Chemical sense..food and tastes

1. Chemical Senses
Taste and Smell

2. Chemoreceptors in the Human Body
• Several types of chemoreceptors are present:
• Taste buds - receptors of taste (gustatation)
• Olfactory receptors- receptors of smell (olfaction)
• Cutaneous nociceptors -Irritating chemicals on skin
• Muscle sensors - burning during heavy exercise,
acidity receptors
• Circulatory sensors - oxygen & CO2 receptors
• Digestive tract sensors - receptors for various
ingested substances

3. Taste & Smell
• We will consider only those dealing with
taste and smell
• These two senses are mediated quite
differently, but are perceived together
• Other receptors in the mouth also
participate:
– Texture
– Temperature

4. Gustation
• Gustation = taste
• Basic tastes:
– Sweetness - outer tip of tongue
– Saltiness - lateral edge of tongue behind tip
– Sourness - lateral edge of tongue behind saltiness
region
– Bitterness - back edge of tongue toward throat
– Umami - taste of glutamate - not localized
• Tastes triggers are not unique
– many chemicals can trigger each receptor type
• Loss of taste is ageusia

5. Organs of Taste
• The tongue is the major organ of gustation
• There are also some tastebuds on the palate,
pharynx, and epiglottis
• Papillae - contain taste buds
– Fungiform papillae (mushrooms) - top surface of
tongue
– Foliate papillae (ridges) - rear, lateral margin of
tongue
– Vallate papillae (pimples) - rear, medial margin of
tongue

6. Mapping Taste

7. Taste Buds
• Only 1% of the epithelial cells on tongue’s
surface are taste buds
• Contain 50 to 75 sensitive cells each
• Taste-sensitive cells have a limited lifetime,
and are constantly being replaced.
• Taste pore permits substances to enter
• Three cell types in each taste bud:
– Taste receptors – chemoreceptors
– Basal cells - source of new taste receptors
– Gustatory afferent fibers - conduct action potentials
to brain

8. Taste Cells - Taste Receptors
• Come from the division and differentiation of the
basal cells
• Not neurons in the strict sense, but have synapses
with gustatory afferent fibers
• Arrival of chemical molecules on surface leads to
change in membrane potential - receptor potential
• Most taste receptors respond to at least two of the
basic tastes
• Sometimes taste receptors generate action
potentials
• Transduction differs for each type of taste
receptor

9. Taste Transduction - Salty
• Sodium in food goes through a sodium
channel (amiloride sensitive sodium
channel) which always sits open
• Sodium influx causes membrane
depolarization (receptor potential)
• Voltage-gated calcium channel opens
• Inward calcium current causes synaptic
release onto gustatory afferent at taste
cell synapse

10. Taste Transduction - Sour
• Hydrogen ions in sour food goes
through the amiloride sensitive sodium
channel and closes an open potassium
channel
• Membrane depolarization (receptor
potential)
• Voltage-gated calcium channel opens
• Inward calcium current causes synaptic
release

11. Taste Transduction - Sweet
• The sweet molecule in food binds to G-
protein coupled receptor
• Effector protein is adenylyl cyclase
• cAMP activated
• Protein Kinase A activated
• PKA closes a potassium channel
• Membrane depolarization (receptor potential)
• Voltage-gated calcium channel opens
• Inward calcium current causes synaptic
release

12. Taste Transduction - Bitter
• Bitter molecule in food blocks potassium
channel
• Membrane depolarization (receptor
potential)
• Voltage-gated calcium channel opens
• Inward calcium current causes synaptic
release

13. Taste Transduction – Bitter II
• Bitter molecule in food binds G-protein
coupled receptor
• Effector protein is phospholipase C
• IP3 produced
• Calcium released from internal stores
• Calcium causes synaptic release
• No change in membrane potential (no
receptor potential)

14. Taste Transduction – Umami
• Glutamate in food binds to transmitter-
gated sodium channel
• Membrane depolarization (receptor
potential)
• Voltage-gated calcium channel opens
• Inward calcium current triggers synaptic
release

15. Neural Pathways for Taste
• Afferents
– Anterior 2/3 of tongue - VII (facial nerve)
– Posterior 1/3 of tongue - IX
(glossopharyngeal nerve)
– glottis, epiglottis, pharynx, palate - X
(vagus nerve)
• All afferent fibers end in gustatory
nucleus in the medulla

16. Pathways to the Brain
• Taste fibers proceed along several pathways to the
medulla oblongata or brain stem, then to the thalamus,
and finally to the taste area on the anterior cortex.
• For taste sensation, gustatory nucleus neurons send
fibers to the ventral posterior medial nucleus (VPM) of
thalamus
• VPM sends fibers to the primary gustatory cortex
(Brodmann's area 43 in ventral parietal lobe
• For autonomic functions, gustatory nucleus sends
fibers to the many brainstem regions involved in
swallowing, salivation, gagging, vomiting, digestion,
respiration and to the hypothalamus and amygdala
– appetite and food preferences

17. Neural Coding for Taste
• Labeled line hypothesis - if each receptor only
responds to a specific flavor, then each axon
would represent a particular taste
• Population coding - if each receptor responds to
many flavors, but differentially, then the
population of activity across all the fibers would
represent a particular taste
• Gustation uses a population coding scheme
• Other inputs participate in the overall perception
of taste as mentioned above

18. Common Ideas from Other Senses
• Taste perception is a result of differences
in neural stimulation
• Different perceptions can arise from the
same cells.
• There are synaptic connections between
neighboring cells, as in the case of vision
and hearing.
• The taste sense exhibits adaptation and
masking, like the other senses.

