SENSE OF SMELL

Odorants are volatile chemical compounds that are carried by inhaled air to the Regio olfactoria (olfactory epithelium) located in the roof of the two nasal cavities of the human nose, just below and between the eyes.

•The odorant must possess certain molecular properties in order to provide sensory properties. It must have some water solubility, a sufficiently high vapor pressure, low polarity, some ability to dissolve in fat (lipophilicity), and surface activity. And to date, no known odorant possesses a molecular weight greater than 294•The olfactory region of each of the two nasal passages in humans is a small area of about 2.5 square centimeters containing in total approximately 50 million primary sensory receptor cells.•The olfactory region consists of cilia projecting down out of the olfactory epithelium into a layer of mucous which is about 60 microns thick.•This mucous layer is a lipid-rich secretion that bathes the surface of the receptors at the epithelium surface. The mucous layer is produced by the Bowman’s glands which reside in the olfactory epithelium.

•The smell receptors are sensitive to seven types of sensations that can be characterized as camphor, musk, flower, mint, ether, acrid, or putrid.

•Physiologically, this convergence increases the sensitivity of the olfactory signal sent to the brain. From the mitral cells the message is sent directly to the higher levels of the central nervous system in the corticomedial amygdala portion of the brain (via the olfactory nerve tract) where the signaling process is decoded and olfactory interpretation and response occurs.

DISORDERS OF OLFACTION:• Anosmia – inability to smell•Cocosmia – things smell like feces• Dysosmia – things smell different than they should• Hyperosmia – an abnormally acute sense of smell•Hyposmia – decreased ability to smell•Olfactory reference syndrome – psychological disorder which causes the patient to imagine he or she has strong body odor.•Parosmia “– things smell worse than they should•Phantosmia – “hallucinated smell”, often unpleasant in nature.

Olfaction and tasteOlfaction, taste and trigeminal receptors (also called Chemesthesis) together contribute to flavor. The human tongue can distinguish only among five distinct qualities of taste, while the nose can distinguish among hundreds of substances, even in minute quantities. It is during exhalation that the olfaction contribution to flavor occurs in contrast to that of proper smell which occurs during the inhalation phase.

Olfaction in other animals

•Having a strong sense of smell is referred to as macrosmatic.• dogs in general have an olfactory sense approximately a hundred thousand to a million times more acute than a human's.•Fish too have a well-developed sense of smell, even though they inhabit an aquatic environment. •Insects primarily use their antennae for olfaction. Sensory neurons in the antenna generate odor-specific electrical signals called spikes in response to odor. 

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