TASTE AND

OLFACTION

DR. TANISHQ HITESH GUPTA

Dr. Baba Saheb Ambedkar

Medical College and Hospital,
Delhi
 Taste & smell
• Chemical senses: Respond to a chemical
stimulus
• Mutually complimentary
• Flavour of food indicates it′s combined
influence on smell & taste
TASTE
• Sense organ for taste: TASTE BUD which
contains the receptor cells.

• Taste buds are present on the PALATE,

EPIGLOTTIS,PHARYNX AND PAPILLAE
OF TONGUE.

• 10,000 taste buds totally there.

• Starts degenerating beyond 45 years.

 PAPILLAE
• FUNGIFORM: Tip of tongue (5 taste buds)
• VALLATE: Arranged in ‘V’ on the back of
tongue (100 taste buds)
• FILLIFORM: Covers the dorsum of tongue,
DO NOT CONTAIN TASTE BUDS
• FOLIATE: Lateral border of tongue
 TASTE BUDS
• Ovoid bodies measuring 50–70 µm
• Made up of SUPPORTING CELLS,
BASAL CELLS & 5 -18 RECEPTOR
CELLS called GUSTATORY RECEPTORS
• Receptor cell has microvilli projecting
into the taste pore & has a life cycle of 10
days
• Unmyelinated nerve endings seen at the
base of receptor cells
 TASTE PATHWAY
• Bilateral pathway.
• Sensory nerve fibres from taste buds are
carried by 3 different nerves-
– From anterior 2/3 of tongue via CHORDA
TYMPANI BRANCH OF FACIAL NERVE (7th)
– From posterior 1/3 of tongue via
GLOSSOPHARYNGEAL NERVE (9th)
– From pharynx, epiglottis & palate via VAGUS
(10th)
 Taste fibres in VII, IX & X nerves unite in

NUCLEUS TRACTUS SOLITARIUS (MEDULLA)

2nd Order N. joins ipsilateral MEDIAL
LEMINISCUS

VPM Nucleus of THALAMUS (3rd Order N.)

Foot of ipsilateral post central gyrus (Broadman’s
area 43,face area)
 Connections
• NUCLEUS TRACTUS SOLITARIUS-
Connected to salivary nucleus
• Insular cortex- anterior to face area.
• Function: conscious perception of taste &
taste discrimination
 Primary somatosensory
cortex (Postcentral gyrus)

Gustatory Insular cortex

Cortex
(Frontal
operculum and
anterior
insular cortex)
 Taste modalities:

• Salt

• Sour

• Sweet

• Bitter

• Umami (“deliciousness”)
 BASIC TASTE SENSATIONS
Produced by Mechanism of Part of
stimulation tongue
most
sensitive
SWEET Sugars,glycols & ↑ CAMP→↓K+ Tip
aldehydes. Conductance

BITTER Alkaloids ↑ IP3→ ↑Ca++ Posterior

(quinine,caffeine) release

SOUR H+ ions Blocking K+ Post ½

channels of lateral
SALT Anions of ionised  Na+ ion Ant ½ of
salts permeability lateral
• Fifth taste sense UMAMI,recently discovered.
• Triggered by monosodium glutamate used in
Asian coking.
• Pleasant & sweet taste but differs from
standard sweet taste.
• Stimulation by activation of glutamate
receptor in taste buds.
 THRESHOLD & ADAPTATION
Order of increasing threshold:
– BITTER lowest,then SOUR followed by
sweet & salt equally
• Taste receptors show adaptation by
elevating the threshold level.
• 30% concentration change necessary to
detect intensity difference.
 AFTER EFFECTS
• Some substances alter the taste of other
substance
• A plant protein MIRACULIN when applied
to tongue makes acids taste sweet
• BETEL NUT & AMLA make water taste
sweeter
 DISORDERS
• AGEUSIA:Loss of taste sensation
- Eg, Common cold,drugs (Captopril,penicillamine)
• HYPOGEUSIA : Less taste sensation (Eg, Old age)
• DYSGEUSIA : Disturbed taste sense (Eg; epileptic
seizure)
• TASTE BLINDNESS : Phenyl thiocarbamide (PTC)
tastes sour to 70% of population but tasteless to
other 30%,autosomal recesive disease.
OLFACTION
 OLFACTORY MUCOUS MEMBRANE
• Location: Roof of nasal cavity.
• Cells present:
– Receptor cells : 10 – 20 millions
– Supporting cells
– Basal cells : Give rise to other types of cells
– BOWMANN′S Glands : Located below the
epithelium, open to the surface & secrete
mucous
OLFACTORY MUCOUS MEMBRANE

• Animals have large olfactory mucous

membrane,eg.in German Shepherd dog
olfactory epithelium area MACROSMOTIC

• Humans are called – MICROSMOTIC

 OLFACTORY RECEPTORS
• Each receptor is
modified neuron.

