physiology of smell and taste

Question 1

taste

Answer 1

sweet- glucose
salty- sodium chloride
savoury- glutamate
sour- hydrogen ions
bitter- alkaloids

Question 2

anatomy of round (posterior to anterior)

Answer 2

• epiglottis
• palatine tonsils
• lingual tonsils
• formane caecum
• circumvallate papillae
• fungiform papillae (carry taste buds)
• filiform papillae (rough surface for manipulation of food)

Question 3

taste buds

Answer 3

On fungiform and circumvallate papillae
2000-8000 of them
Distributed across the tongue (ignore “taste maps”)

Gustatory receptor cell
Surrounded by supporting (sustentacular) cells

substance from food dissolves in saliva

saliva enters taste pore

gustatory receptor cell depolarises in response to substance
G protein-coupled receptor

nerve impulse travels along afferent nerve to brain

Question 4

neurology of taste

Answer 4

Anterior 2/3 of tongue
lingual nerve (trigeminal, CN V)
then chorda tympani (facial, CN VII)
Posterior 1/3 of tongue
glossopharyngeal (CN IX)

Solitary nucleus of the medulla
Thalamus
Gustatory cortex (anterior insula & inferior frontal gyrus)

Question 5

clinical relevance of taste

Answer 5

Ear surgery can cause damage to the chorda tympani
Unilateral – minimal symptoms
Bilateral – unpleasant metallic taste in the mouth

Surgery to sublingual and submandibular salivary glands could damage lingual nerve
unilateral loss of taste and loss of common sensation in floor of mouth and tongue

Dry mouth also affects taste

Question 6

other sensory inputs

Answer 6

Pungency (capsaicin, chilli)
mediated via pain/temperature receptors

Coolness (menthol)
mediated via temperature receptors

Fattiness
specific “fat” taste or just a response to creamy texture? Controversial

Temperature
enhances release of odorants to the nose
improves texture

Texture
Crunchy crisps, creamy foods

Question 7

flavour

Answer 7

Taste is crude (5 receptors)
Other oral sensory inputs are important (texture, temperature)
Also sight, memory, mood, expectation, context
Flavour is mostly due to smell
Retronasal olfaction

Question 8

the nose

Answer 8

Horizontal structure 7cm long
Warms and humidifies incoming air
Quiet breathing = laminar airflow over the inferior turbinate

Sniff = turbulence to mix the air and send odorants to the roof of the nose
Olfactory mucosa sits just below cribriform plate, olfactory bulbs just above

Question 9

olfactory receptors

Answer 9

Odorant molecules in the air dissolve in nasal mucus
Detected by rhodopsin-like detectors on dendrites of olfactory cells
G protein-coupled receptor
Each olfactory cell only expresses one kind of receptor
Olfactory receptor cells supported by sustentacular cells

Question 10

neurology of smell

Answer 10

Olfactory nerve (CN I) projects to
amygdala
hippocampus
parahippocampal gyrus

the only sensory modality that doesn’t relay primarily to the thalamus
straight into the limbic system – emotion and memory

Question 11

clinical relevance of smell

Answer 11

congenital anosmia
Kallmann syndrome (hypogonadotrophic hypogonadism)

acquired
rhinosinusitis
neurological disorders (Alzheimer’s, Parkinson’s, MS)
base of skull fracture
brain tumour
COVID 19

anosmia leads to severe problem with appreciation of food
use chilli, menthol, varied textures

risks to health
smoke alarm
avoid gas appliances
check sell-by dates on food

Huge impact on quality of life
much greater impact than loss of taste