of the brain and spinal cord. Grey matter is found in clusters of neurons in the brain and spinal cord, and in cortical layers that line their surfaces. There is an anatomical convention that a cluster of neurons in the brain or spinal cord is called a [[nucleus (neuroanatomy)|nucleus]], whereas a cluster of neurons in the periphery is called a [[ganglion]].<ref name=DorlandsGanglion/> There are, however, a few exceptions to this rule, notably including the part of the forebrain called the [[basal ganglia]].<ref name=Afifi/>
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| − | {{Clear}}
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| − | ===Human nervous system===
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| − | There are three essential parts of the [[human]] nervous system. These include the [[brain]], the spine, and the nerves. The brain has three main parts that interact with the nervous system: The [[cerebrum]], the [[cerebellum]], and the [[medulla]]. The cerebrum's tasks include high-order thinking and learning, while the cerebellum manages learned automatic bodily functions, including walking, jumping, and running. The medulla processes simple body functions, such as breathing and digestion.
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| − | Reflex reactions occur independent of the brain with the spinal cord being the "center" of the response. Split-second reflex decisions do not involve sensory nerve impulses traveling to the brain and then back to the organ or body part. This would take too long and the nerve impulse may well arrive too late to prevent the stimulus from becoming reality. For instance, if a ball were thrown at an individual's head, the reflex to move out of the way would come from the spine, not the brain, improving reaction time. The spine is also the "highway" which passes orders from the brain to motor nerves.
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| − | There are four kinds of nerves: [[motor nerve|Motor]], [[sensory nerve|sensory]], [[afferent nerve|afferent]], and [[interneuron]]s. Messages carried in all nerve types travel in only one direction.
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| − | ==Invertebrate Nervous Systems==
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| − | ===Porifera: Neural precusors===
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| − | ====Neural precursors in sponges====
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| − | [[Sponge]]s have no cells connected to each other by [[synapse|synaptic junctions]], that is, no neurons, and therefore no nervous system. They do, however, have [[homology (biology)|homologs]] of many genes that play key roles in synaptic function. Recent studies have shown that sponge cells express a group of proteins that cluster together to form a structure resembling a [[postsynaptic density]] (the signal-receiving part of a synapse).<ref name=Sakarya/> However, the function of this structure is currently unclear. Although sponge cells do not show synaptic transmission, they do communicate with each other via calcium waves and other impulses, which mediate some simple actions such as whole-body contraction.<ref name=Jacobs/>
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| − | ====Radiata====
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| − | [[Cnidaria|Jellyfish]], [[ctenophore|comb jellies]], and related animals have diffuse nerve nets rather than a central nervous system. In most jellyfish the nerve net is spread more or less evenly across the body; in comb jellies it is concentrated near the mouth. The nerve nets consist of sensory neurons, which pick up chemical, tactile, and visual signals; motor neurons, which can activate contractions of the body wall; and intermediate neurons, which detect patterns of activity in the sensory neurons and, in response, send signals to groups of motor neurons. In some cases groups of intermediate neurons are clustered into discrete [[ganglion|ganglia]].<ref name=Ruppert/>
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| − | The development of the nervous system in [[radiata]] is relatively unstructured. Unlike [[bilaterians]], radiata only have two primordial cell layers, [[endoderm]] and [[ectoderm]]. Neurons are generated from a special set of ectodermal precursor cells, which also serve as precursors for every other ectodermal cell type.<ref name=Sanes3>{{Cite book|title=Development of the nervous system |publisher=Academic Press |year=2006 |isbn=978-0-12-618621-5 |pages=3–4 |author=Sanes DH, Reh TA, Harris WA}}</ref>
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| − | === Worms ===
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| − | [[Flatworm]]s (phylum Platyhelminthes) have a [[Symmetry (biology)#Bilateral symmetry|bilateral]] nervous system; they are the simplest animals to have one. Two cord-like nerves branch repeatedly in an array resembling a ladder. Flatworms have their sense receptors and nerves concentrated on the anterior end (cephalization). The head end of some species even has a collection of [[ganglia]] acting as a rudimentary [[brain]] to integrate signals from sensory organs, such as [[eyespot]]s.
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| − | [[Image:Tenia_solium_scolex.jpg|thumb|200px|''Tenia solium,'' a [[cestoda|cestode]] ("tapeworm," a type of parasitic [[flatworm]]) showing simple cephalization.]]
