Chapter 16 - The Chemical Senses

Question 1

1- Basic Taste Qualities

Answer 1

Five basic taste qualities
* Salty
* Sour
* Sweet
* Bitter
* Umami: described as meaty, brothy or savory, and associated with MSG

Sometimes “fatty” is added (like cheese)

Tongue map myth
Not true!!

Question 2

2- Some Properties of the
Chemical Senses

Answer 2

• The chemical senses involve three components
• Taste
• occurs when molecules enter the mouth in solid or
  liquid form and stimulate receptors on the tongue.
• Olfaction
• occurs when air-borne molecules enter the nose and stimulate receptor neurons in the olfactory mucosa, located on the roof of the nasal cavity.
• Flavor
• the impression we experience from the combination
  of taste and olfaction and several other factors.

Neurogenesis
* Constant renewal of the receptors unique to taste and smell.
* Due to exposure to harmful
materials.
* cycle of birth,
development, and death
over 5–7 weeks for
olfactory receptors and 1–
2 weeks for taste
receptors.

• Chemical senses as “gatekeepers”.
• (1) identify things that the body needs for survival and
  that should therefore be consumed.
• (2) detect things that would be bad for the body and that
  should therefore be rejected.
• Emotional, components
• things that are bad for us often taste or smell unpleasant
• things that are good for us generally taste or smell good.
• smelling an odor associated with a past place or event
  can trigger memories, which in turn may create
  emotional reactions.

Question 3

3- Connections Between Taste Quality and a Substance’s Effect

Answer 3

• The four taste qualities.
• sodium chloride (salty)
• hydrochloric acid (sour)
• sucrose (sweet)
• quinine (bitter)
• potassium chloride (KCl) has substantial salty and
  bitter components.
• sodium nitrate (NaNO3) results in a taste consisting
  of a combination of salty, sour, and bitter.
• Sweetness is usually associated with substances that have nutritive value.
• Bitter is usually associated with substances that are
  potentially harmful.
• Salty taste indicates the presence of sodium.
• However, there is not a perfect connection between
  tastes and function of substances.
  Ex: why do we sometimes like bitter things like coffee? Why do we sometimes not like healthy things like green juice?

Question 4

4- Structure of the Taste System

Answer 4

Tongue contains papillae
* Filiform: shaped like cones and located over entire surface
* Fungiform: shaped like mushrooms and found on sides and tip
* Foliate: series of folds on back and sides
* Circumvilliate: shaped like flat mounds in a trench located at back
(see images)

Taste buds are located in papillae except for filiform.
* Tongue contains approximately 10,000 taste buds.
* Each taste bud has 50-100 taste cells with tips that extend into the taste pore.
* Transduction occurs when
chemicals contact the receptor sites on the tips.

Signals from taste cells travel along a set of pathways.
* Chorda tympani nerve from front and sides of tongue
* Glossopharyngeal nerve from back of tongue
* Vagus nerve from mouth and throat
* Superficial petronasal nerve from soft palate

These pathways make connections in the nucleus of the solitary tract in the spinal cord. Then, they travel to the thalamus.
* Followed by areas in the frontal lobe:
* Insula
* Frontal opervulum cortex
* Orbital frontal cortex

Question 5

5- Population Coding

Answer 5

• Population coding
• quality is signaled by the pattern of activity distributed across many neurons.
• Experiment by Erickson
• demonstrated this population coding
• Presented a number of different taste stimuli to a rat’s tongue and recorded
  the response of the chorda tympani nerve.
• 13 nerve fibers responded to ammonium chloride (NH4Cl), potassium
  chloride (KCl), and sodium chloride (NaCl).
• across-fiber patterns (population coding)
• Erickson
• shocked rats while they were drinking potassium chloride and then gave them a choice between ammonium chloride and sodium chloride.
• when the rats were shocked for drinking ammonium chloride, they subsequently
  avoided the potassium chloride, as predicted by the electrophysiological results.
• Population coding un humans
• Erickson (1971)
• asked humans to make similarity judgments between a number of different solutions.
• substances that were perceived to be similar were related to patterns of firing for these same substances in the rat.
• Solutions judged more similar psycho-physically had similar patterns of firing, as
  population coding would predict.

