Lecture 15: Sense of smell Flashcards
What higher processing senses is smell strongly linked with?
- Hippocampus and limbic system
Which means it is strongly connected with memories and emotions
How was smell important evolutionary wise?
Allowed for the detection of food quality, danger, mates and assessment of environment
Relative to other sense how sensitive is smell?
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)
Chemically what does the sense of smell allow differentiation of?
Aldehydes, ketones, alcohols, thiols, amines, sulphides etc
(Differing functional groups)
Furthermore we can detect one carbon difference in structures and Enantiomers (mirror image structures)
Do we always consciously smell?
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)
Describe olfactory intensity discrimination
Faster and better than visual intensity discrimination
Do odorants subconsciously change our behaviour?
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
How can smell influence mating and mental health?
Theres is evidence that women partly choose mates based on their smell (pheremones)
Phantosmia (phantom smells) are linked with suicide.
Describe phermones and how they relate to smell?
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
How are phermones important in parenting?
Important for bonding mother and baby
What structure recognises phermones?
The vomeronasal system (different from olfactory bulb)
Describe the vomeronasal organ?
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
Describe vomeronasal receptors;
GPCRs
Describe the reality of the vomernasal organ in humans;
It is poorly developed, 8% or people have developed vomeronasal organs.
These genes are non-functional in humans
Humans dont have accessory olfactory bulb
What does the olfactory system detect?
Airborne molecules called odorants
How does the olfacotry system detect odorants?
Odorants are detected by the olfactory epithelium (contains receptors) in the nose
Contains olfactory receptor neurons, humans have around 12million of these
What structure communicates with ORNs?
The olfactory bulb, located just above the nose…
Not very well developed in humans
What are odorants?
Small molecules translated by the nervous system into the perception of smell
What are the physicochemical properties of odorants?
- Volatile (able to float through the air)
- Small
- Hydrophobic
What happens to olfactory sensitivity with age?
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
How is the sense of smell different in its apparatus?
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
Describe the physical organisation of the olfactory system;
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)
Describe ORN’s
- 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)
Describe olfactory epithelium turnover;
The olfactory epithelium is exposed to pollutants and microbes daily, despite macrophage and neutrophils in mucous turnover is every 6-8 weeks
How is olfactory cells regenerated?
Basal cells (stem cells) are located at the basal part of the olfactory epithelium. These divide and differentiate into ORNs
Where are receptor potentials generated in ORNs?
Receptor potentials are generated in the cilia of neurons
How many types of olfactory receptors do humans have?
Five types
What are the types of odorant receptors in humans?
- (Primarily) Odorant receptors
- Amines
- Pathogen and inflam related compounds
- small volatile compounds and sulphated steroids
- Peptides
Odorant receptor genes and proteins classiy how much of all genes?
5% and is the largest gene family in the genome
although only 40% of these genes are active
Describe the genetic basis of olfactory response;
- 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.
How many genes are expressed in an ORN?
Only one or a few genes are expressed in ORNs
How can the olfactory system detect so many odors?
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
What happens when the odorants binds to the receptor?
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
How do ORN’s reploarise?
phosphodiesterases break down cAMP
Ca binds calmodulin, reducing Cl channel activation
NCX will also extrude Ca
What are the intracellular proteins important in olfactory transduction?
G(olf)
AdenylCyclase 3
cyclic nucleotide gated channels
If you knock out any of the 3 important transduction proteins, what happens
Sense of smell is lost
What is the function of the secondary messanger cascade in olfactory cells?
- 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
In addition to recognising specific odorants, what else is specific about the ORNs?
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
What does the short burst of high HZ AP at high concentrations mean?
That the ORN has adapted to the smell.
Describe the olfactory bulb neural structure;
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)
Whats the first relay structure in the olfactory system?
The olfactory bulb, ORN synapse with mitral cells.
Describe the number of mitral axons in a glomeruli
Around 25 mitral synapses in the golemeruli, receive input from around 25000 ORN
Of all which express the same Odorant gene
Whats the important of ORN convergence;
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
Describe the glomeruli odotopic map;
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
Describe the organisation of the olfactory bulb beyond the mitral cells;
Projections cells;
- Mitral cells
- Tufted cells
Periglomeruli neurons extend b/w glomeruli
What are tufted cells and what is their function?
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
