This document summarizes the chemical senses of taste and smell. It discusses the anatomy and physiology of taste receptors on the tongue and smell receptors in the nose. It describes the transduction processes, neural pathways, and theories of coding for both taste and smell. The main points covered are: - Taste and smell have specialized chemoreceptors that transform chemicals into neural impulses. - Taste receptors are located on papillae on the tongue and smell receptors are in the nasal epithelium. - Neural signals from these receptors travel to the brainstem and thalamus to be interpreted as the sensations of taste and smell. - Coding theories propose that taste is categorized by receptor location

1. Chemical senses
Presented by,
Sameena Latheef. A
II M.Sc. Psychology

2. • Olfaction and gustation are referred to as the chemical senses
• They monitor the chemical content of the environment
• Taste and smell have similar transduction processes
• Each sense has specialized receptors known as chemoreceptors
that transform chemicals in to neural impulses.
• Mainly two type of chemoreceptors ; taste receptors and
smell receptors
• Chemoreceptors for smell and taste are sensitive to stimulation
by chemicals but the chemicals are different in each case
• The chemoreceptors in mouth are sensitive to liquids and
solids
• Chemicals in gaseous form stimulate chemoreceptors in the
nose

3. • Smell: Response of the olfactory system to
airborne chemicals that are drawn by
inhalation over receptors in the nasal passages
• Taste :Response of the gustatory system to
chemicals in the solutions of the oral cavity
• Flavour: the integrated sensory impression
produced by the excitation of both smell and
taste receptors by the food molecules

4. Anatomy of taste
• The sense of taste is mediated by taste receptors cells
which are bundled in clusters called taste buds
• Taste buds are most prevalent on small pegs of
epithelium on the tongue called papillae
• Not all papillae have taste buds.
• Mainly four types of papillae; based on size and its
location
• 1.Filiform papillae:
• Most abundant of the four types .
• Cone shaped and have a hair like or thread like
appearance
• No taste buds present
• Not involved in taste perception

5. • Fungiform papillae:
• Fungus like papillae having dome shaped structures
projecting above the tongue
• Located between the filiform papillae
• Mostly seen at the tip and sides
• Have taste buds on their upper surface
• Come with around 1600 taste buds
• Innervated by the seventh cranial nerve via
submandibular ganglion ,chorda tympani and
geniculate ganglion ascending to the solitary nucleus
in the brain stem

6. • 3.Circumvalate papillae:
• Also called vallate papillae
• These are dome shaped structures
• Only 10-12 in number
• Least in number
• Contains around 250 taste buds
• Not only taste buds they contain serous minor salivary
gland von Ebner’s gland
• 4.Foliate papillae:
• Located in front of the v shaped trough of vallate papillae
• Clustered in to two groups on each side
• Innervated by the facial nerve supplying to the ant.papillae
and glossopharyngeal nerve supplying to the post. papillae
• Presence of von Ebner’s gland
• Contains around 1000 taste buds

8. • Taste buds are found in the walls of tiny
valleys btw papillae
• Each taste bud is derived from a group of
specialized epithelial cells that are connected
to the sensory neurons
• There are two type of cells in each taste bud
:the supporting cells and the receptor cells
• Supporting cells have no neural function and
are considered as degenerated receptor cells
• The receptor cells involved in taste have a very
limited life span , functioning only for a few
days

9. • Like receptor cells in the ear , taste receptors
are equipped with hair like projections called
microvilli
• They project on to the surface of the tongue
• Actual transduction process take place within
the microvilli
• Once the microvilli is stimulated they trigger
generator potentials in the receptor cells that
ultimately summate to produce a neural
impulse in the connecting neural fibres

