Taste Flashcards
Where are the taste receptor cells located
Clustered together in taste buds, which are located in papillae embedded in the epithelium of the tongue
How can the taste papillae be classified
Into circumvallate, foliate, and fungiform papillae
What do receptors in the taste buds on the tongue synapse with
Afferent fibres projecting in the chorda tympani and glossopharyngeal nerves
What do taste buds on the epiglottis synapse with
Superior laryngeal nerve
What are the 4 classical taste qualities
What is the fifth
Could there be a sixth
Bitter
Sweet
Sour
Salt
Umami (the taste of glutamate)
Some suggestion of a sixth quality corresponding to starch
Which taste qualities involve specific receptor mechanisms (3)
What do they use
Bitter
Sweet
Umami
Second messenger cascade
How are sour and salt responses mediated
More directly via modulation of ion channels in the taste cell surface membrane
How can taste cells be subdivided (3)
Receptor cells
Presynaptic cells
Glial like cells
What do receptor cells for taste respond to
They have a narrow specificity expressing receptors for bitter sweet or umami
What do taste presynaptic cell respond to
Respond directly to acid stimulation
What do glial like cells respond to
They carry out potassium and transmitter homoeostasis and may participate in salt transduction
What underlie the umami and sweet responses
Heterodimeric GPCRs
Umami: T1R1 and T1R3
Sweet: T1R2 and T1R3
What underlies bitter responses
Sensed by a family of ~30 T2R receptors which dimerise - accounts for diversity of bitter compounds
Why do we have so many bitter receptors
Bitter indicates food is not right so a wide range allows better sensitivity
What happens to the GPCRs of taste once ligand has bound
β and γ subunits stimulate production of IP3 —> Ca2+ release + TRP5M cation channel opening
Increased [Ca2+] and depolarisation leads to transmitter release from receptor cells via gap junction hemichannels
What does sour transduction involve
Intracellular acidification by weak acid
Mechanism not understood but involves K+ channel which is blocked by intracellular protons, leading to depolarisation, opening of Ca2+ channels and vesicular transmitter release from presynaptic cells
What does salt transduction involve
Na+ entry through epithelial sodium leak channels which depolarised glial like cells
Protons can also enter these channels
How do taste afferents in the cranial nerves VII, IX and X process
Synapse in solitary nuclear complex in the medulla within gustatory nucleus l
In the primate an uncrossed pathway projects to the ventral posterior medial nucleus of the thalamus and then on to gustatory neocortex in the anterior Insula and frontal operculum
How do projections pass from the primary taste cortex (2)
- To the secondary taste area in the orbitofrontal cortex, where units modulate the discharge according to the pleasantness of the taste of the food,
- and to the amygdala, which plays a role in the affective component of taste response
Which taste projections modulate feeding behaviour
Projections to the lateral hypothalamus
Describe the thalamocortical pathway in some primate species such as rodents
Pathway runs via a pontine relay in the parabrachial nucleus where changing physiological conditions may modify feeding behaviour
True or false
Individual taste fibres respond to a range of stimuli but tend to prefer just one
True
Recordings from afferents in the chorda tympani show this
Individual taste fibres respond to a range of stimuli but tend to prefer just one. What does this indicate
How can this discrepancy be resolved
across fibre code, in which taste is identified by the pattern of afferent activity invoked in different fibres
So taste is based on the pattern evoked
By noting that afferent fibres receive broadly tuned input, not only from the receptor cells via ATP but also from the presynaptic cells via serotonin release. Each afferent fibres can therefore respond to a range of taste qualities
How do receptor cells release ATP
Gap junction hemi channels
