L5 - Chemical Senses Flashcards
What are the different levels to sound?
Intensity
Frequency
What are the different levels to light?
Location
Intensity
Wavelength
What are the different levels to odour?
Cant describe the different aspects as you can with sound and light
What is the labelled line code?
Single neurons responds to a specific receptor input
What is the combinatorial code?
Pattern of activity across a whole population of neurons that responds to a specific receptor input
What are the second messengers involved in olfactory sensory transduction in mammals?
G-protein coupled receptors
What are the second messengers involved in olfactory sensory transduction in insects?
Ion channels
What experiment was carried out in Drosophila to show that odour specificity is encoded by receptor repertoire?
Recordings from Drosophila olfactory receptor neurons whose native receptor has been removed by a mutation
Then replaced with an experimenter-selected receptor
Each receptor has a unique profile of odours it binds to
How many receptors are there per neuron?
As olfactory sensory neurons mature, they narrow down to express a single olfactory receptor
The neuron activity will represent the binding data of that receptor
What is convergent activity of olfactory neurons?
Olfactory neuron expressing the same receptor converge on the same glomerulus
- Axons converge
They can still be distributed across the whole membrane
Where do sensory neurons transfer information to second order neurons?
This happens at glomeruli
Receptor-specific matching of sensory neurons to second-order neurons
- This ensures that odour specificity is carried through
What is the function of the first relay synapse?
Transforms the odour code
Found between sensory neurons and second order neurons
What are the features of the first relay synapse?
Synaptic adaptation emphasises the start of the odour
- Synapse does not have as many vesicles to release after a while
Converging sensory neurons onto second order neurons
- Reduces noise
- Strengthens weak responses
What are the two functions of lateral-glomerular cross talk?
Gain control
- Sensitive to both weak and strong odours
De-correlation
- Make neuronal responses to different odours as different as possible
Overview from receptor neurons to lateral tracks?
- Receptor neurons detect odour neurons in air
- Each neuron expresses a particular receptor
- Neurons expressing the same receptor converge on the same glomerulus
- Synapse on to 2nd order neurons
- Changes natures of odour coding - Lateral tracks
- Help gain control
- Go on to higher processing centres in the brain
What is the difference between learned and innate behaviour?
Learned - piriform cortex to mushroom body neurones
Innate - lateral horn neurones to amygdala
What experiments showed that the cortical amygdala is required for innate odour responses?
Mice avoid the smell of foxes (TMT)
If you silence the cortical amygdala - no longer avoid fox odour
Silencing of the brain region shows it is required for a certain behaviour
What experiments showed that the lateral horn is required for innate odour responses?
Fruit flies avoid laying eggs on food that smells of toxic microbes
If you silence this lateral horn neurone - fail to avoid laying eggs on toxic food
Innate behaviour characteristics
Purpose - categorise
What odours? - certain preferred
Connectivity - stereotypes
Activity - dense – can respond to many odours
Learning behaviour characteristics
Purpose - discriminate
What odours? - arbitrary
Connectivity - random
Activity - sparse – only respond to specific odours
Third order neurons sample second order neurons to respond very selectively to odours
Kenyon cells receive input from multiple projection neurons and require multiple simultaneous inputs to fire
- Fre very selectively
- Sample small regions in PN - coding space
- Turns a dense combinatorial code into a sparse selective code
Olfactory search behaviour - bacteria
Biased random walk Bacteria can swim - Straight – runs - Turn – tumble If things improve (increasing nutrients) - run more, tumble less
Olfactory search behaviour - C.elegans
Follow the same rule as bacteria
If things get worse sensory and inter-neurons not active - turns more
Whats an example of a complex olfactory search strategy? - smell
- If you smell something good - fly upwind
- If you lose the odour - wait a little while before turning around
- Strategy for dealing with turbulent odour plumes - When you reach the source - use other sensory cues to reach target
Whats an example of a complex olfactory search strategy? - active sensing
Moving head lets you sample a larger space and generate fast changes of detected odour concentration
- Set behaviour according to adaptation statistics of sensory neurons
Coordinate sniff cycle with how head moves
What is the role of taste transduction?
Uses metabotropic and ionotropic receptors
- Metabotropic – detects sweet, bitter, umami
- Ionotropic – detects sour
- Na channel – detects salt
Amplifies signal
How does lateral inhibition work on taste?
Part of lateral inhibition happens at receptor level
Also a circuit mechanism
GABAergic interneuron
- Receives information from bitter senses
- Then sends information to block sweet sense
What experiment was used to show different tastes activate different parts of the brain?
Optogenetically activating sweet or bitter areas of insula makes mice approach or avoid the stimulus
These areas are called hot spots - in the mouse insula
Example of taste circuits
- Different cranial nerves are activated depending on where the taste buds are found on tongue
- Solitary nucleus of brainstem - hypothalamus and amygdala
- VPM of thalamus
Insula and parietal cortex
