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After studying this chapter, you should be able to:

  • Describe the structure and function of the neural elements in the olfactory epithelium and olfactory bulb.

  • Identify the significance of the family of olfactory receptor genes.

  • Explain how odorant receptors are activated and the mechanism by which signal transduction occurs in these receptors.

  • Label the components of the pathway by which impulses generated in the olfactory epithelium reach five regions of the olfactory cortex.

  • Describe the location and cellular composition of taste buds.

  • Name the five major taste modalities and compare the signal transduction mechanisms in the receptors mediating these different taste modalities.

  • Label the components of the pathways by which impulses generated in taste receptors reach the gustatory region of the insular cortex.

  • Name and discuss abnormalities in odor and taste sensations.


Smell (olfaction) and taste (gustation) are examples of visceral senses because of their close association with gastrointestinal function. Physiologically, they are related to each other as the flavor of food is a combination of its taste and smell. This is why food may taste “different” if one has a cold that depresses the sense of smell. Smell and taste receptors are chemoreceptors that are stimulated by chemical molecules in solution in mucus in the nose (odorants) and saliva in the mouth (tastants). The sensations of smell and taste likely evolved as protective mechanisms to avoid the intake of potentially harmful substances.



The yellowish pigmented olfactory epithelium is a specialized portion of the nasal mucosa that covers an area of 10 cm2 in the roof of the nasal cavity near the septum in humans (Figure 9–1). The olfactory epithelium is the place in the body where the nervous system is closest to the external world. It contains three types of neurons that are important for olfaction: olfactory sensory neurons, supporting cells, and basal stem cells.


Structure of the olfactory epithelium. There are three cell types: olfactory sensory neurons, supporting (sustentacular) cells, and basal stem cells at the base of the epithelium. Each olfactory sensory neuron has a dendrite that projects to the epithelial surface. Numerous cilia protrude into the mucus layer lining the nasal lumen. Odorants bind to specific odorant receptors on the cilia and initiate a cascade of events leading to generation of action potentials in the sensory axon. Each olfactory sensory neuron has a single axon that projects to the olfactory bulb, a small ovoid structure that rests on the cribriform plate of the ethmoid bone. (Reproduced with permission from Kandel ER, Schwartz JH, Jessell TM [editors]: Principles of Neural Science, 4th ed. New York, NY: McGraw-Hill; 2000.)

The bipolar ...

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