Chemical Senses Advance Access originally published online on December 8, 2005
Chemical Senses 2006 31(2):131-144; doi:10.1093/chemse/bjj015
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Anatomical Contributions to Odorant Sampling and Representation in Rodents: Zoning in on Sniffing Behavior
1 Department of Physiology and Program in Neuroscience, University of Massachusetts Medical School, Worcester, MA 01655, USA and 2 Department of Neurobiology and Behavior, Cornell University, Ithaca, NY 14853, USA
Correspondence to be sent to: Thomas A. Schoenfeld, Department of Physiology and Program in Neuroscience, University of Massachusetts Medical School, Biotech 4, 377 Plantation Street, Worcester, MA 01605, USA. e-mail: thomas.schoenfeld{at}umassmed.edu
Odorant sampling behaviors such as sniffing bring odorant molecules into contact with olfactory receptor neurons (ORNs) to initiate the sensory mechanisms of olfaction. In rodents, inspiratory airflow through the nose is structured and laminar; consequently, the spatial distribution of adsorbed odorant molecules during inspiration is predictable. Physicochemical properties such as water solubility and volatility, collectively called sorptiveness, interact with behaviorally regulable variables such as inspiratory flow rate to determine the pattern of odorant deposition along the inspiratory path. Populations of ORNs expressing the same odorant receptor are distributed in strictly delimited regions along this inspiratory path, enabling different deposition patterns of the same odorant to evoke different patterns of neuronal activation across the olfactory epithelium and in the olfactory bulb. We propose that both odorant sorptive properties and the regulation of sniffing behavior may contribute to rodents' olfactory capacities by this mechanism. In particular, we suggest that the motor regulation of sniffing behavior is substantially utilized for purposes of "zonation" or the direction of odorant molecules to defined intranasal regions and hence toward distinct populations of receptor neurons, pursuant to animals' sensory goals.
Key words: odotopic, olfactory airspace, olfactory bulb, olfactory coding space, rhinotopic, zonation hypothesis
1 The upper limit (750) corresponds to the number of glomeruli in each hemisphere of the dorsal MOB of hamsters innervated by 20 mm2 (50%) of central OE (Schoenfeld and Knott, 2004), assuming homotypical innervation (Treloar et al., 2002). The lower limit (500) corresponds approximately to the number of OR genes expressed in the central OE, assuming that hamsters have roughly the same number of functional OR genes as do mice and rats (Young et al., 2002; Zhang and Firestein, 2002; Gibbs et al., 2004; Godfrey et al., 2004; Zhang et al., 2004).
2 The upper limit (250) corresponds to the number of glomeruli in each hemisphere of the ventral MOB of hamsters innervated by 20 mm2 (16.7%) of peripheral OE (Schoenfeld and Knott, 2004), again assuming homotypical innervation (Treloar et al., 2002). The lower limit (167) corresponds approximately to the number of OR genes expressed in a comparable portion of the peripheral OE, again assuming that the number of functional OR genes in hamsters is comparable to that in mice and rats (Young et al., 2002; Zhang and Firestein, 2002; Gibbs et al., 2004; Godfrey et al., 2004; Zhang et al., 2004).
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