Central Processing Of Chemosensory Input

Over the past 20 years, studies of insect olfactory systems have produced a rich literature on the topic of central processing, particularly for pheromonal systems. Work on gustatory systems is far less advanced. The section on insect pheromones provides more information on olfactory processing. This section simply contrasts the gross morphology of the two systems. Both olfactory and gustatory sensory cells are primary neurons; that is, they connect the periphery (sensillum) directly with the central nervous system. Olfactory cells, on the antennae as well as on the palpi, send their axons directly to the antennal lobe, which

FIGURE 5 In insects, both olfactory and gustatory cells send axons (afferents) directly to the central nervous system. The first synapse (information relay point) is in a particular part of the central nervous system for each sensory modality. (a) Olfactory afferents go to the antennal lobe, where the input is organized in a manner resembling a bunch of grapes—glomerular organization. [From Anton, S., and Homberg, U. (1999). In "Insect Olfaction" (B. S. Hansson, ed.), Fig. 6a, p. 110. © Springer-Verlag GmbH & Co. KG, Berlin.] (b) Gustatory afferents from mouthpart sensilla go to the subesophageal ganglion, where they project into a discrete space that is not organized into glomeruli. For both (a) and (b), subsequent processing is done by first-level and higher interneurons. [From Edgecomb, R. S., and Murdock, L. L. (1992). Central projections of axons from the taste hairs on the labellum and tarsi of the blowfly, Phormia regina Melgeri. J. Comp. Neurol. 315, 431-444. Reprinted by permission of Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.]

SOesophageal ganglion of fleshfly

FIGURE 5 In insects, both olfactory and gustatory cells send axons (afferents) directly to the central nervous system. The first synapse (information relay point) is in a particular part of the central nervous system for each sensory modality. (a) Olfactory afferents go to the antennal lobe, where the input is organized in a manner resembling a bunch of grapes—glomerular organization. [From Anton, S., and Homberg, U. (1999). In "Insect Olfaction" (B. S. Hansson, ed.), Fig. 6a, p. 110. © Springer-Verlag GmbH & Co. KG, Berlin.] (b) Gustatory afferents from mouthpart sensilla go to the subesophageal ganglion, where they project into a discrete space that is not organized into glomeruli. For both (a) and (b), subsequent processing is done by first-level and higher interneurons. [From Edgecomb, R. S., and Murdock, L. L. (1992). Central projections of axons from the taste hairs on the labellum and tarsi of the blowfly, Phormia regina Melgeri. J. Comp. Neurol. 315, 431-444. Reprinted by permission of Wiley-Liss, Inc., a subsidiary of John Wiley & Sons, Inc.]

is a part of the insect brain. Gustatory cells, for the most part, send their axons to the ganglion for the segment in which the sensory cell occurs. Figure 5A shows a typical innervation pattern for antennal and mouthpart olfactory cells in a mosquito, and Fig. 5B shows innervation from the gustatory cells in the mouthparts of a blowfly. A striking difference in the organization of the two parts of the central nervous systems receiving these imputs is repeated across many animal phyla. Olfactory systems are characterized by a glomerular arrangement (like a bunch of grapes) of the neural centers

(neuropile) that receive olfactory afferents (input), but gustatory systems have no such patterned arrangement. The distribution of olfactory inputs into glomeruli suggests a strong association of structure with function, and this is most clearly seen in the macroglomeruli, which receive only pheromonal afferents in male moths. There is undoubtedly an association of structure with function in the way gustatory inputs are arranged, but the lack of a glomerular substructure makes any such system far less obvious. The two ways of organizing chemosensory input, throughout animals, may also point to important differences in coding and or evolution.

See Also the Following Articles

Antennae • Brain • Feeding Behavior • Mechanoreception • Mouthparts • Pheromones

Further Reading

Clyne, P. J., Warr, C. G., and Carlson, J. R. (2000). Candidate taste receptors in Drosophila. Science 287, 1830—1834. Hansson, B. S. (ed.) (1999). "Insect Olfaction." Springer-Verlag, Berlin,

Heidelberg. (See especially Chaps. 1, 2, 3 and 4.) Hildebrand, J. G., and Shephers, G. M. (1997). Mechanisms of olfactory discrimination: Converging evidence from common principles across phyla. Annu. Rev. Neurosci. 20, 595—631. Mitchell, B. K. (1994). The chemosensory basis of host—plant recognition in Chrysomelidae. In "Novel Aspects of the Biology of the Chrysomelidae" (P. H. Jolivet, M. L. Cox, and E. Petitpierre, eds.), pp. 141-151. Kluwer, Dordrecht, The Netherlands. Mitchell, B. K., Itagaki, H., and Rivet, M.-P. (1999). Peripheral and central structures involved in insect gustation. Microsc. Res. Technique 47, 401-415. Pollack, G. S., and Balakrishnan, R. (1997). Taste sensilla of flies: Function, central neuronal projections, and development. Microsc. Res. Technique 39, 532-546.

Schoonhoven, L. M., Jermy, T., and van Loon, J. J. A. (1998). "Insect-Plant

Biology." Chapman Hall, London. (See especially Chaps. 5 and 6.) Steinbrecht, R. A. (1997). Pore structures in insect olfactory sensilla: A review of data and concepts. Int. J. Insect Morphol. Embryol. 26, 229-245. Zacharuk, R. Y., and Shields, V. D. (1991). Sensilla of immature insects. Annu. Rev. Entomol. 36, 331-354.

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