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OBJECTIVES

Objectives

After studying this chapter, the student should be able to:

  • Outline the transduction of molecules received by olfactory receptors into signals sent to the olfactory bulb.

  • Diagram the olfactory bulb and indicate its organization.

  • Describe the olfactory projection system involving direct and indirect routes to cortex.

  • Describe the major taste dimensions and their transduction mechanisms.

  • Diagram the taste projection pathway cortex.

OVERVIEW

Smell and taste are called chemical senses because molecules activate the receptors in these systems rather than energy, such as photons in vision and vibrations in hearing. Smell and taste have important gateway functions in approach/withdrawal behaviors. The smell given off by a ripe pineapple induces us to consider eating it, and the ripe, sweet taste prompts consumption of the fruit. However, we are repulsed by the smells and tastes of many bitter and sour substances. Those chemical qualities are often (but not always) associated with spoilage or toxicity. Taste receptors, found mostly on the tongue, exist for sweet, sour, salt, bitter, and, according to some researchers, umami, the savory taste induced by the additive monosodium glutamate (MSG). Taste is also strongly influenced by smell because mastication of food also activates olfactory receptors in the nose. The combination of smell and taste gives rise to the complex perception of flavor. The message encoded by receptors for taste relays through several subcortical structures before reaching the thalamus and then the cortex. Olfaction is the exception among the senses in that neurons in the olfactory bulb project directly to cortex in a pathway whose activation is largely subconscious. However, there is an olfactory projection from cortex back to the thalamus, and then back to cortex. This projection reaches, among other loci, the orbitofrontal cortex, where smell and taste inputs are combined to mediate complex olfactory perceptions such as flavor.

ORGANIZATION OF THE OLFACTORY SYSTEM

The nose has a number of functions besides smell, such as filtering, warming, and humidifying the air you breathe. However, here we concentrate on its function of detecting odorants and reporting their characteristics to the brain. Olfactory receptors exist in the roof of the nose. Figure 14–1 shows the overall internal structure of the nose.

FIGURE 14–1

General structures of the nasal cavities and olfactory mucosa. On the left is a frontal cutaway view of the main nasal chambers (turbinates). The olfactory mucosa is in the uppermost turbinate just inferior to the olfactory bulb. On the right is a side view of the same structures. (Used with permission from David Klemm, Faculty and Curriculum Support, Georgetown University Medical Center, Washington, DC.)

The Nose & Olfactory Epithelium

Internally the nose is divided in the middle by a cartilaginous structure called the septum. On each side of the septum are 3 cavities called “turbinates,” ...

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