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 does each ORN (olfactory receptor neuron) express?
1 receptor gene
Distribution of odorant receptor gene expression
Molecular mechanisms of odorant transduction
Molecular encoding and electrical patterns
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
Molecular encoding and electrical patterns
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
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)
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
