Lecture 15: Sense of smell

Question 1

What higher processing senses is smell strongly linked with?

Answer 1

• Hippocampus and limbic system

Which means it is strongly connected with memories and emotions

Question 2

How was smell important evolutionary wise?

Answer 2

Allowed for the detection of food quality, danger, mates and assessment of environment

Question 3

Relative to other sense how sensitive is smell?

Answer 3

Smell is very sensitive

e.g can detect ethyle mercaptan at 0.2 parts per billion i.e 3 drops in an olympic swimming pool (added to gases, to detect leaks)

Question 4

Chemically what does the sense of smell allow differentiation of?

Answer 4

Aldehydes, ketones, alcohols, thiols, amines, sulphides etc

(Differing functional groups)

Furthermore we can detect one carbon difference in structures and Enantiomers (mirror image structures)

Question 5

Do we always consciously smell?

Answer 5

Subliminal smell:

• Normally unaware of olfactory environment, usually only high concentrations shift attention

Subconscious brain detects olfactory smells (200ms) before we are aware of it (600ms)

Question 6

Describe olfactory intensity discrimination

Answer 6

Faster and better than visual intensity discrimination

Question 7

Do odorants subconsciously change our behaviour?

Answer 7

Yes there is significant data indicating it does

i.e

Ambient scent in shops can increase shopper spending by giving positive perception.

Some smells have direct affects on memories i.e lavender

Question 8

How can smell influence mating and mental health?

Answer 8

Theres is evidence that women partly choose mates based on their smell (pheremones)

Phantosmia (phantom smells) are linked with suicide.

Question 9

Describe phermones and how they relate to smell?

Answer 9

Phermones are species specific odourants that play important roles in behaviour, socially, mating wise and parenting.

Odours from female mice promote male mating behaviour while odors from other males promotes aggression

Question 10

How are phermones important in parenting?

Answer 10

Important for bonding mother and baby

Question 11

What structure recognises phermones?

Answer 11

The vomeronasal system (different from olfactory bulb)

Question 12

Describe the vomeronasal organ?

Answer 12

Located above the nose
Contains two lobes, basal, apical
Each lobe contains their own vomeronasal receptors.

They are connected to the accessory olfactory bulb (different from olfactory bulb used by odorants) these synaptic connections then extend to the amygdala and hypothalamis

Question 13

Describe vomeronasal receptors;

Answer 13

GPCRs

Question 14

Describe the reality of the vomernasal organ in humans;

Answer 14

It is poorly developed, 8% or people have developed vomeronasal organs.

These genes are non-functional in humans

Humans dont have accessory olfactory bulb

Question 15

What does the olfactory system detect?

Answer 15

Airborne molecules called odorants

Question 16

How does the olfacotry system detect odorants?

Answer 16

Odorants are detected by the olfactory epithelium (contains receptors) in the nose

Contains olfactory receptor neurons, humans have around 12million of these

Question 17

What structure communicates with ORNs?

Answer 17

The olfactory bulb, located just above the nose…

Not very well developed in humans

Question 18

What are odorants?

Answer 18

Small molecules translated by the nervous system into the perception of smell

Question 19

What are the physicochemical properties of odorants?

Answer 19

• Volatile (able to float through the air)
• Small
• Hydrophobic

Question 20

What happens to olfactory sensitivity with age?

Answer 20

Olfactory sensitivity declines with age

This is odor specific and some odorants can be remembered for a long time because they are associated with memories

Although women perform better at all ages

Question 21

How is the sense of smell different in its apparatus?

Answer 21

It is attatched to an organ with dual purpose

The nose primarily, warms, humidies and filters the air

It also olfactory epithelium which has the purpose of detecting odorants in inhaled air

Question 22

Describe the physical organisation of the olfactory system;

Answer 22

Olfactory bulb
Cribiform plate (ethmoid bone)
Olfactory epithelium
Nasal cavity (50% resp epi, 50% olf epi)

ORN axons extend through the cribiform plate (18-20bundles of ORN axons)

Question 23

Describe ORN’s

Answer 23

• Bipolar neurons
• Contain olfactory knobs with cilia, which are immersed in mucous (from bowmans gland)
• Olfactory cilia contain the receptors whic are in direct contact with odorants
• Basal cells
• Supporting cells (structure and metabolism)

Question 24

Describe olfactory epithelium turnover;

Answer 24

The olfactory epithelium is exposed to pollutants and microbes daily, despite macrophage and neutrophils in mucous turnover is every 6-8 weeks

Question 25

How is olfactory cells regenerated?

