Lecture 12: Olfactory system

Question 1

How does the olfactory system detect stimuli?

Answer 1

Gas molecules that hit the mucus layer of the nose

Question 2

What kind of model of receptors are used and why?

Answer 2

Key lock model –> many molecules are hydrophobic so they interact with odoran-binding proteins

Question 3

Can a receptor detect the whole gas molecule?

Answer 3

No, it can recognize epitopes

Question 4

What kind of receptors are olfactory receptors?

Answer 4

GPCR (with 7 trans-membrane domains)

Question 5

Where are ORC located?

Answer 5

Nose roof

Question 6

How is is that each of us perceives odor slightly different?

Answer 6

There is big variation in olfactory receptor expression due to low selection pressure of OR genes

Question 7

With what does OCR activation correspond?

Answer 7

Chemical quantity (firing corresponds with quantity)

Question 8

What is anosmia?

Answer 8

Complete or partial loss of the sense of smell

Question 9

How does odor specific anosmia happen?

Answer 9

Dysfunction of commonly expressed olfactory receptors

Question 10

Why do olfactory receptor cells need to be replaced?

Answer 10

Due to direct exposion to noxious environmental stimuli

Question 11

How does signal transduction work in cilia of olfactory receptor cells?

Answer 11

Oderant molecule —> GPCR activation –> activation of adenylate cyclase –> ATP activates cAMP –> cAMP activates Na+/Ca2+ channel –> Ca2+-CAM + Ca2+ gated Cl- channel can open
- Cl- efflux in the cell must be very high for the cell to depolarize

Question 12

How does adaptation in ORC work?

Answer 12

1. Ca2+-CAM binds to CNG channel and reduces its sensitivity to cAMP.
2. Ca2+-CAM activates PDE that destroys cAMP

Question 13

What is the function of ORC adaptation?

Answer 13

Helps in detection of new odors in environment.
- Olfaction is a difference detector and not a constant chemical sensor

Question 14

How is the S/N ratio in second neurons improved (neuronal quality)?

Answer 14

Many ORCs expres the same receptor

Question 15

How can we smell different odors even though a lot of OCRs express the same receptors?

Answer 15

ORCs respond to related molecules with different affinities –> olfactory coding space is high-dimensional - different odors activate different combinations of ORCs

Question 16

The zonal expression of OR genes in the epithelium is also present in the olfactory bulb. In the bulb are glomuli present. Which cells play an excitatory role and thereby convey information to the olfactory cortex?

Answer 16

Mitral and Tuffled cells (M/T cells)

Question 17

Which cells are mainly and which fully GABAergic?

Answer 17

Peri-glomerular cells - Granule cells

Question 18

Which cells make distant lateral connections?

Answer 18

Mitral and granule cells

Question 19

Which interneurons are also mainly replaced?

Answer 19

PG and Granule cells
(is to some extend activity dependend)

Question 20

What are possible functions of interneuron replacement (rewiring)?

Answer 20

• Adjust to high ORC turnover
• Adjust sensitivity to changes in stimulus environment
• Support short-term and long-term odor memory

Question 21

Odors induce synchronized oscillations. Which 3 rhythms do we have?

Answer 21

Fast: Beta and Gamma
Slow: Theta

Question 22

Why are PNs (second order neurons) important?

Answer 22

They make spatio-temporal encoding of olfactory information possible.
- Odor-evoked activity is distributed in time and space —> activity pattern

Question 23

Functions of Theta, Beta, Gamma oscillations:

Answer 23

• Theta: Phase is locked to breathing, present in bulb and cortical areas.
• Beta: increases with repeated presentation of odors, associated with learning, also present in bulb and cortical areas
• Gamma: increase in power correlated with olfactory acuity (detection, distinguishing,identification of different odors)

Question 24

What does the primary olfactory cortex, specifically piriform cortex encode?

Answer 24

Molecular features of odors, associates odors with behavioral features

Question 25

What is the role of the amygdala in the olfactory system?

Answer 25

• Emotional response to odors
• Innate behavioral reponse (pheromones)
  NOTE: input from MOB and accessory OB

Question 26

What happens in the orbitofrontal cortex?

Answer 26

• Consciousness of smell!
• Associative area of olfaction and taste
• Modulation by physiological state
• Subjective classes pleasent and unpleasent encoded in space

Question 27

Which brain structure is largely bypassed?

Answer 27

Thalamus