Ventral nerve cord

The ventral nerve cord is a major structure of the invertebrate central nervous system. It is the functional equivalent of the vertebrate spinal cord.[2] The ventral nerve cord coordinates neural signaling from the brain to the body and vice versa, integrating sensory input and locomotor output.[1] Because arthropods have an open circulatory system, decapitated insects can still walk, groom, and mate—illustrating that the circuitry of the ventral nerve cord is sufficient to perform complex motor programs without brain input.[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 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.[4] 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.[4][5]

In the small worm Meara stichopi there is a pair of dorsal nerve cords instead.[6]

Function

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.[3] 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.[7]

Evolution

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.[5] 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.[4] In organisms with fused neuromeres, the connectives are still there but are very reduced in length.[4]

Development

The insect ventral nerve cord develops according to a body plan based on a segmental set of 30 paired and one unpaired neuroblasts.[8] 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.[9][10] 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.[11] Research in the fruit fly D. melanogaster suggests that all neurons of a given hemilineage release the same primary neurotransmitter.[12]

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.[13]

See also

References
  1. Tuthill JC, Wilson RI (October 2016). "Mechanosensation and Adaptive Motor Control in Insects". Current Biology. 26 (20): R1022–R1038. doi:10.1016/j.cub.2016.06.070. PMC 5120761. PMID 27780045.
  2. Hickman C, Roberts L, Keen S, Larson A, Eisenhour D (2007). Animal Diversity (4th ed.). New York: McGraw Hill. ISBN 978-0-07-252844-2.
  3. Venkatasubramanian L, Mann RS (June 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.
  4. Niven JE, Graham CM, Burrows M (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. PMID 17803455.
  5. Court R, Namiki S, Armstrong JD, Börner J, Card G, Costa M, et al. (September 2020). "A Systematic Nomenclature for the Drosophila Ventral Nerve Cord". Neuron. 107 (6): 1071–1079.e2. doi:10.1016/j.neuron.2020.08.005. PMC 7611823. PMID 32931755.
  6. Martín-Durán JM, Pang K, Børve A, Lê HS, Furu A, Cannon JT, Jondelius U, Hejnol A (January 2018). "Convergent evolution of bilaterian nerve cords". Nature. 553 (7686): 45–50. Bibcode:2018Natur.553...45M. doi:10.1038/nature25030. PMC 5756474. PMID 29236686.
  7. Agrawal S, Dickinson ES, Sustar A, Gurung P, Shepherd D, Truman JW, Tuthill JC (December 2020). Calabrese RL, Marder E, Fujiwara T (eds.). "Central processing of leg proprioception in Drosophila". eLife. 9: e60299. doi:10.7554/eLife.60299. PMC 7752136. PMID 33263281.
  8. Thomas JB, Bastiani MJ, Bate M, Goodman CS (1984). "From grasshopper to Drosophila: a common plan for neuronal development". Nature. 310 (5974): 203–207. Bibcode:1984Natur.310..203T. doi:10.1038/310203a0. PMID 6462206. S2CID 4321262.
  9. Harris RM, Pfeiffer BD, Rubin GM, Truman JW (July 2015). "Neuron hemilineages provide the functional ground plan for the Drosophila ventral nervous system". eLife. 4: e04493. doi:10.7554/eLife.04493. PMC 4525104. PMID 26193122.
  10. Broadus J, Doe CQ (December 1995). "Evolution of neuroblast identity: seven-up and prospero expression reveal homologous and divergent neuroblast fates in Drosophila and Schistocerca". Development. 121 (12): 3989–3996. doi:10.1242/dev.121.12.3989. PMID 8575299.
  11. Truman JW, Moats W, Altman J, Marin EC, Williams DW (January 2010). "Role of Notch signaling in establishing the hemilineages of secondary neurons in Drosophila melanogaster". Development. 137 (1): 53–61. doi:10.1242/dev.041749. PMC 2796924. PMID 20023160.
  12. Lacin H, Chen HM, Long X, Singer RH, Lee T, Truman JW (March 2019). "Neurotransmitter identity is acquired in a lineage-restricted manner in the Drosophila CNS". eLife. 8: e43701. doi:10.7554/eLife.43701. PMC 6504232. PMID 30912745.
  13. Joly W, Mugat B, Maschat F (January 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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