Lecture 17: Taste and Smell

Question 1

Taste

Answer 1

• Taste receptors have different thresholds (most sensitive for bitter to protect against ingestion of toxic substances)
• Taste receptors adapt rapidly (3-5 sec for partial adaptation and complete adaptation in 1-5 min

Question 2

Three types of papillae

Answer 2

Fungiform
Vallate
Foliate

Question 3

Fungiform

Answer 3

Mushroom-shaped over entire surface of tongue, 1-5 taste buds each

Question 4

Vallate

Answer 4

Largest and jave many taste buds each; 8-12 vallate papillae total that make a V at the back of the tongue

Question 5

Foliate

Answer 5

Laterally on tongue - many taste buds in each during childhood but their numbers decrease with age

Question 6

What is the sensory neuron in the gustatory pathway?

Answer 6

Gustatory epithelial cell

Question 7

Explain the structure and function of a gustatory epithelial cell.

Answer 7

A taste bud will contain up to 100 gustatory epithelial cells. These are receptor cells to taste - not neurons but highly specialized cell types which can generate graded potentials to release neurotransmitter. Receptors are located on the cilia of the cells - food chemicals must be soluble ie must be able to dissolve in saliva in order to bind to the receptor. There are also receptors for pain, temperature and mechanical stimuli. Basal cells are stem cells which can divide and differentiate into supporting cells.

Question 8

Basal Epithelial cells

Answer 8

Turnover of taste cells is 7-10 days

Question 9

5 basic taste modalities

Answer 9

1. Salty - metal ions (inorganic salts); sodium chloride tastes saltiest
2. Sour - acids (hydrogen ions in solution)
3. Sweet - sugars, saccharin, alcohol, some amino acids
4. Bitter - alkaloids such as quinine and nicotine, caffeine, morphine, strychnine and nonalkaloids such as aspirin
5. Umami - amino acids glutamate and aspartate; example: beef (meat) or cheese taste, and monosodium glutamate (food additive)

Question 10

1 Sweet

Answer 10

To ensure adequate intake of carbohydrates for energy in the body

Question 11

2 Sour

Answer 11

To prevent intake of toxic substances into body. Also helps detect ripeness

Question 12

3 Salty

Answer 12

To ensure adequate intake of salt to regulate the amount of water in body

Question 13

4 Bitter

Answer 13

To prevent intake of poisonous substances into the body

Question 14

5 Umami

Answer 14

To ensure adequate intake of proteins for proper growth and maintenance of the body

Question 15

Tatse receptors

Answer 15

• Different thresholds (most sensetive for bitter)
• Adapt rapidly: 3-5 Seconds Partial , 1-5 min complete

Question 16

Mechanism

Answer 16

1. Salt: influx of Na+ through channels – depolarizes gustatory epithelial cells
2. Sour: H+ can directly go in and/or block leaky K+ channels leading to
   depolarization
3. Sweet, Bitter, Umami: All bind to appropriate cell surface receptors
   (coupled to the G protein gustducin and use second messengers to open
   channels leading to depolarization and release of the neurotransmitter ATP

Question 17

Mouth also contains ________ , ________ , and _______.

Answer 17

• thermoreceptors, mechanoreceptors, and nociceptors

Temperatyre and texture can influence our perception of food taste

Question 18

Two main cranial nerves carry taste impulses from tongue to brain

Answer 18

• Facial nerve (VII) transmits impulses from taste receptors in the anterior two-thirds of the tongue
• Glossopharyngeal (IX) carries impulses from posterior one-third of tongue and from pharynx
• Vagus nerve (X) transmits impulses from epiglottis and lower pharynx (very minor)

Question 19

Smell

Answer 19

• receptors are also chemoreceptors
• olfactory epithelium (pseudostratified columnar cells) located in roof of nasal cavity - not the best location to catch all smells
• The olfactory epithelium covers the superior nasal concha on each side of the nasal septum, and contains millions of bowling pin-shaped receptor cells-the olfactory sensory neurons.

Question 20

Olfactory sensory neurons

Answer 20

Several long cilia projecting from single apical dendrite
Not motile
Axons gather into smal fasicles to form filaments of olfactory nerve

Question 21

Specific Olfactory Receptors

Answer 21

• each smell may contain hundreds of different odorants
• humans have ~350 different odorant receptors - sensation of smell is not as tidy as for taste
• each receptor responds to one or more odorants and each odorant binds to several different receptor types
• but each receptor cell expresses only one type of receptor
• very sensitive - often only a few odorant molecules required to activate the receptor cell
• pain and temperature receptors are also in nasal cavities
• respond to irritants, such as ammonia, or can “smell” hot or cold(chili peppers, menthol)

Question 22

in order to ____ substance, it must be ________ as it enters the nasal cavity

Answer 22

in order to smell substance, it must be volatile as it enters the nasal cavity and then it must be able to dissolve in the mucus-rich fluid that coats the olfactory epithelium in order to have access to the receptors of the olfactory cilia

Question 23

Olfactory Transduction

Answer 23

1. Na+ influx leads to depolarization and impulse transmission
2. Ca2+ influx causes transduction process to adapt, decreasing its response to a sustained stimulus (suggested to open Cl-channels)

Question 24

Olfactory tracts (axons of mitral cells) have 2 destinations:

Answer 24

1. the olfactory cortex where smells are identified and interpreted (surprisingly, only some of this travels via the thalamus; most of it does not)
2. The other pathway flows to the hypothalamus, amygdaloid body, and other regions of the limbic system. There, emotional responses to odors are elicited. Smells associated with danger-smoke, cooking gas, or skunk scent- trigger the sympathetic fight-or-flight response. Appetizing odors stimulate salivation and the digestive tract, and unpleasant odors can trigger protective reflexes such as sneezing or choking

Question 25

Example of olfactory tracts

Answer 25

Smell smoke → fight or flight reaction; smell delicious food cooking → activate salivation; smell something unpleasant → sneezing, choking, even vomiting