The olfactory perception or olfactory perception , and sense of smell or olfactory sense (of Latin olfacere , 'smell ), is the perception of odors . Osmology or osphresiology investigates the interrelationships between the complex sense of smell .
In humans, the sense of smell often seems to play less of a role than sight, hearing or touch. However, its achievements are noticeable when olfactory perception is lost, for example with a cold .
Such a condition would be life-threatening for many wild animals as they are dependent on their sense of smell in several ways. Because the smells or fragrances that can only be perceived in this way are used to identify food , spoilage ( putrefaction ) or decay (carrion odor), to distinguish one's own body odor from that of familiar group members ( stable odor ) and from foreign conspecifics as well as from that of other species , the warning of enemies ( predator ) or the assumption of prey ( prey ).
The olfactory perception is therefore not only important for food intake, but also plays an essential role in social behavior and mating behavior. The sexual maturity of female animals is signaled to male conspecifics by pheromones ( sex attractants). In addition, fragrances also serve as spatial orientation . Many animals set scent marks to demarcate a territory or, like ants , follow the scent trail of their predecessors. In addition, chemical signal substances can also be used for communication between different species. For example, the flowers of many plants emit fragrant substances which attract insects , which they now pollinate ( Allomon ) or just collect nectar ( Kairomon ) or do both ( Synomon ). When it comes to pest control in fruit growing , the effect of pheromones can be used, for example to limit the pairing of plum moths.
Various sensory systems can be involved in olfactory perception : in addition to the actual olfactory system ( olfactory stimuli ), the nasal-trigeminal system (tactile and chemical stimuli) as well as influences of the gustatory system ( taste stimuli ). The sense of smell is the most complex chemical sense . The sensory cells of smell are equipped with specific odor receptors and in vertebrates are usually located in the nose . Some smells are not perceived consciously (see also Jacobson organ ).
Properties in mammals
The reception zone of the olfactory system is located inside the nose . In each nasal cavity, three bulge-like structures protrude inward from the outer walls of the nose, the turbinates ( Conchae nasales ), which direct the flow of air. The olfactory area is limited to the mucous membrane above the nasal concha, the olfactory mucous membrane of the olfactory region , and is also known as the olfactory organ ( organum olfactus ).
This area, which has a yellow to brown color and is around 2 × 5 cm² in humans - 2 × 25 cm² in dogs - contains the sensory cells that specialize in odorous molecules . Specific receptors of a certain type are located in the cell membrane of extensions of the individual olfactory cells , each of which responds to particular chemical properties of the odorous substances . There are around 400 different molecular olfactory receptors in humans , with a certain receptor cell usually carrying only one type. In dogs or rats , a total of more than 1000 different types of receptors are formed.
The olfactory nerve ( olfactory nerve , 1st cranial nerve ) is responsible for the sensory innervation of the olfactory mucosa , while the trigeminal nerve (5th cranial nerve) sensibly innervates the remaining mucous membrane of the inside of the nose and can be addressed by mechanical and chemical stimuli. During normal breathing, only small amounts of partial air reach the olfactory region . In the sensory analysis, the air flow is intensified and air is sucked in through the nose in short bursts (sniffing) or moved here from the oral cavity (tasting).
The sensory cells of the olfactory sense, the olfactory cells, have a ( dendritic ) process from which several cilia arise, which lie parallel to the surface in the mucus of the olfactory mucous membrane. Embedded in their membrane, they each carry specific receptor proteins for the absorption of stimuli. If odorous substances reach these membrane proteins, they can - depending on their chemical properties - be bound and thus change the receptor.
Via changes in the odor receptor proteins , subsequent activation of adenylate cyclase , subsequent activation of cAMP- controlled ion channels and further steps, a receptor potential is built up and converted into a series of action potentials. These signals from the olfactory receptor cells are passed on to centrally located nerve cells of the olfactory system via their neuritic cell processes.
The axons of olfactory cells pull in bundles of nerve fibers as fila olfactoria of the olfactory nerve through the holes of the screen plate ( lamina cribrosa ) of the ethmoid ( ethmoid ) in the cranial cavity for overlying the olfactory bulb ( olfactory bulb ) of the brain , where the central nervous system processing begins. The stimulus patterns of smells are processed and analyzed in the two bulbs . The olfactory bulb is nervously linked to the hypothalamus , which is, among other things, significantly involved in the control of food intake and sexual behavior .
The actual sense of smell, which can be strongly connected with emotions , memories and hedonic judgments, then arises in rather unspecific, evolutionarily old cortical brain centers. In this area, both the chemosensory analysis of the breath and the retronasal analysis of food aromas are carried out. Specific odor or pheromone perception is possible via the vomeronasal organ , which is assigned to an additional (accessory) also olfactory system . In addition, there is occasionally talk of a hematogenic smell, which is understood to mean the perception of odorous substances that have been injected into the blood .
Odor-active substances must be volatile . The relationships between the chemical-physical properties of the odorous substances and the resulting olfactory sensations have not yet been adequately researched. Most of the substances to be smelled are carbon compounds .
