Chemical Senses Flashcards
Olfaction compared to other senses.
Can link to memories and emotions.
Dogs can smell tumours.
Multidimensional coding space and they are hard to categorise.
Vision can be categorised easily by colour and brightness. But smell cannot.
Labelled line vs combinatorial code
Labelled line is where a single neuron responds to a specific molecule.
Combinatorial code is where sugar would be encoded across many channels and it’s not as simple as labelled line. And this is the case most of the time.
Olfactory amplification
All of the secondary messenger signalling pathways are designed to amplify signals so even a very small amount of odour molecules can cause a response.
In mammals they are G protein
In insects they are ionitropic.
Olfactory receptors in different animals
And when they mature
There are many types of olfactory receptor. Each type binds to a unique profile of odourants.
Flies have 50 types.
Humans have 300/400
Mice have 1000
Each neuron has only one type of receptor. As olfactory neurones mature they narrow down to express a single type of receptor. but at the start they can have many.
Glomerulus and where it is
Means little ball of thread.
Sensory neurons with the same type of receptor all go to the same glomerulus.
In drosophila the sensory neurons send their axons to the antennal lobe (called olfactory bulb in humans)
Pictures can show the axons which come from sensory neurons expressing the same receptor and converge into one glomerulus.
Sensory neurons vs receptor neurons
And where does the info go from here
In mammals it is called sensory neurons.
In drosophila is it called receptor neurons.
It goes to the second order neurons at the glomeruli.
Second order neurons.
They are called mitral tufted cells in mammals.
They are called projections neurons in drosophila.
And each mitral cell receives input from one glomerulus.
The first relay synapse
The synapse connecting the sensory neuron to the second order neuron.
Sensory and second order neuron spiking.
Sensory neuron spikes evenly when the odour occurs.
Second order spikes a lot at the start and then less later on. This is due to adaptation.
The sensory neuron has vesicles saved up that it releases all at once to the second order neuron at the start of stimulation.
This allows the NS to respond to changes in the odour concentration.
How do sensory neurons join to second order neurons.
Many sensory neurons converge into one second order neuron.
This reduces noise by having one neuron to average together the info from many sensory neurons.
It will strengthen weak responses because the second order neuron is listening to many sensory neurons at once.
Lateral interglomerular cross talk
There are neurons that carry info between the different glomeruli.
Many of these are inhibitory.
They allow us to be sensitive to very weak and very strong odours.
They decorrelate population responses. They make responses to different odours as different as possible.
Innate and learned odours.
Animals innately know that food smells good. Humans use the amygdala for this and insects use the lateral horn.
They can learn new odours.
In humans this is done by the piroform cortex
In insects it’s the mushroom body that contains Kenyon cells.
Silencing nice amygdala means they no longer avoid smells associated with danger.
Innate neurons vs learned ones.
Innate neurons have a more dense activity because a neuron will respond to many smells if they are all in the same category.
They will have preferred odours and a more strereotyped activity.
Learning neurons have more sparse activity and are specific to a small number of odours.
They respond to arbitrary odours as they don’t know what possibilities are out there and they will have random connectivity.
Bacteria movement
They can swim straight (runs) or turn (tumble) by spiralling their flagella.
They follow a simple rule to reach nutrients.
If nutrient conc increases then run more. If it decreases then tumble and swim another direction.
They have a nutrient receptor.
C elegans rule
Their cillia detect an odour increasing and this causes suppression of the worm turning another direction.
When there is no odour then the suppression will not be there and they can turn away.
Drosophila smelling in the wind
Searching for smells in the air is a lot more complicated because of gusts of wind creating odour plumes.
If they smell something nice they go upwind.
If you lose the odour wait before turning around because plumes are turbulent.
Mechanoreceptors test wind direction.
How we do active sensing.
Moving your head around lets you sample a larger space and helps to detect changes in odour conc best.
Mammals can coordinate the sniff cycle with how you move your head.
Taste 4
Secondary messenger cascades work to amplify the signals.
Bitter sweet umami have G protein.
Salt and sour have ligand.
Taste buds send axons through the cranial nerves to the brain stem. Then to the ventral posterior medial nucleus of the thalamus and then to the cortex.
Lateral inhibition in taste.
Helps with distinguishing tastes.
The sensory neurons are inhibited by bitter compounds.
A GABA interneuron will inhibit sweet neuron activation and stop feeding behaviour in response to bitter stimuli.
The GABA interneuron will stimulate the bitter neuron which cause avoidance behaviour.
Mouse insula
There are hot spots in the mouse insula that are responsive to particular tastes.
Optogenetically expressed channel rhodopsin which activates neurons when illuminated. This can be used to activate sweet or bitter areas of the insula. Which made the mice approach or avoid the light which was activating their brain areas.
what are some examples of how we do object localisation
the orienting reflex where we move the head and eyes to focus stimuli on the fovea, this can happen for auditory and visual stimuli.
smooth pursuit is where we follow a moving object.
we predict future motions during prey capture.
saccadic movements/ constant movement of eyes during object inspection, this is not random and the eyes are looked at most when we are studying a face.
what leads to disappearance of the orienting reflex
ablation of the optic tectum (superior colliculus in mammals)
how long does the phototransduction cascade take and why is this important
it takes 60ms so we are actually seeing the recent past, this is why we need motion prediction because some things move so fast we cannot follow them.
