What is gustation?
-the process of eating
What is taste?
-not flavour! -it is the sensation detected by the mouth -quite crude
What is flavour?
-the sensory experience of food and drink
What does flavour contain?
-dominated by smell and taste -can include texture,appearance, temperature, pain (chilli), fat
How does appearance come into flavour?
-we eat with our eyes, appearance is very important to what the food tastes like for us
How does temperature come into flavour?
-if something is the wrong temperature then its flavour is wrong (like warm coke)
How does pain come into flavour?
chilli= humans really like it, active ingredient is capsaicin, neurotoxin= kills some sensory neurons and activates pain fibres and those sensitive to chemical burn
How does fat come into flavour?
-if not the right level of fat then food lacks interest, fat sensitive receptors in mouth that feed into the gustatory pathways
What are the 5 well defined sensory stimuli that taste is confined to? And the 2 less well defined?
-sweet, sour, salty, bitter, umami -less well defined: metallic, piquant or pungent
How many odours can humans detect and how many can we discriminate?
-more than 2000 -each of the odours has a specialised receptor -sense of smell is more subtle -but can only discriminate 5 odours, can get interesting interesting combinations but pair wise they can only discriminate 5
What contributes to flavour?
-both olfaction and taste
What does food in the mouth activate?
-activates taste (gustatory) afferents (running towards the brain -activates olfactory afferents as everything that is in the mouth has access to the nasal cavity (olfaction via diffusion of volatile odours into nasal cavity)
What separates the nasal cavity and the mouth?
-the palate -if have a blocked nose= that is why we can't taste as well, need olfactory input too
Where are the fatty acid receptors located?
-around the soft palate, on the side of the mouth, all the other receptors are on the tongue
Does the tongue have specialised part that respond to one flavour?
-sort of -has regions that will respond more to one -we have different bits sensitive to one in particular but practically any part of the tongue can respond to any part of the tongue -typically if you touch sth with the tip of your tongue then it will determine sweet as the predominant taste
Where on the tongue is sweet located and why is it important to detect?
-tip of the tongue -important as it indicates presence of sugars= energy
What are the two flavours that all animals seek out?
-sweet and salty
Where on the tongue is salty located and why is it important to detect?
-forward edges of the tongue -important as salt is an essential part of the diet and we need it for proper function of muscles and the nervous system
Where on the tongue is bitter located and why is it important to detect?
-back edges and back centre of the tongue -bitter= indication that something is toxic, plants produce bitter flavour to warn predators not to eat
Where on the tongue is umami located and why is it important to detect?
-not clear -it is the amino acid glutamate -essential amino acid, used in the brain, it enhances all the other tastes and the olfactory -when we get given unknown meet= it has acids activating umami = that is part of teh chicken flavour -so we say it tastes like chicken -can add umami= can make vegetarian food tastes like meat
Where on the tongue is sour located and why is it important to detect?
-central edges of the tongue -sour=if sth is sour it is intermediate (of bitter= then you want to spit it out) with sour also indicates that food probs not good for you -acid indicates it is starting to ferment= rotting
What are the taste sensitive structures called?
papillae, they are the little ridges on the tongue
What is a taste bud?
-collection of taste-receptive cells -selection of different taste cells
How many taste buds does a person usually have?
How long does it take for taste cells to be replaced?
What is the structure of a taste bud?
-the apical end is exposed to the environment and the basal end is connected to the gustatory afferent neurons
Are taste cells in a taste bud all of the same type?
-no, they have different sensitivities, respond differently to different flavours
Does each taste bud provide more than one type of output?
-Final output from each taste bud must be integrated centrally to achieve tongue’s regional selectivity -so only one!
What does combing sweet and salty do?
-combining sweet and salty flavour= results in more intesne sweet and salty flavour not the taste experience =this integration doesn't happen at the level of sensory perception but much higher in the brain
What is the location of this taste bud?
-predominant experience coming from a taste bud located on the forward edges= biggest response to NaCl, as both of the taste cells are depolarised by NaCl
-when taste cells depolarised by the chemical interacting with its apical surface can release neurotransmitter which in turn activate sthe nerve terminals at that taste cell,
-each taste cell has a collection of receptors that allow it to respond to different chemicals
How is salty taste sensorily transducted?
-salt= sodium flavour, is detected easily, sodium is outside of the cell and passes into the apical surface of the taste cell down a Na channel, different form voltage channel, and this depolarises the cell, = then open voltage dependant Ca2+ channels= release secretary granule = then neurotransmitter release and activates the primary sensory neuron= AP in that neuron -salt detects sodium ions= depolarize taste cell via amiloride sensitive Na channels
What is the sour sensory transduction?
H+ (from the acid) is outside of the cell and passes into the apical surface of the taste cell down a H+ channel, different form voltage channel, and this depolarises the cell, = then open voltage dependant Ca2+ channels= release secretary granule = then neurotransmitter release and activates the primary sensory neuron= AP in that neuron
Do sweet and umami have one mechanism of sensory transduction?
-no, probably more
What receptor does sweet sensory transduction operate trough?
What receptor does umami sensory transduction operate trough?
What is are G-protein coupled receptors?
-trimer that when activated dissociate and sets of a cascade of actions
How does the sensory transduction work in sweet and umami?
