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Perception 2: Chemical Senses- Smell and Taste - Coggle Diagram
Perception 2:
Chemical Senses-
Smell and Taste
.
involve taking airborne (odorants) and water-soluble chemical compounds (tastants) into the body.
gatekeeper senses:
play an important role in signalling the safety of the environment and food.
often generate positive and negative affective responses (linked to emotional processing that are adaptive:
aid identification of beneficial and dangerous environments and substances that should be approached/avoided/consumed/rejected respectively.
unique feature- neurogenesis:
unlike any other types of receptors, chemical receptors regenerate themselves continuously throughout the lifespan
taste receptors regenerate roughly ever 1-2 weeks
olfactory receptors regenerate roughly ever 5-7 weeks
Smell Perception
(Olfaction)
sensations evoked by airborne chemical compounds (odorants that are able to stimulate olfactory receptors in the nose)
distance sense:
provides information about chemicals suspended in the air around us > also strongly linked to emotional and memory processing.
can get information about the environment around us from a distance > smelling things far away.
physical stimuli
odorant:
a molecule that is capable of stimulating olfactory receptors.
to be smelled:
they have to be
volatile
(able to float through the air)
they have to be
hydrophobic
(repellant to water)
they have to have a
small molecular weight
: roughly between 15 g/mol and 300 g/mol
exceptions:
there are two chemical compounds (both extremely dangerous in closed spaces) with a small molecular weight that we cannot smell (methane- 16.04 g/mol, and carbon monoxide- 18.01 g/mol)
for safety reasons, a safety taggant is added as an odorant to natural gas, so we can detect it in the environment.
receptors, transduction, and neural processing
transduction
: odorants enter the nasal passage
odorants enter the nasal cavity via the nose or mouth (retronasal passage)
the nose contains two types of internal linings: the respiratory epithelium and olfactory epithelium, which contain olfactory sensory neurons (receptors)
the role of respiratory epithelium is to filter, humidify, and warm the air we breathe.
molecules become trapped in the olfactory mucus and stimulate the receptor sites located on the hairlike protrusions on the dendrites of olfactory receptor neurons.
it takes seven or eight odour molecules binding to a receptor to initiate an action potential, to be able to detect the odour.
more molecules are needed in order to be able to identify and discriminate between odours
activated sensory neurons activate neurons in the olfactory bulb, before that information is sent to the primary olfactory cortex.
signals from the olfactory bulb are sent to both the primary olfactory cortex and the amygdala and limbic system (involved in emotional reactions to odours)
from the primary olfactory cortex, signals then travel to the secondary olfactory (orbitofrontal) cortex in the frontal lobe, where integration of sensory information from multiple sensory modalities takes place.
individual receptors across different species are equally sensitive; the difference in overall sensitivity lies in the difference of the number of receptors they have.
in all mammals, pretty much the same set of 1000 genes is involved in the regulation and expression of olfactory receptors.
a certain proportion of these genes are non-functional pseudogenes.
in dogs and mice, about 20% are pseudogenes
in humans, between 60 and 70% are pseudogenes > each person has a different number of pseudogenes, resulting in individual differences in sensitivity to smells
the relationship between
the properties of chemical molecules and the smells they evoke
is still unclear
shape-based coding:
assumed simple correspondence between molecular shape and perceived smell.
population coding:
odorants are coded by combinations of olfactory receptors
specific receptors may be part of the code for multiple odorants
the specific time order of activation of olfactory receptors might be important
the intensity of the odorant might change which receptors will be activated.
vibrational coding:
the odours associated with different chemical molecules or compounds are determined not by their shape but by their molecular vibrations.
olfactory identification in humans is poor
despite good discrimination and detection sensitivity, humans cannot label odours with great accuracy, even very familiar odours.
recognition threshold:
concentration needed to determine the quality of an odorant is higher for recognition (about 3 times as high) than that for detection and discrimination.
both odour sensitivity and identification increase in childhood and early adulthood, but decrease starting in middle age.
earlier findings have suggested that humans can discriminate between 100,000 odours, but the lates computational estimates suggest that we can differentiate between one trillion smells.