10. • Soluble chemicals in food dissolved by saliva known as tastants ,
spread throughout the mouth ,enter the gap btw the papillae and
pass in to the taste pores
• There are two basic category of tastants :
• 1. Ions that enter the gustatory cells through ion channels in the
microvilli ( salt ( sodium ions ) and sour taste ( hydrogen ions )
• 2. Molecules that bind to gustatory receptors on the cell ( sweet
(sugars and some amino acids ) , bitter, and umami).
• These molecules act via G protein coupled receptors . The ligand-
receptor complex would cause a series of biochemical reaction
within the gustatory cell , which would lead to release of
neurotransmitters.
• The taste hairs extending to the taste pores detect these tastants
and stimulate their connected taste receptor cells to pass signals on
to sensory neurons in the tissue deep to the taste bud
• These signals are passed on to the gustatory region of the brain
where the sense of taste is interpreted

12. Neural pathways
• Three cranial nerves take part in the transmission
process that carries impulses from taste receptors in
the brain
• 1. facial nerve (C7)
• 2.glossopharyngeal nerve (C9)
• 3.Vagus nerve(C10)
• The facial nerve innervates 2/3 portion of the tongue
and glossopharyngeal nerve innervates the 1/3 portion
• The vagus nerve innervates the taste buds located in
the pharynx
• These three cranial nerves are multipurpose . Only a
portion of them is concerned with taste

14. • The three cranial nerves conducting the impulses that
eventually produce the taste sensation enter the brain
at the medulla
• There they synapse at the solitary nucleus {The nucleus
solitarius is the nucleus in the medulla that receives
afferent information from the larynx (via cranial nerve
X) and posterior pharynx and mediates the gag and
cough reflexes (cranial nerves IX and X)}
• From there , the fibres either cross the brainstem or
remain on the same side .
• Both ascend to the thalamus in the same neural tracts
that also carry sensory info.from below the neck: the
medial lemniscus
• At the thalamus, the neural fibres carrying the taste
signals synapse again in the arcuate nucleus before
continuing to the somatosensory cortex ( Area 1)

15. • The neural pathways that define the route of
taste signals from tongue to cortex follow the
same pattern as those conducting signals from
visual and auditory receptors

16. Coding for taste
• Taste has been divided in to four basic
sensations : Sweet , sour , salty and bitter
• Each is produced by a different group of
chemicals ,and each begins the same way
• The chemicals stimulate the taste buds and
change the membrane permeability of the
receptor cells , initiating a generator potential
• What properties of the chemicals or why the
membrane undergoes any change ?
• Vincent Dethier’s study on blowfly

17. • Stereochemical theory ;
• Edward Hodgson and Kenneth Roeder(
neurophysiologists)
• Used blowfly to unravel the relationships btw
various chemical stimuli and their abstractions
into the neural code for taste
• Hodgson found that the critical feature that
determines how taste receptors will react to
stimuli is the molecular shape of a particular
chemical rather than its content
• Stimulated hair cells with four different types of
alcohol and found that one alcohol (inositol)
produces a distinct neural response
• Inositol differs from others most clearly in its
molecular shape

18. • On this basis the Stereochemical theory , or
lock-and –key theory of chemical-receptor
relationships was advanced
• The idea is that the membrane surrounding
the receptor has very distinct structural slots
and can be filled only with chemicals of a
particular shape
• Only when these particular slots are filled , a
chemical reaction ensues , triggering a neural
generator potential

19. Coding at the receptor level
• Place theory of coding described the type of
coding for taste that occur at the receptor
level
• The receptors for each of the four basic taste
sensations tend to be concentrated on
different parts of the tongue
• Sweet and salt receptors are located in the
front part of the tongue
• Sour receptors on the sides and bitter
receptors at the back of the tongue

20. • Taste involves the intermingling of the four
primary sensations to produce all the taste
sensations (as in colour coding )
• Many experiments support the place code for
taste
• And according to this theory the same
receptor will give the same sensation no
matter what the stimulation is
• Also the sensitivity threshold of receptors on
the tongue varies from person to person

21. • Some people may be overly responsive and
under responsive to certain tastes and some
people can even be taste blind
• Substances like monosodium glutamate
heighten the taste sensation in people and
excess use can have side-effects

