The Olfactory System - Introduction

Tutorial 30: The Olfactory System

Smell is based on physical stimulation of hair like receptor cells by volatile, chemical substances. Smell is the only sense that is not processed within the thalamus before reaching the cerebral cortex. Most olfactory receptors respond to a wide range of odors. The perception of odor may, therefore, be dependent on the pattern of activation across many receptors. In addition, the sense of smell shows a high degree of sensory adaptation. The strength of any newly introduced odor decreases within approximately 4 minutes to less than half of its original strength; an advantage when one encounters an unpleasant smell.

The classification of primary odors is difficult in humans. This is reflected in the lack of a good vocabulary available to describe odors. Although we can discriminate among many thousands of odors, our ability to describe them is limited to comparing one to another. "I like your perfume. It smells like lily-of-the valley and citrus." Thus, for humans the perception of olfaction is limited to the identification of objects as opposed to the analysis of particular qualities.

The pervasive role of smell in everyday life is often overlooked. Olfaction plays an important part in the human experience, from the detection of unpleasant smells emitted by potentially dangerous substances, to the role of odors in the experience of flavor, and the role of odor in sexual behavior and emotional memory. Figure 30 will illustrate and describe the basic structures and functions associated with olfaction in humans.

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In addition to bypassing the thalamus before cortical processing, the olfactory system is the only sense modality that has no connections to more recently evolved areas of the cortex. In fact, the older portion of the mammalian forebrain is called rhinencephalon or smell brain because of this. The inability to classify odors much beyond a scale of pleasant to unpleasant is likely a reflection of this very old system's role in the approach-avoidance behavior essential to survival. Individual people differ greatly in sensitivity to odors.

It is unclear how different odors are distinguished by receptor activity. It may be that different proteins in the receptors are responsive to specific molecules of a given odor. Different receptors may have different distributions of these receptor proteins. The olfactory system is highly sensitive. For example, one form of musk can be detected by a normal person at a dilution of less than one ten-millionth of a milligram per liter of air. It has been proposed that molecules stimulate olfactory receptor cells via two mechanisms, direct and indirect or from a distance. The direct action theories are, however, better supported by modern research.

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Tutorial 30: The Olfactory System

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