Chemical Senses Flashcards
How are the chemical senses and auditory sense different?
multidimensional coding space compared to sound for e.g. where theres only a few things you can characterise a sound by
What is a labelled line and combinatorial code?
Labelled line = neuron pathway specifically and exclusively for a specific function
Combinatorial code = Combination of neural pathways in different intensities to perform a function (detecting an odour)
Describe olfactory sensory transduction
- Sensory neurons in olfactory epithelium in nose
1. Odour detected by olfactory receptor which is a g protein coupled receptor
2. Activates G protein which activates adenylate cyclase
3. Adenylate cyclase then produces cAMP (amplifies signal)
4. cAMP binds to channels and Ca2+ ions flood into cell
5. Ca2+ gated channel is opened from calcium influx which causes more current
What is important about olfactory sensory transduction?
It is a process of amplifying a signal through signalling cascade
e.g. lots of cAMP produced from one odourant binding means to lots of Ca2+ channels opening
What is the difference between olfactory sensory transduction in mammals and insects?
Mammals odorant receptor = G protein coupled receptor
Insects = ion channel
How do we tell apart different cells from each other?
- Each odorant receptor is a ‘different shaped lock’ which responds to a different shaped ‘key’ (odorant molecule)
- Odorant binds to protein (receptor) doesn’t have to be a perfect match, many similar molecules can still bind and activate
EVERY RECEPTOR HAS A DIFFERENT PROFILE OF ODORANTS THAT IT RESPONDS TO
How many different odorant receptors do humans have?
300-400
How does molecular level specificity get translated into neural specificity?
each neuron expressing a single type of receptor
What happens during neural maturation with olfactory processing?
Immature neurons= express range of different receptors
Mature neurons= Narrow down to pick one receptor and suppress expression of other receptors
What is glomeruli convergence?
- Olfactory sensory neurons that express the same receptor will converge to the same anatomical location called the ‘glomerulus’
-Will converge no matter if the neurons are scattered - Which neurons converge at which glomeruli is conserved across mammals and insects
What is the antennal lobe?
- ‘Olfactory bulb’ but its its called an antennal lobe in insects
- where axons of the same receptor expressing converge in a same sort of glomerulus structure
What do second order neurons do and what are they called in mammals and insects?
- Second order neurons carry information from glomeruli to higher brain centres
Drosophila = projection neurons
Mammals = Mitral/tufted cells
What are neurons that pass information between glomeruli called?
Interneurons
How do second order neurons preserve specificity?
Each second order neuron is specific to receive input from one specific glomerulus in order to preserve specificity when transferring information to higher brain centres
What is the purpose of the synapse between the sensory neurons and the second order neurons?
- Synaptic adaptation emphasises the start of the odour which is essential in animals as in the wild it is essential to detect change
- Converged signal at the glomerulus. reduces noise and strengthens a weak response
(e.g. the projection neuron is listening to a crowd of sensory neurons rather than just one)
What is the point in glomeruli communicating with each other?
‘Lateral information flow’
1. Gain control - helps be sensitive to both very strong and very weak odours
Glomeruli will communicate with each other to ‘tamper’ down the signalling when strong so it can detect differences with other odours
- De-correlation - odour patterns need to be as different as possible to detect differences, lateral inhibition neurons help with this
Where does the olfactory information go to in the brain?
Humans:
- Mitral can send info to piriform cortex (holds memories of smells)
- Instinctual responses kept in amygdala
Flies:
- Innate responses = lateral horn
- Instinctual responses= mushroom body
So the distinction is conserved!!
How do we test innate responses in mice?
- Mice avoid the odorant TMT which is found in fox urine
- If you put the fox odour in one quadrant the mice avoid it
- Silencing part of the amygdala using an optogenetic silencer means the mice are less avoidant of TMT showing its important in the innate response
How do you test innate responses in flies?
- Fruit flies avoid laying eggs on food that smells of toxic microbes
- If you silence the lateral horn neuron the flies fail to avoid laying eggs on toxic food
Why aren’t instinctual behaviours hard-wired?
Learning behaviours not helpful like this - don’t know what’s coming so neurons wired randomly
Innate behaviours stereotypical to react to food/predators
What is Biassed random walk?
Walk in random direction and if smell is getting better keep going but if not turn around in random direction and walk again
How do bacteria do a biassed random walk?
They can swim straight using flagella or turn (‘tumble)
How does motor control work with olfactory searched behaviour?
C.elelgans have a sensory neuron that detects favourable odours:
- When activated it inhibits motor neuron controlling turning behaviour to stop it and find the source of the odour easily
- If odour neuron is decreasing, sensory neuron is quiet which means motor neuron is active and the C.elegan will head in a new direction
How do flies use olfactory searched behaviour?
- Flow in wind to find smell
- But wind is turbulent and blows in different directions so you lose the smell but might be going in the right direction
- Flies olfactory system has to ‘smooth’ out signals in and around odour plume
