One of the chemical senses, specifically the sense of smell. Olfaction registers chemical information in organisms ranging from insects to humans, including marine organisms. For terrestrial animals, its stimuli comprise airborne molecules. The typical stimulus is an organic chemical with molecular weight below 300 daltons; about a half million such substances exist. A few inorganic chemicals can also stimulate olfaction, notably hydrogen sulfide, ozone, ammonia, and the halogens. For marine organisms, amino acids and proteins, which derive largely from the decomposition of organic matter, form particularly good stimuli.
Anatomy and physiology
The anatomy of olfactory structures and the neurophysiology of olfaction differ significantly among different animal groups.
Insect olfactory receptors exist within sensory hairs on the antennae. Each hair contains thousands of pores in its cuticle. Stimulating molecules must pass through the pores in order to stimulate the outer segment of a receptor cell. This process of transduction has been studied through the measurement of electric potentials from the receptor neurons. The best known potential, the electroantennogram, reflects the action of many receptor neurons.
Certain insect olfactory receptors are specialists, and respond to a very narrow range of relevant chemicals, such as sex attractants secreted by females but registered only by males. Generalist receptors, on the other hand, respond to a wide range of chemicals. One generalist cell differs somewhat from another in the variety of materials to which it responds, permitting discrimination of different materials in an across-receptor pattern of induced neural activity. See also: Chemical ecology; Insect physiology
The olfactory organ of fishes resides typically in tubular chambers on either side of the mouth (Fig. 1). As the fish swims, water passes through the chamber and comes into contact with the olfactory epithelium that lines the folds of a structure called the olfactory rosette. This epithelium has great similarity to that of terrestrial vertebrates. In addition to proteins and amino acids, fishes respond to many of the odorants that form the stimuli for terrestrial olfaction. Hence, fishes respond to stimuli that humans characterize as floral, fruity, putrid, and so on.
In terrestrial vertebrates, the olfactory receptors reside within a sac or cavity more or less similar to the human nasal cavity (Fig. 2). The olfactory mucosa patch in the cavity characteristically contains millions of receptor cells; however, in some olfactory-dominated (or macrosmatic) mammals, such as the dog and rabbit, it contains tens of millions. The location of the olfactory mucosa relative to air currents in the cavity plays some role in the ongoing olfactory vigilance of the organism. In the human, a microsmatic organism, the mucosa sits out of the main airstream. During quiet breathing, eddy currents may carry just enough stimulus to evoke a sensation, whereupon sniffing will occur. Sniffing amplifies the amount of stimulus reaching the receptors by as much as tenfold.
Reception of the chemical stimulus and transduction into a neural signal apparently occur on the olfactory receptor cilia (Fig. 2). The ciliary membrane contains receptor protein molecules that interact with stimulating molecules through reversible binding. Nonprotein portions of the membrane may also have some chemoreceptive role, through modulation of the conformation or orientation of the receptor proteins.
Vertebrate receptor cells, like the generalist cells of insects, show broad tuning, that is, they respond to many odorants. The breadth of tuning depends in part on cell age—receptor cells have a life span of just a few weeks, and show broader tuning in their first two to three weeks than thereafter. Since new cells emerge from basal cells beneath the receptor cells, a receptor cell of any age may appear anywhere in the olfactory mucosa. However, there do exist some regional differences in the tuning. In frogs and the tiger salamander, organisms commonly used to study vertebrate olfaction, cells in the anterior portion of the mucosa show a bias toward water-soluble odorants, whereas those in the posterior portion show a bias toward lipid-soluble odorants. An anterior-posterior gradation, seen easily in regional recordings of the receptor potential known as the electroolfactogram, also reveals itself in recordings from the olfactory bulb of the brain.
Adjacent points in the mucosa project generally, though not exclusively, to adjacent points in the olfactory bulb (Fig. 3). The synapses between the incoming olfactory nerve fibers and the second-order cells, mitral cells, occur in basketlike structures called glomeruli. On average, a glomerulus receives about 1000 receptor cell fibers for each mitral cell. The tangled arrangement of fibers within a glomerulus invites the conclusion that it must operate as a functional unit. Second-order cells, which are also influenced by collateral neurons, such as the interior granule cell, nevertheless show very similar breadth of tuning to that seen in individual receptor cells. The location of cells within the bulb seems to play a role in encoding odor quality: each odorant stimulates a more or less unique spatial array.
Second-order cells, unlike first-order cells, may change their responsiveness with the state of need of the organism. For instance, mitral cells will respond more vigorously to food odors in a hungry organism. Such modulation presumably results from interplay of incoming neural signals with outgoing activity which originates in more central structures.
