1. THE CHEMICAL SENSES

2. THE CHEMICAL SENSES
Animals depend on the chemical senses to identify nourishment
Chemical sensation
Oldest and most common sensory system with the aim to detect
environmental chemicals
Chemical senses
Gustation & Olfaction (separate but processed in parallel)
Chemoreceptors

3. TASTE
The Basics Tastes
Saltiness, sourness, sweetness, bitterness, and umami.
Innate preferences and rejections for particular tastes (sweet and
bitter) have a survival reasons
Usually there is correspondence between chemical ingredients and
taste:
Sweet—sugars like fructose, sucrose, artificial sweeteners
(saccharin and aspartame)
Bitter—ions like K+ and Mg2+, quinine, and caffeine
Salty—salts
Sour—acids
How to distinguish the countless unique flavors of a food
1) Each food activates a different combination of taste receptors
2) Distinctive smell (it combines with taste to give the flavor)
3) Other sensory modalities (texture and temperature)

4. TASTE
The Organs of Taste
Tongue, mouth, palate, pharynx, and epiglottis
Nasal cavity for smell

5. TASTE
Areas of sensitivity on the tongue (but most of the tongue is sensitive to all basics
tastes)
Tip of the tongue: Sweetness
Back of the tongue : Bitterness
Sides of tongues: Saltiness and sourness
Papillae (taste receptors)
Foliate
Vallate
Fungiform
At threshold concentration
(just enough exposure of
single papilla to detect taste)
they respond to only one taste.
More concentrations lead to
less selectivity

6. TASTE
Tastes Receptor Cells
Apical end is the chemically sensitive part. It has small extensions called microvilli
that project into the taste pore.
Receptor potential: Voltage shift – depolarization of the membrane cause CA++
entering the cell and release of transmitter

7. TASTE
Transduction: process by an environmental stimulus cause an electrical response
in a sensory receptor.
In the case of taste, chemical stimuli (tastants) may:
1)Pass directly through ion channels
2)Bind to and block ion channels
3)Bind to G-protein-coupled receptors
Slightly different mechanisms for saltiness, sourness, bitterness, sweetness and
umami (amino acids)

8. TASTE
Saltiness
Special Na+ selective channel.
The ion pass directly through channel
causing deporalization
Sourness
Sourness- acidity – low pH
H + binds to and block ion channels
causing deporalization

9. TASTE
Bitterness
Bitter substances are detected by different types
T1R and T2R receptor. They work as G-protein
coupled receptors
Sweetness
It also detected by receptors T1R2+T1R that
have the same signaling mechanism (cf. bitter
taste)
The expressed in different taste cells allow the
system not to be confused about the taste
Umami
Umami receptors T1R1+T1R3 detect amino
acids

10. TASTE
Bitterness Sweetness Umami

11. TASTE
VII Facial nerve
IX Glossopharyngeal nerve
X Vagus nerve
They carry primary gustatory
axons
Gustatory nucleus
Point where taste axons bundle and
synapse
Ventral posterior medial nucleus
(VPM)
Deals with sensory information from
the head
Primary gustatory cortex (Insula)
Receives axons from VPM taste
neurons
Lesion in VPM and Gustatory cortex
can cause ageusia- the loss of taste
perception

12. SMELL
Smell is not only important for taste but also for social communication
Pheromones are important signals
• Reproductive behavior
• Territorial boundaries
• Identification
• Aggression

13. SMELL
The Organs of Smell
1)Olfactory epithelium: contains olfactory receptor cells, supporting cells (produce mucus),
and basal cells (source of new receptor cells)
2)Olfactory axons constitute olfactory nerve
3)Cribriform plate: A thin sheet of bone through which small clusters of axons penetrate,
coursing to the olfactory bulb
Anosmia: Inability to smell

14. SMELL
Olfactory Transduction
Receptor potential: if strong enough generates APs in the cell body and
spikes will propagate along the axon

15. SMELL
Adaptation: decreased response despite continuous stimulus. Common features of sensory
receptors across modalities
Each receptor cell express a single
olfactory receptor protein.
They responds to different odours
but with preferences.
Many different cells are scattered
into the epithelium

16. SMELL
Central Olfactory Pathways
Mapping of receptor cell into glomeruli is extremely precise

17. SMELL
Axons of the olfactory tract branch and enter the forebrain (unconscious perception)
bypassing the thalamus
Neocortex (conscious perception) is reached by a pathway that synapses in the medial
dorsal nucleus of the thalamus