Antennal Lobe

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The antennal lobe is a dense neuropil structure of the medial part of the brain in which the olfactory receptor neuron axons terminate. The axons together with local neurons and antennal lobe projection neurons form globular clusters in which synaptic interactions occur, the glomeruli [Anton & Homberg, 1999]．There are also some efferent neurons from other brain areas that innervate the antennal lobe, presumably forming a feedback loop.
　Depending on the species, there are about 10000-100000 olfactory receptor neurons per antenna, 300-750 local interneurons, and 200-1000 antennal lober projection neurons. The convergence ratio in the antennal lobe is thus about 1/100 [Schneider, 1964]．In vertebrates, the organisation of the primary olfactory processing center, the olfactory bulb, is surprisingly similar to the antennal lobe, also having a glomerular organisation [Hildebrand & Shepherd, 1997]．As for the number of glomeruli, the mouse olfactory bulb contains about 1800 [Royet et al., 1988] while the number is one order of magnitude lower in most insects in which the glomeruli are conserved among individuals of a species (fruit fly: 43, silkmoth: ca. 60, honey bee: 166, but a larger number in the locust) [Anton & Homberg, 1999]．In Drosophila, it has been shown that olfactory receptor neurons that express the same odorant receptor always project to the same glomerulus (to 2 glomeruli in some cases). The antennal lobe of insects is a convenient model system to study olfactory information processing.

In the antennal lobe of some insects, for instance moths, conspicuous sexual dimorphism is found. The males posess a specific region in the antennal lobe composed of very large glomeruli. This is the macroglomerular complex, which is concerned with the processing of sex pheromone information. The other, ordinary glomeruli involved in the processing of general odour information are markely smaller.

Information from the antennal lobe is conveyed to the protocerebrum via antennal lobe projection neurons. These project to the mushroom body and the lateral protocerebrum (in particular the lateral horn).

• Antennal lobe structure

• Antennal lobe function

• Antennal lobe neurons

• Antennal lobe neurotransmitters

• Antennal lobe imaging

References

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

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

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 270:559–568.

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