Chemical Senses Flashcards
Labelled line code model for smell detection?
This idea suggests that smells are detected from a single receptor channel, activating a single neurone in the CNS, which is detected by a single decoder. This is the output
What does the combinational code for smell detection tell us?
A combination of neurones are activated by multiple signal receptors. Multiple detectors detect this stimulus and this accumulation of knowledge helps to decide a smell.
Odour detections is conserved across species, what does this tell us?
It tells us that the way nerves are laid out must be for some purpose hence the layout being conserved between species
How are weaker signals turned into much stronger signals?
This is in ref to a signal being detected from an odour
Sensory neurones can be used for signal amplification - this is done with a g protein as remember these allow for cascades of signals
What is the process of odour detection?
An odourant molecule will bind to an olfactory receptor
This activates a G protein
This then binds to adenylyl cyclase
This enzyme turns ATP into cAMP
This cAMP opens a cation channel bringing in some calcium and sodium into the cell
This calcium then activates a calcium 2+ activated chloride channel, allowing calcium to move out of the cell.
How can odour signals be amplified?
The enzyme adenylyl cyclase activated by G proteins turn many ATP molcules into cAMP if activated continuously / for a longer time than usual.
This can cause cAMP to open cation channels for longer / more time
This leads to an increase in calcium via amplification meaning more calcium activated chloride channels can be opened / opened for longer, meaning more chlorides leave the cells.
This is how you can cause a single odour binding to a receptor (which in theory shouldnt create a big response) to create a detectable signal.
How do insects detect odours?
Note insects dont have g proteins in their odour detection systems
They rely on ion channels
So suggests signal amplification happens DIFFERENTLY in insects
What was found in odour detection experiments on drospholia?
Some olfactory neurones respond to specific odours whilst others respond to a variety of odours.
What are the different types of olfactory neurones receptors ?
Progenitors, precursors, immature and mature cells
These cells detect different odours. Some detect on odour, others detect multiple.
What happens as olfactory sensory neurones mature?
They choose which receptor they want. I.e. progenitors, precursors.
Where do sensory olfactory neurones which have the SAME receptors all converge?
They synapse on a structure called a glomerlus
Where do odour molecules bind when you at the top of the nasal cavity?
They bind to the olfactory epithelium
This is where the dendrites from the sensory olfactory neurones are found
Where do the axons of all the sensory neurones go to?
They go to the olfactory bulb (this is inwards from the epithelial membrane)
How do olfactory neurones ensure all of their neurones synapse to the same location
These neurones faciliculate (bundle their axons) their axons so they reach the same location.
This same location is the OLFACTORY bulb
In humans sensory information from olfactory neurones go to olfactory bulbs
Where does sensory information go to in drosophila?
These go the antenna lobe
What is important to note about olfactory bulbs and the anntenna lobe in drosophila?
These are all bilateral structures
They are also make of little balls (glomeruli)
What is the conserved circuit of neurones in drosophila and human olfactory systems?
You have primary olfactory receptor neurones, which all synapse onto glomeruli (either the olfactory bulb of the antenna lobes.
You then have interneurones and second order neurones.
In drosophila what are their second order neurones and interneurones called?
Remember their primary order neurones are called olfactory neurones
Their interneurones are called local neurones
Their second order neurones are projection neurones
What are the second order neurones and the interneurones in mammals?
Remember you have olfactory neurones which synpase onto glomeruli i.e. the olfactory bulb
You then have granule cells / perigolmerular cells as your interneurones
You then have second order neurones which are mitral cells and tufted cells.
Where do second order neurones get input from?
A single glomerulus
Why do we have primary and second order neurones? And what is important to note about glomeruli like the olfactory bulb? And what is a relay synapse?
Due to all the processing that is needed to encode a smell we have primary and secondary neurones
The olfactory bulb (golmeruli) is important for transforming how odourant information is encoded.
The synpase between an primary and a secondary neurone is a relay synapse. This is also IMPORTANT for changing how information is encoded in the brain.
