1. Show photos or films of various animals that possess one or more of the specific types of receptors.

1. Discuss the concept of absolute threshold for each sense organ and use Figure 43.4 to illustrate the general theory of a threshold.

1. Show Figure 43.5 and discuss the papillae, the sense of taste and also how it is related to the sense of smell.

2. Hand out PTC taste papers and neutral taste papers when discussing taste receptors. Mention that the ability to taste PTC is an autosomal dominant condition (both homozygotes and heterozygotes are tasters) and add that the tastes reported by various persons vary (e.g. bitter, sour, etc.).

3. Show Figure 43.6 to illustrate the various locations of cells that contribute to the sense of smell.

4. Ask students to explain why they have more difficulty tasting their food when they have a bad cold with a "stuffy" nose.

5. Have two students pinch their noses closed and blindly sample two types of citrus (e.g. orange and grapefruit); without the sense of smell, can they distinguish between the two?

1. Use Figure 43.8 to introduce the idea of a mechanoreceptor and discuss areas on the human body that are particularly responsive to touch.

2. Use Figure 43.10 to introduce stretch reflexes and demonstrate the knee-jerk reaction.

1. Show Figure 43.11 and discuss how hair cells play an important role in mechano reception.

2. Ask students to give examples of how this works (e.g. a female student may not feel a mosquito on her leg the day she shaved, but would feel the mosquito the next day).

3. Discuss the importance of whiskers for navigation in mice and other animals (the house mouse apparently cannot navigate when its whiskers are cut off).

4. Mention the statocyst of invertebrates and the lateral line system of fish as two examples of hair cell systems critical to navigation (Figures 43.12 and 13).

1. Show Figure 43.15 and discuss the operation of the vertebrate ear.

1. Discuss the heat and infrared receptors in snakes and show Figure 43.17, along with videos of snakes responding to infrared stimuli.

1. Discuss photopigments and the nerve impulse generated when they absorb light and change structure (Figure 43.19).

2. Survey a few of the photoreceptors in various lower phyla, and point out the evolutionary trends in this area (Figure 43.18).

1. Show Figures 43.22—43.24 and review the anatomy of the human eye.

2. Discuss myopia and hyperopia and review the various methods available for correcting sight disorders. Draw figures to show how myopia hyperopia are corrected by changing the focal point of the light with a lens.