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Introduction

THROUGH THE SENSES OF SMELL and taste, we are able to perceive a staggering number and variety of chemicals in the external world. These chemical senses inform us about the availability of foods and their potential pleasure or danger. Smell and taste also initiate physiological changes required for the digestion and utilization of food. In many animals, the olfactory system also serves an important social function by detecting pheromones that elicit innate behavioral or physiological responses.

Although the discriminatory ability of humans is somewhat limited compared with that of many other animals, odor chemists estimate that the human olfactory system may be capable of detecting more than 10,000 different volatile chemicals. Perfumers who are highly trained to discriminate odorants can distinguish as many as 5,000 different types of odorants, and wine tasters can discern more than 100 different components of taste based on combinations of flavor and aroma.

In this chapter, we consider how odor and taste stimuli are detected and how they are encoded in patterns of neural signals transmitted to the brain. In recent years, much has been learned about the mechanisms underlying chemosensation in a variety of animal species. Certain features of chemosensation have been conserved through evolution, whereas others are specialized adaptations of individual species.

A Large Family of Olfactory Receptors Initiate the Sense of Smell

Odorants—volatile chemicals that are perceived as odors—are detected by olfactory sensory neurons in the nose. The sensory neurons are embedded in a specialized olfactory epithelium that lines part of the nasal cavity, approximately 5 cm2 in area in humans (Figure 29–1), and are interspersed with glia-like supporting cells (Figure 29–2). They are relatively short lived, with a life span of only 30 to 60 days, and are continuously replaced from a layer of basal stem cells in the epithelium.

Figure 29–1

The olfactory system. Odorants are detected by olfactory sensory neurons in the olfactory epithelium, which lines part of the nasal cavity. The axons of these neurons project to the olfactory bulb, where they terminate on the dendrites of mitral and tufted cell relay neurons within glomeruli. In turn, the axons of the relay neurons project to the olfactory cortex, where they terminate on the dendrites of pyramidal neurons whose axons project to other brain areas.

Figure 29–2

The olfactory epithelium.

A. The olfactory epithelium contains sensory neurons interspersed with supporting cells as well as a basal layer of stem cells. A single dendrite extends from the apical end of each neuron; sensory cilia sprout from the end of the dendrite into the mucus lining the nasal cavity. An axon extends from the basal end of each neuron to the olfactory bulb.

B. A scanning electron micrograph of the olfactory epithelium shows the dense mat of ...

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