ventral nerve cord

The ventral nerve cord runs down the ventral ("belly", as opposed to back) plane of the organism. It is made of nervous tissue and is connected to the brain.

Ventral nerve cord neurons are physically organized into neuromeres that process signals for each body segment.{{cite journal | vauthors = Niven JE, Graham CM, Burrows M | title = Diversity and evolution of the insect ventral nerve cord | journal = Annual Review of Entomology | volume = 53 | issue = 1 | pages = 253–271 | date = 2008 | pmid = 17803455 | doi = 10.1146/annurev.ento.52.110405.091322 }} Anterior neuromeres control the anterior body segments, such as the forelegs, and more posterior neuromeres control the posterior body segments, such as the hind legs. Neuromeres are connected longitudinally, anterior to posterior, by fibrous nerve tracts called connectives. Pairs of hemisegments, corresponding to the left and right side of the ventral nerve cord, are connected horizontally by fibrous tracts called commissures.{{cite journal | vauthors = Court R, Namiki S, Armstrong JD, Börner J, Card G, Costa M, Dickinson M, Duch C, Korff W, Mann R, Merritt D, Murphey RK, Seeds AM, Shirangi T, Simpson JH, Truman JW, Tuthill JC, Williams DW, Shepherd D | display-authors = 6 | title = A Systematic Nomenclature for the Drosophila Ventral Nerve Cord | journal = Neuron | volume = 107 | issue = 6 | pages = 1071–1079.e2 | date = September 2020 | pmid = 32931755 | pmc = 7611823 | doi = 10.1016/j.neuron.2020.08.005 | doi-access = free }}

In the small worm Meara stichopi there is a pair of dorsal nerve cords instead.{{cite journal | vauthors = Martín-Durán JM, Pang K, Børve A, Lê HS, Furu A, Cannon JT, Jondelius U, Hejnol A | title = Convergent evolution of bilaterian nerve cords | journal = Nature | volume = 553 | issue = 7686 | pages = 45–50 | date = January 2018 | pmid = 29236686 | pmc = 5756474 | doi = 10.1038/nature25030 | bibcode = 2018Natur.553...45M }}

Like the vertebrate spinal cord, the function of the ventral nerve cord is to integrate and transmit nerve signals. It contains ascending and descending neurons that relay information to and from the brain, motor neurons that project into the body and synapse onto muscles, axons from sensory neurons that receive information from the body and environment, and interneurons that coordinate circuitry of all of these neurons. In addition to spiking neurons which transmit action potentials, some neural information is transmitted via non-spiking interneurons. These interneurons filter, amplify, and integrate internal and external neural signals to guide and control movement and behavior.{{cite journal | vauthors = Agrawal S, Dickinson ES, Sustar A, Gurung P, Shepherd D, Truman JW, Tuthill JC | title = Central processing of leg proprioception in Drosophila | journal = eLife | volume = 9 | pages = e60299 | date = December 2020 | pmid = 33263281 | pmc = 7752136 | doi = 10.7554/eLife.60299 | veditors = Calabrese RL, Marder E, Fujiwara T | doi-access = free }}

Ventral nerve cords are found in some phyla of the bilaterians, particularly within the nematodes, annelids and the arthropods. Ventral nerve cords are well-studied within insects, have been described in over 300 species covering all the major orders, and have remarkable morphological diversity. Many insects have a rope-ladder-like ventral nervous cord, composed of physically separated segmental ganglia. In contrast, in Drosophila, the thoracic and abdominal neuromeres are contiguous and the whole ventral nerve cord is considered to be one ganglion. The presumed common ancestral structure is rarely observed; instead the ventral nerve cords of most insects show extensive modification as well as convergence. Modifications include shifts in neuromere positions, their fusion to form composite ganglia, and, potentially, their separation to revert to individual ganglia. In organisms with fused neuromeres, the connectives are still there but are very reduced in length.

The insect ventral nerve cord develops according to a body plan based on a segmental set of 30 paired and one unpaired neuroblasts.{{cite journal | vauthors = Thomas JB, Bastiani MJ, Bate M, Goodman CS | title = From grasshopper to Drosophila: a common plan for neuronal development | journal = Nature | volume = 310 | issue = 5974 | pages = 203–207 | date = 1984 | pmid = 6462206 | doi = 10.1038/310203a0 | bibcode = 1984Natur.310..203T | s2cid = 4321262 }} A neuroblast can be uniquely identified based on its position in the array, its pattern of molecular expression, and the suite of early neurons that it produces.{{cite journal | vauthors = Harris RM, Pfeiffer BD, Rubin GM, Truman JW | title = Neuron hemilineages provide the functional ground plan for the Drosophila ventral nervous system | journal = eLife | volume = 4 | pages = e04493 | date = July 2015 | pmid = 26193122 | pmc = 4525104 | doi = 10.7554/eLife.04493 | doi-access = free }}{{cite journal | vauthors = Broadus J, Doe CQ | title = Evolution of neuroblast identity: seven-up and prospero expression reveal homologous and divergent neuroblast fates in Drosophila and Schistocerca | journal = Development | volume = 121 | issue = 12 | pages = 3989–3996 | date = December 1995 | pmid = 8575299 | doi = 10.1242/dev.121.12.3989 }} Each neuroblast gives rise to two hemilineages: an "A" hemilineage characterized by active Notch signalling, and a "B" hemilineage characterized by an absence of active Notch signalling.{{cite journal | vauthors = Truman JW, Moats W, Altman J, Marin EC, Williams DW | title = Role of Notch signaling in establishing the hemilineages of secondary neurons in Drosophila melanogaster | journal = Development | volume = 137 | issue = 1 | pages = 53–61 | date = January 2010 | pmid = 20023160 | pmc = 2796924 | doi = 10.1242/dev.041749 }} Research in the fruit fly D. melanogaster suggests that all neurons of a given hemilineage release the same primary neurotransmitter.{{cite journal | vauthors = Lacin H, Chen HM, Long X, Singer RH, Lee T, Truman JW | title = Neurotransmitter identity is acquired in a lineage-restricted manner in the Drosophila CNS | journal = eLife | volume = 8 | pages = e43701 | date = March 2019 | pmid = 30912745 | pmc = 6504232 | doi = 10.7554/eLife.43701 | doi-access = free }}

Engrailed is a transcription factor that helps regulate the gene frazzled in order to separate neuroblasts during embryonic development. The segregation of neuroblasts is essential for the formation and development of the ventral nerve cord.{{cite journal | vauthors = Joly W, Mugat B, Maschat F | title = Engrailed controls the organization of the ventral nerve cord through frazzled regulation | journal = Developmental Biology | volume = 301 | issue = 2 | pages = 542–554 | date = January 2007 | pmid = 17126316 | doi = 10.1016/j.ydbio.2006.10.019 | doi-access = free }}

• Dorsal nerve cord in chordates
• Supraesophageal ganglion, the arthropod "brain"
• Nerve net in cnidaria and echinodermata phyla
• Hemichordates, who have both dorsal and ventral nerve cords

{{Reflist}}

• [http://www.prokop.co.uk/Research/Drosi-Info/nerve-cords.html Comparison of spinal cord and ventral nerve cord]
• [http://www.lobsters.org/tlcbio/biology6.html Nervous system of a lobster]
• [https://web.archive.org/web/20110303060627/http://www.entomology.umn.edu/cues/4015/morpology/ Insect morphology]

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Category:Annelid anatomy

Category:Arthropod anatomy

Category:Nematode anatomy

Category:Invertebrate nervous system