Question 6

6- Specificity Coding

Answer 6

Experiment by Mueller et al.
* Genetic cloning was used to
determine if mice could be created that possessed a human receptor that responds to PTC.
* Normally, mice don’t have this receptor or respond to this substance.
* The experiment was successful, but not all data show the same results.
(See results in graph)

• Recordings from neurons
  at the beginning of the
  taste systems of animals
• Some neurons are
  specialized to respond to
  specific stimuli. Some
  neurons respond to a number of different types of stimuli.
• Applying amiloride to the
  tongue
• blocks flow of sodium to
  taste receptors.
• Causes decrease in the
  responding of neurons in
  rat’s brainstem that respond
  to salt, but not to those that
  respond to salty and bitter.

Question 7

7- Individual Differences in Taste

Answer 7

There are different responses to phenylthiocarbamide
(PTC) and to 6-n-propylthiouracil (PROP):
* Tasters, nontasters, and supertasters
- Tasters have more taste buds than nontasters.
- Tasters have specialized receptors for these
compounds.
- Supertasters appear more sensitive to bitter
substances than tasters.
*The presence of specialized receptors.
- genes on human chromosomes that are
associated with taste and smell receptors.
- PROP and PTC tasters have specialized receptors that are absent in nontasters

Question 8

8- The Functions of Olfaction

Answer 8

• Many animals are macrosmatic: having a keen sense of smell that is necessary for survival (dogs, bears…)
• Humans are microsmatic: a less keen sense of smell
  that is not crucial to survival.

Question 9

9- Detecting Odors

Answer 9

Measuring the detection threshold
* Yes/no procedure: participants are given trials with odors along with
“blank” trials.
- They respond by saying yes or no.
- This can result in bias, in terms of when the participant decides to
respond.
* Forced-choice: two trials are given,
one with odorant and one without.
- Participant indicates which smells strongest.
(see human detection threshold graph)

Rats are 8 to 50 times more sensitive to odors than humans.
Dogs are 300 to 10,000 times more sensitive.
However, individual receptors for all these
animals are equally sensitive.
The difference lies in the number of receptors
they each have.
* Humans have ten million and dogs have one billion olfactory receptors.

Question 10

10- Identifying Odors

Answer 10

Humans can discriminate more than one trillion different odors.
* Find it difficult to identify odors
* Only successful half of the time

Question 11

11- The Puzzle of Olfactory Quality

Answer 11

Researchers have found it difficult to map perceptual experience onto physical attributes of odorants.
* There is no specific language for odor
quality.
* Some molecules that have similar structure smell different, and some that have different structures smell the same.
Links have been found between the structure of
molecules, olfactory quality, and patterns of
activation in the olfactory system.
(See images)

Question 12

12- The Olfactory Mucosa

Answer 12

Olfactory mucosa is located at the top of the nasal cavity.
* Odorants are carried along the mucosa coming in contact with the
olfactory receptor neurons (ORN).
* These neurons contain molecules called olfactory receptors.
* Humans have about 350 types of receptors; each have a protein that crosses the membrane seven times.

Question 13

12- Odor objects

Answer 13

How we identify different odor objects?

Processing odors
* Perceiving odor objects involves olfactory
processing that occurs in two stages.
1) takes place at the beginning of the
olfactory system in the olfactory mucosa
and olfactory bulb, involves analyzing.
- In this stage, the olfactory system analyzes the different chemical components of odors and transforms
these components into neural activity at specific places in the olfactory bulb.
2) takes place in the olfactory cortex and beyond, involves synthesizing.

(see images)

Question 14

14- The Search for Order in the Olfactory Bulb

Answer 14

Representing Odors in
the Cortex
Signals from the olfactory bulb are sent to:
* Primary olfactory (piriform) cortex in the temporal lobe and amygdala
- Amygdala plays a role in emotional reactions to odors.
* Then to secondary olfactory (orbitofrontal) cortex in the frontal lobe

How Odorants Are
Represented in the
Piriform Cortex
Experiment by Rennaker
* Used multiple electrodes to measure neural responding in the piriform cortex, and found that isoamyl
acetate causes activation across the cortex.

a) odor objects (odorant molecules) b) olfactory bulb (chemotopic map activated) c) piriform cortex (scattered activation) d) piriform cortex after learning (pattern for odor object)

Experiment by Wilson
* Measured response of neurons in the rat’s piriform cortex to two odorants
- A mixture: isoamyl acetate and peppermint
- A compound: isoamyl acetate alone
* Results showed that with enough exposure, the piriform cortex could discriminate between the mixture and the compound.