26- The Chemical Senses

Question 1

how does our body detect CO2 and O2 levels?

Answer 1

chemoreceptors in arteries of the neck measure partial pressure of CO2/O2 in the blood

Question 2

how does our body detect chemical irritants?

Answer 2

nerve endings in skin/ mucous membranes warn us of chemical irritants

Question 3

how does our body detect acidity?

Answer 3

sensory nerve endings in muscle respond to acidity

e.g. lactic acid production from exercise causes a burning feeling

Question 4

how is taste perceived?

Answer 4

there are five tastes - sweet, sour, umami, bitterness and salt

touch, texture, temperature and smell of food - information is sent to the brain and combined to provide a unique tasting experience and distinct flavour to foods

Question 5

importance of the palate in taste?

Answer 5

palate - roof of the mouth separating oral and nasal cavities. has taste buds.

Question 6

importance of the epiglottis in taste?

Answer 6

leaf-shaped cartilage that covers the laryngeal inlet and prevents food entering the trachea. has taste buds

Question 7

importance of the pharynx and nasal cavity in taste?

Answer 7

odours pass via the pharynx to the nasal cavity – detected by olfactory receptors

Question 8

structures involved in taste

Answer 8

epiglottis - has taste buds, prevents food entering the trachea by blocking the laryngeal inlet

palate - has taste buds

pharynx & nasal cavity - smell of food from the pharynx to the nasal cavity, detected by olfactory receptors which send information to the olfactory bulb via CN1

tongue:
- different regions of the tongue are more sensitive to different tastes - e.g. the tip for sweetness, the back for bitterness. still produce a combination of tastes and distinct flavours for a specific food
- papillae on the tongue have taste buds, taste buds have taste receptor cells, with taste paws as the chemically sensitive ends of taste buds
- different taste receptor cells have different types of taste receptors, thus have different sensitivities for different tastes. increase AP firing for that taste.
- taste buds are surrounded by their precursors/ basal cells and gustatory axons which send gustatory info to the brain

Question 9

name the mechanisms of transduction for the fve tastes

Answer 9

saltiness and sourness = ion channel mechanisms

bitterness, sweetness and umami = Gq coupled GCPR mechanisms

Question 10

what ion determines saltiness?

Answer 10

Na+

Question 11

what ion determines sourness?

Answer 11

protons determine acidity and sourness

Question 12

describe the ion channel transduction mechanism for saltiness

Answer 12

Na+ passes through an amiloride controlled Na+ selective channel down a conc grad from outside to inside the taste cell

Na+ influx causes depolarisation = triggers opening of v.g. Na+ and Ca2+ ICs = influx

release of neurotransmitter from synaptic vesicles - acts on gustatory afferent axons

increased Na+ = increased saltiness

Question 13

describe the ion channel transduction mechanism for sourness

Answer 13

H+ passes through proton channels down conc. grad - binds and blocks K+ sensitive channels

H+ influx causes taste cell depolarisation = triggers opening of v.g. Na+ and Ca2+ ICs = influx

triggers release of NT from synaptic vesicles - acts on gustatory afferent axons

Question 14

what Gq-coupled T1R/T2R receptors detect bitterness, sweetness and umami?

Answer 14

bitterness = 25 subtypes of T2 receptors. bitterness is often a sign of poison, we’ve adapted to have a large range of receptors to detect different poisonous substances

sweetness = one receptor with T1R2 and T1R3 protein subunits

umami = one receptor with T1R1 and T1R3 protein subunits

Question 15

describe the GCPR transduction mechanism for bitterness

Answer 15

bitterness detected by 25 different subtypes of T2R receptors

bitter tastants bind to T2R - activates Gq coupled protein

activates phospholipase C = PIP2 converted to IP3 and DAG as intracellular messengers

IP3 activates a specific Na+ channel unique to taste cells causing a Na+ influx AND releases Ca2+ from intracellular stores

Na+ and Ca2+ release depolarises taste cell = ATP reelased and passes through ATP permeable membrane

gustatory afferents activated

Question 16

why don’t we confuse bitterness, umami and sweet tastes despite them all being Gq coupled pathways?

Answer 16

taste cells express either bitter, sweet or umami receptors

their taste cells associate with different gustatory afferent axons

Question 17

describe the GCPR transduction mechanism for sweetness

Answer 17

T1R2 and T1R3 subunits dimerise - sweet tastant binds to dimer receptor

activates Gq protein = phospholipase C= IP3 and DAG

IP3 acts on Na+ selective channel unique to taste buds = Na+ influx AND Ca2+ release from intrac. stores causing Ca”+ influx

depolarisation = ATP released and passes through ATP permeable channel

stimulates gustatory afferent axons

Question 18

flow of information from taste cells to the brain - the gustatory pathway

Answer 18

taste cells - LGIC/ Gq coupled mechanism

gustatory afferent axons

gustatory nucleus in medulla

ventral posterior medial nucleus in thalamus

gustatory cortex - different regions of cortex respond specifically for different tastes

Question 19

what are pheromones?

Answer 19

olfactory stimuli for chemical communication between individuals

used as a mode of communication with animals, for making territory or reproductive behaviours

Question 20

structures related to olfaction

Answer 20

nasal cavity contains olfactory epithelium - tissue with cells we used to smell

olfactory epithelium contains - olfactory receptor cells, supporting cells and basal cells

olfactory receptor cells = genuine cells with axons that project to the CNS, one of the few neuronal cells types replaced throughout life

basal cells = immature olfactory receptor cells which are replaced continuously in cycles

supporting cells = produce mucus layer which flows continuously. mucus contains many proteins, enzymes and antibodies - protective function. odorants dissolve in mucus before being detected by cilia of olfactory receptor cells.

Question 21

describe the transduction mechanism of olfactory cells and neurons

Answer 21

odorant molecules dissolve in mucus produced by supporting cells

odorant binds to cilia of olfactory receptor neuron - activates olfactory receptor protein G-olf

G-olf is adenylyl cyclase coupled = stimulates cAMP production and opening of cAMP activated channels

Na+ and Ca2+ influx via channels

Ca2+ stimulates opening of Cl- ICs = Cl- efflux

depolarisation of olfactory receptor neuron = initiates receptor potential that propagates along the olfactory receptor neuron dendrite as a graded electrical signal

receptor potential triggers a series of action potentials within the soma - propagate along olfactory nerve axon of CN1 (olfactory nerve) to the olfactory bulb

olfactory bulb separated into glomeruli (cluster of nerve endings). olfactory receptor cells that express the same odorant receptor proteins project to the same glomeruli of the olfactory bulb

to higher brain regions:
- hippocampus for memory
- hypothalamus and amygdala for emotional aspects
- frontal cortex for the olfactory cortex region = perception of smell

Question 22

what is population coding?

Answer 22

temporal and spatial frequencies of sensory signals from multiple neurons

combined activity of a large number of neurons used to represent specific sensory info = important for complex sensory stimuli

Question 23

how does population coding work with gustation?

Answer 23

taste receptor cells in taste buds express only one specific receptor protein - each receptor is fine tuned to detect a specific taste quality

taste receptor cell axons and neurons in CNS respond more broadly to a range of taste stimuli - overlap of different responses, brain combines info from various taste receptors to create distinct flavours

individual taste receptor cells - responds with high affinity/sensitivity to one specific taste = has a specific taste receptor protein

Question 24

how does population coding work with olfaction?

Answer 24

individual olfactory receptor cell has one type of olfactory receptor protein = responds to various odours

olfactory receptor axons and neurons overlap - combination of responses to the brain, information allows brain to identify specific smells