Kenyon cell

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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]

Contents

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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Neurotransmitter receptor

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Olfactory bulb

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Mushroom bodies

The mushroom bodies or corpora pedunculata are a pair of structures in the brain of insects, other arthropods, and some annelids. They are also known to play a role in olfactory learning and memory. In most insects, the mushroom bodies and the lateral horn are the two higher brain regions that receive olfactory information from the antennal lobe via projection neurons. They were first identified and described by French biologist Félix Dujardin in 1850.

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Neurotransmission

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Mitral cell

Mitral cells are neurons that are part of the olfactory system. They are located in the olfactory bulb in the mammalian central nervous system. They receive information from the axons of olfactory receptor neurons, forming synapses in neuropils called glomeruli. Axons of the mitral cells transfer information to a number of areas in the brain, including the piriform cortex, entorhinal cortex, and amygdala. Mitral cells receive excitatory input from olfactory sensory neurons and external tufted cells on their primary dendrites, whereas inhibitory input arises either from granule cells onto their lateral dendrites and soma or from periglomerular cells onto their dendritic tuft. Mitral cells together with tufted cells form an obligatory relay for all olfactory information entering from the olfactory nerve. Mitral cell output is not a passive reflection of their input from the olfactory nerve. In mice, each mitral cell sends a single primary dendrite into a glomerulus receiving input from a population of olfactory sensory neurons expressing identical olfactory receptor proteins, yet the odor responsiveness of the 20-40 mitral cells connected to a single glomerulus is not identical to the tuning curve of the input cells, and also differs between sister mitral cells. Odorant response properties of individual neurons in an olfactory glomerular module. The exact type of processing that mitral cells perform with their inputs is still a matter of controversy. One prominent hypothesis is that mitral cells encode the strength of an olfactory input into their firing phases relative to the sniff cycle. A second hypothesis is that the olfactory bulb network acts as a dynamical system that decorrelates to differentiate between representations of highly similar odorants over time. Support for the second hypothesis comes primarily from research in zebrafish.

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Calyx of Held

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Nonsynaptic plasticity

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The lateral horn is one of the two areas of the insect brain where projection neurons of the antennal lobe send their axons. The other area is the mushroom body. Several morphological classes of neurons in the lateral horn receive olfactory information through the projection neurons.

Insect olfaction

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An axo-axonic synapse is a type of synapse, formed by one neuron projecting its axon terminals onto another neuron’s axon.

References

  1. 1 2 3 4 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 31 October 2021). ISSN   1072-0502. PMC   311242 . PMID   10454370.CS1 maint: DOI inactive as of October 2021 (link)
  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. S2CID   86229892.
  4. 1 2 3 4 5 6 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. S2CID   18521720.
  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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