So where is the first relay synapse found of the olfactory system? What is a key feature of it? What happens to it over time?
Note. Interneurones are found in glomeruli. They are between first and second order neurones and they are between glomeruli. The relay neurone is between 1st and 2nd order neurones.
The first relay synapse is found in the brain, it is found between the PRIMARY and the secondary neurones. In drosophila this is between the local neurones and the projection neurones
This is rapidly adapting!
Overtime THIS synapse weakens - as time in MILLI seconds increases the signal released across the relay neurone decreases
What does the relay synapse signalling weaken over time (after milli seconds)
This is because the presynaptic terminal at the primary neurone gets exhausted from sending signals to the post synaptic neurone.
This is because the presynaptic neurone loses it’s readily available synaptic vesicles
This suggests that initially alot of neurotransmitter is released from the synaptic vesicle, but overtime the amount of neurotransmitter decreases thus less of a signal is passed on.
How does the relay neurone - which depletes its neurotransmitter resources very quickly, show synaptic adaptation?
Note. Interneurones are found in glomeruli. They are between first and second order neurones and they are between glomeruli. The relay neurone is between 1st and 2nd order neurones.
The fact that the presynaptic PRIMARY neurones runs out of neurotran very quickly means that the secondary neurones respond more to the start of an odour signal rather than to a continuous odour signal.
This is beneficial as the odour may disappear very quickly from the environment stopping stimulation.
The relay neurone (interneurone) helps to send signals to the secondary neurone, even when the first order neurone isnt detecting the signal anymore
Synaptic adaptation is shown by the second order neurones responding to the start of a signal from the relay / first order neurone, instead of a full signal.
As the signals from the relay neurone decreases over time due to neurotransmitter loss, a response to the start of a neurotransmitter signal is optimal.
Where are interneurones found?
In drosophila (Called local cells) and in mammals (called granule cells or periglomerular cells) are found in the glomeruli.
That is the olfactory bulb or the antenna lobe. These collectively are called neuropil structures.
Note. Interneurones are found in glomeruli. They are between first and second order neurones and they are between glomeruli. The relay neurone is between 1st and 2nd order neurones.
What are inter neurones roles?
Note. Interneurones are found in glomeruli. They are between first and second order neurones and they are between glomeruli. The relay neurone is between 1st and 2nd order neurones.
What is the importance of interneurones?
Note. Interneurones are found in glomeruli. They are between first and second order neurones and they are between glomeruli. The relay neurone is between 1st and 2nd order neurones.
They help stimuli to be processed at different intensities. Note these are the inter neurones between glomeruli.
They allow you to detect very dilute or distinct odours
So importantly they link glomeruli! And affect whether inhibitory neurones which are attached to the glomeruli turn on or off by providing increased or decreased stimulation
What are inhibitory INTER neurones? What happens when they’re activated?
Note. Interneurones are found in glomeruli. They are between first and second order neurones and they are between glomeruli. The relay neurone is between 1st and 2nd order neurones.
As the stimulation between different glomeruli increases (caused by lots of weak odours and this information is transferred by interneurones) which helps to detect weaker MORE dilute odours
You get inhibitory interneurones being activated, which inhibits the synapses between MOST OF the primary and second order neurones.
The main role of this is to help us pick out individual odours from lots of weak odours.
Note when there isnt lots of weak odours the inhibitory neurones dont work.
Thus interneurones connect glomeruli and help with the detection of strong (no inhibition) and weak odours (inhibition)
What do lateral inhibitory interneurones do? And what happens if they are knockout?
Note. Interneurones are found in glomeruli. They are between first and second order neurones and they are between glomeruli. The relay neurone is between 1st and 2nd order neurones.
The lateral inhibitory interneurones are important for separating the different populations of odours. So again fits with idea that they help for detection of a single neurone due to inhibition of most primary and secondary neurones
When these are knocked out they cause odours to be less easily distinguished.
