Lecture 11: Special Senses II Flashcards
What is taste?
the sensation evoked by chemical “tastants” acting on the tongue
Why are tastants water soluble?
because they have to mix with saliva to reach their receptors
What are the five defined tastes? What is significant about these tastes?
salt (NaCl), sweet (sugar), umami (meatiness), sour (acid), bitter
all have specific receptors in the tongue
What is kokumi?
mouth feel and is a recently identified “taste”
What are examples of taste structures?
papillae, taste buds and central pathways
What are the papillae innervated by?
specific primary afferent neurons that run with axons in the 7th, 9th and 10th cranial nerves
Where do axons for taste run?
run through to the nucleus of the solitary tract in the medulla and from there to the thalamus and through to the gustatory cortex where the first level of taste sensation is processed
What do different types of taste receptor cells express?
different receptors
Which methods of transduction occur in TRC1, TRC2 and TRC3?
TRC1: simple
TRC2: via G-protein coupled receptors
TRC3: via otopetrin-1
What is the most likely transmitter of TRC2 and what does it act upon?
likely to be ATP which acts via ionotropic P2X2 / P2X3 receptors
How is ATP released from TRC2?
release mechanism not via conventional vesicle exocytosis
How is a signal passed from a taste cell to a gustatory afferent?
depolarisation of taste cell releases ATP which depolarises gustatory afferent terminals via heterotrimeric P2X2 / P2X3 purinoceptors
What does olfaction and taste contribute to?
flavour
What can food/drink in mouth activate?
taste (gustatory) afferents and olfactory afferents
How does olfaction work?
volatile odorants diffuse into the nasal cavity
What does the roof of the nasal cavity contain?
olfactory receptor cells (neurons) that turn over continuously
have receptive cilia in nasal mucous
Where do olfactory receptor cells send axons?
through Cribiform plate to the olfactory bulb
How many different olfactory receptor proteins are there?
up to 2000 different proteins and each has its own preferred odorants
What is the basic olfactory transduction process?
all olfactory receptor proteins activate Golf to activate adenylate cyclase
cyclic AMP activates cAMP dependent cation channel to depolarise membrane and produce a receptor potential
What does identification of odorants depend on? What does processing involve?
convergence within pathway and processing involves identity of each olfactory neuron responding
What do olfactory neuron precursors look like?
epithelial cells
What is the role of the olfactory bulb?
consists of second order olfactory neurons which have branching dendritic trees that form glomeruli with terminals of olfactory receptor cells
What do granule cells act as?
tuning interneurons
What do individual neurons respond to? What do individual glomeruli encode?
more than one odorant
only one odour
What is flavour?
the sensory experience of food and drink
What is the experience of flavour dominated by?
smell and taste but also includes texture, appearance, temperature, pain (chilli), fat
How many odours can humans detect?
more than 2,000 different odours
What are the two sensations detected by the ear?
hearing and a component of balance
What does hearing and balance both involve detection of?
movement via hair cells in cochlea or semicircular canals
What detects hearing and balance?
not neurons, but modified epithelial cells
What is the role of the organ of Corti?
sites on basilar membrane and contains hair cells, which are mechanosensitive
What is the role of the basilar membrane?
tuned to pass high frequency vibrations close to oval window, low frequency vibrations closer to helicotrema
How does vibration pass through the cochlea?
enters and then passes through the scala media, tectorial membrane and basilar membrane before leaving
What is the local structure that detects mechanical stimulation in the ear?
inner hair cells
How does sensory transduction occur in auditory and vestibular hair cells?
cilia of hair cells are connected by tip links and vibration moves cilia opening stretch sensitive K+ channels in the tips of the cilia ->
depolarisation leads to glutamate release from basal membrane onto afferent terminals that are also depolarised
Why does influx of K+ result in depolarisation?
K+ concentration in scala media is very high
therefore, movement in the opposite direction hyperpolarizes
What do inner hair cells receive terminals from?
several auditory afferents with different thresholds
What can individual auditory afferents contact?
several outer hair cells
What is the role of efferent input received by outer hair cells?
“tunes” the local stiffness of the transduction system, thereby sharpening the frequency response of the neighbouring inner hair cells
