Numerical modeling of turbulent and laminar airflow and odorant transport during sniffing in the human and rat nose

Human sniffing behavior usually involves bouts of short, high flow rate inhalation (>300 ml/s through each nostril) with mostly turbulent airflow. This has often been characterized as a factor enabling higher amounts of odorant to deposit onto olfactory mucosa than for laminar airflow and thereby aid in olfactory detection. Using computational fluid dynamics human nasal cavity models, however, we found essentially no difference in predicted olfactory odorant flux (g/cm2 s) for turbulent versus laminar flow for total nasal flow rates between 300 and 1000 ml/s and for odorants of quite different mucosal solubility. This lack of difference was shown to be due to the much higher resistance to lateral odorant mass transport in the mucosal nasal airway wall than in the air phase. The simulation also revealed that the increase in airflow rate during sniffing can increase odorant uptake flux to the nasal/olfactory mucosa but lower the cumulative total uptake in the olfactory region when the inspired air/odorant volume was held fixed, which is consistent with the observation that sniff duration may be more important than sniff strength for optimizing olfactory detection. In contrast, in rats, sniffing involves high-frequency bouts of both inhalation and exhalation with laminar airflow. In rat nose odorant uptake simulations, it was observed that odorant deposition was highly dependent on solubility and correlated with the locations of different types of receptors.     

The sniff is part of the olfactory percept

In this review, we use data obtained primarily from humans to argue that sniffs are not merely a stimulus carrier but are rather a central component of the olfactory percept. We argue that sniffs 1) are necessary for the olfactory percept, 2) affect odorant intensity perception and identity perception, 3) drive activity in olfactory cortex, 4) are rapidly modulated in an odorant-dependent fashion by a dedicated olfactomotor system, and 5) are sufficient to generate an olfactory percept of some sort even in the absence of odor.     

The fluid mechanics of arthropod sniffing in turbulent odor plumes

Many arthropods capture odorant molecules from the environment using antennae or antennules bearing arrays of chemosensory hairs. The penetration of odorant-carrying water or air into the spaces between these chemosensory hairs depends on the speed at which they are moved through the surrounding fluid. Therefore, antennule flicking by crustaceans and wing fanning by insects can have a profound impact on the odorant encounter rates of the chemosensory sensilla they bear; flicking and fanning are examples of sniffing. Odors are dispersed in the environment by turbulent wind or water currents. On the scale of an antenna or antennule, an odor plume is not a diffuse cloud but rather is a series of fine filaments of scent swirling in odor-free water. The spatiotemporal pattern of these filaments depends on distance from the odor source. The physical interaction of a hair-bearing arthropod antennule with the surrounding fluid affects the temporal patterns of odor concentration an animal intercepts when it sniffs in a turbulent odor plume.     

Reproductive and hormonal changes during the estrous cycle and pregnancy in Asian elephants (Elephas maximus)

Serum progesterone and urinary total estrogen concentrations were determined weekly to bi-weekly in 2 female Asian elephants for 96 weeks. The mean estrous interval was approximately 16 weeks in the nonpregnant animal. A total of 5 cycles were observed in the 96 week study period. The serum progesterone concentration ranged from 150 pg/ml to greater than 350 pg/ml during the luteal phase of the estrous cycle. The serum progesterone was elevated for 8–12 week weeks of the 16 week estrous cycle. The urinary total estrogen concentration ranged from less than 10 to greater than 300 pg/μg creatinine. The second animal was pregnant at the beginning of the study period. The serum progesterone concentration was elevated (> 100 pg/ml) in the pregnant animal until parturition. The urinary total estrogens increased from approximately 50 pg/μg creatinine to greater than 400 pg/μg creatinine during the first year of pregnancy and remained elevated until parturition. Estrous cycling had not resumed by 3 months post partum.     

Sniff magnitude test: relationship to odor identification, detection, and memory tests in a clinic population

Recently a novel measure of olfactory function, the Sniff Magnitude Test (SMT), was developed that relies on changes in inhalation in response to an odor. The relationship of this unique test to that of other olfactory tests has received little investigation. In this study, we assessed, in 132 patients presenting to a chemosensory disorders clinic, the relationship of SMT scores to those from 3 standardized psychophysical tests: the University of Pennsylvania Smell Identification Test (UPSIT), a phenyl ethyl alcohol odor detection threshold test, and a short-term odor memory/discrimination test. SMT scores were roughly related to olfactory dysfunction categories defined for the UPSIT and correlated moderately with the other tests. Malodors (1% and 3% methylthiobutyrate [MTB], 1% ethyl 3-mercaptoproprionate) exhibited stronger correlations than nonmalodors (3% phenyl ethyl alcohol [PEA], 3% amyl acetate, 3% n-butanol) and elicited greater sniff suppression. In a principal component analysis, the SMT measures loaded on components different from those of the other tests, which loaded on a separate component. Anticipatory responses (i.e., smaller sniffs) occurred across trials for the first malodor (1% MTB), but not for the first nonmalodor (3% PEA), that was encountered. These results, along with those of an earlier factor analysis, suggest that sniff magnitude is influenced by odorant quality and intensity, as well as by cognitive factors.