To survive, animals must properly sense their surrounding environment. The types of sensation that allow for detecting these changes can be categorized as tactile, thermal, aural, or olfactory. Olfaction is one of the most primitive senses, involving the detection of environmental chemical cues. Organisms must sense and discriminate between abiotic and biogenic cues, necessitating a system that can react and respond to changes quickly. The nematode, Caenorhabditis elegans, offers a unique set of tools for studying the biology of olfactory sensation. The olfactory system in C. elegans is comprised of 14 pairs of amphid neurons in the head and two pairs of phasmid neurons in the tail. The male nervous system contains an additional 89 neurons, many of which are exposed to the environment and contribute to olfaction. The cues sensed by these olfactory neurons initiate a multitude of responses, ranging from developmental changes to behavioral responses. Environmental cues might initiate entry into or exit from a long-lived alternative larval developmental stage (dauer), or pheromonal stimuli may attract sexually mature mates, or repel conspecifics in crowded environments. C. elegans are also capable of sensing abiotic stimuli, exhibiting attraction and repulsion to diverse classes of chemicals. Unlike canonical mammalian olfactory neurons, C. elegans chemosensory neurons express more than one receptor per cell. This enables detection of hundreds of chemical structures and concentrations by a chemosensory nervous system with few cells. However, each neuron detects certain classes of olfactory cues, and, combined with their synaptic pathways, elicit similar responses (i.e., aversive behaviors). The functional architecture of this chemosensory system is capable of supporting the development and behavior of nematodes in a manner efficient enough to allow for the genus to have a cosmopolitan distribution.
Douglas K. Reilly and Jagan Srinivasan
Daniel W. Wesson, Sang Eun Ryu, and Hillary L. Cansler
The perception of odors exerts powerful influences on moods, decisions, and actions. Indeed, odor perception is a major driving force underlying some of the most important human behaviors. How is it that the simple inhalation of airborne molecules can exert such strong effects on complex aspects of human functions? Certainly, just like in the case of vision and audition, the perception of odors is dictated by the ability to transduce environmental information into an electrical “code” for the brain to use. However, the use of that information, including whether or not the information is used at all, is governed strongly by many emotional and cognitive factors, including learning and experiences, as well as states of arousal and attention. Understanding the manners whereby these factors regulate both the perception of odors and how an individual responds to those percepts are paramount for appreciating the orchestration of behavior.
Quentin Gaudry and Jonathan Schenk
Olfactory systems are tasked with converting the chemical environment into electrical signals that the brain can use to optimize behaviors such as navigating towards resources, finding mates, or avoiding danger. Drosophila melanogaster has long served as a model system for several attributes of olfaction. Such features include sensory coding, development, and the attempt to link sensory perception to behavior. The strength of Drosophila as a model system for neurobiology lies in the myriad of genetic tools made available to the experimentalist, and equally importantly, the numerical reduction in cell numbers within the olfactory circuit. Modern techniques have recently made it possible to target nearly all cell types in the antennal lobe to directly monitor their physiological activity or to alter their expression of endogenous proteins or transgenes.
Asymmetry of bilateral visual and auditory sensors has functional advantages for depth visual perception and localization of auditory signals, respectively. In order to detect the spatial distribution of an odor, bilateral olfactory organs may compare side differences of odor intensity and timing by using a simultaneous sampling mechanism; alternatively, they may use a sequential sampling mechanism to compare spatial and temporal input detected by one or several chemosensors. Extensive research on strategies and mechanisms necessary for odor source localization has been focused mainly on invertebrates. Several recent studies in mammals such as moles, rodents, and humans suggest that there is an evolutionary advantage in using stereo olfaction for successful navigation towards an odor source. Smelling in stereo or a three-dimensional olfactory space may significantly reduce the time to locate an odor source; this quality provides instantaneous information for both foraging and predator avoidance. However, since mammals are capable of finding odor sources and tracking odor trails with one sensor side blocked, they may use an intriguing temporal mechanism to compare odor concentration from sniff to sniff. A particular focus of this article is attributed to differences between insects and mammals regarding the use of unilateral versus bilateral chemosensors for odor source localization.
Chemoreception is the physiological capacity whereby organisms detect the varied external and internal chemical information required for survival and is the most primitive sensory process. Fish living in water have respiratory, gustatory, and olfactory chemosensory systems that detect water-soluble chemical cues. Respiratory chemoreception mainly in the gills detects changes in the levels of three respiratory gases: oxygen (O2), carbon dioxide (CO2), and ammonia (NH3). Gustatory chemoreception (gustation), which involves several taste receptor genes, is primarily involved in the tasting of foods. Olfactory chemoreception (olfaction), which involves between 15 and 150 olfactory receptor genes, is involved in a variety of important biological functions such as procuring foods, recognizing hazards (predators, contaminants, and toxic and alarm substances), discriminating species (individual, kin, and conspecific), controlling social behavior (dominance hierarchies, symbiotic behavior, territorial behavior, and schooling behavior), and reproductive and migratory behavior (mating, search for spawning site, imprinting, and homing). The olfactory functions are primarily controlled by hormones secreted from various endocrine glands that are the key mediators and integrators of external and internal information in organisms. Conversely, olfactory stimuli cause changes in hormone conditions. One good example is the amazing olfactory abilities of salmon. They can memorize information related to their natal stream odors during downstream migration in juveniles so that, after they travel thousands of kilometers in the ocean over many years during feeding migration, they are able to use their homing abilities to migrate precisely to their natal stream for reproduction in adults. Olfactory memory formation and retrieval of natal stream odors in salmon, which are primarily controlled by the brain–pituitary–thyroid hormones and brain–pituitary–gonad hormones, respectively, are essential to imprinting and homing migration. Salmon olfactory systems can discriminate seasonally and yearly stable compositions of dissolved amino acids in their natal streams produced by biofilms in the riverbed. Ocean and freshwater ecosystems may have been affected by climate change-related CO2-induced acidification that impairs olfactory-mediated neural and behavioral responses in fish.
Richard L. Doty
Decreased ability to smell is common in older persons. Some demonstrable smell loss is present in more than 50% of those 65 to 80 years of age, with up to 10% having no smell at all (anosmia). Over the age of 80, 75% exhibit some loss with up to 20% being totally anosmic. The causes of these decrements appear multifactorial and likely include altered intranasal airflow patterns, cumulative damage to the olfactory receptor cells from viruses and other environmental insults, decrements in mucosal metabolizing enzymes, closure of the cribriform plate foramina through which olfactory receptor cells axons project to the brain, loss of selectivity of receptor cells to odorants, and altered neurotransmission, including that exacerbated in some age-related neurodegenerative diseases.