The superior turbinate, the cribriform plate, the upper surface of the middle turbinate and the opposed part of the nasal septum are covered by a specialized epithelium containing receptors cells. The sense of smell is mediated via stimulation of these olfactory receptors by volatile chemicals. Five different types of cells form the olfactory epithelium: the bipolar olfactory neuron, which is a primary sensory neuron with an olfactory knob from which several olfactory cilia extend; the basal cell, which replaces the bipolar neuron cells every 7 weeks; sustentacular cell, which acts as a support cell supplying nutrients for bipolar neuron cells; microvillar cell, which have no clearly defined role except to perhaps assist olfaction; Bowman's glands, which provide a serous component to the mucous layer covering the olfactory epithelium (Rice and GluckMAN 1995).
The exact mechanism of olfaction is somewhat vague. Multiple theories have been proposed but none have really been supported scientifically. There is some suggestion that different odors produce different patterns of activity across the olfactory mucosa. Whatever the explanation at the molecular level, depolarization of the bipolar neurons occurs, resulting in an action potential that is transmitted along the olfactory nerve, and the information is processed centrally in the olfactory tubercle, pyri-form cortex, amygdaloid nucleus, and hypothalamus. Interestingly enough, olfactory receptor cells are the only nerve cells capable of regeneration, allowing for (at least theoretically) the possibility of regeneration after severe injury (LAffORT et al. 1974).
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