Where do second order neurones send their information?
Remember second order neurones also have glomeruli like olfactory bulbs
In mammals and drosophila
Mammals:
So from the second order neurones glomeruli info is sent to the piriform cortex where behaviour is learned. this is where you associate a particular smell with an event
Dros:
- this region is called the mushroom body in drosophila and is where kenyon cells are
Mammals:
- they also have the amygdala - this is for innate behaviours. Helps for basic smell association with food or predators for example. This is inbuilt and is ALREADY there hence the name innate
Dros:
This is the lateral horn in drosophila
How did we find out about the role of the amygdala / lateral horn being related to innate behaviours?
Remember innate behaviours are already there. They aren’t learned
Mice are afraid of foxes
Mice avoid the smell of foxes
Mice had to move around a quadrant split into four. In one box there was fox odour. The mice avoided this quadrant
How did we know the amygdala was involved?
- inhibitory channel rhodopsin was put into the mices amygdala and so was a light diode
Then a light is turned on it causes the channel rhodospin to become activated by the breakdown of rhodospin, inhibiting neurones in the amygdala. If these neurones where inhibited then the mice wouldn’t avoid the fox smell as odour detection wouldn’t occur.
The mice thus as normal avoided fox smell when there was not light - thus no inhibition of neurones. But when the light was turned on it caused the mice to avoid certain areas.
Fly example of innate behaviours associated with the lateral horn?
In the context of laying eggs and avoiding toxic microbes
Flies avoid laying eggs on toxic microbes
Silencing olfactory neurones and neurones in the fly lateral horn causes them to lay their eggs on toxic microbes.
Thus this behaviour was innate
What are the different features of the amygdala (innate bahviour) and piriform (learned behaviour) complexes?
Amygdala / innate learning sees:
Odours categoried (such as food) Activity is dense so there are lots of neurone responsible Certain preferred odours are detected Connectivity is stereotyped
In the piriform complex / learned behaviours:
the prurpose is to discrimate odours
Activity is sparse - meaning not many neurones are involved
many different kinds of odours can bet detected
Connectivity is also random
Where do stereotyped connections go and how is this different to random connections?
Stereotyped connections always go to the higher brain centres in the brain
Random connections send responses to odours to DIFFERENT places in the brain.
How do flying insects detect food?
These insects smell something good and upwind - so they fly
If they fly and they lose the odour, they wait a while to see if the wind blows to see if the odour comes back - this is a strategy to deal with turbulent odour plumes
When the fly reaches the source it uses other sensory cues to find the food.
How do mice perform active sensing?
To detect a smell mice move their head back and forth. This helps them to sample a larger space to detect the smell
Allows for faster changes in the environment to detect odours
This also allows the mice to observe the change in odour concentration in the environment so they coordinate where a smell is coming from..
What happens if you block a nostril?
Less odour detection ability - cant sample a large area
How to detect taste and how is taste detection amplified?
Taste transduction uses metabotropic and inotropic receptors
The metaboTropic receptors are used to detect sodium levels and amplifies signals
What is the process of taste detection?
Taste buds send signals via different cranial nerves to the brainstem. Here information is sent to the hypothalamus / amygdala.
Or it is sent to further regions of the thalamus such as the insula or the parietal cortex
Which taste buds link to which cranial nerves?
The epiglottis taste buds link to cranial nerve X (10)
The anterior taste buds on two thirds of the tongue link to cranial nerve VII (7)
The taste buds on the posterior one third of the tongue link to cranial nerves IX (9)
What is similar between the mechanism of taste and smell?
Both have first, second order neurones and interneurones
What are the interneurones in the taste mechanism pathway? And what do they do?
The interneurones between the first and second order neurones are the GABAergic neurones.
These allow for inhibition of MOST first and second neurone synapses in order to distinguish a certain taste from lots of different tastes.
Remember the mechanisms of smell and taste are the same
What are mice more drawn to ? Sweet or bitter?
Sweet