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| − | For example, [[planarian|planaria]], a type of flatworm, have dual [[nerve cord]]s running along the length of the body and merging at the tail. These nerve cords are connected by transverse nerves like the rungs of a ladder. These transverse nerves help coordinate the two sides of the animal. Two large [[ganglia]] at the head end function similar to a simple [[brain]]. [[Photoreceptor]]s on the animal's eyespots provide sensory information on light and darkness.
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| − | [[Nematode]]s (roundworms, phylum Nematoda) have a simple nervous system, with a main nerve cord running along the [[ventral]] side (the "belly" side). Sensory structures at the anterior or head end are called amphids, while sensory structures at the posterior end are called phasmids.
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| − | The nervous system of the roundworm ''Caenorhabditis elegans'' has been mapped out to the cellular level. Every neuron and its cellular lineage has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is sexually dimorphic; the nervous systems of the two sexes, males and [[hermaphrodites]], have different numbers of neurons and groups of neurons that perform sex-specific functions. In ''C. elegans,'' males have 383 neurons, while hermaphrodites have 302 neurons (Hobert 2005).
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| − | In [[annelid]]s (segmented worms, phylum Annelida), the nervous system has a solid, ventral nerve cord from which lateral nerves arise in each segment. Every segment has an autonomy; however, they unite to perform as a single body for functions such as locomotion.[[File:Earthworm nervous system.png|thumb|250px|right|Earthworm nervous system. ''Top:'' side view of the front of the worm. ''Bottom:'' nervous system in isolation, viewed from above]]
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| − | [[Worm]]s are the simplest bilaterian animals, and reveal the basic structure of the bilaterian nervous system in the most straightforward way. As an example, [[earthworm]]s have dual [[ventral nerve cord|nerve cord]]s running along the length of the body and merging at the tail and the mouth. These nerve cords are connected by [[transverse plane|transverse]] nerves like the rungs of a ladder. These transverse nerves help [[coordinate]] the two sides of the animal. Two [[ganglion|ganglia]] at the head end function similar to a simple brain. [[Simple eyes in arthropods|Photoreceptor]]s on the animal's eyespots provide sensory information on light and dark.<ref name=Adey>{{Cite journal|author=ADEY WR |title=The nervous system of the earthworm Megascolex |journal=J. Comp. Neurol. |volume=94 |issue=1 |pages=57–103 |year=1951 |month=February |pmid=14814220 |doi= 10.1002/cne.900940104|url=}}</ref>
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| − | The nervous system of one very small worm, the [[roundworm]] ''[[Caenorhabditis elegans]]'', has been mapped out down to the synaptic level. Every neuron and its [[fate mapping|cellular lineage]] has been recorded and most, if not all, of the neural connections are known. In this species, the nervous system is [[sexually dimorphic]]; the nervous systems of the two sexes, males and [[hermaphrodites]], have different numbers of neurons and groups of neurons that perform sex-specific functions. In ''C. elegans'', males have exactly 383 neurons, while hermaphrodites have exactly 302 neurons.<ref name=Wormbook/>
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| − | === Arthropoda ===
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| − | [[Arthropod]]s, such as [[insect]]s and [[crustacean]]s, have a nervous system made up of a series of ganglia, connected by a [[ventral nerve cord]] which is made up of two parallel connectives running along the length of the belly. Typically, each body segment has one [[ganglion]] on each side, though some ganglia are fused to form large ganglia like the brain.
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| − | The head segment contains the [[brain]], also known as the supraesophageal ganglion. In the insect nervous system, the brain is anatomically divided into the protocerebrum, deutocerebrum, and tritocerebrum. Immediately behind the brain is the [[subesophageal ganglion]], which controls the mouth parts.
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| − | Many arthropods have well-developed [[sense|sensory]] organs, including [[compound eye]]s for vision and [[antenna (biology)|antennae]] for [[olfactory]] and [[pheromone]] sensation. The sensory information from these organs is processed by the brain.
