Properties and Types of Sensory Receptors

Objectives

When you have completed this section, you should be able to

• define receptor and sense organ;
• list the four kinds of information obtained from sensory receptors and describe how the nervous system codes for each type; and
• outline three ways of classifying receptors.

Think About It

Although you may find it difficult to immerse yourself in a hot tub of water or a cold lake, you soon adapt and become more comfortable. In light of this, do you think cold and warm receptors are phasic or tonic?

Key Point Review

1. What is the difference between a receptor and a nerve ending?

2. Distinguish between general and special senses.

3. Three schemes of receptor classification were presented in this section. In each scheme, how would you classify the receptors for a full bladder? How would you classify taste receptors?

4. What does it mean to say sense organs are transducers? What form of energy do all receptors have as their output?

5. Nociceptors are tonic rather than phasic receptors. Speculate on why this is beneficial to homeostasis.

The General Senses

Objectives

When you have completed this section, you should be able to

• list several types of receptors for the general senses;
• describe the somesthetic projection pathways in the CNS; and
• explain the mechanisms of pain.

Key Point Review

6. What stimulus modalities are detected by free nerve endings?

7. Name any four encapsulated nerve endings and identify the stimulus modalities for which they are specialized.

8. Where do most second-order somatosensory neurons synapse with third-order neurons?

9. Explain the phenomenon of referred pain in terms of the neural pathways involved.

10. Explain the roles of bradykinin, substance P, and endorphins in the perception of pain.

The Chemical Senses

Objectives

When you have completed this section, you should be able to

• explain how taste and smell receptors are stimulated; and
• describe the structure and projection pathways of olfactory and gustatory receptors.

Think About It

Which taste sensations could be lost after damage to (1)the facial nerve or (2)the glossopharyngeal nerve? Fractures of which cranial bone would most likely disrupt the sense of smell?

Key Point Review

11. What is the difference between a lingual papilla and a taste bud?

12. List the four primary taste sensations and discuss the adaptive significance (survival value) of each.

13. Which cranial nerves carry gustatory impulses to the brain?

14. What part of an olfactory cell bears the binding sites for odor molecules?

15. Which regions of the brain receive and process input from the olfactory cells?

Hearing and Equilibrium

Objectives

When you have completed this section, you should be able to

• identify the physical properties of sound waves that account for pitch and loudness;
• describe the gross and microscopic structures of the ear;
• explain how the ear converts vibrations to nerve impulses and discriminates between sounds of different intensity and pitch;
• explain how the vestibular apparatus enables the brain to interpret the body's position and movements; and
• describe the pathways taken by auditory and vestibular signals to the brain.

Think About It

What type of muscle fibers–Type I or II (see chapter 12)–do you think constitute the stapedius and tensor tympani? That is, which type would best suit the purpose of these muscles?

Think About It

The semicircular ducts do not detect motion itself, but only acceleration–a change in the rate of motion. Explain.

Key Point Review

16. What physical properties of sound waves correspond to the sensations of loudness and pitch?

17. What is the benefit of having three auditory ossicles and two muscles in the middle ear?

18. Explain how vibration of the tympanic membrane ultimately results in fluctuations of membrane voltage in a cochlear hair cell.

19. How does the brain recognize the difference between high C and middle C of a piano? Between a loud sound and a soft one?

20. How does the function of the semicircular ducts differ from the function of the saccule and utricle?

21. How is the mode of sensory transduction in the semicircular ducts similar to that in the saccule and utricle?

Vision

Objectives

When you have completed this section, you should be able to

• describe the anatomical features of the eye and the structures that serve it;
• describe the structure of the retina and its rod and cone cells;
• explain how the optical system of the eye creates an image on the retina;
• explain how the retina converts this image to nerve impulses;
• explain why different types of receptor cells and neuronal circuits are required for day and night vision; and
• trace the visual projection pathways in the brain.

Think About It

Which extrinsic muscles of the eyes are the prime movers in convergence?

Think About It

If you look directly at a dim star in the night sky, it disappears, and if you look slightly away from it, it reappears. Why?

Key Point Review

22. Why can't we see wavelengths below 350 nm or above 750 nm?

23. Why are light rays bent (refracted) more by the cornea than by the lens of the eye?

24. List as many structural and functional differences between rods and cones as you can.

25. Explain how the absorption of a photon of light leads to depolarization of a bipolar retinal cell.

26. Discuss the duplicity theory of vision, summarizing the advantage of having two types of retinal photoreceptor cells.

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