Chemical senses Flashcards
What do we mean when we say that humans are “microsmatic”?
We do have some capacity to orient ourselves using smell, but it is not crucial to our survival, only to our quality of life
Animals are MACROSMATIC i.e. smell used to orient in space, detect predators, mark territory, pheromones etc
How is the process of smelling very different to sight and hearing?
Very few words to describe specific smells - some smells can be the same i.e. same receptors activated but receptor structures very different, or some smells might be very different but produced by molecules with very similar structures
What is a “detection threshold”?
The number of molecules needed to smell something (usually measured in parts per billion)
Our individual olfactory receptors actually are equally as sensitive as dogs’ but we just have fewer of them
What are the properties of olfactory receptors?
Each one is specific to a narrow range of chemicals - we have ~350 different types of receptors as a result
What is a recognition profile of a smell?
The pattern of activation of different olfactory receptor neurons by a certain odorant - this explains why molecular structures can be similar while still producing a different smell i.e. their recognition profile is different
What are the stages of smell transduction?
Receptors activated and depolarised –> signals transmitted to a glomerulus in the olfactory bulb
From here signals are sent straight to higher cortical areas with no need to relay in thalamus as the olfactory bulb is already so close to the brain areas (touches frontal cortex)
What is the chemotopic map?
Similar gist to the tonotopic maps, different molecular features are represented by different regions along the olfactory bulb e.g. functional groups determine general area of bulb activated, molecules with longer carbon chains generally synapse at glomeruli further left along the bulb
This coding of odourants reliably allows specific “hardwired” behavioural responses e.g. recognition of a pheromone triggers certain behaviour
What higher-order processing is involved in smell?
- We can perceive a single smell e.g. coffee even when made up of many different molecules i.e. we can combine them as one whole
- We can distinguish between multiple of these complex smells in the environment
- Past experience and learning can affect our odour perception
What is the pathway once signals are at the olfactory bulb?
Bulb –> olfactory tract –> periform and periamygdaloid cortex (i.e. PRIMARY OLFACTORY CORTEX) –> orbitofrontal olfactory area
What happens in the primary olfactory cortex?
individual compounds cause widespread activity with substantial overlap between patterns of activity for different compounds (not as strictly localised as in the bulb)
Given enough time, neurons here can “learn” to discriminate between odours i.e. stay in a room long enough, olfactory system will be able to adapt
What are “odour objects”?
The ability to categorise an odour even though exact molecular composition varies
It works by pattern completion following more exposure to an odour:
- Odorant molecules activate specific parts of the chemotopic map on the olfactory bulb
- Scattered activation in the periform cortex
- After learning, these scattered activations are able to sum up to the specific pattern for that odour
What happens in the orbitofrontal cortex?
Higher level processing e.g. labelling of smells
Labels result in changes in perception of that smell e.g. its pleasantness
What happens in the amygdala?
This is even higher level processing relating smells to emotions (also bring in memory here to recall associated emotions)
What is the DIFFERENCE THRESHOLD?
In addition to detection threshold i.e. smallest difference in concentration of 2 odours that can be detected
Can be precisely measured using an olfactometer
What is the RECOGNITION THRESHOLD?
Concentration at which quality of smell recognised rather than simply detected as present
When we struggle to identify an odour it is not an olfactory deficiency but rather an inability to retrieve its name from memory stores
e.g. at the recognition threshold we would recognise individual qualities of a smell, and once a name is added we smell it as that one concept rather than individual qualities
What 3 methods can be used to measure activity of the olfactory system?
CALCIUM IMAGING - Measuring activity of neurons, fluorescent green glow is reduced when more calcium in the neurons i.e. when they are more activate
OPTICAL IMAGING - Measures olfactory bulb, active areas appear less red as less oxygen remaining in blood
2-DEOXYGLUCOSE TECHNIQUE - radioactive glucose taken up by active neurons and measure radioactivity in various parts to measure activity
What are the 4 important roles of taste (and smell)?
1) Prepare body for digestion
2) Gatekeeper function - identify what need for survival and what is potentially dangerous
3) Contribute to “flavour” (aspect of life quality)
4) Strong connections to memory e.g. important for potentially damaging things
How can taste demonstrate the plasticity of modules?
We can learn to modify our responses to certain tastes and develop a taste for foods we initially found unappealing
What are the 4 types of papillae?
