Editing Nerve net (section)

==Evolution==

The emergence of true nervous tissue was once thought to have followed the [[Genetic divergence|divergence]] of last common ancestor of [[Porifera]] (sponges) and [[Cnidaria]] and [[Ctenophora]]. Recent taxonomic divisions, however, suggest that [[Ctenophora]] is sister to the other extant [[metazoans|Metazoa]].<ref name="MorozKocot2014">{{cite journal|last1=Moroz|first1=Leonid L.|last2=Kocot|first2=Kevin M.|last3=Citarella|first3=Mathew R.|last4=Dosung|first4=Sohn|last5=Norekian|first5=Tigran P.|last6=Povolotskaya|first6=Inna S.|last7=Grigorenko|first7=Anastasia P.|last8=Dailey|first8=Christopher|last9=Berezikov|first9=Eugene|last10=Buckley|first10=Katherine M.|last11=Ptitsyn|first11=Andrey|last12=Reshetov|first12=Denis|last13=Mukherjee|first13=Krishanu|last14=Moroz|first14=Tatiana P.|last15=Bobkova|first15=Yelena|last16=Yu|first16=Fahong|last17=Kapitonov|first17=Vladimir V.|last18=Jurka|first18=Jerzy|last19=Bobkov|first19=Yuri V.|last20=Swore|first20=Joshua J.|last21=Girardo|first21=David O.|last22=Fodor|first22=Alexander|last23=Gusev|first23=Fedor|last24=Sanford|first24=Rachel|last25=Bruders|first25=Rebecca|last26=Kittler|first26=Ellen|last27=Mills|first27=Claudia E.|last28=Rast|first28=Jonathan P.|last29=Derelle|first29=Romain|last30=Solovyev|first30=Victor V.|last31=Kondrashov|first31=Fyodor A.|last32=Swalla|first32=Billie J.|last33=Sweedler|first33=Jonathan V.|last34=Rogaev|first34=Evgeny I.|last35=Halanych|first35=Kenneth M.|last36=Kohn|first36=Andrea B.|title=The ctenophore genome and the evolutionary origins of neural systems|journal=Nature|volume=510|issue=7503|year=2014|pages=109–114|issn=0028-0836|doi=10.1038/nature13400|pmid=24847885|pmc=4337882|bibcode=2014Natur.510..109M|doi-access=free}}</ref><ref name="WhelanKocot2015">{{cite journal|last1=Whelan|first1=Nathan V.|last2=Kocot|first2=Kevin M.|last3=Moroz|first3=Leonid L.|last4=Halanych|first4=Kenneth M.|title=Error, signal, and the placement of Ctenophora sister to all other animals|journal=Proceedings of the National Academy of Sciences|volume=112|issue=18|year=2015|pages=5773–5778|issn=0027-8424|doi=10.1073/pnas.1503453112|pmid=25902535|pmc=4426464|bibcode=2015PNAS..112.5773W|doi-access=free}}</ref><ref name="BorowiecLee2015">{{cite journal|last1=Borowiec|first1=Marek L.|last2=Lee|first2=Ernest K.|last3=Chiu|first3=Joanna C.|last4=Plachetzki|first4=David C.|title=Extracting phylogenetic signal and accounting for bias in whole-genome data sets supports the Ctenophora as sister to remaining Metazoa|journal=BMC Genomics|volume=16|issue=1|year=2015|page=987|issn=1471-2164|doi=10.1186/s12864-015-2146-4|pmid=26596625|pmc=4657218|doi-access=free}}</ref><ref name="WhelanKocot2017">{{cite journal|last1=Whelan|first1=Nathan V.|last2=Kocot|first2=Kevin M.|last3=Moroz|first3=Tatiana P.|last4=Mukherjee|first4=Krishanu|last5=Williams|first5=Peter|last6=Paulay|first6=Gustav|last7=Moroz|first7=Leonid L.|last8=Halanych|first8=Kenneth M.|title=Ctenophore relationships and their placement as the sister group to all other animals|journal=Nature Ecology & Evolution|volume=1|issue=11|year=2017|pages=1737–1746|issn=2397-334X|doi=10.1038/s41559-017-0331-3|pmid=28993654|pmc=5664179|bibcode=2017NatEE...1.1737W }}</ref>
 