detection sensitivity
differs across different chemical compounds: some chemicals need lower concentrations to be detected than others (lower absolute thresholds/better sensitivity)
factors that affect sensitivity:
women generally have lower thresholds than men
sensitivity is worse in smokers and drinkers
sensitivity is better in the morning than later in the day
by age 85, 50% of the population is unable to detect most smells.
pheromones:
chemicals released by one animal, detected by another, that shapes the second animal's behaviour or physiology.
in other species, pheromones are detected by the specialised vomeronasal system, which does not seem to be present in humans.
the role of pheromones is not clear > alteration of moods, menstrual synchrony, not likely as sexual attractants.
olfactory adaptation:
sense of smell is essentially a change detector > you can adapt to the smell in a room after a bit of time.
disorders:
anosmia:
the total inability to smell, most often resulting from various infections in the nasal cavity, or head trauma.
can be congenital in rare cases, can be a cause of loss of taste.
estimated that as many as 1 in 20 suffer from a reduced sense of smell.
associated with decreased well-being and a decrease in quality of life.
specific anosmia:
the inability to smell one specific compound amid otherwise normal smell perception
Taste Perception
(Gustation)
sensations evoked by solutions in the mouth that contact the receptors n the tongue and the roof of the mouth.
tastant: any stimulus that can stimulate taste receptors.
physical stimuli
taste buds:
on average, there are about 10,000 taste buds in a person's mouth.
grouped in structures called papillae
most on the tongue, some on the roof of the mouth, and some on the back of the throat.
send information to the brain via cranial nerves
unlike the descriptions seen in most texts, the distribution of receptor types on the tongue is even, at least in humans.
receptors, transduction, and neural processing
the relation between physical stimuli and perceived tastes is much clearer.
labelled-line coding:
modality-specific receptors transfer the information through a line to higher levels of processing.
the taste pathway:
signals from taste cells travel along various cranial nerves.
these pathways first synapse in the spinal cord and then in the thalamus.
finally, they terminate in the primary olfactory cortex and the project to the orbitofrontal cortex
taste thresholds
detection thresholds:
sweetness > one part in 200
saltiness > one part in 400
sourness > one part in 130,000
bitterness > one part in 2,000,000
survival value:
bitter > might signal poisons
sour > configured to detect acidic solutions that might harm the body
sweet and salty > our bodies need sodium and sugar to survive
most sensitive to bitter substances: all animals need protection from toxins (bitter)
higher preference for sweet and sour substances
individual differences in taste perception
arthur fox (1931) discovered that certain bitter substances (PTC/PROP) taste dramatically different to different peope.
bitter to some, but not others.
the gene for PTC/PROP receptors was discovered in 2003.
individuals with
two recessive
genes are non-tastes of PTC/PROP.
individuals with
one or more
of the genes are tastes of PTC/PROP.
these genetic differences are also associated with differences in the density of fungiform papillae.
tasters, hypotasters, and supertasters
hypo: indifferent or likes bitter. seeks out spicy food. adventurous eater.
normal: tolerates bitter. tolerates spicy food. moderately adventurous eater.
super: avoids bitter food. avoids spicy food.
picky eater.
more supertasters and fewer non-tasters among females.
more supertasters and fewwer non-tasters among asians compared to causasians
flavour perception
perception of taste is significantly affected by other inputs > olfactory, visual, and somatosensory.
taste + smell + temp + texture + vision + audition = flavour
red-coloured food gives an increased perception of sweetness and the colour of the plate the food is on can also have an impact.
playing music or sounds appropriate to the food we are eating can also accentuate its taste.
the role of somatosensory input in taste and smell
pain in taste perception:
the hotness of chillies is a somatosensory sensation of capsaicin burn on the tongue
acquisition of chilli pepper preference depends on social influence
variability across individuals, depending on the number of papillae.
somatosensation in olfactory perception:
the trigeminal nerve carries information from somatosensory receptors in the nose and other areas of the face to the thalamus and then on to the somatosensory cortex
your eyes tear when you chop onions because chemicals in the onions irritate the trigeminal nerve, causing a reflex reaction.