Lecture 11: Sensing the World III - Chemical Sensation

Question 1

Why are odours and flavours important

Answer 1

Motivated and emotional behavioural
responses are greatly influenced by presence of chemical signals.
* Odours are key in detecting potential dangers
and opportunities for food or social
interactions.
* Short- and long-range signaling.
* Flavours help in identification of specific foods and food quality, and support learning
associations between tastes and emotional
events (i.e. malaise).
* Short-range signaling

Question 2

Why are olfactory cues important

Answer 2

Olfactory cues support diverse
behaviours:
* Food or mate seeking.
* Feeding.
* Co-specific identification (group or non-
group member).
* Marking territories.
* Reproduction.
* Aggression and early warning.

Question 3

How relevant is smell to humans compared to dogs

Answer 3

Is generally accepted that animals rely more on olfaction than
humans.
* Dogs can detect odors 100x less concentrated than humans.
* But, human and dog olfactory receptors are equally sensitive =
respond to one single odor molecule!
* Dogs have 100 times more receptors

Question 4

are we able to use scents as dogs do?

Answer 4

Scent tracking: bloodhound tracking a pleasant scent and humans smelling cookies

Question 5

Describe the human nose, how it works and its structure

Answer 5

Primary function: humidify and warm air going into the lungs.
* Secondary function: olfaction.
* Air flows into the nose cavity.
* Odorants interact with the olfactory epithelium.
* Mucus in the epithelium captures odorants
structure:
- olfactory cleft
- olfactory bulb
- olfactory epithelium
- air and odorants
- turbinates

Question 6

Describe the structure of the olfactory epithelium

Answer 6

• olfactory epithelium
  underneath has olfactory mucosa
  -supporting cells
• Olfactory cilia
• Olfactory sensory neurons
• Basal cell

Question 7

What do olfactory sensory neurons do

Answer 7

Detect odours and produce mucus

Question 8

How are odours recognised in a molecular level

Answer 8

Odorants are recognised by
specific receptors in the cilia of OSNs.
* Olfactory receptors are G-coupled proteins whose activation opens Na+/Ca2+ channels.
* OSN is depolarized by Na+/Ca2+ influx, firing action potentials.

Question 9

Describe the olfactory pathways

Answer 9

Axons from OSNs pass through the tiny holes in the cribriform plate (bone) to enter the brain.
* Each type of OSN projects its axon to a single
glomerulus within the olfactory bulb.
* OSN axons make synapsis with mitral and tufted cells, that project to the
primary olfactory cortex and other brain regions.

Question 10

How do human olfactory receptors work

Answer 10

Humans have around 1000 different odor receptors but can perceive more than a trillion (!!) odorants.
* Shape-pattern theory: each scent—as a function of odorant-shape to OR-shape fit
—activate unique arrays of olfactory receptors in the olfactory epithelium.
* These various arrays produce specific firing patterns of neurons in the olfactory bulb,
which then determine the scent we perceive.

Question 11

Is olfactory the same for everyone

Answer 11

It’s a subjective experience - Detection threshold can be affected by several factors:
* Gender: women generally lower threshold than men, especially during
ovulatory periods of menstrual cycles, but their sensitivity is not heightened during pregnancy.
* Training: professional perfumers and wine tasters can distinguish up to
100,000 odorants.
* Age: By 85, 50% of population is effectively anosmic (sense of smell loss).

Question 12

Why is taste important

Answer 12

Short range information (inside the mouth).
* Taste recognition guide appetite and trigger physiological processes for absorbing nutrients.
* Important for identifying nutrients and avoiding chemical threats.
* Good taste = usually good.
* Bad or bitter taste = potentially harmful.
-> Greatly influenced by culture.
* Taste liking/disliking already present in newborns.

Question 13

Newborn/rat innate reactions to sweet and bitter taste

Answer 13

Tongue protrusions - likes the sweet taste
gaping - dislikes the bitter taste

Question 14

Define taste, retronasal olfactory sensation and flavour:

Answer 14

• Taste: detection of chemical compounds in the mouth by direct contact with chemoreceptors on the tongue and the roof mouth. Taste categories: sweet, salty, sour, bitter, and umami.
• Retronasal olfactory sensation: perception of odorants while chewing and swallowing food.
• Flavour: combination of taste and olfaction (retronasal)

Question 15

Give structure and function of taste sensors

Answer 15

Taste receptors are arranged in taste buds, distributed along the tongue, palate, pharynx, epiglottis, and upper third of the oesophagus.
* Taste buds arranged in three kind of papillae, distributed in specific regions of the tongue.
* Receptors for different tastes group together in the same bud.
- Receptors in the papillae.
* Receptor activation sends neural signal through taste nerves.
structure:
- circumvallate papillae
- foliate papillae
- fungiform papillae

Question 16

Define and describe the structure of receptor cells

Answer 16

Each taste bud contains several types of taste receptor cells.
* Receptor cells are specialised cells containing microvilli (extensions of
cellular membrane) containing receptor proteins.
Three main receptor types:
- Type I: support function.
- Type II: detect bitter, sweet and umami. Chemical signal to neighboring cells
(including Tipe III).
- Type III: detect sour. Synaptic communication
with afferent fibers.
Salty…?

Question 17

What are the receptors in the different types of cells

Answer 17

G-coupled protein receptors (like in olfactory neurons) T1R and T2R.
* T1R detects sweet and umami taste.
* T2R detects bitter taste.
* Ion (Na+) channel ENaC detects salty taste.
* Sour receptors are yet to be described

Question 18

Describe the taste neural pathways

Answer 18

Three cranial nerves collect taste information:
* Chorda tympani
* Glosso-pharyngeal
* Vagus
* Synapse at nucleus of the solitary tract (medulla) → hypothalamus → insula or gustatory primary cortex → orbitofrontal cortex.

Question 19

Taste mediated behaviours

Answer 19

• Odour detection helps us find food, but
  taste determines if the food is nutritious or a potential chemical threat.
• Bitter taste might signal poisonous food.
  While intense sour might be related to
  harming acidic substances.
• Sweet and salty tastes normally induce
  seeking behaviour since such
  substances increase survival.
• Infants’ behaviour and facial expressions
  reveal innate taste preferences.