[[Porifera]] is an extant phylum within the [[Animal|animal kingdom]], and species belonging to this phylum do not have nervous systems.  The placement of [[Ctenophora]] implies that either nervous systems were lost in the ancestor of Porifera, or they evolved independently in the ancestors of Ctenophora and [[ParaHoxozoa]].
Although Porifera do not form [[synapse]]s and [[myofibril]]s which allow for neuromuscular transmission, they do differentiate a proto-neuronal system and contain [[homology (biology)|homologs]] of several genes found in [[Cnidaria]] which are important in nerve formation.<ref name=Sakarya>{{Cite journal|author1=Sakarya O|author2=Armstrong KA|author3=Adamska M|editor1-last=Vosshall|editor1-first=Leslie |title=A post-synaptic scaffold at the origin of the animal kingdom |journal=PLOS ONE |volume=2 |issue=6 |pages=e506 |year=2007 |pmid=17551586 |pmc=1876816 |doi=10.1371/journal.pone.0000506|display-authors=1|bibcode=2007PLoSO...2..506S|last4=Adamski|last5=Wang|last6=Tidor|last7=Degnan|last8=Oakley|last9=Kosik|doi-access=free}}</ref> Sponge cells have the ability to communicate with each other via calcium signaling or by other means.<ref name=Jacobs>{{Cite journal|title=Evolution of sensory structures in basal metazoa |journal=Integr Comp Biol |volume=47|issue=5 |year=2007 |pages=712–723 |doi=10.1093/icb/icm094 |vauthors=Jacobs DK, Nakanishi N, Yuan D, etal |pmid=21669752 |doi-access=free }}</ref> Sponge larvae differentiate [[Sensory receptor|sensory cells]]  which respond to stimuli including light, gravity, and water movement, all of which increase the [[Fitness (biology)|fitness]] of the organism. In addition to sensory cells differentiated during development, adult Porifera display contractile activity.<ref name=Galliot>{{Cite journal|vauthors=Galliot B, Quiquand M|editor1=Ernest |title=A two-step process in the emergence of neurogenesis |journal=European Journal of Neuroscience |volume=34 |issue=6 |pages=847–862 |year=2011 |pmid=21929620 |doi=10.1111/j.1460-9568.2011.07829.x |s2cid=41301807 }}</ref>  

The emergence of nervous systems has been linked to the evolution of [[Voltage-gated sodium channel#Voltage-gated|voltage-gated sodium (Nav) channels]]. The Nav channels allow for communication between cells over long distances through the propagation of action potentials, whereas [[Voltage-dependent calcium channel|voltage-gated (Cav) calcium channels]] allow for unmodulated intercellular signaling. It has been hypothesized that Nav channels differentiated from Cav channels either at the emergence of nervous systems or before the emergence of multicellular organisms, although the origin of Nav channels in history remains unknown. Porifera either came about as a result of the divergence with Cnidaria and Ctenophora or they  lost the function of the gene encoding Nav channels. As a result, Porifera contain Cav channels which allows for intercellular signaling, but they lack Nav channels which provide for the conductance of action potentials in nerve nets.<ref name=Liebeskind>{{Cite journal|vauthors=Liebeskind BJ, Hillis, DM, Zakon HH|title=Evolution of sodium channels predates the origin of nervous systems in animals | journal=Proceedings of the National Academy of Sciences of the United States of America |volume=108 |issue=22 |pages= 9154–9159 |year=2011 |pmid=21576472 |pmc=3107268 |doi=10.1073/pnas.1106363108|bibcode= 2011PNAS..108.9154L|doi-access=free }}</ref>

Nerve nets are found in species in the phyla Cnidaria (e.g. [[scyphozoa]], [[box jellyfish]], and [[sea anemone]]s), [[Ctenophore|Ctenophora]], and [[Echinoderm]]ata. Cnidaria and Ctenophora both exhibit radial symmetry and are collectively known as coelenterates. [[Coelenterata|Coelenterates]] diverged 570 million years ago, prior to the [[Cambrian explosion]], and they are the first two phyla to possess nervous systems which differentiate during development and communicate by synaptic conduction. Most research on the evolution of nervous tissue concerning nerve nets has been conducted using cnidarians. The nervous systems of coelenterates allow for sensation, contraction, locomotion, and hunting/feeding behaviors. Coelenterates and bilaterians share common neurophysiological mechanisms; as such, coelenterates provide a model system for tracing the origins of [[neurogenesis]]. This is due to the first appearance of neurogenesis having occurred in [[eumetazoa]], which was a common ancestor of coelenterates and bilaterians. A second wave of neurogenesis occurred after the divergence of coelenterata in the common ancestor of bilateria.<ref name=Galliot /> Although animals with nerve nets lack a true brain, they have the ability to display complex movements and behaviors. The presence of a nerve net allows an organism belonging to the aforementioned phyla of Cnidaria, Ctenophora, and Echinodermata to have increased fitness as a result of being able to respond to their environment.