21.2. Olfaction

Structure

The sense of smell, or olfaction, is responsive to chemical stimuli. The olfactory receptor neurons are located in a small region within the superior nasal cavity. This region is referred to as the olfactory epithelium and contains bipolar sensory neurons. Each olfactory sensory neuron has dendrites that extend from the apical surface of the epithelium into the mucus lining the cavity. As airborne molecules are inhaled through the nose, they pass over the olfactory epithelial region and dissolve into the mucus. These odorant molecules bind to proteins that keep them dissolved in the mucus and help transport them to the olfactory dendrites. The odorant–protein complex binds to a receptor protein within the cell membrane of an olfactory dendrite. These receptors are G protein–coupled, and will produce a graded membrane potential in the olfactory neurons.

The axon of an olfactory neuron extends from the basal surface of the epithelium, through an olfactory foramen in the cribriform plate of the ethmoid bone, and into the brain. The group of axons called the olfactory tract connect to the olfactory bulb on the ventral surface of the frontal lobe. From there, the axons split to travel to several brain regions. Some travel to the cerebrum, specifically to the primary olfactory cortex that is located in the inferior and medial areas of the temporal lobe. Others project to structures within the limbic system and hypothalamus, where smells become associated with long-term memory and emotional responses. This is how certain smells trigger emotional memories, such as the smell of food associated with one’s birthplace. Smell is the one sensory modality that does not synapse in the thalamus before connecting to the cerebral cortex. This intimate connection between the olfactory system and the cerebral cortex is one reason why smell can be a potent trigger of memories and emotion.

The nasal epithelium, including the olfactory cells, can be harmed by airborne toxic chemicals. Therefore, the olfactory neurons are regularly replaced within the nasal epithelium, after which the axons of the new neurons must find their appropriate connections in the olfactory bulb. These new axons grow along the axons that are already in place in the cranial nerve.

Sensitivity and Roles of Olfaction

The odor detection threshold is the lowest concentration of a certain odor compound that is detectable by the an organism's sense of smell. The threshold of a chemical compound is determined in part by its shape, polarity, partial charges, and molecular mass.

Most mammals have a good sense of smell, whereas most birds do not, except the tubenoses (e.g., petrels and albatrosses), certain species of vultures, and the kiwis. Among mammals, it is well developed in the carnivores and ungulates, which must always be aware of each other, and in those that smell for their food, such as moles. Having a strong sense of smell is referred to as macrosmatic.

Based on experiments in which animals animals are exposed to aromas in known extreme dilutions, dogs are estimated to have an olfactory sense approximately ten thousand to a hundred thousand times more acute than a human's. Bloodhounds were bred for the purpose of tracking humans and they can detect a scent trail a few days old. Bears are also highly sensitive, and this is essential for locating food underground. Using their elongated claws, bears dig deep trenches in search of burrowing animals and nests as well as roots, bulbs, and insects.

Fishes have a well-developed sense of smell. Salmon utilize their sense of smell to identify and return to their home stream waters. Catfish use their sense of smell to identify other individual catfish and to maintain a social hierarchy. Many fishes use the sense of smell to identify mating partners or to alert to the presence of food.

The sense of smell is less developed in the primates, including humans. In many species, olfaction is highly tuned to pheromones. Comparisons between species of mammals have revealed some correlation between the size of the nasal cavity, the elaboration of the ethmoturbinates, the area lined with olfactory epithelium and the olfactory sensitivity of the animal. There are many exceptions to this pattern, however, and few species have been studied in detail.

Food Detection and analysis

Animals vary greatly in diet, method of foraging (searching for food) and sensory abilities. Some foods are more easily detected through olfaction, while others might be easier to find visually. For example, ripe cherries may be located visually by a diurnal animal whereas the nectar of a flower that opens at night may be better detected through olfactory cues.

The evaluation of olfactory sensitivity for food detection should not be restricted to the ability of detecting odorants at low concentration. It can be equally important to quantify the number of odorants to which a species is sensitive and its ability to discriminate between the combinations of odors that emanate from various sources.

In addition to simply locating a source of food, it is frequently important to analyze its quality before ingesting it. Many plants keep fruits protected by high levels of indigest alkaloids until the fruits are ripe and the seeds ready for dispersal. Other resources such as ripe sweet fruits falling from a tree or a recently killed prey may decompose quickly and it may be important for an opportunistic consumer to evaluate, before consumption, if the nutrient gain will be worth the associated load of toxins.

