Olfaction is the sense of smell. This sense is mediated by specialized sensory cells of the nasal cavity of vertebrates, and, by analogy, sensory cells of the antennae of invertebrates. Many vertebrates, including most mammals and reptiles, have two distinct olfactory systems the main olfactory system, and the accessory olfactory system (mainly used to detect pheremones). For air-breathing animals, the main olfactory system detects volatile chemicals, and the accessory olfactory system detects fluid-phase chemicals. For water-dwelling organisms, e.g., fish or crustaceans, the chemicals are present in the surrounding aqueous medium. Olfaction, along with taste, is a form of chemoreception. The chemicals themselves which activate the olfactory system, generally at very low concentrations, are called odorants.
The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures about 1 square centimetre (on each side) and lies on the roof of the nasal cavity about 7 cm above and behind the nostrils. The olfactory epithelium is the part of the olfactory system directly responsible for detecting odors.
The peripheral olfactory organ or organ of smell consists of two parts: an outer, the external nose, which projects from the center of the face; and an internal, the nasal cavity, which is divided by a septum into right and left nasal chambers.
Olfactory epithelium consists of three distinct types of cells:
- Olfactory Cells
- Supporting Cells
- Basal Cells
The olfactory cells of the epithelium are bipolar neurons which congregate to form the olfactory nerve (cranial nerve I).
Analogous to neural glial cells, the supporting cells (a.k.a. sustencular cells ) of the olfactory epithelium function as metabolic and physical support for the olfactory cells. Histologically, the supporting cells are tall columnar cells featuring microvilli and a prominentterminal web. The nuclei of supporting cells are more apically located than those of the other olfactory epithelial cells.
Resting on the basal lamina of the olfactory epithelium, basal cells are stem cells capable of division and differentiation into either supporting or olfactory cells. The constant divisions of the basal cells leads to the olfactory epithelium being replaced every 2–4 weeks.
Basal cells can be divided on the basis of cellular anatomy histological markers into two populations: the horizontal basal cells which line the olfactory epithelium and the slightly more superficial globose basal cells.
SENSE OF SMELL
The role of the sense of smell is translate the molecular imput in smell sensation.
- only volatile substances that can be sniffed into the nostrilcan be smelled.
- the stimulating substances must be slightly water soluble.
- substances must be at least slightly lipid soluble.
MECHANISM OF TRANSDUCTION THE MOLECULAR IMPUT
ABNORMALITIES OF OLFACTION
- Anosmia is a lack of functioning olfaction, or in other words, an inability to perceive odors. Anosmia may be either temporary or permanent.
- Hyposmia is a reduced ability to smell and to detect odors.
- Dysosmia , also known as olfactory dysfunction, is the impairment of olfactory stimuli processing leading to an altered sense of smell.
These dysfunctions can present in a variety of ways, such as the stimuli not activating the olfactory bulb, some odors being interpreted as other odors, or hallucinations of smells.
The olfactory organ is, due to its immediate, relatively simple use in everyday life, often considered to be of lesser importance by the medical community. As a matter of fact, it does play a major role since, especially as far as diagnosis is concerned, it may turn out to be a valuable aid. Research has recently shown that the defects in the sense of smell may represent signs of serious diseases. For instance, the studies below can be suggested:
Furthermore, the following article about schizophrenia is suggested:
Vincenzo Emanuele Berardi e Trecca Fabrizio