Chemical Senses

1. 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. 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. Olfactory Receptors Figure 15.21

4. 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 Ca2+ channels, causing depolarization of the receptor membrane that then triggers an action potential

5. 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. Taste Buds Figure 15.23

7. 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. 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. General sensations

10. 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. 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. 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. 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. Interoceptors • Respond to stimuli arising within the body • Found in internal viscera and blood vessels • Sensitive to chemical changes, stretch, and temperature changes