Answer 25

Basal cells (stem cells) are located at the basal part of the olfactory epithelium. These divide and differentiate into ORNs

Question 26

Where are receptor potentials generated in ORNs?

Answer 26

Receptor potentials are generated in the cilia of neurons

Question 27

How many types of olfactory receptors do humans have?

Answer 27

Five types

Question 28

What are the types of odorant receptors in humans?

Answer 28

• (Primarily) Odorant receptors
• Amines
• Pathogen and inflam related compounds
• small volatile compounds and sulphated steroids
• Peptides

Question 29

Odorant receptor genes and proteins classiy how much of all genes?

Answer 29

5% and is the largest gene family in the genome

although only 40% of these genes are active

Question 30

Describe the genetic basis of olfactory response;

Answer 30

• All mammals have ~1000 genes although some are non-functional
• Each individual has their own set of pseudogenes resulting in individual variability to sensitivity to smell.

Question 31

How many genes are expressed in an ORN?

Answer 31

Only one or a few genes are expressed in ORNs

Question 32

How can the olfactory system detect so many odors?

Answer 32

The capacity of the olfactory system to detect and encode a large variety of odors in the environment can be explained by the molecular diversity of the odorant receptors and the cellular diversity of the ORN’s

Question 33

What happens when the odorants binds to the receptor?

Answer 33

Depolarisation

Binding will active the g protein (g olf protein, one type!)

G(olf) will activate Adenylate cyclase = increased cAMP

cAMP will activate cAMP gated Na/Ca channels

Ca will activate Cl channels

Cell depolariszes

Question 34

How do ORN’s reploarise?

Answer 34

phosphodiesterases break down cAMP

Ca binds calmodulin, reducing Cl channel activation

NCX will also extrude Ca

Question 35

What are the intracellular proteins important in olfactory transduction?

Answer 35

G(olf)
AdenylCyclase 3
cyclic nucleotide gated channels

Question 36

If you knock out any of the 3 important transduction proteins, what happens

Answer 36

Sense of smell is lost

Question 37

What is the function of the secondary messanger cascade in olfactory cells?

Answer 37

• Amplification
• Intergration of receptor-odour binding events

i.e

1 membrane receptor activates tens of g proteins

AC produces thousands cAMP per second

3 cAMP open a channel

Hundreds of thousands of ions can cross the membrane via one channel

Therefore one odorant is capable of producing an electrical event in one ORN

Question 38

In addition to recognising specific odorants, what else is specific about the ORNs?

Answer 38

They can identify the concentration(intensity) of odour.

frequency of AP change with concentration, also length of time.

Very high concentration response is a limited burst of very high Hz AP

Question 39

What does the short burst of high HZ AP at high concentrations mean?

Answer 39

That the ORN has adapted to the smell.

Question 40

Describe the olfactory bulb neural structure;

Answer 40

Olfactory neurons of similar type innervate the same mitral cells in the glomeruli. (may need to look at diagram to understand what this looks like), sort of like ball of synapses

(~8000 glomeruli)

Question 41

Whats the first relay structure in the olfactory system?

Answer 41

The olfactory bulb, ORN synapse with mitral cells.

Question 42

Describe the number of mitral axons in a glomeruli

Answer 42

Around 25 mitral synapses in the golemeruli, receive input from around 25000 ORN

Of all which express the same Odorant gene

Question 43

Whats the important of ORN convergence;

Answer 43

The degree of convergence presumably increases sensitivity of mitral cells to ensure maximum fidelity of odour detection

Also contributes to the elimination of background smell

Question 44

Describe the glomeruli odotopic map;

Answer 44

Receptors that recognise similar odours tend to map the same general area in the olfactory bulb (glomeruli)

Interesting complex smells such as coffee which have 100’s of odourants tend to innervate only a few glomeruli

Therefore the olfactory system appears to employ a sparse coding mechanism that cues on a small number of dominant molecules in a mixture

Question 45

Describe the organisation of the olfactory bulb beyond the mitral cells;

Answer 45

Projections cells;

• Mitral cells
• Tufted cells

Periglomeruli neurons extend b/w glomeruli

Question 46

What are tufted cells and what is their function?

Answer 46

Tufted cells are part of the lateral inhibition system and are there to sharpen olfactory signal to the olfactory cortex

Usually through lateral olfactory tract