22. THE OLFACTORY SYSTEM
• The olfactory neuro epithelium is located at the upper area
of each nasal chamber adjacent to the cribriform plate,
superior nasal septum, and superior-lateral nasal wall.
• It is a specialized pseudostratified neuro epithelium
containing the primary olfactory receptors.
• As humans age, the number of olfactory neurons steadily
decrease.
• In addition to the olfactory neurons, the epithelium is
composed of supporting cells, Bowman glands and ducts
unique to the olfactory epithelium, and basal cells that
allow for the regeneration of the epithelium.
• Odorants diffuse into the mucous and are transported to
the olfactory receptor with the help of odorant-binding
proteins. Important determinants of an odors' stimulating
effectiveness include duration, volume, and velocity of a
sniff

24. The olfactory receptor cells
• Each olfactory receptor cell is a primary sensory bipolar
neuron.
• The average nasal cavity contains more than 100
million such neurons.
• They are generated throughout life by the underlying
basal cells.
• New receptor cells are generated approximately every
30-60 days.
• Each regenerating receptor cell extends its axon into
the CNS as a first-order olfactory neuron and forms
synapses with target mitral and tufted cells in the
olfactory bulb.

25. • The bipolar olfactory neurons’ peripheral process
extends to the mucosal surface to end in an
olfactory knob, which has several immobile cilia
forming a dense mat at the mucosal surface.
• The family of odor receptor proteins are G-
protein coupled receptors (GPCRs) associated
with adenylate cyclase.
• GPCRs mediate most of our physiological
responses to hormones , neurotransmitters and
environmental stimulants
• The genes that encode them were discovered in
1991 by Linda Buck and Richard Axel, culminating
in the Nobel Prize awarded in 2004.

26. • Once an odorant binds to its receptor, a signalling cascade
depolarizes the neuron, which sends the signal along its
axon, which then converges together within the bundled
axons of the olfactory nerves deep to the epithelium.
• These axons project through the cribriform plate to the
olfactory bulb.
• The olfactory bulb cells contacted by the olfactory receptor
cells include the mitral and tufted cells, arranged in
specialized areas termed glomeruli.
• The axon terminals of receptor like neurons synapse within
the same glomeruli, forming an early topographical odorant
map.
• The corresponding glomeruli of the olfactory bulbs are in
turn activated, creating a unique pattern of excitation in the
olfactory bulb for each odorant.

27. • The glomerular cells are the primary output
neurons of the olfactory bulb. Axons from these
cells travel to the olfactory cortex, which is
divided into 5 parts, including
• (1) the anterior olfactory nucleus, connecting the
two olfactory bulbs through the anterior
commissure,
• (2) the olfactory tubercle,
• (3) the pyriform cortex, which is the main
olfactory discrimination region,
• (4) the cortical nucleus of the amygdala, and
• (5) the entorhinal area, which projects to the
hippocampus.

28. • The olfactory pathway does not involve a thalamic
relay prior to its cortical projections.
• Relays from the olfactory tubercle and the pyriform
cortex project to other olfactory cortical regions and to
the medial dorsal nucleus of the thalamus and
probably involve the conscious perception of odors.
• Conversely, the cortical nucleus of the amygdala and
the entorhinal area are limbic system components and
may be involved in the affective, or pleasurable,
components of odors.
• Regional cerebral blood flow (measured with positron
emission tomography) is significantly increased in the
amygdala with introduction of a highly aversive
odorant, and it is associated with subjective ratings of
perceived aversiveness.

30. • The vomeronasal organ (VNO), or Jacobson organ, is a
bilateral membranous structure located within pits of the
anterior nasal septum, deep to the nasal respiratory
mucosa and next to the septal perichondria.
• Its opening in the nasal vestibule is visible in 91-97% of
adult humans, and it is 2 cm from the nostril at the junction
of the septal cartilage with the bony septum..
• The VNO is believed by some to detect external chemical
signals termed pheromones or vomeropherins through
neuroendocrine-type cells found within the organ.
• These signals are not detected as perceptible smells by the
olfactory system and may mediate human autonomic,
psychologic, and endocrine responses.
• Free trigeminal nerve endings, which are stimulated by
aversive or pungent stimuli (eg, ammonia), exist in the
nasal mucosa. These are processed via separate pathways
from those in the olfactory system.