The perception of scents is strongly influenced by the hormone status and motivation. For example, hypogonadism often leads to extensive anosmia (the loss of the sense of smell), a high level of estrogen leads to an increased sensitivity to smell or a satiety with food leads to a change in the hedonic evaluation of smells.
For olfactory perception, as for gustatory perception, a vector coding of the impressions is assumed. This coding explains the extraordinary variety of olfactory impressions and also how much the diversity of the perceptual world of a living being increases if there is only one more receptor type (about 7 instead of 6) and a higher resolution (about 30 instead of 10 differentiable levels) is achieved. Small differences in the resolution of the receptors also have such a strong impact between people. In the past, humans and other primates were considered “microsmats” (“low-smelling”) in contrast to “ macrosmats ” such as dogs and rats. It is now known, however, that the olfactory performance of primates can exceed that of dogs and rats with regard to some scents. Dogs are extremely sensitive to the smell of fatty acids (sweat from prey), but are less sensitive to fruit scents than some primates.
Properties in humans
The olfactory mucous membrane of a person is located on the roof of the nasal cavity and has a total area of 5 cm². It contains around 20–30 million olfactory cells that carry around 400 different receptors . A single sensory cell usually only carries one specific type of receptor. Thus there are several thousand olfactory cells of the same type, but they are distributed over the entire olfactory mucosa. Odor substances are recognized on the basis of chemical structural features. A single odorous substance usually addresses several specific receptor types and thus also different olfactory cells. Sensory cells of a certain type are excited by chemically similar compounds with the same characteristics, although the sensitivity for such classes can be quite different. By combining the simultaneous activation of different receptors, humans can distinguish around 10,000 different smells. An alternative theory proposed by Malcom Dyson in 1928 sees a connection with the molecular vibration of odorous substances. The theory was taken up by Luca Turin in 1996 and has been controversial since then.
The sense of smell is largely developed at birth. The olfactory cells in humans are renewed every 30 to 60 days. In the process, olfactory cells die off ( apoptosis ) and are replaced by young new olfactory cells resulting from the division of basal cells . Their neurites grow site-specifically and usually move to the vacated areas in the olfactory bulb .
The 6–20 fine hairs ( cilia ) of the dendrite of an olfactory cell end in the mucous layer formed by Bowman's glands , which covers the olfactory mucous membrane. Molecules of odorous substances dissolve in the mucous layer and attach themselves to the membrane of olfactory cells via specific receptor molecules . A G protein is activated by the binding of an odorous substance molecule to the receptor molecule in the cell membrane of the cilia . This initiates a signal cascade within the olfactory cell , with cAMP ensuring that the Ca 2+ level in the cytosol increases by opening (CNG) ion channels . This leads to an opening of Cl - - ion channels and thus a Cl - efflux, whereby the cell now depolarized and an action potential is triggered.
The action potentials of the olfactory cells are sent to the brain as signals via their neurites . The totality of the neurites forms the olfactory threads ( Fila olfactoria ). This bundle of around 20 olfactory nerves (Nervi olfactorii) is also considered to be the first cranial nerve . They pull through the holes in the sieve plate of the ethmoid bone into the inside of the skull to the olfactory bulb ( olfactory bulb ). The first neuron of the olfactory tract ends here. This is where complex interconnection points are located, the synapses of the olfactory clusters ( glomeruli olfactorii ). Here often more than 1,000 axons, namely those from olfactory receptors of the same type, converge on a single subsequent second neuron, which is called the mitral cell . Cells adjacent to the mitral cells (periglobular and granule cells) increase the selectivity of olfactory perception by inhibiting or amplifying the signal.
In addition to connections between the two olfactory bulbs, which are already assigned to the olfactory brain ( rhinencephalon ) or endbrain (telencephalon), there are projections to the primary olfactory cortex , the part of the cerebral cortex responsible for processing olfactory information, as olfactory tracts ( tractus olfactorius ) . From there there are also connections to other brain regions, in particular to the hypothalamus and the limbic system .
The absorption of stimuli in the unconscious perception of pheromones can differ .
Perception and recognition threshold
Most odor-active substances have a molar mass below 300 g / mol. For the perception of particularly odor-active substances, 10–100 million molecules are sufficient , that is 10–15 to 10–14 mol of a substance. The amount from which a substance can be smelled is called the odor threshold . A distinction is made between the perception or absolute threshold and the recognition threshold for the respective fragrance (see also olfactometry ).
- Perception threshold
- Only four micrograms of the methyl mercaptan contained in garlic in 10 6 m³ of air (corresponding to a hall of 500 × 100 × 20 meters) or 4 · 10 −15 g / dm³ are enough to make a person feel “it smells like something”.
- Even olfactory stimuli below the threshold of attention-dependent conscious perception can develop effects as so-called subliminal stimuli that can be used, for example, for “ subliminal advertising ”.
- Detection threshold
In order to be able to recognize a certain substance by its odor, the odor concentration must be significantly higher; for methyl mercaptan, this recognition threshold is fifty times the absolute threshold of perception and is thus around 0.2 picograms per liter of air (2 · 10 −13 g / dm³).
