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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. All parts of the nervous system are made of [[nervous tissue]], which conducts electrical impulses. Prominent components in a nervous system include [[neuron]]s and [[nerve]]s.
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| − | This major coordinating system is found in both [[invertebrate]]s and [[vertebrate]]s, but is most complex in vertebrate animals.
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| − | 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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| − | ''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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| − | ==Overview==
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| − | ===The need for a nervous system===
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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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| − | ===Basic components and functions===
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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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| − | |}
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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]] 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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| − | ===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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| − | === 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.
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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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| − | === 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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| − | * 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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| − | * Kimball, J. W. 2006.[http://users.rcn.com/jkimball.ma.ultranet/BiologyPages/C/CNS.html The human central nervous system.] ''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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