Smell

Mechanism of the sense of smell

It is still rather badly known, but considerable progresses were accomplished these last years in the comprehension of this mechanism following the discovery of the genes which code receiving proteins of the odorous ones. Those are localized in the membrane of the lashes of the olfactive Neuron S. These neurons are bipolar specialized neurons ( they present ramifications called lashes on the dendritic side (i.e. towards the nasal cavity), a cellular body located in the first third of the Muqueuse, and a Axone communicating with the olfactive Bulbe at the Vertébré S alive in medium aérien' ) of which the external part, ciliée, leads in the olfactive epithelium. Cells glandular, present in the mucous membrane and the submucosa secrete the Mucus papering the olfactive epithelium. Mucus ensures a permanent washing of the mucous membrane. The odorous molecules (the Ligand S) arrive either directly by diffusion in mucus, or are " dealt with by Protéine S of transport (known as " odor binding protein" OBP) which make it possible the molecules Hydrophobe S (majority) to penetrate in mucus recovering the epithelium, and thus to reach the membrane receivers of the olfactive neurons. These proteins of transport would concentrate the odorous molecules (it is hypothetical) on the membrane receivers. Each type of olfactive receiver seems to have a particular sensitivity, which recovers partially, but not completely, those of the other cells. That means that definite Molécule activates a single whole of receivers (each one of these receivers answering with an intensity which is clean for him). The axons of the olfactive neurons carrying the same receiver converge towards the same synaptic structure (cluster) localized within the olfactive bulb. This “geographical” activation then results in a reason spatiotemporel nervous private individual within the bulb olfactive and interpreted like an odor by the brain.

The olfactive neurons, contrary to the other neurons, for example those of the Hearing, renew constamment.
It is then on the level of the cerebral Cortex, in the area préfrontale of the Cerveau, that all this information (with the Goût) is processed by the organization.

The human sense of smell is regarded as one of the least developed directions. However, smell is of great importance in the determination conscious or unconscious of our behaviors. There exists, in practice, two perceptive thresholds. Weakest corresponds to the detection of an odor, but that the subject cannot identify. The second threshold corresponds to the identification of the odor in question. Certain molecules, like the Thiol S, are detected atrates much lower than of others. Certain animals are able to detect molecules a billion times more diluted than the threshold of our sense of smell.

Lastly, there exists a presumption that certain molecules, like the Phéromone S, are detected by the olfactive system, even if their perception is not translated into term of “conscious” odor.

Dysosmies: disorders of the sense of smell

The anosmia: deficiency of the sense of smell

The loss or the substantial reduction in the sense of smell is called Anosmie, generally which had with traumatisms, or badly neat infections (rhinitis,…). The anosmia can relate to all the odors or only some of them (specific anosmias). The anosmia is also often accompanied by agueusy (its equivalent related to the taste), though one tardily notes especially a reduction in the taste among people having lost the sense of smell.

The anosmia or the difficulty of recognition of the odors can be precursory signs of degenerative diseases neuro, the such disease of Alzeimer. There exist screening tests early of these diseases based on the sense of smell.

The loss of sense of smell can have effects varied on the organization of people who are reached by it. One often notes a period of strong depression related to the anosmia, accompanied by various symptoms, among which a diminution of the appetite and sexual excitation.

Hyperosmy: exacerbated sensitivity of the sense of smell

The hyperosmy appears in certain states nevrotic.

Cacosmy

Disorder of the sense of smell which leads the patients to like or perceive stinking odors, putrid or considered unpleasant. The cacosmy can have an origin physiological (rhinitis, sinusitis, tumoral) or psychological. Often confused with the cacostomy which indicates the exhalation of odors désagéables. Those Ci come from functional disorders (of the mouth or the digestive system).

Appearance of the sense of smell

At the Man : In the years 1960, the research undertaken by professor Lipsitt made it possible to show that there exist capacities of detection and training of the odors at the newborn. Even " in utero" , the olfactive system of the fetus is one of the first directions to be set up between 11 and 15 weeks. The exposure of the fetus to the odorous substances transported by the amiotic liquid gives him a first olfactive experiment which is likely to influence its preferences after the birth.

Anatomical elements

At the anatomical level, the olfactive system is composed of two structures, the principal olfactive system and the system trigéminal. There exists a called troisème left " Body voméronasal " , which is located in withdrawal in the opening of the nostrils. It does not have a precise function established at the man, but is related to the animal for its reproduction and the marking of its territory.

References

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