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| − | ====Arthropods====
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| − | [[File:Spider internal anatomy-en.svg|thumb|right|250px|Internal anatomy of a spider, showing the nervous system in blue]]
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| − | [[Arthropod]]s, such as [[insect]]s and [[crustacean]]s, have a nervous system made up of a series of [[ganglion|ganglia]], connected by a [[ventral nerve cord]] made up of two parallel connectives running along the length of the [[Abdomen|belly]].<ref name=Chapman>{{Cite book|title=The insects: structure and function |author=Chapman RF |publisher=Cambridge University Press |year=1998 |isbn=978-0-521-57890-5 |chapter=Ch. 20: Nervous system |pages=533–568}}</ref> Typically, each body segment has one [[ganglion]] on each side, though some ganglia are fused to form the brain and other large ganglia. The head segment contains the brain, also known as the [[supraesophageal ganglion]]. In the [[Insect#Nervous system|insect nervous system]], the brain is anatomically divided into the [[protocerebrum]], [[deutocerebrum]], and [[tritocerebrum]]. Immediately behind the brain is the [[subesophageal ganglion]], which is composed of three pairs of fused ganglia. It controls the [[Arthropod mouthparts|mouthparts]], the [[salivary glands]] and certain [[muscle]]s. Many arthropods have well-developed [[sense|sensory]] organs, including [[compound eye]]s for vision and [[antenna (biology)|antennae]] for [[olfaction]] and [[pheromone]] sensation. The sensory information from these organs is processed by the brain.
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| − | In insects, many neurons have cell bodies that are positioned at the edge of the brain and are electrically passive—the cell bodies serve only to provide metabolic support and do not participate in signalling. A protoplasmic fiber runs from the cell body and branches profusely, with some parts transmitting signals and other parts receiving signals. Thus, most parts of the insect brain have passive cell bodies arranged around the periphery, while the neural signal processing takes place in a tangle of protoplasmic fibers called [[neuropil]], in the interior.<ref>Chapman, p. 546</ref>
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| − | === Mollusca ===
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| − | Most [[mollusk]]s, such as [[snail]]s and [[bivalve]]s, have several groups of intercommunicating neurons called [[ganglion|ganglia]]. The nervous system of the sea hare ''Aplysia'' has been extensively used in [[neuroscience]] experiments because of its simplicity and ability to learn simple associations.
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| − | The [[cephalopod]]s, such as [[squid]] and [[octopus]]es, have relatively complex [[brain]]s. These animals also have complex [[eye]]s. As in all [[invertebrate]]s, the [[axon]]s in cephalopods lack [[myelin]], the insulator that allows fast saltatory conduction of action potentials in vertebrates. (In saltatory conduction, the action potentials do not pass continuously along the nerve, but rather "hop" from node to node in the myelin sheath along the nerve.) To achieve a high enough conduction velocity to control [[muscle]]s in distal [[tentacle]]s, axons in the cephalopods must have a very wide diameter in the larger species of cephalopods. For this reason, the squid giant axons were used by neuroscientists to work out the basic properties of the action potential.
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| − | ==References==
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| − | * Burns, C. P. E. 2006. Altruism in nature as manifestation of divine ''energeia.'' ''Zygon'' 41(1):125-137.
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| − | * Finger, S. 2001. ''Origins of Neuroscience: A History of Explorations Into Brain Function''. Oxford Univ. Press. ISBN 9780195146943.
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| − | * Hobert, O. 2005. [http://www.wormbook.org/chapters/www_specnervsys/specnervsys.html Specification of the nervous system.] ''Wormbook.'' Retrieved April 28, 2007.
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| − | * Kandel, E. R., J. H. Schwartz,and T. M. Jessel (Eds.). 2000. ''Principles of Neural Science''. McGraw-Hill Professional. ISBN 9780838577011.
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| − | * Kimball, J. W. 2006.[http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CNS.html The human central nervous system.]
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| − | ''Kimball's Biology Pages.'' Retrieved April 28, 2007.
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| − | * Kimball, J. W. 2006. [http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/P/PNS.html Organization of the nervous system.] ''Kimball's Biology Pages.'' Retrieved April 28, 2007.
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| − | * Towle, A. 1989. ''Modern Biology.'' Austin, TX: Holt, Rinehart and Winston. ISBN 0030139198.
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| − | {{organ_systems}}
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| − | {{nervous_system}}
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| − | [[Category:Life sciences]][[Category:Anatomy and physiology]]
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| − | {{credit|Nervous_system|125360742|Cephalization|123469818}}
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