The central neural pathways of the olfactory system have a complexity unmatched among the sensory systems. One pathway carries information to the pyriform cortex (paleocortex of the temporal lobe), to a sensory relay in the thalamus (dorsomedial nucleus), and to the frontal cortex (orbitofrontal region) [Fig. 4]. This pathway seems rather strictly sensory. Another pathway carries information to the pyriform cortex, the hypothalamus, and other structures of the limbic system. The latter have much to do with the control of emotions, feeding, and sex. The strong affective and motivational consequences of olfactory stimulation seem compatible with projections to the limbic system and with the role of olfaction in certain types of physiological regulation.
In many organisms, odors from other members of the same species have powerful motivational effects. Such stimuli are called pheromones. Sex attractants, trail pheromones, and maternal pheromones are examples. In many vertebrate species, reception of pheromones occurs via an important accessory olfactory organ, known as the vomeronasal organ, which characteristically resides in the hard palate of the mouth or floor of the nasal cavity (Fig. 4). See also: Pheromone
Sensitivity and functional properties
At its best, human absolute sensitivity exceeds that of the most sensitive physical instruments, but sensitivity varies from odorant to odorant over several orders of magnitude. For instance, the threshold for the potent odorant 3-methoxy-3-isobutyl pyrazine (green bell pepper odor) equals about 1 part per 1012 parts of air, whereas that for the far less effective odorant methanol (wood alcohol) equals about 100 parts per 106. A common range of thresholds for materials used in fragrances and flavors is 1 to 100 parts per 109. Physicochemical determinants of molecular deposition and penetration through mucus correlate rather well with this threshold range. In aliphatic series in which physicochemical properties vary progressively with molecular chain length, threshold characteristically changes progressively. Such a correlation holds true for terrestrial vertebrate species other than humans, though absolute values differ. The available data suggest that rats and dogs have best sensitivity, about a thousandfold better than that of humans.
Thresholds gathered from various groups of human subjects permit certain generalities about how the state of the organism affects olfaction. For instance, persons aged 70 and above are about tenfold less sensitive than young adults. Males and females have about equal sensitivity, except perhaps in old age, where females are more sensitive. Females show a small fluctuation in sensitivity during the menstrual cycle. Sensitivity increases prior to ovulation. Smokers have about the same sensitivity as nonsmokers. Persons with certain medical disorders, such as multiple sclerosis, Parkinson's disease, paranasal sinus disease, Kallmann's syndrome, and olfactory tumors, exhibit decreased sensitivity (hyposmia) or complete absence of sensitivity (anosmia).
Above its threshold, the perceived magnitude of an odor changes by relatively small amounts as concentration increases. A tenfold increment in concentration will cause, on average, about a twofold change in perceived magnitude. The perceived magnitude of an odor is often greatly influenced by olfactory adaptation, a process whereby during continuous short-term exposure to a stimulus its perceived magnitude falls to about one-third of its initial value. Concentrations below this conditioning level may then prove imperceptible, but concentrations above the conditioning level will be affected relatively little. Recovery from such adaptation begins as soon as the stimulus is removed, and is generally complete in a few minutes.
Adaptation is largely specific to the conditioning odorant. That is, exposure-induced changes in sensitivity to one odorant will leave sensitivity to most other odorants roughly unchanged. Instances where exposure to one odorant does alter the sensitivity to another exemplify cross-adaptation. Experiments on cross-adaptation serve as a means to explore chemical or structural commonalities among stimuli.
The stimuli for olfaction are commonly complex, that is, they are mixtures. Such products as coffee, wine, cigarettes, and perfumes contain at least hundreds of odor-relevant constituents. Only rarely does the distinctive quality of a natural product, such as a vegetable, arise from only a single constituent. A chemical analysis of most products will not usually allow a simple prediction of odor intensity or quality. One general rule, however, is that the perceived intensity of the mixture falls well below the sum of the intensities of the unmixed components. This suppression of intensity works against the weaker components, which may lose their separate identity in the mixture but may still contribute to overall perceptual quality. A pleasant-smelling mixture of unknown composition may actually be found upon analysis to contain some very unpleasant constituents in small amounts. Their presence is often partially masked by stronger-smelling constituents, yet may make an essential contribution. Perfumery, which plays a wide role in the “deodorization” (often through addition of odor) and reodorization of many manufactured products, entails a careful blending of pleasant- and sometimes unpleasant-smelling materials into a final acceptable odor.
General notions about the properties that endow a molecule with its quality have spawned more than two dozen theories of olfaction, including various chemical and vibrational theories. Most modern theories hold that the key to quality lies in the size and shape of molecules, with some influence of chemical functionality. For molecules below about 100 daltons, functional group has obvious importance: for example, thiols smell skunky, esters fruity, amines fishy-uriny, and carboxylic acids rancid. For larger molecules, the size and shape of the molecule seem more important. Shape detection is subtle enough to enable easy discrimination of some optical isomers (for example, D-carvone versus L-carvone, which smell like spearmint and caraway, respectively). Progressive changes in molecular architecture along one or another dimension often lead to large changes in odor quality. No current theory makes testable predictions about such changes. See also: Chemical senses; Chemoreception