Smell (olfaction) Flashcards
LO
- Be able to describe the specialized sense organs involved in olfaction/gustation at a molecular and cellular level
- Be able to describe how olfactory/gustatory information can be coded
- Be able to describe the structures of the brain that processes this type of sensory information in the CNS
Sensory systems: general features
What are the multiple layers?
What are the functional categories of sensory receptors?
What are chemoreceptors?
Chemoreceptors: receptors that generate a signal when they bind to chemicals in the external environment
What is the thought about chemo sensation?
Chemo sensation is thought to be one of the oldest senses- evolutionarily conserved from simple organisms (C. elegans) through to flies (drosophila) and mammals
Whats olfaction?
Olfaction= information about airbourne molecules (odorants)
Whats gustation?
Gustation= information about ingested substances (chemical and physical qualities)
What behaviour do lower organisms produce?
seeking and avoidance behaviour
Whats olfaction distance chemosensation?
Olfaction: distance chemosensation (smell from long distances from source of smell)
Distance chemosensation
threshold varies between different molecules
- Ethanol 2mM
- 2-trans-6-cis nonadenial 0.07nm
The interpretation of a smell can be concentration dependent. Give an example
Indole
What are natural odours a combination of?
different molecules
What is olfaction a reflection of?
Olfaction is a reflection of the pattern of different cells that are activated by the different molecules– interpreted at higher centres in the CNS
Where are sensory receptors located?
Sensory receptors are located in the olfactory epithelium
Label this olfactory epithelium
Tell me about the polarity of olfactory neurones?
Olfactory neurons are bipolar
Tell me about the myelination of olfactory neurons?
unmyelinated sensory afferent
Whats embedded within the mucous layer of the olfactory epithelium?
specialised cilia
What does the mucus concentrate in the olfactory epithelium?
Mucus concentrates the chemicals and brings them into contact with the cilia
What is the mucus layer of the olfactory epithelium produced by?
The bowmans gland
Why do olfactory neurons only last 6-8 weeks?
olfactory neurons are prone to damage
sniffing out ways to overcome paralysis
What are cilia sensitive to?
odours
What do GPCRs serve as?
chemoreceptors
- The role of GPCRs as olfactory receptors is evolutionarily conserved
- Large amount of sequence variability – allows different GPCRs to detect different chemicals
What does each ORN (olfactory receptor neuron) express?
1 receptor gene
Distribution of odorant receptor gene expression
Molecular mechanisms of odorant transduction
What is the structure that houses the bipolar olfactory receptor neurons?
The nasal epithelium is the structure that houses the bipolar olfactory receptor neurones
Where are olfactory receptor proteins concentrated?
Olfactory receptor proteins are concentrated in the cilia of olfactory receptor neurones
What do individual olfactory receptor genes appear to have?
Individual olfactory receptors genes appear to have a selective distribution in the nasal epithelium
What do odorant molecules bind to?
Odorant molecules bind to GPCRs and this leads to the activation of a cyclic nucleotide gated (CNG) channel
What spreads to the intial segment of the afferent axon- threshold- AP?
A graded receptor potential passively spreads to the initial segment of the afferent axon – threshold – Action potential
What do the olfactory receptor processes pass through?
The olfactory receptor processes pass through the cribriform plate and enter the olfactory bulb
Molecular encoding and electrical patterns
How can odours be encoded?
Coding mechanisms can be explored by taking a chemical, for example an alcohol, and changing the length of the molecule.
Tell me about across-fibre pattern coding
- The responses of three sensory neurons are plotted against the length of the alcohol molecule. What is depicted is how many molecules of odour are required to evoke a single action potential in a sensory neuron for each alcohol.
- Each of the three sensory neurons respond best to different alcohols, and the range which they respond to also differs. In fact, the pattern of activity of all three define which alcohol is present. With only three sensory neurons some discrimination is possible.
- Therefore, to discriminate a particular odour the central neurons must read the different patterns of action potentials carried by very many sensory neurons – called the “across fibre pattern”.