Territoriality - Scent Marking

Scent marking, also known as territorial marking or spraying when this involves urination, is a behavior used by animals to identify their territory. Most commonly, this is accomplished by depositing strong-smelling substances contained in the urine, faeces, or, from specialized scent glands located on various areas of the body. Often, the scent contains pheromones or carrier proteins such as the major urinary proteins to stabilize the odours and maintain them for longer. The animal sniffing the scent frequently displays a flehmen response to assist in detecting the mark.

Felids such as leopards and jaguars mark by rubbing themselves against vegetation. Prosimians and New World monkeys also use scent marking, including urine washing (self-anointing the body with urine), to communicate. Many ungulates, for example the blue wildebeest, use scent marking from two glands, the preorbital gland and a scent gland in the hoof.

Territorial scent marking may involve behaviors specific to this activity. When a wolf marks its territory, it lifts a hind leg and urinates on a scent post (usually an elevated position like a tree, rock, or bush). This raised leg urination is different from normal urination, which is done while squatting.

In the Eastern carpenter bee, Xylocopa Virginica, both sexes have glands that evolved for marking the nest. Males, although they have the gland, are unable to produce the marking substance. Females secrete it near the nest site entrance to establish their territory.

Mate attraction

Many vertebrates use chemical signals (pheromones) to attract mates. Pheromones are chemical substances released in the urine or feces of animals or secreted from sweat glands that are perceived by the olfactory system and that elicit both behavioral and endocrine responses in conspecifics. Sex pheromones are used to arouse, attract, and elicit specific behavioral responses from the opposite sex. In mammals, chemical signals and the scent glands that secrete them are frequently found only in adults of one sex, are often only secreted during the breeding season and are used exclusively in mating.

Mammalian pheromones can elicit both long-lasting effects that alter the hormone levels of the recipient animal, and short-term effects on its behavior. For example, detection of male pheromones by female mice has been found to encourage onset of puberty, however the detection of female pheromones have been found to delay the onset of puberty.

There is vast evidence for the use of pheromones in mating behaviors. For example, when boars become sexually aroused, they salivate profusely dispersing pheromones into the air. These pheromones attract receptive sows, causing them to adopt a specific mating posture known as standing, which allows the male boar to mount and copulate.

Major histocompatibility complex (MHC) proteins are membrane molecules that are expressed in every cell of an individual and are used by the immune system to distinguish between self cells and invasive cells. An organism usually contains many loci with MHC genes and there are many alleles of the gene. In addition to its role in immune function, studies suggest that the MHC is also involved in mate choice for many vertebrates through olfactory cues. Several studies have detected odor-based mate choice in rats and other mammals resulting in the selection of mates with different MHC background than the individual making the selection. These mating preferences could be adaptive and maintain the enormous allelic diversity of the MHC complex.

Individual or kin recognition

Social cooperation is frequently mediated by cue-based and context-based mechanisms, such as familiarity, imprinting and phenotype matching. The cues are most frequently sensory information gathered from visual, olfactory and auditory stimuli. The belding ground squirrel kin produce similar odors in comparison to non-kin. The squirrels spent longer investigating non-kin scents suggesting recognition of kin odor. They produce at least two scents arising from dorsal and oral secretions, giving two opportunities for kin recognition. Among arthropods, studies suggest that the bald-faced hornet, Dolichovespula maculata, can recognize nest mates by their cuticular hydrocarbon profile, which produces a distinct smell.

Summary

Olfactory receptor cells are found in the olfactory epithelium located on the roof of the nasal cavity. When stimulated by odorants, these cells produce action potentials which travel down the axon through the cribriform plate of the ethmoid bone and into the brain. Olfactory information is processed in various parts of the brain, but mostly in the olfactory cortex of the cerebrum. The sense of olfaction tends to be well developed in nocturnal animals and least developed in visually-oriented diurnal animals such as most birds. This sense is used for food detection and analysis, and species and individual recognition such as in territoriality, mate attraction and kin recognition.

Key terms

Olfaction, odorant, cribriform plate, ethmoid bone, olfactory tract, olfactory cortex, olfactory receptor, olfactory neuron, smell, olfactory epithelium, scent trail, scent marking, pheromone, territory, scent glands, Elehmen response, major histocompatibility complex, kin recognition, imprinting.

Figure credits

Figure 1 by OpenStax College – Anatomy and Physiology. https://cnx.org/contents/FPtK1zmh@8.79:s3XqfSLV@8/Sensory-Perception. Fig. 3.