Function of Peripheral Olfactory Organs

The function of the olfactory receptor organs of insects is to send the central nervous system (CNS), on a split-second basis, a rendering of the relative abundance of single odorants that together comprise odour blends. The organs’ olfactory receptor neurons (ORNs) generate action potentials (spikes) that are transmitted to the antennal lobe (AL) (Chaps. 4 and 5), which then sends the processed signal on to higher integrative centers of the brain (Chap. 9). When an ORN is hit by molecules of an active stimulus, the molecules are thought to interact with receptor proteins situated in the dendritic membrane (Chap. 2). These interactions form a receptor potential that travels down the dendrite to a site where action potentials are triggered when the receptor potential reaches a threshold level. The action potentials travel down the length of the ORN axon into the AL (Chap. 5). The action potentials of particular ORNs have characteristic sizes and waveforms (Kaissling 1974) that often differ greatly enough among ORNs that researchers can discriminate them and assess their activities separately. Spike sizes may be related to the size of the ORNs generating them (Hansson et al. 1994b), but the size of the spike is not known to carry any extra information to the AL. It is the frequency of spikes rather than their size that is important for generating behavioural responses.