Optical Methods in Studies of Olfactory System - Bioelectronic Nose: Integration of Biotechnology and Nanotechnology - page 187

Fig. 11.1 Functional anatomy and structure of the olfactory system. a Localization of the olfactory
apparatus in a human. b Structure of the olfactory epithelium showing the different cell types and
the connecting from axons to glomeruli of the olfactory bulb. Bipolar olfactory sensory neurons
are embedded in a small area of OE. These neurons project axons to the olfactory bulb of the
brain. (Reprinted from Ref. [ 5 ] with permission from NPG). c Schematic diagram of olfactory
signal transduction pathway. When an odorants binds to an OR protein (OR) in the olfactory cilia
membrane a G protein (G α olf ) is activated, leading to an increases in the generation of cAMP from
ATP by adenylyl cyclase (ACIII). Olfactory cyclic nucleotide-gated channels (CNGC) conduct
Na + and Ca 2+ into the cilia. Ca 2+ in turn opens Ca 2+ -activated Cl − channels (CaCC), which as
intracellular Cl − concentration is high, leads to Cl − efflux. (Reprinted from Ref. [ 34 ] with permis-
sion from NPG)
tory sensory system can efficiently distinguish among numerous odorant molecules,
specifically hydrophobic organic molecules at very low concentrations [ 2 - 5 ]. Un-
like unique receptor-ligand interactions, the olfactory sensory system is based on
combinatorial codes where a single receptor type can respond to many different
odorants [ 6 - 8 ]. The broad tuning of olfactory cells with overlapping odorant re-
sponse profiles allows for the identification and discrimination of odor signals for
ORs, revealing a combinatorial receptor coding scheme in the entire system [ 9 - 11 ].
Numerous ligand-binding assays have been developed and used to figure out
binding specificity with various ORs. These assays have generally utilized olfactory
neurons or heterologous cells to express recombinant ORs and allow detection of
OR-ligand pairs for the purpose of research in the olfaction mechanism. Researchers
have also developed a much broader range of olfactory biosensor platforms such as
surface plasmon resonance (SPR) [ 12 - 15 ], quartz crystal microbalance (QCM) [ 16 -
19 ], nanomaterial based-field effect transistor (FET)-type sensors [ 20 - 25 ], light-ad-
dressable potentiometric sensor (LAPS) [ 26 - 29 ], and microelectrode array [ 30 - 32 ]
that utilize characterized OR-ligand interactions to validate the sensors [ 33 ].