Chapter 15: The Chemical Senses

1. Which structure separates the olfactory epithelium from the olfactory bulbs?

d. Cribiform plate

2. The olfactory bulb does not project to the

a. thalamus.

b. amygdala.

c. entorhinal cortex.

d. olfactory tubercle.

e. pyriform cortex.

Answer: a

3. Which brain region is dedicated solely to olfaction perception?

b. Pyriform cortex

4. A room is filled with the scents of lavender and roses during a woman's fMRI. Where would a researcher expect to see increased brain activity?

d. Both orbitofrontal and cingulate cortex

5. In which disease or disorder is olfaction dysfunction or anosmia part of the diagnostic criteria?

e. Eating disorders, schizophrenia, Parkinson's disease, and Alzheimer's disease

6. The mucus layer and the epithelium, with neural and supporting cells is(are) called

c. nasal mucosa.

7. A scientist creates a knockout rat that has nonfunctional Bowman's glands. Which functional effect would this most likely have on the rat?

d. Increased susceptibility to infection

8. Which cell types are not involved with proper generation of olfactory receptor neurons throughout an animal's lifetime?

b. Sustentacular cells

9. Which conclusion can be drawn regarding the high number of olfactory pseudogenes in humans?

e. Reliance on olfaction has decreased throughout human evolution.

10. A scientist creates a mouse model with dysfunctional cyclic nucleotide-gated channels. Which of the following would not occur in this model?

b. Influx of calcium

11. Which protein is important for modifying receptor sensitivity in olfaction?

a. β-arrestin

12. Which cell type does not synapse within glomeruli?

d. Granule

13. Exposure to a single odor activates _______ glomeruli, while exposure to a complex odor (150 compounds) activates _______ glomeruli.

c. a few; a few

14. A patient sustains a head injury resulting in damage of some glomeruli. Which effect would most likely result from this damage?

a. Inability to detect certain smells

15. Which type cell type primarily forms the lateral olfactory tract, which projects to many areas in the brain?

d. Mitral

16. Most projections from the lateral olfactory tract are _______, and they _______ activate the pyriform cortex, _______ the activation of glomeruli to different odors.

b. ipsilateral; broadly; unlike

17. Projections from the pyriform cortex to other brain regions allow olfaction to influence which other behavior(s) and/or function(s)?

e. Sexual, appetitive, memory, and visceral

18. A _______ would be least likely to show a change in behavior in response to species-specific pheromones.

a. human

19. Neurons from the _______ project to the hypothalamus and the _______.

b. accessory olfactory bulb; amygdala

20. A scientist studying the vomeronasal system creates a V2R knockout rat. Measurement of which variable would best evaluate the behavior of the knockout animal compared to controls?

d. Distance traveled away from cat spray (kairomones)

21. Taste cells are not present in which part of the mouth?

c. Gums

22. _______ projections are topographically represented along the rostral-caudal axis in the _______.

c. Cranial nerve; gustatory nucleus

23. _______ papilla, located posteriorly on the tongue, would respond most strongly to a _______ tastant.

e. Circumvallate; bitter

24. A scientist creates a TRP channel knockout mouse model for all TRP channels. These mice would not be expected to respond to which of the following tastants in their drinking water?

a. Sweet, bitter, umami, sour

25. The T2R gene specifically codes for which taste?

c. Bitter

26. A decrease in extracellular H+ would decrease the ability to detect which taste?

b. Sour

27. The G-protein gustducin is in involved in which of the following taste sensations?

c. Bitter

28. Which evidence supports labeled line coding in the taste system?

d. Mice re-expressing PLCβ2 in T2-expressing taste cells in a PLCβ2 mutant mouse are able to respond to bitter tastes.

29. What would be the expected behavior of a TRPM5-/- knockout mouse compared to a wild type mouse?

c. Increased drinking of water with quinine

1. What features of the olfactory system make it highly sensitive to low concentrations of odorants?

Answer: Low concentrations of odorants are detectable due to the high degree of convergence in the olfactory system. Many olfactory axons (25,000 in the mouse) that almost exclusively express a single odorant receptor gene project to one glomerulus. This glomerulus only synapses with a few (25 in the mouse) mitral cells. This convergence allows for amplification of olfactory receptor neuron signals.

2. How does olfactory information reach the brain region(s) that perceive(s) and interpret(s) odors?

Answer: Mitral cells in the olfactory bulb project to the pyriform cortex, which is the primary olfactory cortex, via the lateral olfactory tract.

3. What is the difference between pheromones and kairomones?

Answer: Pheromones are stimuli that mediate behaviors such as mating and social behavior among members of the same species. Kairomones are stimuli that modulate behavior between different species, such as predator and prey.

4. Describe the steps in sensory transduction in olfactory neurons.

Answer: An odorant enters the nasal cavity and binds to an odor receptor protein located on the olfactory cilia. This triggers Golf to dissociate its α subunit to activate adenylyl cyclase III (ACIII). This increases cAMP, which opens cyclic nucleotide-gated channels to permit an influx of cyclic Na+ and Ca2+ for depolarization. When this depolarization reaches the axon hillock, action potentials are generated. This signal travels to a glomerulus where it synapses on mitral cells. These mitral cells then project to the pyriform cortex for processing.