-the chemicals acivate the G coupled protein (here coupled with alpha gustducin) -this activates phospholipase C which activated a series of depolarising mechanisms (activating Ca2+ channels, and closing K+ channels)=this depolarises the cell -leads to release of neurotransmitter and activation of the primary sensory neuron (action potentials)
What is special about the T1R1/T1R3 (umami) receptors?
-respond to glutamate and are enhanced by inosine -also have specialised metabotropic glutamate receptor with similar coupling to membrane potential
What are the TR channels?
-TRPM5- TR chnnels are transient receptor channels, the classic TRPV1= the one detecting chilli, they all actively depolarise the cell TRPN channels respond to menthol basically channels that allow Ca2+ in= depolarising the cell, widespread in the neural system
How does bitter sensory transduction work?
-same as umami and sweet except the receptors are T2R and they activate either alpha gustducin or alpha transducin
What is the neurotransmitter used in sensory transduction in taste?
-controversial -used to think serotonin (5-HT) but now know it is a variety of them: including glutamate, acetylcholine, Noradrenaline, GABA
What happens when a taste cell is depolarized?
-leads to release of excitatory transmitter that depolarizes gustatory afferent terminal
What is the central taste pathway?
-gustatory afferents coming from the tongue synapse into gustatory nucleus part of medulla (ipsilateral projection), the solitary nucleus (gets lot of sensory information)
-main goes then ventral posterior medial nucleus of thalamus (VPM)
-side ones go to hypothalamus and amygdala
-then insula(gustatory cortex) and frontal cortex (gustatory cortex, frontal operculum)
What is perception of taste mediated by?
mediated via gustatory projection to ventral posterior medial nucleus of the thalamus -for purposes of perceiving tastes= goes to the thalamus
Where is the gustatory cortex?
-insula and frontal operculum
What interactions of the taste pathway lead to flavour?
-olfactory interactions lead to sensation of flavour
What are the two olfactory organs in most animals?
-olfactory epithelium -vomeronasal organs (for pheromone detection) -these two sort of merged in humans and pheromone detection not very strong
Where is the olfactory epithelium?
-lies on roof of nasal cavity
What does the olfactory epithelium consist of?
-contains olfactory receptor cells (neurons) that are continually turning over
-layer of olfactory receptor cells, supporting cells and basal cells
How long do we keep our olfactory cells?
-about 2 weeks
-replaced by the stem cells produced in the brain -the cells develop and then send their axons through cribiform plate to the nasal cavity and to the olfactory bulb
How do the odour chemicals get to the receptor cells in the nasal cavity?
-odourants dissolve in the mucus layer and contact the cillia of the olfactory cells, axons of olfactory cells penetrate the bony cribiform plate on their way to the central nervous system
Why is mucus in the nasal cavity important?
-provides chemical environment that dissolves the chemicals coming in and this way can present them to the olfactory neurons
-there are cillia and there are the receptor proteins to be found
How selective are receptors in the olfactory system?
-many different odourants can be detected -selectivity depends on odourant receptor molecule -pathway common after activation of the receptor
How does sensory transduction work in olfaction?
-many different odourants can be detected,
-pathway common after activation of the receptor
-G coupled protein receptors, the G proetin= G olf, when dissociated= activates adenylyl cyclase
= that increases cyclic AMP concentration,
=activates class of receptors called cyclic nucleotide activated cation channels (if increase cyclic AMP the open a channel that allows Na and Ca2+ in= depolarising the membrane,
and also then activates Cl- channel that further depolarises the membrane
-that sends of AP down the axon to the olfactory bulb
What G protein is involved in olfaction?
-G-olf -G olfactory
What is the receptive field of an olfactory receptor neuron?
-the chemicals that activate the neuron are its receptive field
Are different olfactory receptor neurons specific?
-have different response profiles to arrays of odourants
-differs from cell to cell
What is the information leaving the olfactory bulb like?
-only a single odour will be associated with a signalling neuron, somehow the smells are picked , fine tuned so only one odour is represented eventhough they are on a spectrum (the response)
What happens in the olfactory bulb?
-very primitive part of the brain
-second order olfactory neurons have branching dendritic trees that form glomeruli with terminals of olfactory receptor cells
-so have olfactory receptor cells they send axons through the cribiform plate and synapse with the second order olfactory neurons in their glomeruli, the second order continue into the brain but pass thriugh the mitral, tufted and granular cells that also receive some input from the sensory neurons
What are he olfactory bulb mitral cells?
-receive excitatory input from olfactory sensory neurons (+ tufted cells) -receive olfactory input and project into the brain
How do the glomeruli in the olfactory bulb behave?
-encode only one odour -always receive info from cells reporting just a single odour (so ones responding to bananas will respond only to those not to almonds)
What is the specific mapping of olfactory neurons to the olfactory bulb?
-receptor cells synapsing within a particular glomerulus all have the same receptive field (express the same odourant receptor)
What is the central olfactory pathway?
1. Olfactory receptors
2.Olfactory bulb from there goes to several targets=
a)Pyriform cortex from there goes to orbitofrontal cortex (=olfactory cortex)
c)amygdala from there to orbifrontal cortex thalamus and hypothalamus (also thalamus and orbitofrontal cortex exchange information)
d)entorhinal cortex and from there to hippocampal formation
What are the main points from the olfactory pathway?
-projection neurons of olfactory bulb project directly to olfactory cortex and then to the thalamus -also have projection via olfactory tubercule to medial dorsal thalamus and the orbitofrontal cortex -integration with mood and affect via amygdala