Editing Nervous system (section)

{{Short description|Part of an animal that coordinates actions and senses}}
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{{Infobox anatomy
| Name        = Nervous system
| Latin       = systema nervosum
| Image       = TE-Nervous system diagram.svg
| Caption     = The human nervous system
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In [[biology]], the '''nervous system''' is the [[complex system|highly complex]] part of an [[animal]] that coordinates its [[behavior|actions]] and [[sense|sensory]] information by transmitting [[action potential|signals]] to and from different parts of its body. The nervous system detects environmental changes that impact the body, then works in tandem with the [[endocrine system]] to respond to such events.<ref>{{Cite book |title=Principles of Anatomy and Physiology |last1=Tortora |first1=G.J. |last2=Derrickson |first2=B. |publisher=J. Wiley |year=2016 |isbn=978-1-119-34373-8 |edition=15th}}</ref> Nervous tissue first arose in [[Ediacara biota|wormlike organisms]] about 550 to 600 million years ago. In [[Vertebrate|vertebrates]], it consists of two main parts, the [[central nervous system]] (CNS) and the [[peripheral nervous system]] (PNS). The CNS consists of the [[brain]] and [[spinal cord]]. The PNS consists mainly of [[nerve]]s, which are enclosed bundles of the long fibers, or [[axon]]s, that connect the CNS to every other part of the body. Nerves that transmit signals from the brain are called [[motor nerve]]s (efferent), while those nerves that transmit information from the body to the CNS are called [[sensory nerve]]s (afferent). The PNS is divided into two separate subsystems, the [[somatic nervous system|somatic]] and [[autonomic nervous system|autonomic]] nervous systems. The autonomic nervous system is further subdivided into the [[sympathetic nervous system|sympathetic]],  [[parasympathetic nervous system|parasympathetic]] and [[enteric nervous system|enteric]] nervous systems. The sympathetic nervous system is activated in cases of emergencies to mobilize energy, while the parasympathetic nervous system is activated when organisms are in a relaxed state. The enteric nervous system functions to control the [[gastrointestinal system]]. Nerves that exit from the brain are called [[cranial nerve]]s while those exiting from the spinal cord are called [[spinal nerve]]s.

The nervous system consists of [[nervous tissue]] which, at a cellular level, is defined by the presence of a special type of cell, called the [[neuron]]. 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 impulses traveling along thin fibers called [[axon]]s, which can be directly transmitted to neighboring cells through [[electrical synapse]]s or cause chemicals called [[neurotransmitter]]s to be released at [[chemical synapse]]s. A cell that receives a synaptic signal from a neuron may be [[depolarization|excited]], [[hyperpolarization (biology)|inhibited]], or otherwise [[neuromodulation|modulated]]. The connections between neurons can form [[neural pathway]]s, [[neural circuit]]s, and larger [[large-scale brain networks|networks]] that generate an organism's perception of the world and determine its behavior. Along with neurons, the nervous system contains other specialized cells called [[glial cell]]s (or simply glia), which provide structural and metabolic support. Many of the cells and vasculature channels within the nervous system make up the [[neurovascular unit]], which regulates cerebral blood flow in order to rapidly satisfy the high energy demands of activated neurons.<ref>{{Cite journal |last=Iadecola |first=Costantino |date=2017-09-27 |title=The Neurovascular Unit Coming of Age: A Journey through Neurovascular Coupling in Health and Disease |journal=Neuron |volume=96 |issue=1 |pages=17–42 |doi=10.1016/j.neuron.2017.07.030 |issn=1097-4199 |pmc=5657612 |pmid=28957666}}</ref>

Nervous systems are found in most [[multicellular animal]]s, 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 the [[radially symmetric]] organisms [[ctenophores]] (comb jellies) and [[cnidarians]] (which include [[Sea anemone|anemones]], [[hydras]], [[coral]]s and [[jellyfish]]) consist of a diffuse [[nerve net]]. All other animal species, with the exception of a few types of [[worm]], have a nervous system containing a brain, a central cord (or two cords running in 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 around 300 billion cells in [[African elephant]]s.<ref>{{cite journal |vauthors=Herculano-Houzel S, Avelino-de-Souza K, ''et al''. |title=The elephant brain in numbers |journal=Front Neuroanat |volume=8 |pages=46 |year=2014 |pmid=24971054 |pmc=4053853 |doi=10.3389/fnana.2014.00046 |doi-access=free}}</ref>

The central nervous system functions to send signals from one cell to others, or from one part of the body to others and to receive feedback. Malfunction of the nervous system can occur as a result of genetic defects, physical damage due to trauma or toxicity, infection, or simply [[senescence]]. The medical specialty of [[neurology]] studies disorders of the nervous system and looks for interventions that can prevent or treat them. In the peripheral nervous system, the most common problem is the failure of nerve conduction, which can be due to different causes including [[diabetic neuropathy]] and demyelinating disorders such as [[multiple sclerosis]] and [[amyotrophic lateral sclerosis]]. [[Neuroscience]] is the field of science that focuses on the study of the nervous system.