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where does the superior colliculus receive information from and what does it do
what is its main function and what does a lesion cause
ganglion cells, auditory system, somatosensory system.
it integrates information from different sense to make a decision on where to move.
its main function is to regulate saccidic movements
lesion leads to disappearance of the orienting reflex.
what is retinotopic map organisation
what is motor map organisation
how are they linked
neighboring cells in the retina feed info to neighboring places in their target structures (superior col, LGN etc)
the neruons in the superior colliculus are also organsied into maps, stimulation of a specific neuron will shift the eye to a specific angle, these are motor maps.
the retinotopic maps and the motor maps are aligned with eachother and when you see something in a particular visual field, a particular neuron is activated in the superior colliculus.
if this activation is too large it will lead to activation in the lower neurons which are responsible for moving the eye in a particular direction.
which areas are involved in stimulus localisation and motion processing
the analysis of the info starts in the retina, there are direction orientation selective ganglion cells and neurons for motion anticipation.
the dorsal stream in the cortex
the superior and inferior colliculus.
where does motion processing start to happen and where does the info go
what are the neurons like in the path
why do we need motion processing
it starts in the magnocellular ganglion cells and then down the dorsal stream.
the neurons are motion sensitive and retinotopically organised.
we need it to see coffee pour into a cup, if not everything would look static.
what is the direction of movement that causes the biggest response in a neuron called
what is the direction giving no response called
where is this direction selectivity seen
preferred direction
null direction
in the ganglion cells in the retina
PIC OF CIRCLE GRAPH
is the foveation hypothesis valid and what does it say
it is not valid
it says that interaction between the retinotopic maps initiates the orienting reflex
what is the morphology like of direction selective cells
highly asymmetric, they only have dendrites in one particular direction and this helps with responding to motion in one particular direction.
what types of input do the retinal ganglion cells receive
what happens when there is motion in the preferred direction compared to the null
they get excitatory input from bipolar cells and inhibitory input from amacrine cells.
when there is motion in the preferred direction the cell depolarises and fires an AP. the excitation input is larger than the delayed inhibitory input.
when the motion is null the cell depolarises but doesnt fire an AP. the excitation is smaller and delayed and the inhibition is larger.
PICTURE OF SMALL BAD WAVES
what is the difference between how static and moving objects are processed
the retina predicts the location of moving objects, they start spiking before the object approaches the receptive field, 80ms before.
for static objects spiking will occur 60ms after it has entered the receptive field.
what is the difference between declaritive and non declaritive memory
long term vs short term memory
declaritive is memories of facts and events.
non declaritive is accessed without conscious thought like walking or biking.
long term memory lasts longer than short term.
synaptotagmin
triggers ntm release when ca binds
what are the types of synaptic vesicle
readily releasable vesicles are attached to the active zone and are released as soon as the Ca conc rises
proximal pool vesicles are not in the active zone yet and are not ready to be released
reserve/resting pool of vesicles which is where most of the vesicles are, far from the active zone.
what are the types of glutamate receptor
which ones are permeable to Ca
there are two ionotropic types-
NMDA and non NMDA.
there is one type of metabotropic-
mGlut.
NMDA receptor is permeable to Ca but the non NMDA receptor is not.
why is it good to use invertebrates for study
they have larger neurons sizes than mammals so electrophysiology is easier.
they have simpler circuits but we cannot directly correlate it to humans.
most of them are temperature dependant so you dont have to worry about what temperature to keep the animals at.
they have characteristic neurons so you can make neuron maps and each neuron can be defined.
what are some simple forms of memory in invertebrates
habituation- a form of adaptation where a repeated stimulus will result in the response decreasing, but only if the stimulus is not harmful. after a while you will become desensitized to the stimulus.
sensitisation- repeated presentation of a stimulus results in increased behavioral response.
aplysia- what do they look like and what reflexes do they have.
how do the reflexes change with repeated stimulation
they have a gill for breathing and a siphon for locomotion.
they have a gill withdrawal reflex when poked on siphon, a sensory neuron on the siphon detects something and sends info to the gill by motor neuron to withdraw.
when you touch the siphon many times the reflex is reduced.
they are happy being cold
why does habituation occur
there is a reduction in synaptic strength.
there is reduced transmitter release, the readily releasable group of vesicles is much fewer, a 50% lower quantal release.
example of sensitisation in the aplysia
what are the different neurons involved and how do they cause a bigger response
pairing an electric shock with poking the siphon causes an increase in the response.
after a while, poking with no shock will still give a large response.
the L29 neuron will detect the electric shock and synapse with the presynaptic side of the sensory neuron.
the L29 neuron releases seratonin from its pre synapse onto the sensory neuron axon. This will activate the G protein and then adenylyl cyclase and then pka.
pka inactivates K channels and means there will be more depolarisation for longer and more vesicle release.
PICTURE
discuss the siphon touch and shock pairing in terms of pavlovs dogs
the siphon touch is the conditioned stimulus
the shock is the unconditioned stimulus
the L29 neuron and the sensory neuron are activated at the same time.
associative learning vs sensitisation
associative learning gives a bigger response than sensitistion because in associative learning both L29 and the sensory neuron are active together.