Tell me about another approach to across-fibre pattern coding
- Another approach is to take naturally occurring substances (banana, lemon, cheese, and bread) and test these on the receptors, simply as a way of narrowing down the substances likely to activate the receptor.
- What do we find? The responses of seven sensory neurons to naturally occurring odours are shown. Not one sensory neuron responds to only one chemical alone, instead one sensory neuron responds to a range of substances
- Some strongly, some weakly and some not at all. Using this approach, we can therefore define the odours that a sensory neuron responds to.
What do the olfactory neurons project directly back to?
The olfactory bulb
Label the olfactory bulb neural network
What have experiments shown about olfactory neurons that express the same odorant receptor?
Experiments have shown that olfactory neurons expressing the same odorant receptor send their projections to the same bilateral glomeruli
What can a single glomerulus contain?
A single glomerulus can contain dendrites from up to 25 mitral cells and 25,000 olfactory cells
What do the axons of the mitral cells project from?
The axons of the mitral cells project from the olfactory bulb to accessory olfactory nuclei
Molecular encoding and electrical patterns
What does the relationship between one type of ororant neuron and one glomerulus enable?
The relationship between one type of odorant neuron and one glomerulus enables specific regions of the olfactory bulb to respond to different chemicals
What will different odours and chemicals activate?
Different odours and chemicals will activate a unique spatial pattern in the olfactory bulb depending upon their chemical composition
What will chemically distinct single odours maximally activate?
one or only a few glomeruli
Central processing of olfactory system
Central processing in the olfactory system
Tell me about pheromon detection in mammals
A number of different organisms release pheromones – perform an important role in modulating behavior (mood, reproductive behavior)
What do vomeronasal organ express?
The vomeronasal organ expresses a specific subset of molecules called vomeronasal receptors (eg:VR1)
Tell me what AOB projects
AOB – projects to amygdala and hypothalamus = regulate reproductive behaviour & mood
How do insects use pheromones?
The insect attractants that are released are generally a mixture, and are collectively called pheromones – chemical attractants used to find a mate.
Tell me about pheromones in cockroaches
In cockroaches the pheromone is called periplanone. So far this has been shown to have at least two components
A component (7-Methylene-4-isopropyl-12-oxa-tricyclo[4.4.2.01,5]-9-dodecen-2-one)
B component ((1S, 2S,5S,10S)-(6E)-8-Methylidene-5-propan-2-ylspiro[11-oxabicyclo[8.1.0]undec-6-ene-2,2′-oxirane]-3-one). These have been synthesised and are used in insecticides to attract insects to a particular place.
Why two components? Behavioural tests show that either component can release the same behaviour from the male but suggest that Periplanone B is responsible for long distance attraction, while the A component influences the behaviour of the male near the female.
It is probable that the female also releases other pheromones during contact and that the male does as well.
Tell me about labelled line coding
- A male insect detects incredibly low concentrations of pheromone (0.6 pg (very tiny amount!) upto 30m away) by virtue of masses of specialised receptors on its antennae – these receptors are not present in females or in immature males.
- Information about the pheromone is carried in specialised pathways to the brain – neuronal cells dedicated to the detection of only one molecule, in this case the pheromone.
- this is sometimes called labelled line coding as the neurons are ‘labelled’ to respond to only one odour, that of the female pheromone
Summary: smell encoding
- Olfactory receptor cells expressing the same receptor protein send axons to the same glomerulus
- Different molecular components in a complex odour activate a distinctive population of glomerular neurones
- Therefore, odours are thought to be represented by a spatial map in the olfactory bulb
- It is not currently understood how this activity is then organised in the higher order centres such as the pyriform cortex as there is not a clear and obvious map like in other sensory systems, eg. touch.
- The vast majority of olfactory neurons can respond to more than a single odour molecule
- An odour is coded by the activity pattern across many sensory neurones – this type of coding is called across-fibre-coding
- In insect’s pheromone detection is coded by labelled line coding. Where a neurone is labelled to respond to only a single odour molecule: the pheromone
