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what is sound related to?
intensity
frequency
what is light related to?
location
intensity
wavelength (colour)
labelled line
each sensory neuron is dedicated to one specific stimulus
specific neuron responding to only one stimulus
particular sensory perception and there’s a dedicated neural channel for that perception
—> specific neural channels dedicated for that perception
combinatorial code
many neurons fire together in response to a stimuli
particular sensory stimulus that is encoded by multiple neurons - a population of neurons
each neuron responds differently to different situations
Basic Mechanism of Olfactory Transduction - the way a single odorant molecule gets turned into an electrical signal that travels to the brain
this mechanism has to do with how a single odorant molecule is turned into an electrical signal that travels to the brain (triggering a nerve impulse)
Odorant is detected - an odorant molecule reaches the olfactory epithelium and binds to a specific G-protein- coupled olfactory receptor on the cilia of an olfactory sensory neuron
The binding changes the receptor’s shape and activates its attached G-protein (Golf)
Activated Golf then activates adenylyl cyclase
Adenylyl cyclase converts ATP into many cAMP molecules
Rising cAMP binds to and opens cAMP-gated cation channels - letting Na+ and Ca2+ flow into the cell
The incoming Ca2+ opens Ca2+-activated Cl- channels, Cl- exits, enhancing depolarisation
The combined ion movements depolarise the neuron to threshold (makes the inside of the cell positive enough to hit its trigger voltage which starts an action potential)
The neuron fires an action potential that travels through the olfactory nerve to the brain’s olfactory bulb
are different olfactory receptors specific to different odorants or not?
the different olfactory receptors are specific to different odorants
each olfactory receptor responds to a unique profile of odorants
Experiment done on Drosophila to support the idea that different olfactory receptors are specific to different odorants (Hallem and Carlson) :
removed the natural olfactory receptors from Drosophila
artificially placed olfactory receptors in one specific neuron
took electrical recordings from that neuron
then presented a panel of different odours to record how the particular receptor responds to different odours
—> example, receptor Or7a responds most to E2-hexenal whereas receptor Or10a to methyl salicylate
—> each olfactory receptor has a particular profile of odours that it will respond to
Single-cell whole transcriptome sequencing (Hanchate et al.)
as olfactory sensory neurons mature - they narrow their gene expression down - to express a single olfactory receptor each
shows that at the beginning when neurons are immature they start off by expressing lots of different receptors - but over time as they mature - 1 receptor takes over and cancels out the expression of all other receptors through negative feedback loops
The pathway of sensory neurons transferring information to second-order neurons at glomeruli in Drosophila:
Drosophila (fruit fly)
Olfactory receptor neurons (ORNs) detect odour
their axons enter the antennal lobe
ORNs with the same receptor meet in one glomerulus
In that glomerulus they excite second-order neurons:
projection neurons (PNs) - carry signal out
and
interneurons:
local interneurons - give lateral inhibition
PNs send axons to mushroom body and lateral horn
so the sensory neurons transfer information to second-order neurons - which then carry the infro from the glomeruli into higher brain centres
The pathway of sensory neurons transferring information to second-order neurons at glomeruli in Mammals:
Mammals
Olfactory sensory neurons (OSNs) in nose detect odour
Axons bundle as olfactory nerve to the olfactory bulb
OSNs with the same receptor join one glomerulus
In that glomerulus they excite second-order neurons:
mitral and tufted cells (projection)
and
interneurons:
periglomerular and granule cells (inhibition)
mitral/tufted axons form the lateral olfactory tract to:
piriform cortex, amygdala/tubercle, hippocampus
what does receptor specific matching of sensory neurons to second-order neurons ensure?
ensures that odour specificity is carried through
where do second order neurons carry info from and to?
second order neurons carry the information from the glomeruli into higher brain centres
second order neurons rceive input from only one glomerulus to maintain specificity
what are the key computations carried out in the first olfactory relay in early olfactory information processing?
adaptation
reducing noise
gain control
de-correlation
lateral inhibition
Adaptation at the synapse
there’s synaptic adaptation that emphasises the start of the odour
allows the nerve system to focus on changes in odours
Converging sensory neurons onto second-order neurons
reduces noise
strengthens weak responses
allows the second order neurons to integrate and average together the activity of lots of sensory neurons
what are the functions of the lateral inter-glomerular cross-talk?
gain control - neurons to be sensitive to both very weak and very strong odours
de-correlation - make responses of neuronal population to different odours as different as possible
what are odours detected by?
olfactory sensory neurons
all the sensory neurons that express the same receptor converge on what location?
glomerulus
the second order neurons receive input from what?
a single glomerulus
what does the synapse between the sensory neurons and the 2nd order neurons transform?
transforms the ododur code to emphasise the start of the odour
reduces noise does what?
amplifies/strengthens weak responses
the local neurons between the sensory neurons and 2nd order neurons carry out what?
gain of control
to tell apart both weak and strong odours
de-correlation
seperate out different odours
What are the kinds of higher olfactory behaviour that the different areas of the brain regulate?
learned behaviour (what is picked up)
innate behaviour (born knowing how to do)
where does learned behaviour/responses happen in humans?
cortex (piriform)
where does innate behaviour/responses happen in humans?
amygdala
where does learned behaviour/responses happen in insects?
mushroom body
where does innate behaviour/responses happen in insects?
lateral horn
experiment showing cortical amygdala is require for innate odour responses (Root et al.)
mice are afraid of predators
the experimental odour is TMT = an odour specific to foxes
mice are placed in a behavioural arena and put the fox odour in one of the quadrants
blue line seen indicates the tracks of the mice
mice avoids the smell of the foxes = mice spent less time in the odour quadrant
then an optogenetic silencer is used to silence the cortical amygdala
and then use an optical fibre to shine a light into that part of the brain
it prevents the behavioural response = so mice don’t avoid fox odour anymore
TMT = less time spent in quadrant
Silencer + optical fibre = equal amount of time spent in each quadrant
—> this shows that specific region is required for certain behaviour and when blocked the behaviour changes/doesn’t happen
state the differences between innate vs learned circuitry
purpose
innate - categorise
learned - discriminate
activity
innate - dense
learned - sparsely
what odours?
innate - certain preferred
learned - arbitrary —? respond to any odours
connectivity
innate - stereotyped
learned - random
strategies for innate behaviour
hardcoded rules, FSM, genetic code
—> fixed, automatic, unchanging
strategies for learned behaviour
supervised, reinforced learning, neural networks
—> flexible, adaptive, experience-based
what does taste transduction use?
metabotropic and ionotropic receptors
metabotropic receptors can be used to amplify signals
key taste circuit
taste information comes into the tongue through cranial nerves (taste buds)
taste information then goes into the brain stem to the part of the brain called the solitary nucleus
this then goes to the ventral posterior medial nucleus (VPM) and the thalamus
then to the insula and parietal cortex
lateral inhibition on taste (Chu et al.)
(fruit-fly experiment)
bitter and sweet sensing neuron
bitter sensing neuron activates GABAergic inhibitory interneuron - which releases GABA onto postsynaptic terminals of the sweet sensing neurons
so when the bitter and sweet are tasted at the same time the bitter taste will directly inhibit the sweet taste - so fly won’t want to keep eating it