Sense of smell

Of the Sense of smell of humans is also referred to as olfactory perception and, with the olfactory epithelium, the olfactory threads and the upstream part of the olfactory brain, is divided into three different anatomical structures that are responsible for the perception and processing of olfactory stimuli.

Although the human sense of smell is much less developed than the sense of smell of primates, this system of olfactory perception allows a distinction to be made between a billion different odor mixtures and eight different odor qualities.

A disturbed, absent or increased odor perception is usually related to either neurological diseases or mental illness phenomena.

What is the sense of smell?

The sense of smell or olfactory perception is the human sensory channel that is responsible for smells.

The sense of smell or olfactory perception is the human sensory channel that is responsible for smells. It is divided into three different structures:

The olfactory epithelium in the main nasal cavity absorbs the odor. The olfactory threads, the so-called lamina cribrosa with the fila olfactoria, lie above the ethmoid bone and transmit the smells that have been absorbed. The olfactory bulb, i.e. the upstream part of the brain, processes the stimuli transmitted in this way.

The olfactory brain, the so-called olfactory cortex, overlaps the center for taste information in its secondary center, which inseparably links these two areas of perception.

In contrast to most animal species, the human sense of smell is barely developed. Regardless of this, even humans are able to differentiate between around a trillion different smells.

Function & task

The sense of smell is used to perceive and differentiate between smells. For example, humans identify eight different odor qualities and can differentiate odor sources into the groups flowery, 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. The absorption of stimuli takes place via the penetration of odor molecules into the olfactory mucous membrane.

To increase the perception of the sense of smell, intermittent nasal breathing can serve, which swirls the breath and thus allows more fragrance molecules to reach the olfactory cleft. Here the olfactory stimuli reach around 30 million sensory cells in the nose.

These sensory cells on the nasal mucous membrane bind odorant molecules to receptors and activate a G protein in the process. In this way, an intracellular signal cascade is initiated, which leads to the opening of the ion channels. This opening ensures an outflow of Cl which depolarizes the cells and thus triggers an action potential.

The resulting action potentials get through holes in the sieve plate of the ethmoid into the olfactory brain, from where they are passed on to the brain areas of memory storage, emotion and motivation and smell identification. This transmission takes place via the fibers and olfactory tracts of the three-layer olfactory brain and directs the perceptions, for example, directly to the limbic system and the hypothalamus.

In these areas of the brain, the storage of odor perceptions and the odor identification take place, which is often evidenced by the direct connection to the limbic system in an emotional and motivated manner.

Like the sense of hearing, the human sense of smell can compare two directions of smell through the nasal cavities, which are separated in the middle. This means that humans are not only able to identify odor sources, but can also approximate these odor sources.

The odor identification takes place in the thalamus. Only the processing of perceptions in the adjoining hippocampus stores the individual odor perceptions permanently.

The olfactory memory of humans can be divided into a presemantic and a semantic memory. Presemantic memory creates a spontaneous relationship between smells and places where people have increasingly perceived the smell.

The olfactory system of humans thus overlaps not only with the gustatory, but also with the visual sensory system, which allows visual and olfactory perceptions to be visualized by linking visual memories and olfactory memories. The semantic memory makes the verbalization of smells possible, since the perceptions are stored in it under individual names.

While the sense of smell is of much greater importance for primates, it is less important for humans and not particularly well developed. Nevertheless, the sense of smell can also help humans, together with gustatory perception, in identifying toxic and non-toxic substances and potential sources of danger. For example, certain smells often trigger a gag reflex, which evolutionarily has primarily fulfilled a protective function.

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Illnesses & ailments

Various neurological diseases can impair the sense of smell or even lead to anosmia, i.e. the complete loss of the sense of smell. In particular, damage to the cells of the olfactory cortex is associated with odor disorders.

Cell damage in this area is often caused by degenerative diseases such as Parkinson's or Alzheimer's disease, which can destroy entire areas of the brain. Strokes or inflammatory processes in the brain can also damage the structures of the olfactory brain and lead to faulty or absent odor perception.

Disturbed odor perception does not always have to be related to a physiological cause. In the context of certain diseases of the psyche, for example phantosmias, odor perceptions occur despite the absence of a source of irritation.

Neurology, on the other hand, calls defective odor perceptions with regard to odor quality as parosmia or cacosmia. A reduced olfactory performance due to cell loss is again known as hyposmia, while excessive olfactory performance is known as hyperosmia.