Physiology of Taste and Smell

Question 1

the receptors for taste and smell are what type of receptors?

Answer 1

chemoreceptors

stimulated by binding to particular chemicals

Question 2

why is taste and smell important?

Answer 2

stimulation of taste and smell receptors induce a pleasurable or objectionable sensation
- poisons taste bitter
- off food smells bad etc
taste and smell influences flow of gastric juice

Question 3

what is gustation?

Answer 3

taste

Question 4

what are the organ of taste?

Answer 4

taste buds
contain sensory receptor cells of taste and support cells
- arranged like orange segments
also contain basal cells which continuously replace the sensory receptor cells

Question 5

what binds the chemicals to receptor cells?

Answer 5

microvilli on surface

Question 6

describe the life span of taste receptor cells?

Answer 6

usually last around 10 days

then replaced from basal cells within taste buds

Question 7

what do taste receptor cells do and where are they found?

Answer 7

synapse with afferent nerve fibres

present in tongue, palate, epiglottis and pharynx

Question 8

where are most taste buds found in the tongue?

Answer 8

in papillae in the tongue

Question 9

what are papillae and what 4 types are present in the tongue?

Answer 9

finger like structures which give rise to the rough appearance of the dorsum of the tongue
(seen as little red dots or raised bumps on the tongue)
4 types:
- filiform (most numerous, don’t contain taste buds)
- fungiform
- vallate
- follate

Question 10

what are the steps in production of taste sensation?

Answer 10

1. binding of taste provoking chemical (tastant) to receptor cells alters cell ionic channels and produces depolarising receptor potential
2. receptor potential initiates action potentials in afferent nerve fibres which synapse with receptor cells
3. signals conveyed by cranial nerves via brainstem and thalamus to cortical gustatory areas

Question 11

via which cranial nerves do afferent taste fibres reach the brainstem?

Answer 11

CN VII (chorda tympani branch) = from anterior 2/3rds of tongue
CN IX = from posterior 1/3rd of tongue
CN X = from areas other than tongue (e.g epiglottis and pharynx)

Question 12

what are the 5 primary tastes?

Answer 12

salty(stimulated by chemical salts)
sour (stimulated by acids - H+)
sweet (stimulated by configuration of glucose)
bitter (stimulated by groups of tastants - alkaloids, poison and toxic substances)
umami (meaty or savory) (stimulated by amino acids - esp glutamate)

Question 13

name 3 types of taste abnormalities

Answer 13

ageusia
hypogeusia
dysgeusia

Question 14

what is ageusia?

Answer 14

loss of taste function

can be caused by nerve damage, local inflammation, some endocrine disorders

Question 15

what is hypogeusia?

Answer 15

reduced taste function

can be caused by chemotherapy and medications

Question 16

what is dysgeusia?

Answer 16

distortion of taste function

can be caused by infections, tooth decay, GORD, URTI, medications, neoplasms, chemotherapy, zinc deficiency)

Question 17

what 3 cell types are found in olfactory epithelium?

Answer 17

olfactory receptor cells
supporting cells
basal cells

Question 18

what are olfactory receptor cells and describe their structure?

Answer 18

specialised endings of renewable afferent neurons

each neuron has a thick short dendrite and expanded end called the olfactory rod

Question 19

what do olfactory rods do?

Answer 19

cilia project from the olfactory rods to the surface of the olfactory mucosa
odorants bind to the cilia

Question 20

lifespan of olfactory receptors?

Answer 20

around 2 months

replaced by basal cells

Question 21

what do olfactory receptors do?

Answer 21

axons from olfactory receptors join to collectively form afferent fibres of the olfactory nerve
these pierce the cribiform plate of the ethmoid bone and enter the olfactory bulbs in the inferior surface of the brain

Question 22

what are olfactory bulbs?

Answer 22

ganglia which transmit smell information to the brain via the olfactory tract to reach the temporal lobe and olfactory areas

Question 23

how can breathing affect olfaction?

Answer 23

during quiet breathing odorants only reach smell receptors by diffusion as olfactory mucosa is above the normal path of airflow
the act of sniffing enhances olfaction by drawing air currents upwards within the nasal cavity

Question 24

what is required for a substance to be smelled?

Answer 24

substance must be:

• sufficiently volatile (some of its molecules can enter nose with air)
• sufficiently water soluble (so it can dissolve in mucous coating of olfactory mucosa)

Question 25

how can humans differentiate between different odours?

Answer 25

olfactory receptors have different sensitivity to different substances

Question 26

name 3 abnormalities of smell

Answer 26

anosmia (inability to smell) - allergy, injury, infection
hyposmia (reduced ability to smell) - parkinsons disease
dysosmia (altered sense of smell)