1. Chemical Senses
Taste and Smell
PREPARED BY:-
SHEIKH ATEEQ
MOHD YASEEN

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 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. 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.

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

31. 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.

32. Taste & Smell

33. 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.