31. Coding for smell
• Before a stimulus can produce a sensation of
smell , it must excite a neural impulse in the
olfactory system
• What is required to elicit an olfactory
response is a gaseous stimulus that can
change the permeability of the dendrite type
receptor cells on the surface of the epithelium
• But the way the receptors in the olfactory
system transduce airborne chemicals into
neural impulses is not so clear

32. • The properties of smell:
• With taste researchers were able to break
down sensations into four primary categories
• But the problem in smell research is that the
research has been unable to isolate the
physical properties and sensations that
accompany smell
• Why do certain airborne molecules stimulate
one sensation and the others stimulate
another stimulation was mainly explained by
two theories in the early time

33. • The infrared theory and the Raman shift theory
• Infrared theory maintained that the heat of airborne
molecules stimulates certain olfactory receptors to
produce smell.
• Raman shift theory held that ultraviolet rays are the
stimulating factor.
• But these two theories have been disproved
• When receptors are shielded from stimulation by a thin
membrane that prevents physical contact between
odorous molecules and receptors but allows heat and
ultraviolet light to penetrate ,no olfactory sensations
are produced
• This indicate that the role played by ultraviolet rays in
the olfactory process is minor

34. • The Stereochemical theory
• Proposed by Robert W. Moncrieff
• The relationship btw airborne molecules and
smell receptors is essentially chemical .
• Different smells results from different ways in
which certain chemicals affect certain receptors
• Moncrieff dealt with the fact that chemically
similar substances have diff. smell and that
chemically diff. substances have the same smell
by suggesting that the size and shape of the
chemical molecules are the key to its capacity to
stimulate diff. receptors .
• Thus chemically dissimilar molecules could be
similar in shape and size and ,for this reason ,
produce similar smell sensations

35. • Moncrieff reasoned that receptor sites could vary in
shape , so that only chemicals of a particular size or
shape could fit the membrane and cause a generator
potential- a kind of lock –and –key concept proposed
by Hodgson for taste .
• Categorizing Odors
• Few years after Moncrieff introduced his theory , an
American scientist , John Amoore , elaborated on it
extensively.
• Amoore chemically synthesised different molecules of
the same size and shape
• He demonstrated that different chemicals do smell the
same if they are identical in size and shape

36. • He examined more than 600 chemical
compounds and then classified them in to
seven primary odors
• The seven primary odors are amphoraceous,
musky, floral , pepperminty, ethereal, pungent
and putrid
• Each of these sensations according to Amoore
was produced by molecules that has a specific
shape and size.
• He was able to predict the smell sensation
simply by noting the size and shape of a
specific molecule

37. • But their exist ambiguities in Amoore’s
findings
• And it was found impossible to replicate
Amoore’s findings

38. Taste disturbances
• Hypogeusia – reduced sense of taste
• Dysgeusia – taste sensation disturbance (pleasant or
unpleasant taste)
• Phantogeusia – perception of an unpleasant taste in
the absence of a stimulus
• Ageusia – absence of taste – it is rare and, most of the
times, is the result of a central nervous system
• Glossodynia – burning sensation.

39. Smell disorders
• Hyposmia is a reduced ability to detect odors.
• Anosmia is the complete inability to detect odors.
In rare cases, someone may be born without a
sense of smell, a condition called congenital
anosmia.
• Parosmia is a change in the normal perception of
odors, such as when the smell of something
familiar is distorted, or when something that
normally smells pleasant now smells foul.
• Phantosmia is the sensation of an odor that isn’t
there .

40. References
• Levinthal, C. F. (1990). Introduction to
physiological psychology. Englewood Cliffs, NJ:
Prentice Hall.
• Schneider, A. M., & Tarshis, B. (1980). An
Introduction to physiological psychology. New
York: Random House.
• Pinel, J. P., &Mana, M. J.(1997). Biopsychology.
Boston, MA: Allyn and Bacon.