19. Olfaction - Smell
• Inside each side of the nose is an air chamber,
the nasal cavity.
• Air comes in through the nostril and flows down,
around the rear of the roof of the mouth, into the
throat.
• When you sniff, air swirls up into the top of the
cavity.
• A small patch of about 10 million specialized
olfactory (smelling) cells are in the cavity
• Loss of smell is anosmia

20. Organs of Smell
• Olfactory epithelium in roof of nasal cavity
• Olfactory receptors – chemoreceptors
• Supporting cells - secrete mucous
• Basal cells - generate new olfactory receptors
• Size of the olfactory epithelium is a measure
of keenness of smell
– humans have about 10 sq. cm
– dogs have 170 sq. cm and dogs may also have
100 times the olfactory receptor density

21. Visualizing Smell

22. Olfactory Receptor Cells
• The olfactory sensors are located on each side of
the inner nose.
• True neurons:
• Dendrite ends in knob with multiple cilia
(containing receptors) at surface of epithelium
• Odor particles in the air stick to the cilia
• Unmyelinated axon leaves base of epithelium thru
cribiform plate and ends in olfactory bulb
• Born, live, and die with a 4-8 week cycle - only
neurons regularly replaced throughout life
• The olfactory cells produce nerve signals, which
travel to the olfactory bulb

23. Mechanisms of Olfactory Transduction
• Odorant binds to G-protein coupled receptor protein
– There are 500 - 1000 different olfactory receptor proteins
coded genetically
• Effector protein is adenylyl cyclase
• cAMP produced and binds to cation channel (sodium
+ calcium)
• Calcium influx opens calcium-sensitive chloride
channel
• Calcium and chloride cause membrane
depolarization (receptor potential)
• Action potentials in olfactory receptor fibers

24. The Olfactory Bulb
• A pre-processing center that sorts the signals
before they travel along the olfactory tract to
the brain
• Axons from receptor cells project to the
olfactory bulbs
• Here the glomeruli, receive signals from
26,000 receptors
• The olfactory bulbs on either side are cross-
connected.

25. Pathways to the Brain
• Nerve fibers project from the olfactory
bulb to the olfactory tubercle
• Olfactory tubercle neurons project to
medial dorsal nucleus of thalamus
• Thalamic neurons project to
orbitofrontal cortex
• reach the olfactory areas in the
neocortex for the sensation of smell

26. Neural Coding in the Olfactory System
• Olfactory receptors respond to a variety
of odorants
• Population code is used
• Both spatial distribution and timing of
action potentials is important

27. Other Pathways
• Olfactory bulb neurons also end in other
places
• Olfactory cortex that is not part of neocortex
– old system
• Effects many brain systems
– Odor discrimination
– Odor perception
– Motivation
– Emotions
– Reproduction, feeding imprinting, memory

28. Olfactory Senses in Other Animals
• In insects, the olfactory sense is located on the
antennae.
• Snakes and lizards possess a Jacobson's organ in
the front of the mouth that is directly connected to
the olfactory center in the brain.
– The flicking tongue transfers scents to this organ for
analysis.
• Scents seem to have a strong influence on the
social interactions of many animals
• Birds have a well-developed olfactory sense, which
was not appreciated until recently.

29. Smell in a Blow-fly

30. Some Facts About Smell
• The olfactory sensations are given as fruity,
flowery, resinous, spicy, foul, and burned.
• Smell is 10,000 times as sensitive as taste
• Smell is primarily responsible food flavors.
• The two nostrils receive slightly different chemical
signatures
– allows finer discrimination of odors.
• There is strong adaptation; one soon becomes
accustomed to an odor and unaware of it
• One odor can be masked by another
– the theory of perfume.
• Need a 20% increase in concentration to cause a
perceptible increase in the strength of perception.

31. Odor & Memory
• Odors call up memory.
• Smell is the only sense with direct
access to the amygdala, the 'emotional
center' of the brain.

32. Interaction of Taste & Smell
• Information on taste is organized in the brain
separately from that of smell
• When the brain processes this information,
nerve signals from the two senses unite and
create a third, different representation.
• The latter represents flavor
the combination of taste and smell.
• Flavor is handled in a distinct region of the
brain, separate from those where smell and
taste are processed.

33. Taste & Smell

34. Poisons
• Taste and smell are not reliable guides to
poisons
– only to identification of known substances.
• Some innocuous substances taste terrible,
while some poisons taste delightful.
– Lead acetate, or sugar of lead, tastes pleasantly
sweet, but is a powerful cumulative poison.
– The aromatic compounds benzene and toluene
are fragrant, but benzene is dangerously
carcinogenic, while toluene is relatively safe.

35. Taste, Smell, & Memory
• The chemical senses are sometimes
associated with vivid mental images and
recollections
– an unexpected connection to higher mental
processes.
• Scientists investigated the way in which both
taste and smell sensations are stored in
memory.