Insects olfactory processing

Fig-1 Schematic view of the overall olfactory processing in insects. Pheromones and other semiochemicals are detected by specialized sensilla on the antennae, where the chemical signal is transduced into nervous activity. The olfactory receptor neurons in the semiochemi-cal-detecting sensilla are connected directly to the antennal lobe. Here the semiochemical-derived electrical signals are processed and sent out (through projection neurons) to the protocerebrum. Olfactory information is then integrated with other stimulus modalities, a decision is made, and the motor system is told what to do... 

P americana is one of just a few species of insects in which both peripheral and central olfactory processing have been studied. In contrast to many short-lived lepidopterans, in which the male antenna is highly specialized for sex pheromone reception, the antennae of male cockroaches contain numerous food-responsive sensilla. In addition to olfactory sensilla, the antennae also house mechano-, hygro-and thermoreceptors, as well as contact chemoreceptors (Schaller, 1978 review Boeckh et al., 1984). Extensive ultrastructural and electrophysiological evidence has demonstrated that morphologically defined sensillum types house receptor cells of specific functional types (Sass, 1976, 1978, 1983 Schaller, 1978 Selzer, 1981, 1984 review Boeckh and Ernst, 1987). Boeckh and Ernst (1987) defined 25 types of cell according to their odor spectra, but of the 65 500 chemo- and mechanosensory sensilla on the antenna of adult male P. americana, an estimated 37 000 house cells that respond to periplanone-A and periplanone-B. 

Recent findings with terrestrial crustaceans seem to indicate that their olfactory systems have adapted to detect and process airborne volatiles (and thus are quite similar to insect olfactory systems), while maintaining the general olfactory... 

The concept that an enantiomeric pair forms a diastereomeric pair when bound, even if only transiently, to a chiral receptor, fits in with some of the results mentioned previously in which it is found that activity is brought about by one of a pair of enantiomers, and appears to be a plausible explanation of how an insect can discriminate between enantiomers. The initial olfactory process for each enantiomer is different. 

As is the case for all sensory pathways, the capacity to perceive and respond to olfactory cues (odorants) is the combined result of events that take place in both peripheral and central processing centers. These steps, which will be discussed in detail below, begin with the molecular transduction of chemical signals in the form of odorants into electrical activity by olfactory receptor neurons (ORNs) in the periphery whose axonal projections form characteristic synaptic connections with elements of the central nervous system (CNS). Within the CNS, complex patterns of olfactory signals are integrated and otherwise processed to afford recognition and ultimately, the behavioral responses to the insect s chemical environment. Within the context of pheromone recognition these responses would likely be centered on various elements of the insect s reproductive cycle. 

Figure 16.1 The three levels of molecular recognition in the pheromone olfactory system of insects. Pheromone adsorbs on the cuticle, where it enters the sensillum lymph through pores (1). The first level of molecular recognition occurs when the PBP binds and desorbs the pheromone from the cuticle (2). PBP transports the pheromone through the lymph to the receptor, where the second level of recognition occurs (3). The third level of recognition involves the pheromone-degrading enzymes, which rapidly inactivate pheromone that has dissociated from the PBP (4). PBP-pheromone and/or pheromone alone may also be removed by an endocytotic process, possibly mediated by SNMP (5). Finally, intracellular enzymes may be involved in further removal of pheromone (6).

Figure 20.1 Olfactory sensory cells of both insects and vertebrates are primary sensory cells, i.e. they are bipolar neurons extending a sensory dendritic process towards the odorous environment and projecting an unbranched axon directly to specialized target regions in the central nervous system.

The olfactory system must not only be able to detect and process information about the quality of a stimulus but also accurately code changes in concentration. At the peripheral level, insect ORNs can detect and code differences in stimulus concentration over several orders of magnitude (Todd and Baker, 1999). 

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