5. What are considered to be the primary taste categories?

Answer: The primary taste categories are salty, sour, sweet, bitter, and umami.

6. List some differences between olfaction and taste in central processing.

Answer: The olfactory system projects to its primary cortex before projecting to the thalamus, unlike other senses. Olfaction activation in the pyriform cortex is widespread, whereas each individual taste is distinctly represented in its own domain in the cerebral cortex.

7. Compare the taste receptors for sour and salty with those for sweet, bitter, and umami.

Answer: Sour and salty use ion channels, the amiloride-sensitive Na+ channel, and H+-permeant nonselective channel, respectively. Sweet, bitter, and umami all use G-protein coupled receptors. Sweet uses a T1R2 and T1R3 heterodimer, whereas umami uses a T1R1 and T1R3 heterodimer. Bitter uses only a T2R receptor for transduction.

8. How does taste information reach the neocortex? What part(s) of the cortex receive(s) input from the taste pathway?

Answer: Taste information from taste cells is transmitted to the nucleus of the solitary tract via cranial nerves VII, IX, and X. This information is then relayed to the ventral posterior medial nucleus of the thalamus and then projected to the insular and frontal taste cortices.

9. Are individual odors recognized by a labeled line strategy or by computing the activity from ensembles of neurons? Which system is involved in recognition of tastes?

Answer: Individual odors are recognized by computing the activity from ensembles of neurons, whereas tastes are recognized by labeled line coding.

1. Which structure is not part of the pathway by which olfactory signals first reach cortex?

c. Olfactory thalamus

2. In terms of scent detection and tracking capabilities in humans,

c. humans (like dogs) can follow scent trails by frequent sniffing with orthogonal digressions.

3. Which statement about human odorant perception is false?

a. The compound ethyl mercaptan is added to natural gas because this odorant is universally detectable by healthy humans.

4. Regarding a zinc nasal spray treatment to prevent the spread of the deadly polio virus,

e. in the 1990s, some thought zinc could prevent the common cold and nasal sprays with zinc salts were created and sold but were not monitored by the FDA.

5. Which is not an established odorant-mediated response or capability of humans?

a. Pheromone-based sexual attraction mediated by the vomeronasal organ

6. Olfactory receptors are found at the highest concentration in

b. the olfactory cell cilia.

7. Olfactory receptors most closely resemble which protein in structure and mechanism of action?

d. Muscarinic acetylcholine receptor

8. Odorant receptor genes and/or proteins

d. constitute approximately 3-5% of the genome of mammals.

9. Which of the following statements about the olfactory capabilities of dogs is false?

e. The reliability of dogs in detecting cancers has led to their widespread use in oncology clinics.

10. Listed below are the various components of the olfactory transduction process:

1. Golf G-protein

2. Voltage-gated sodium channel

3. Calcium-activated chloride channel

4. cAMP-gated ion channel

5. Adenylyl cyclase III

b. 1; 5; 4; 3; 2

11. A mouse model with anosmia has been generated using gene knockout techniques. Removing which gene would lead to anosmia?

b. Adenyl cyclase III

12. The Drosophila olfactory lobe, the mushroom body, appears similar in structure and function to the mammalian olfactory

c. bulb.

13. In humans, the olfaction-responsive brain area that responds to multimodal stimuli (such as the sight and smell of food) is located in

b. orbitofrontal cortex.

14. Which statement about the pheromone-based sensory systems of mammals is false?

a. Pheromones are usually detected in an organ that is distinct from the primary olfactory epithelium.

b. Pheromone receptors project to a specialized (accessory) region of the olfactory bulb.

c. The accessory olfactory bulb projects to the hypothalamus and amygdala.

d. Vomeronasal receptors are highly expressed in all primates except humans.

e. Pheromones mediate behaviors with conspecifics, whereas kairomones mediate behaviors with other animals.

Answer: d

15. Stephanie has damage to her insular cortex. Which sense would have difficulty processing?

e. Taste

16. Samuel is having difficulty distinguishing bitter tastes. In which location might he have damage?

a. Cranial nerve IX

17. Sour substances activate taste cells by

d. the depolarizing effect of protons entering taste cells.

18. The sweet transduction system involves

a. heterodimeric G-protein-coupled receptors.

b. activation of G-proteins.

c. activation of phospholipase C.

d. IP3 activation of TRPM5 channels.

e. All of the above

Answer: e

19. Which mechanism for the neural encoding of taste perception has the strongest experimental support?

c. Labeled lines

20. According to one study, wild type mice cannot taste aspartame, a compound that humans find sweet (it does not activate mouse sweet receptors). A transgenic mouse line is created in which a bitter receptor, T2R19, has been knocked out. Additionally, the human sweet receptor gene has been engineered to be expressed selectively in mouse T2R19 cells. If the mice were then tested for lick rate preference when given different concentrations of aspartame in their water, which behavioral responses would you expect?

b. The mice would prefer water over the aspartame solution and would decrease lick rate as the concentration of aspartame increased.