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Antennal Lobe

The antennal lobes (ALs) are a pair of glomerular neuropiles receiving input from the antennal nerve. The AL consists of local interneurons (LNs) whose neurites arborise in the AL and in some cases in the entire deutocerebrum, of projection neurons (PNs) that receive inputs within the AL and relay information to high olfactory centers in the protocerebrum, and of the axon terminals of antennal olfactory receptor neurons (ORNs). Dense regions of synaptic neuropil in which these neurons connect are called glomeruli. Numerous glomeruli form a grape-like structure (Anton and Homberg, 1999). Besides, there are a few centrifugal neurons that transmit descending (feedback) information from higher centers to the AL.

The absolute numbers of these different types of neurons differ widely across species. The number of ORNs ranges from 104 to 105, of local interneurons from 300 to 750, and of projection neurons from 200 to 1000. Information from ORNs converges onto projection neurons at a ratio of approximately 100 : 1. The olfactory glomerulus is one of the most conspicuous and well-delineated anatomical modules in both vertebrates and invertebrates (Hildebrand and Shepherd, 1997). In insects, the number of glomeruli in the primary olfactory neuropil is generally much lower than in the mouse (mouse: ca. 1800; Drosophila: 43; honeybee: 166; silkmoth: 60, but locust: ca. 1000) so that each glomerulus can be identified easily in an insect brain (Anton and Homberg, 1999). In Drosophila, ORNs that express the same odorant receptor project to one or at most two identified glomeruli. Therefore, insects are excellent models to investigate the general principles of olfactory information processing.

In moths and cockroaches, sexual dimorphism of the Als at glomerular level is well studied. Males possess a macroglomerular complex (MGC) that is a group of large male-specific glomeruli processing sex pheromone information. Ordinary glomeruli are smaller glomeruli that process general odors. In female moths, “large female glomeruli” are present instead (Rössler et al. 1998; Berg et al. 2002), processing host-plant (Shields and Hildebrand 2001) or pheromone and general plant odor information, (Hillier et al. 2006).

Olfactory information processed in the AL is transmitted by the PNs to second order centers, the mushroom body (MB) and other protocerebral (PC) areas, in particular the lateral horn (LH). Projection neurons from the same glomerulus have also the same target areas in the PC, implying that odorant maps are preserved up to that stage.



Reference

Anton S, Homberg U (1999) Antennal lobe structure. In: Hansson BS (ed)
Insect olfaction. Springer, Berlin, pp 97-124.

Berg BG, Galizia CG, Brandt R, Mustaparta H (2002) Digital atlases of the antennal lobe in two species of tobacco budworm moths, the Oriental Helicoverpa assulta (male) and the American Heliothis virescens (male and female).

Hildebrand JG, Shepherd GM (1997) Mechanisms of olfactory discrimination: converging evidence for common principles across phyla. Annu Rev Neurosci. 20:595-631.

Hillier NK, Kleineidam C, Vickers NJ (2006) Physiology and glomerular projections of olfactory receptor neurons on the antenna of female Heliothis virescens (Lepidoptera: Noctuidae) responsive to behaviorally relevant odors. J Comp Physiol A Neuroethol Sens Neural Behav Physiol. 192:199-219.

Rössler W, Tolbert LP, Hildebrand JG (1998) Early formation of sexually dimorphic glomeruli in the developing olfactory lobe of the brain of the moth Manduca sexta. J Comp Neurol. 396:415-428.

Royet JP, Souchier C, Jourdan F, Ploye H. (1988) Morphometric study of the glomerular population in the mouse olfactory bulb: numerical density and size distribution along the rostrocaudal axis. J Comp Neurol. 270:559-68.

Schneider D. (1964) Insect antennae. Annu Rev Entomol 9:103-122.

Shields VD, Hildebrand JG (2001) Responses of a population of antennal olfactory receptor cells in the female moth Manduca sexta to plant-associated volatile organic compounds. J Comp Physiol A. 186:1135-51.


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