1. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Chemical Senses  Chemical senses – gustation (taste) and olfaction (smell)  Their chemoreceptors respond to chemicals in aqueous solution  Taste – to substances dissolved in saliva  Smell – to substances dissolved in fluids of the nasal membranes

2. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Sense of Smell  The organ of smell is the olfactory epithelium, which covers the superior nasal concha  Olfactory mucosa contains 10–20 million olfactory cells (bipolar neuron), each of which bears 10–20 cilia called olfactory hairs.  Olfactory receptor cells are bipolar neurons with radiating olfactory cilia  Olfactory receptors are surrounded and cushioned by supporting cells  Basal cells lie at the base of the epithelium

3. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Olfactory Receptors Figure 15.21

4. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Physiology of Smell  Olfactory receptors respond to several different odor-causing chemicals  When bound to ligand these proteins initiate a G protein mechanism, which uses cAMP as a second messenger  cAMP opens Na + and Ca 2+ channels, causing depolarization of the receptor membrane that then triggers an action potential

5. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Taste Buds  Most of the 10,000 or so taste buds are found on the tongue  Taste buds are found in papillae of the tongue mucosa  Papillae come in three types: filiform, fungiform, and circumvallate  Fungiform and circumvallate papillae contain taste buds

6. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Taste Buds Figure 15.23

7. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Taste Sensations  There are five basic taste sensations  Sweet – sugars, saccharin, alcohol, and some amino acids  Salt – metal ions  Sour – hydrogen ions  Bitter – alkaloids such as quinine and nicotine  Umami – elicited by the amino acid glutamate

8. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Physiology of Taste  In order to be tasted, a chemical:  Must be dissolved in saliva  Must contact gustatory hairs  Binding of the food chemical:  Depolarizes the taste cell membrane, releasing neurotransmitter  Initiates a generator potential that elicits an action potential

9. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings General sensations

10. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings  General senses (also called somatic, somatosensory, or somesthetic) have receptors that are widely distributed throughout the body.  These detect touch, pressure, heat, cold, and pain, as well as many other stimuli that we do not consciously perceive.  The special senses are limited to the head, including vision, hearing, equilibrium, taste, and smell.

11. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Functional classification of Receptors  Chemoreceptors respond to chemicals.  Thermoreceptors respond to temperature changes.  Nociceptors are pain receptors and sense tissue damage.  Mechanoreceptors respond to a physical change in their shape.  Photoreceptors respond to light.

12. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Receptors can be classified according to the origins of their stimuli:  Interoceptors detect stimuli from internal organs.  Proprioceptors sense position and movement of the body or its parts.  Exteroceptors detect external changes

13. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Exteroceptors  Respond to stimuli arising outside the body  Found near the body surface  Sensitive to touch, pressure, pain, and temperature  Include the special sense organs

14. Copyright © 2006 Pearson Education, Inc., publishing as Benjamin Cummings Interoceptors  Respond to stimuli arising within the body  Found in internal viscera and blood vessels  Sensitive to chemical changes, stretch, and temperature changes