The sense of smell in humans is also called olfactory perception and is divided into three different anatomical structures with the olfactory epithelium, the olfactory filaments and the upstream part of the olfactory brain, which are jointly responsible for the perception as well as processing of odor stimuli. Although the sense of smell in humans is much less pronounced than the sense of smell in primates, this system of olfactory perception allows the discrimination of a trillion different odor mixtures and eight different odor qualities. Impaired, absent, or heightened olfactory perception is usually related to either neurological disorders or mental illness phenomena.
What is the sense of smell?
The sense of smell or olfactory perception is the human sensory channel responsible for odors. The sense of smell or olfactory perception is the human sensory channel responsible for odors. It is divided into three different structures:
The olfactory epithelium in the nasal cavity picks up the odor. The olfactory filaments, the so-called lamina cribrosa with the fila olfactoria, lie above the ethmoid bone and transmit the absorbed odors. The bulbus olfactorius, i.e. the upstream part of the brain, processes the stimuli passed on in this way. The olfactory brain, the so-called olfactory cortex, overlaps the center for taste information in its secondary center, which inextricably links these two perceptual areas. Unlike most animal species, the sense of smell in humans is barely developed. Notwithstanding, even humans are capable of discriminating between about a trillion different odors.
Function and task
The sense of smell is used to perceive and distinguish odors. For example, humans identify eight different odor qualities and can thus differentiate odor sources into the groups floral, earthy, animal, woody, green, spicy, resinous and fruity. The tasks of the sense of smell are ultimately divided into two basic functions: stimulus reception and stimulus processing. Stimulus reception takes place via the penetration of odor molecules into the olfactory mucosa. To increase the perceptions of the sense of smell, intermittent nasal breathing can be used, which swirls the respiratory air and thus allows more odor molecules to enter the olfactory cleft. Here, olfactory stimuli reach the approximately 30 million sensory cells of the nose. These sensory cells on the nasal mucosa bind odorant molecules to receptors and activate a G protein in the process. This initiates an intracellular signaling cascade that leads to the opening of ion channels. This opening provides a Cl efflux that depolarizes the cells, triggering an action potential. The resulting action potentials enter the olfactory brain through holes in the sieve plate of the ethmoid bone, from where they are transmitted to the brain areas of memory storage, emotion and motivation, and odor identification. This transmission takes place via the fibers and olfactory pathways of the three-layered olfactory brain and directs the perceptions, for example, without detours to the limbic system and the hypothalamus. In these brain areas, the storage of olfactory perceptions and odor identification takes place, which is often emotionally and motivationally occupied by the direct connection to the limbic system. Like the sense of hearing, the sense of smell in humans can compare two directions of odor via the centrally separated nasal cavities. Thus, humans are not only able to identify odor sources, but can additionally perform an approximate localization of these odor sources. Odor identification takes place in the thalamus. However, it is not until the processing of perceptions in the subsequent hippocampus that the individual odor perceptions are permanently stored. The olfactory memory of humans can be divided into a presemantic and a semantic memory. The presemantic memory establishes a spontaneous link between odors and places where the person has increasingly perceived the odor. Thus, the olfactory system of humans overlaps not only with the gustatory, but also with the visual sensory system, which, by binding visual memories and olfactory memories, allows for the visualization of odor perceptions that are actually formless. The semantic memory makes the verbalization of odors possible, since the perceptions are stored in it under individual names.While the sense of smell is of incomparably greater importance to primates, it is less important and not particularly strong for humans. Nevertheless, the sense of smell, together with gustatory perception, can also help humans to identify toxic and non-toxic substances and potential sources of danger. Often, for example, certain odors trigger a gag reflex, which evolutionarily has served primarily a protective function.
Diseases and ailments
Various, neurological diseases can impair the sense of smell or even lead to anosmia, the complete loss of the sense of smell. In particular, damage to the cells of the olfactory cortex is associated with olfactory dysfunction. Cell damage in this area is often caused by degenerative diseases such as Parkinson’s or Alzheimer’s disease, which can lead to the destruction of entire brain areas. Strokes or inflammatory processes in the brain can also damage the structures of the olfactory brain and lead to faulty or absent olfactory perceptions. However, impaired olfactory perception does not always have to be related to a physiological cause. In the context of certain diseases of the psyche, for example phantosmia, olfactory perceptions occur despite the absence of a stimulus source. In contrast, neurology refers to defective olfactory perceptions with respect to odor quality as parosmias or cacosmias. Decreased olfactory performance due to cell loss is again known as hyposmia, whereas excessive olfactory performance is known as hyperosmia.
