After studying this chapter, you should be able to:
Describe the basic features of the neural elements in the olfactory epithelium and olfactory bulb.
Describe signal transduction in odorant receptors.
Outline the pathway by which impulses generated in the olfactory epithelium reach the olfactory cortex.
Describe the location and cellular composition of taste buds.
Name the five major taste receptors and signal transduction mechanisms in these receptors.
Outline the pathways by which impulses generated in taste receptors reach the insular cortex.
Smell (olfaction) and taste (gustation) receptors are chemoreceptors that are stimulated by molecules in a solution within the nasal mucus and saliva in the mouth. The sensations of smell and taste allow individuals to distinguish between estimates of up to 30 million compounds that are present in food, predators, and mates and to convert the information received into appropriate behaviors. Odors from food enter our nasal passages at the same time that the taste receptors in our mouth are stimulated by the food. Smell and taste are separate senses but interact with each other to account for the flavor we experience when we eat and drink. Flavor refers to the overall perception that results from a combination of taste and smell.
OLFACTORY EPITHELIUM & OLFACTORY BULBS
Olfactory sensory neurons are located in the yellowish pigmented olfactory epithelium of the nasal mucosa (Figure 11–1A). These sensory neurons are interspersed with glia-like supporting (sustentacular) cells and basal stem cells. New olfactory sensory neurons are generated by basal stem cells to replace those damaged by exposure to the environment. The olfactory epithelium is covered by a thin layer of mucus secreted by the supporting cells and Bowman glands, which lie beneath the epithelium.
Sensory components for smell and taste. (A) There are three cell types in the olfactory epithelium: olfactory sensory neurons (odorant receptors), supporting (sustentacular) cells, and basal stem cells. 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. (B) Taste buds are composed of basal stem cells and three types of taste cells (dark, light, and intermediate). Taste cells extend from the base of the taste bud to the taste pore, where microvilli contact tastants dissolved in saliva and mucus. (Modified with permission from Kandel ER, Schwartz JH, Jessell TM [editors]: Principles of Neural Science, 4th ed. New York, NY: McGraw-Hill; 2000.)
Each olfactory sensory neuron has a short, thick dendrite that projects into the nasal cavity where it terminates in a knob containing 6–12 cilia (...