Circumvilliate (flat mounds posterior on tongue)
Foliate (folded, along very edge)
Filiform (cone shaped along lateral edge, only type to not contain taste buds, function is more to increase SA in contact with food)
Fungiform (mushroom shape at tip of tongue)
Why is there no taste sensation if you stimulate the centre of the tongue
No papillae present there
What is the structure of a taste bud?
50-100 taste cells in a globe shape meeting up towards the taste pore which projects onto the surface of the tongue
At the base of the bud taste fibres transmit electrical signals in a number of nerves
What nerves do signals get transmitted down?
CHORDA TYMPANI - from front and sides of tongue
GLOSSOPHARYNGEAL - back of tongue
VAGUS - Mouth and throat
SUPERFICIAL PETROSAL - from soft palette
What 4 different types of hair cell receptors are on taste cells? (there will only be a single receptor type on a given cell)
Bitter (quinine) Sweet (glucose) Sour (HCl) Salty (NaCl) Umami (monosodium glutamate)
These compounds come closest to having one of the 4 basic tastes
How do taste cells protect themselves from potentially damaging molecules?
Constant cycle of neurogenesis i.e. new cells generated every 1-2 weeks for taste cells and 5-7 weeks for smell
What happens at the receptors on the gustatory hair cells?
Transduction –> nerve fibres from tongue and mouth synapse in the nucleus of the solitary tract (medulla oblongata)
From here to thalamus –> frontal operculum and insula (GUSTATORY CORTEX)
Fibres serving taste system also reach orbitofrontal cortex which also receive olfactory signals (area for decision-making)
What does flavour perception involve?
Taste + smell
RETRONASAL OLFACTION - Odorant molecules from oral cavity and pharynx reaching olfactory mucosa
ORTHONASAL OLFACTION - via nose
Why does pinching nostrils affect flavour perception?
Prevents vapours reaching olfactory receptors by eliminating air flow through the channel
Mainly affects complex flavours - simple ones can very accurately be detected by taste buds alone
How can flavour as a process be described?
MULTIMODAL
Even incorporates vision, touch and sound
Why has the orbitofrontal cortex been suggested as the primary flavour processing centre?
Responses from taste and smell combine here first
Consists of BIMODAL NEURONS which are able to respond to more than one sense e.g. taste+smell
What are OFC neurons capable of doing?
Responding to congruent qualities in different modalities e.g. sweet smell and sweet taste (tuned to respond most efficiently to those modalities that occur together most commonly) - congruency of colour can further enhance a taste, more and more as colour intensity increases
How is the OFC also involved in hunger?
Firing of neurons in this area reflects the extent to which an animal will consume a particular food - if hunger decreases for certain foods firing in response to that food odour decreases (causally probably the other way around
OFC neurons, remember, are essentially reflecting food’s perceived pleasantness and in this way helping to modulate food intake
What is meant by “distributed coding” for taste?
Taste quality signalled by pattern of activity distributed across many neurons
Similar patterns =similar perception of taste quality
How did electrophysiological results support the idea of distributed coding?
NH4Cl and KCl have the same patterns of activation
Rats received a shock when drinking KCl
Then avoided NH4Cl when then given choice between that and NaCl suggesting that same patterns of activation = same perceived taste
What is meant by specificity coding of taste?
Taste quality is signalled by activity in neurons tuned to respond to specific qualities
What evidence supports specificity coding?
Most evidence has come from taste receptors - specificity of receptors has been supported by loss of activity in response to certain tastes when receptors eliminated, and also in avoidance behaviours
Situation may be a bit more complicated - some receptors may have a PREFERENCE for a taste, but others will still respond to it so we wouldn’t completely lose our response to that taste
What is a current proposed conclusion regarding taste receptor coding?
Basic taste qualities determined by specificity
Subtle differences between tastes in a category determined by distributed coding
What are some genetic differences between “tasters” and “non-tasters”?
Higher density of taste buds
Specialised receptors present
Different taste preference may actually stem from different food EXPERIENCE i.e. more or less intense taste that could be caused by different types and numbers of taste receptors on tongue
Describe the ascending gustatory pathway
Afferent fibres of cranial nerves from the tongue collect in the rostral solitary nucleus in medulla oblongata
Axons ascend ipsilaterally in the central tegmental tract of brainstem and terminate at the thalamus
From here neurons project to insular cortex and operculum
