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The sense of smell is unique among the sensory systems in that its central connections first project to phylogene- tically older portions of the cerebral cortex before reaching the thalamus and eventually the neocortex. Smell and taste also have access to neural circuits that control both emotional states of body and certain memories. The neural systems that convey smell and taste are remarkably sensitive,capable of detecting and discriminating stimuli at extremely low concentrations.

SOME ADDITIONAL INFORMATION

We are continuously bombarded by molecules released into our environment.Through the senses of smell and teaste these molecules provide us with important information that we use constantly in our daly lives.
They signal pleasure or danger and inform us about food and drink,or the presense of something to seek or avoid. Thus, like the other senses we have so far considered (somatic sensibility, vision,and hearing), smell and taste inform us about the external world. In addition, however, they also connect that perception with information about our internal environment, its needs,and its satisfactions: hunger,thirst,sex and satiety.

Abnormalities of Olfaction

Olfactory acuity varies from person to person.In medical literature the term hyposmia (dimiminished sense of smell) is favored,such as occur during the common cold. Total loss or an absence of the sense of smell is known as anosmia,such as mechanical blockage of the airway, infecction or the presence of a tumor. And cacosmia is a repugnant smell.

SMELL

Smell, or olfaction, happens when chemicals in the air enter the nose during the breathing process. Smell receptors lie in the top of the nasal passage. They send impulses along the olfactory nerve to the olfactory bulb at the base of the brain. Researchers theorize that there are a great many types of olfactory receptors. People perceive particular smells when different combinations of receptors are stimulated.
The sense of smell (olfaction) is carried by receptors that lie deep within the nasal cavity. In humans these receptors are confined to a patch of specialized ephithelium (olfactory epithelium,coverly roughly 5cm2 of dorsal posterior recess of the nasal cavity and lying over the turbinate cartilage.
The receptors are bipolar neurons that have a short peripheral process extends to the surface of the mucosa,where it ends in an expanded olfactory knob.