Odorant transport in a large scale bullfrog nasal cavity model

In order to conduct a detailed study of odorant transport, a large scale (52x) model of the bullfrog nasal cavity was constructed from CAT scans. The concentration of odorants (presented initially in the vapor phase) was measured at various points within the nasal cavity to determine experimentally the mucosal activity patterns of soluble and insoluble odorants. The odorants carvone, geraniol, and d-limonene, which are known to have high, medium, and low solubility in bullfrog olfactory mucosa, were studied at two flow rates (6.6 and 11.4 L/min corresponding to 128 and 220 cc/min in the real bullfrog). Velocity profiles inside the model were also measured at the two flow rates using a hot-film anemometer probe. From the velocity profiles, it was found that approximately 60% of the air entering the external naris is directed along the medial trough, to flow over the eminentia, and exits via the internal naris. These velocity profiles and the airphase odorant concentration measurements were used to determine the odorant mass transfer rates. The average mass transfer coefficients was found to be 1.04 and 1.08 cm/sec for flow rates 128 and 220 cc/min, respectively, with the highest mass transfer coefficients occurring around the eminentia. The odorant collections showed that, similar to their solubility in real bullfrog olfactory mucosa, carvone was most soluble, geraniol medium soluble, and d-limonene least soluble in the mucus simulant. The airphase odorant collection results showed that the odorants carvone, geraniol, and d-limonene would give the same relative response as that found by Mozell et al. (Chemical Senses 16:631-649, 1991) assuming that the odorants transferred to the model surface act as the olfactory stimulus. Just as Mozell et al. determined a relationship between the flow rate and odorant solubility, we found that for the most soluble odorant, positive flow effects were seen as flow increased. Overall, the imposed patterning of odorant distribution is fairly well simulated within the large scale bullfrog nasal cavity model and the model results show that odorant mass transfer to the model surface represents the stimulus that is proportional to the actual physiological recording suggesting that mass transfer theory can suitably explain olfactory response recordings