Ventral nerve cord

The ventral nerve cord (VNC) is a major structure of the invertebrate central nervous system. It is the functional equivalent of the vertebrate spinal cord. The VNC coordinates neural signaling from the brain to the body and vice versa, integrating sensory input and locomotor output [2][3]

Left, a schematic of the Drosophila central nervous system, including the brain and ventral nerve cord. Right, a cross section of the ventral nerve cord, illustrating sensory input and motor output. Adapted with permission from [1]

Structure

The ventral nerve cord usually consists of segmental ganglia with the nerve cords running down the ventral ("belly", as opposed to back) plane of the organism. Ventral nerve cords from anterior to posterior (the thoracic and abdominal tagma in the arthropods) are made up of segmental ganglia that are connected by a tract of nerve fibers passing from one side to the other of the nerve cord called commissures . The complete system bears some likeness to a rope ladder. In some animals the bilateral ganglia are fused into a single large ganglion per segment.[3]

Evolution

Ventral nerve cords are found in some phyla of the bilaterians, particularly within the nematodes, annelids and the arthropods. They are well-studied within insects, and VNCs have been described in over 300 species covering all the major orders. The presumed common ancestral structure is rarely observed; instead the VNCs 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.[4]

Development

The insect VNC develops according to a body plan based on a segmental set of 30 paired and one unpaired neuroblasts[5]. 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[6][7]. 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[8]. Research in the fruit fly D. melanogaster suggests that all neurons of a given hemilineage release the same primary neurotransmitter[9].

"engrailed" is a transcription factor that helps regulate the gene "frizzled" in order to separate neuroblast during embryonic development. The segregation of neuroblast is essential for the formation and development of the ventral nerve cord.[10]

See also

References
  1. Tuthill, John C; Wilson, Rachel I (Oct 24, 2016). "Mechanosensation and adaptive motor control in insects" (PDF). Current Biology. 26: R1022–R1038. doi:10.1016/j.cub.2016.06.070. PMID 27780045.
  2. Venkatasubramanian, L; Mann, RS (2019). "The development and assembly of the Drosophila adult ventral nerve cord". Current Opinion in Neurobiology. 56: 135–143. doi:10.1016/j.conb.2019.01.013. PMC 6551290. PMID 30826502.
  3. Hickman, Cleveland; Roberts, L; Keen, S.; Larson, A.; Eisenhour, D. Animal Diversity (4th ed.). New York: McGraw Hill. ISBN 978-0-07-252844-2.
  4. Niven, Jeremy E.; Graham, Christopher M.; Burrows, Malcolm (2008). "Diversity and Evolution of the Insect Ventral Nerve Cord". Annual Review of Entomology. 53 (1): 253–271. doi:10.1146/annurev.ento.52.110405.091322. ISSN 0066-4170. PMID 17803455.CS1 maint: ref=harv (link)
  5. Thomas, John B.; Bastiani, Michael J.; Bate, Michael; Goodman, Corey S. (1984). "From grasshopper to Drosophila: a common plan for neuronal development". Nature. 310 (5974): 203–207. Bibcode:1984Natur.310..203T. doi:10.1038/310203a0. ISSN 0028-0836. PMID 6462206.
  6. Harris, Robin M; Pfeiffer, Barret D; Rubin, Gerald M; Truman, James W (2015-07-20). "Neuron hemilineages provide the functional ground plan for the Drosophila ventral nervous system". eLife. 4: e04493. doi:10.7554/eLife.04493. ISSN 2050-084X. PMC 4525104. PMID 26193122.
  7. Broadus, J.; Doe, C. Q. (1995). "Evolution of neuroblast identity: seven-up and prospero expression reveal homologous and divergent neuroblast fates in Drosophila and Schistocerca". Development (Cambridge, England). 121 (12): 3989–3996. ISSN 0950-1991. PMID 8575299.
  8. Truman, J. W.; Moats, W.; Altman, J.; Marin, E. C.; Williams, D. W. (2010-01-01). "Role of Notch signaling in establishing the hemilineages of secondary neurons in Drosophila melanogaster". Development. 137 (1): 53–61. doi:10.1242/dev.041749. ISSN 0950-1991. PMC 2796924. PMID 20023160.
  9. Lacin, Haluk; Chen, Hui-Min; Long, Xi; Singer, Robert H; Lee, Tzumin; Truman, James W (2019-03-26). "Neurotransmitter identity is acquired in a lineage-restricted manner in the Drosophila CNS". eLife. 8: e43701. doi:10.7554/eLife.43701. ISSN 2050-084X. PMC 6504232. PMID 30912745.
  10. Joly, Willy; Mugat, Bruno; Maschat, Florence (2007). "Engrailed controls the organization of the ventral nerve cord through frazzled regulation". Developmental Biology. 301 (2): 542–554. doi:10.1016/j.ydbio.2006.10.019. PMID 17126316.


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