olifaction and gustation

Question 1

where are taste receptors found?

Answer 1

Taste receptor cells are located in papillae mostly localized to the tongue epithelium however some exist around the throat.

Question 2

name the different pappile types in order of predominance from back of the throat to the front

Answer 2

circumvallate - foliate - fungiform

C+Fo ~100 taste pores Fu~5

Question 3

what nerves innovate taste receptors on the toung

Answer 3

grossopharangeal (facial nerve 9) and chorda tympani (facial nerve 7)
both go up the solitary tract to the thalmus

Question 4

how often do taste receptor cells die and replenish?

Answer 4

every 250 hours ~10.5 days

Question 5

how is sour taste transduced?

Answer 5

Sour transduction is known to involve intracellular acidification by weak acids; it is thought that this blocks plasma membrane K+ channels causing depolarisation and Ca2+ release

Question 6

how is the taste of salt transduced?

Answer 6

occurs in ‘glial like cells’ and is thought to be purely down to greater levels of Na+ entering the cell and thus causing membrane depolarization.

Question 7

explain how sweet, umami and bitter signals are transduced?

Answer 7

all GPCRs (Gi) Cause an increase in Ca2+ levels leading to opening of panexin channels allowing ATP to leave as a transmitter.

sweet - T1R2+T1R3 dimer only 3 can function alone- not well

umami - T1R1+T1R3 dimer

bitter - family of 30 T2Rs

Question 8

how where GPCR taste receptors first discovered

Answer 8

comparative genome studies in mice and humans of A-typical taste

Question 9

explain and evaluate the labeled line theory of taste compared with ensemble coding approach

Answer 9

Labeled line hypothesis 1:1 relationship between taste cell type relation

However:
None of the taste cells are selective
Nerve innovates multiple taste cells

ensemble coding better model

Question 10

explain the route of taste afferents to the cortex

Answer 10

Nerves→ solitary nucleus→ thalamus→ insular cortex

Question 11

how long to olfactory neurons live?

Answer 11

~60 days

Question 12

approximately how many olfactory receptor receptor types are present in humans?

Answer 12

~ 400types in humans

Question 13

what is significant about the molecules we tend to smell

Answer 13

we tend to only smell large molecules

Question 14

how do action potential firing rate and cAMP production relate?

Answer 14

Action potential firing rate is proportional to cAMP production

Question 15

olfactory receptors are not selective on a molecular level so how is molecule recognition achieved?

Answer 15

All non selective but only express 1 type of receptor

Lock and key system demonstrates population coding
Combination of subdomains at post glomerular level receptors are only specific for one subdomain

Lateral inhibition between similar receptor glomeruli refines signal

multiple subdomain reception gives our brains a complete odorant profile

Question 16

where are olifactory receptors located?

Answer 16

Olfactory system receptors are located on the olfactory epithelium within each nasal cavity. Odorants dissolve in the mucus film covering all approximately 5cm^2 of human olfactory epithelium.

Question 17

structure of an olfactory receptor cell

Answer 17

bipolar
this cillia with receptors extend into mucus
axons extend through cribriform plate to receptor specific glomeruli in olfactory bulb

Question 18

what are basal cells how do they function

Answer 18

basal cells; which function as olfactory receptor specific stem cells, located under supporting cells

Question 19

what is the olfactory receptor binding site?

Answer 19

Hypervariable regions in membrane spans 3,4,5 of these receptors form a pocket believed to be the odorant binding site.

Question 20

most common anosmia

Answer 20

12% of the population have a selective deficit in perception of musk

Question 21

how do olfactory G proteins function?

Answer 21

alpha subunits dissociate to stimulate adenylyl cyclase. Adenylyl cyclase then converts ATP into cAMP. cAMP then opens nucleotide gated cation channels generating inward receptor current as Na+ and Ca2+ diffuse into the cell. If there is sufficient stimulation to depolarise the cell past the threshold potential it will induce action potential firing.

Question 22

what are TAARS

Answer 22

a family of receptors in the olfactory system called TAARS. TAARS are sensitive to amines particularly ones that are diferntialy sexulay expressed leading to the belief that these receptors have a separate role in social cues.

Question 23

how are constant stimuli filtered out in the olfactory system?

Answer 23

Olfactory receptors rapidly attenuate with exposure to constant stimuli; this is demonstrated. This feature enables filtering out information that is no longer silent to the individual, preventing for instance one’s own smell drowning out salient cues in the environment.

Question 24

what is the role of periglomerular cells and granule cells in the olfactory bulb

Answer 24

Periglomerular cells and granule cells mediate lateral inhibition in the olfactory bulb. If an odorant excites multiple similar receptors to varying degrees lateral inhibition can be used to filter the signal ensuring only the most prominent signal is passed on

Question 25

how is contra-lateral inhibition used in the olfactory system?

Answer 25

Contralateral inhibition is a technique that the olfactory system uses to encode directionality of an olfactory stimulus. Inhibitory axons travel across the anterior commissure to corresponding parts of the olfactory bulb.

Question 26

what is significant about olfactory processing in the brain compared with other senses?

Answer 26

many of its projections bypass the thalamus on the way to the cortex giving smell its pervasive nature as a sense.
many projections also go via the amygdala lending to the emotional weight of olfactory stimuli also the entorinal cortex tying it closely to memory perception.