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. 2010 Mar 31;4:6. doi: 10.3389/fncel.2010.00006

Figure 2.

Figure 2

Mechanisms of odor processing in the peripheral olfactory system of the larval and adult Drosophila. (A) The larval olfactory system is composed of two dorsal organs located at the tip of the head. Inset: Each dorsal organ is a central, multiporous “dome”. Odorant receptors (ORs) reside in the dendritic membranes of 21 olfactory receptor neurons (ORNs) which densely innervate the dome. (B) In adults, the 3rd antennal segments and maxillary palps are covered with olfactory sensilla. Inset: ORs reside in the dendritic membranes of ORNs housed within an individual porous sensillum. (C) The binding domains of ORs determine the specificity and affinity of an OR for specific odors. (D) Increasing the number of ORs in the ORN dendrites increases the number of available odor-binding sites, possibly enhancing ORN sensitivity. (E) Larval ORNs project ipsilaterally to discreet glomeruli within the larval antennal lobe (LAL) and synapse onto second-order olfactory projection neurons (PNs). In larvae, the ORN to glomerulus (GLOM) to PN ratio is 1:1:1. (F) Adult ORNs project bilaterally to discreet glomeruli within both the left and right antennal lobes (AL) and synapse onto PNs. In adults, the ORN to GLOM to PN ratio is ∼26:1:3. (G) In adults, increasing the number of ORN inputs enhances the PN signal-to-noise ratio (SNR). The maximum SNR increase achievable by pooling n inputs is √n. (H) In adults and larvae, inhibitory local interneurons (LNs) connect many glomeruli in the AL. These are thought to suppress PN output downstream of strong ORN input (inset, adapted from Olsen and Wilson, 2008). (I) In adults, excitatory LNs connect many glomeruli throughout the AL and enhance PN output downstream of weak ORN input (inset, adapted from Olsen et al., 2007). (J) The properties of OR specificity and affinity combined with lateral processing in the AL contribute to concentration invariant odor perception and behavior. Wild type flies may theoretically show stable behavioral attraction (positive response index (RI)) across a range of stimulus intensities (red). Detection over a range of concentrations might involve an array of narrowly tuned OR/ORN classes (say, 3) and individuals with only a single functional OR may be expected to respond across a narrow range of stimulus intensities (blue, green, maroon).