After all, contamination caused by smells can be differentiated with a simple "nose test" using smell strips . Even if the olfactory strip odor thresholds vary from person to person, there are typical limit values. So 50 ppm of diesel in ethanol (after training also 10 ppm), 100 ppm of fusel oil (1-pentanol) in bio-alcohol and 100 ppm of acetic acid and (also) butyl acetate in ethyl acetate (ethyl acetate) were "smoked". In 2018, a research paper by Veronika Schöpf, Psychology, University of Graz was published. 27 typical bacterial strains are found in the noses of 67 test persons. In people with a less sensitive sense of smell, an increasing number of bacteria were found that excrete strongly smelling butyric acid.
Many mammals have a much finer olfactory perception than humans - in the case of a German Shepherd, for example, by a factor of 1000.
Central nervous interconnections for identification and memory
In most cases, intense experiences with the smell at a certain location or events associated with the smell (episodic- autobiographical memory ) play a role in the ability to remember. The evaluation of an odor takes place before the actual odor detection.
A distinction is often made between an implicit presemantic and a semantic memory for smells. In the case of presemantic memory, the connection between a smell and a place is spontaneously remembered. This is often done with the help of the visual system by visualizing the place and remembering an atmosphere that we smell (for example "Christmas"). Since the olfactory cortex does not contain any image of the individual fragrances, olfactory sensations are anchored with spatial assignment and, in the case of the sighted, are also represented by parts of the visual cortex, making them pictorial. A second, semantic reference is also required for the verbal reproduction of a smell, with which a name (for example “cinnamon”) can be assigned and identified verbally. When processing olfactory stimuli, there is a difference between explicit semantic and presemantic implicit memory.
Nerve fibers run from the olfactory cells in direct connection to the olfactory bulb, which is our primary olfactory center. Sensory odor discrimination occurs primarily via the projection of the olfactory bulb via the lateral stria to the area prepiriformis (primary olfactory cortex) and the thalamus . This is followed by transmission to the orbitofrontal cortex. The connection via the medial stria via the olfactory tubercle to the thalamus is also used for odor identification.
From the olfactory bulb there are connections via the lateral stria to the prepiriform area and then further to the hippocampus . Processing in the hippocampus means that memory contents are permanently stored. The hippocampus works with few resources, which means it sorts out practically no information on the way to long-term memory. For this reason, smells do not have to be learned like vocabulary, but can be saved promptly.
Central nervous connections and emotions
The following connections stand for the emotional component of olfactory perception: From the olfactory bulb via the lateral stria there is a connection with the amygdala ( limbic system ), the lateral hypothalamus , then the basal forebrain and the orbitofrontal cortex . There are also projections over the stria medialis to the tuberculum olfactorium and further to the septum. This circuit is primarily responsible for conveying the feeling that we experience when we smell a scent. The amygdala in particular is involved in conveying feelings, while the basal forebrain and the orbitofrontal cortex play a role in motivational functions. Information that is linked to emotions can be learned better because it is not only to be stored explicitly in the semantic memory, but also implicitly stored with the emotional background via the episodic memory.
In humans, some unpleasant smells can trigger protective reflexes such as gag reflexes. The close connection of the sense of smell with the limbic system and the hypothalamus leads to a special position in learning processes: In contrast to classical conditioning , the time intervals between unconditioned stimulus and conditioned stimulus can be extremely extended. Despite long intervals, this can lead to a conditioned reaction (e.g. nausea and vomiting as a result of disgust ), triggered by an originally neutral stimulus (e.g. a certain smell), which now causes this reaction as a conditioned stimulus.
The hedonic assessment of fragrances, in contrast to flavorings , is largely learned in humans in the first 5–10 years of life. While newborns show clear reactions of pleasure or displeasure to stimuli from sucrose ( sweet ) or caffeine ( bitter ) through facial reactions , the reactions to smells are often indifferent. The odor of faeces , fruit or sweat is not differentiated hedonically.
A distinction is made between quantitative and qualitative odor disorders . The quantitative disturbances include the complete absence of the olfactory sense as anosmia , the insufficient olfactory performance as hyposmia and the excessive olfactory performance as hyperosmia . The high-impaired smell is in the range neurological Kakosmie or Parosmia and in the psychiatric field, the phantosmia as an olfactory hallucination .
It is estimated that humans should be able to differentiate between over 1 billion different mixtures of fragrances . However, the lack of linguistic expressions for smells limits our ability to differentiate olfactory nuances. While inexperienced people recognize around 50% of the odors that are repeatedly presented and also name them correctly, those who have been trained can increase their hit rate to 98%. In contrast to other sensory impressions such as color designations as part of visual perception, there are no abstract basic terms in olfactory perception. In various proposals for systematisation and classification, the basic odors are based on the material names.
Dealing with the history of smell as well as researching it is part of the history of the senses and the history of science . It can also be understood as part of cultural history , especially when it comes to researching linguistic expressions and social or activity-specific differences.
For ancient authors, olfactory perception is usually of secondary interest, while more attention is paid to seeing and hearing . The focus of the examinations is usually the person and his olfactory perception. In more recent research in antiquity, the preoccupation with olfactory perception is increasingly coming into focus.
History of science
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