• Have short thick

dendrites with
expanded ends–
OLFACTORY RODS
having cilia.

• Renewed once in few

weeks
Criteria for odoriferous substances

• Olfactory receptors respond only to

substances that are in contact with
olfactory epithelium
• These substances:
- Must be volatile
- Must be water soluble as well as lipid
soluble
 MECHANISM OF STIMULATION
Odoriferous molecules combine with cilia

Via Cyclic AMP & Phospholipase C formation

 Na+ & Ca++ Influx

Development of RECEPTOR POTENTIAL

When FIRING Level reached AP formed

Conducted to OLFACTORY CORTEX

Interpretation of smell sensation
 SMELL PATHWAY
Olfactory
receptor cell axons
↓
Pierce the
cribriform plate of
ethmoid
↓
Enters olfactory
bulb
↓
 SMELL PATHWAY CONT’D
↓
synapse with
dendrites of
mitral cells to
form olfactory
glomeruli
↓
Tufted cells &
periglomerular
cells also
participate.
• Granule cell make reciprocal synapses between
with lateral dendrites of Mitral & Tufted cells

• Mitral & Tufted cells excitatory to Granule cell

by releasing glutamate

• Granule cell in turn inhibits mitral & tufted cell

by releasing GABA
 Mitral Cell

Tufted Cell

Granule cell

Periglomerular Cell
 axons of mitral & tufted cells pass posteriorly
through olfactory stria
↓
olfactory cortex of same side
(Ant olfactory nucleus, olfactory tubercle,
prepyriform cortex, amygdala, entorhinal cortex)
Anteromedial surface of temporal lobe (Area 28)
primary olfactory area
↓
Signals reach
↓
– ORBITO FRONTAL CORTEX
– HYPOTHALAMUS
– HIPPOCAMPUS
• Thus olfactory impulses projected both to:
– NEOCORTEX ( Orbitofrontal cortex & Frontal
cortex)
• Perception & discrimination of odours.

– LIMBIC SYSTEM

a) Amygdala & Hypothalamus:

• Emotional, motivational, behavioral &
physiological effects of odours

b) Entorhinal cortex:
• Concerned with olfactory memories
 SPECIAL FEATURES

• Pathway involves only two sets of neurons.

• Only place where neurons are in direct

contact with external environment.

• Undergo degeneration periodically &

replaced by new cells
SPECIAL FEATURES CONT’D

• No relay in Thalamus
• Olfactory signals do not reach
somatosensory cortex (3,1,2) but reach
Orbitofrontal cortex
• Impulses reach Limbic system,so related
with food & sex related behabiour
 IMPORTANCE OF OLFACTION
• In food & sex motivated behaviour olfaction
plays an important role in sub human
species & minor role in humans
– Pheromones are odoriferous hormone like
substances secreted by animals to attract
mating partner.

– Sensed by VOMERONASAL ORGAN

present in animals .
• Protective role in humans
 PRIMARY SMELL SENSATIONS
According to Amoor 7 Primary smell senations:
– Camphoraceous
– Floral
– Etheral
– Pepperminty
– Musky
– Pungent
– Putrid
 ADAPTATION
• Smell sensation shows COMPLETE
ADAPTATION

• Function of both RECEPTORS & CNS

• Mediated by Ca++ ion channels

 DISORDERS
• ANOSMIA : Loss of smell sensation (eg;
head injury)
• HYPOSMIA : Diminished sensitivity
• DYSOSMIA : Disturbed smell sense
• PAROSMIA : Abnormal smell sense (eg;
tumour of olfactory cortex)
• OLFACTORY HALLUCINATIONS may
precede epileptic seizures
• KALLMAN’S SYNDROME : Hypogonadism
with loss of smell sensation
T
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