Lecture 11: Special Senses II

Question 1

What is taste?

Answer 1

the sensation evoked by chemical “tastants” acting on the tongue

Question 2

Why are tastants water soluble?

Answer 2

because they have to mix with saliva to reach their receptors

Question 3

What are the five defined tastes? What is significant about these tastes?

Answer 3

salt (NaCl), sweet (sugar), umami (meatiness), sour (acid), bitter
all have specific receptors in the tongue

Question 4

What is kokumi?

Answer 4

mouth feel and is a recently identified “taste”

Question 5

What are examples of taste structures?

Answer 5

papillae, taste buds and central pathways

Question 6

What are the papillae innervated by?

Answer 6

specific primary afferent neurons that run with axons in the 7th, 9th and 10th cranial nerves

Question 7

Where do axons for taste run?

Answer 7

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

Question 8

What do different types of taste receptor cells express?

Answer 8

different receptors

Question 9

Which methods of transduction occur in TRC1, TRC2 and TRC3?

Answer 9

TRC1: simple
TRC2: via G-protein coupled receptors
TRC3: via otopetrin-1

Question 10

What is the most likely transmitter of TRC2 and what does it act upon?

Answer 10

likely to be ATP which acts via ionotropic P2X2 / P2X3 receptors

Question 11

How is ATP released from TRC2?

Answer 11

release mechanism not via conventional vesicle exocytosis

Question 12

How is a signal passed from a taste cell to a gustatory afferent?

Answer 12

depolarisation of taste cell releases ATP which depolarises gustatory afferent terminals via heterotrimeric P2X2 / P2X3 purinoceptors

Question 13

What does olfaction and taste contribute to?

Answer 13

flavour

Question 14

What can food/drink in mouth activate?

Answer 14

taste (gustatory) afferents and olfactory afferents

Question 15

How does olfaction work?

Answer 15

volatile odorants diffuse into the nasal cavity

Question 16

What does the roof of the nasal cavity contain?

Answer 16

olfactory receptor cells (neurons) that turn over continuously
have receptive cilia in nasal mucous

Question 17

Where do olfactory receptor cells send axons?

Answer 17

through Cribiform plate to the olfactory bulb

Question 18

How many different olfactory receptor proteins are there?

Answer 18

up to 2000 different proteins and each has its own preferred odorants

Question 19

What is the basic olfactory transduction process?

Answer 19

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

Question 20

What does identification of odorants depend on? What does processing involve?

Answer 20

convergence within pathway and processing involves identity of each olfactory neuron responding

Question 21

What do olfactory neuron precursors look like?

Answer 21

epithelial cells

Question 22

What is the role of the olfactory bulb?

Answer 22

consists of second order olfactory neurons which have branching dendritic trees that form glomeruli with terminals of olfactory receptor cells

Question 23

What do granule cells act as?

Answer 23

tuning interneurons

Question 24

What do individual neurons respond to? What do individual glomeruli encode?

Answer 24

more than one odorant

only one odour

Question 25

What is flavour?

Answer 25

the sensory experience of food and drink

Question 26

What is the experience of flavour dominated by?

Answer 26

smell and taste but also includes texture, appearance, temperature, pain (chilli), fat

Question 27

How many odours can humans detect?

Answer 27

more than 2,000 different odours

Question 28

What are the two sensations detected by the ear?

Answer 28

hearing and a component of balance

Question 29

What does hearing and balance both involve detection of?

Answer 29

movement via hair cells in cochlea or semicircular canals

Question 30

What detects hearing and balance?

Answer 30

not neurons, but modified epithelial cells

Question 31

What is the role of the organ of Corti?

Answer 31

sites on basilar membrane and contains hair cells, which are mechanosensitive

Question 32

What is the role of the basilar membrane?

Answer 32

tuned to pass high frequency vibrations close to oval window, low frequency vibrations closer to helicotrema

Question 33

How does vibration pass through the cochlea?

Answer 33

enters and then passes through the scala media, tectorial membrane and basilar membrane before leaving

Question 34

What is the local structure that detects mechanical stimulation in the ear?

Answer 34

inner hair cells

Question 35

How does sensory transduction occur in auditory and vestibular hair cells?

Answer 35

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

Question 36

Why does influx of K+ result in depolarisation?

Answer 36

K+ concentration in scala media is very high

therefore, movement in the opposite direction hyperpolarizes

Question 37

What do inner hair cells receive terminals from?

Answer 37

several auditory afferents with different thresholds

Question 38

What can individual auditory afferents contact?

Answer 38

several outer hair cells

Question 39

What is the role of efferent input received by outer hair cells?

Answer 39

“tunes” the local stiffness of the transduction system, thereby sharpening the frequency response of the neighbouring inner hair cells