Kenyon cell

Kenyon cells are the intrinsic neurons of the mushroom body,[1] a neuropil found in the brains of most arthropods and some annelids.[2] They were first described by F. C. Kenyon in 1896.[3] The number of Kenyon cells in an organism varies greatly between species. For example, in the fruit fly, Drosophila melanogaster, there are about 2,500 Kenyon cells per mushroom body, while in cockroaches there are about 230,000.[4]

Structure

While the exact features of Kenyon cells can vary between species, there are enough similarities to define their general structure. Kenyon cells have dendritic branches that arborize in the calyx or calyces, cup-shaped regions of the mushroom body. At the base of the calyces, Kenyon cell axons come together and form a bundle known as the pedunculus. At the end of the pedunculus, Kenyon cell axons bifurcate and extend branches into the vertical and medial lobes.[4]

Kenyon cells are mainly postsynaptic in the calyces, where their synapses form microglomeruli. These microglomeruli are made up of Kenyon cell dendrites, cholinergic boutons, and GABAergic terminals. Antennal lobe projection neurons are the source of the cholinergic input, and the GABAergic input is from protocerebral neurons.[4]

Kenyon cells are presynaptic to mushroom body output neurons in the lobes. However, the lobes are not only output regions; Kenyon cells are both pre and postsynaptic in these regions.[1]

The cells are subdivided into subtypes; for example, those that have their cell bodies outside of the calyx cup are called clawed Kenyon cells.[5]

Development

Kenyon cells are produced from precursors known as neuroblasts. The number of neuroblasts varies greatly between species. In Drosophila melanogaster, Kenyon cells are produced from only four neuroblasts, while in the honey bee they are the product of thousands of neuroblasts. Differences in neuroblast number between species are related to the final number of Kenyon cells in an adult.[4]

The positioning of Kenyon cells depends on their birth order. The somata of early-born Kenyon cells are pushed outward as more Kenyon cells are created. This results in a concentric pattern of cell bodies, with the somata of the last-born cells in the center, where the neuroblast had been, and the somata of the first-born cells at the outermost margins of the cell body area.[1] Where a Kenyon cell sends its dendrites in the calyces and which lobes it projects its axons to varies based on its birth-order.[4] Distinct types of Kenyon cells form at specific times during development.[1]

Function

Mushroom bodies are essential for olfactory learning and memory. Odor information is represented by sparse combinations of Kenyon cells. Learning is facilitated by dopamine-driven plasticity of the odor response of Kenyon cells.[6] The cAMP signaling cascade, especially protein kinase A, must function properly in Kenyon cells for learning and memory to occur.[4]

Information about odors may be encoded in the mushroom body by the identities of the responsive neurons as well as the timing of their spikes.[7] Experiments in locusts have shown that Kenyon cells have their activity synchronized to 20-Hz neural oscillations and are particularly responsive to projection neuron spikes at specific phases of the oscillatory cycle.[8]

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References
  1. Farris, Sarah M.; Sinakevitch, Irina (2003-08-01). "Development and evolution of the insect mushroom bodies: towards the understanding of conserved developmental mechanisms in a higher brain center". Arthropod Structure & Development. Development of the Arthropod Nervous System: a Comparative and Evolutionary Approach. 32 (1): 79–101. doi:10.1016/S1467-8039(03)00009-4. PMID 18088997.
  2. Strausfeld, Nicholas J.; Hansen, Lars; Li, Yongsheng; Gomez, Robert S.; Ito, Kei (1998-05-01). "Evolution, Discovery, and Interpretations of Arthropod Mushroom Bodies". Learning & Memory. 5 (1): 11–37. doi:10.1101/lm.5.1.11 (inactive 2020-03-25). ISSN 1072-0502. PMC 311242. PMID 10454370.
  3. Kenyon, F. C. (1896-03-01). "The brain of the bee. A preliminary contribution to the morphology of the nervous system of the arthropoda". Journal of Comparative Neurology. 6 (3): 133–210. doi:10.1002/cne.910060302. ISSN 1550-7130.
  4. Fahrbach, Susan E. (2005-12-06). "Structure of the mushroom bodies of the insect brain". Annual Review of Entomology. 51 (1): 209–232. doi:10.1146/annurev.ento.51.110104.150954. ISSN 0066-4170. PMID 16332210.
  5. Strausfeld NJ (August 2002). "Organization of the honey bee mushroom body: representation of the calyx within the vertical and gamma lobes". J. Comp. Neurol. 450 (1): 4–33. doi:10.1002/cne.10285. PMID 12124764.
  6. Owald, David; Waddell, Scott (2015-12-01). "Olfactory learning skews mushroom body output pathways to steer behavioral choice in Drosophila". Current Opinion in Neurobiology. Circuit plasticity and memory. 35: 178–184. doi:10.1016/j.conb.2015.10.002. PMC 4835525. PMID 26496148.
  7. Gupta, Nitin; Stopfer, Mark (6 October 2014). "A temporal channel for information in sparse sensory coding". Current Biology. 24 (19): 2247–56. doi:10.1016/j.cub.2014.08.021. PMC 4189991. PMID 25264257.
  8. Gupta, Nitin; Singh, Swikriti Saran; Stopfer, Mark (2016-12-15). "Oscillatory integration windows in neurons". Nature Communications. 7: 13808. Bibcode:2016NatCo...713808G. doi:10.1038/ncomms13808. ISSN 2041-1723. PMC 5171764. PMID 27976720.
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