physiology lecture on the taste and smell

Physiology lecture on taste and smell

1. Taste and Olfaction
Dr Shamshad

2. •Describe primary taste sensation
•Explain the structure and mechanism of
stimulation of taste buds
•Describe taste pathway
•Discuss mechanism of stimulation of olfactory cells
•Describe olfactory pathway
•Explain role of CNS in olfaction
•Clinical correlation of taste and smell

3. Functions of Taste and smell
•Taste & smell help to separate undesirable/ lethal
foods from pleasant to eat and nutritious foods.
•Elicit physiological responses ,involved in the
digestion & utilization of foods.
•Smell allows animals to recognize the proximity
of other animals or even individual animals.
•Both senses are strongly tied to primitive
emotional and behavioral functions of our
nervous systems.

4. Some substances that initially taste sweet have a bitter aftertaste
EX: Saccharin
High intensity causes the person/ animal to reject the food.
Protective mechanism as many deadly toxins are bitter

5. Pr.
tastes
Chemical substance
SourAcid ,[H+]
Saltyionized salts ,Na+ Cations>anions
BitterOrganic substances.2 classes:
(1)long-chain organic substances with nitrogen
(2) alkaloids. Drugs in medicines, EX quinine, caffeine,
strychnine &nicotine.
SweetGp.of organic chemicals : sugars, glycols, alcohols,
aldehydes, ketones, amides, halogenated acids, &inorganic
salts of lead & beryllium, esters,aminoacids suphunic acids
Umami
Delicious or
pleasant
L-glutamate, EX: meat extracts & aging cheese,

6. THRESHOLD FOR TASTE
Taste Substance Threshold
sour taste HCL 0.0009 M
Sour NaCl 0.01 M
Sweet Sucrose 0.01 M
Bitter Quinine 0.000008 M

7. Taste Blindness
Some people are taste blind for certain substances, Ex: Different
types of thiourea compounds.
Phenylthiocarbamide used to demonstrate taste blindness.
15 - 30 % of all people exhibit taste blindness.

8. Taste buds and cells
Dia. :130 mm, L: 116 mm
50 modified epithelial cells.
Types of cells : Supporting/sustentacular and taste
Mature cells lie toward the center of the bud; these cells soon
break up and dissolve.
Life span :Around 10 days in lower mammals & unknown in
humans.
AGE: Adults: 3-10,000 buds
Children more than adult
As age advance after 45 years reduce in number.

9. Taste buds located on 3 types of tongue papillae :
(1) Large no. walls of the troughs that surround the circumvallate
papillae, as V line : on post. tongue.
(2) On the fungiform papillae, on anterior tongue.
(3) On the foliate papillae located in the folds along lateral part of
tongue.
Also on the palate, a few on the tonsillar pillars, epiglottis, & in
the proximal esophagus.

11. Structure of taste buds
Outer tips of the taste cells arranged around a minute taste pore.
From the tip several microvilli /taste hairs protrude outward into the
taste pore to approach the cavity of the mouth.
microvilli provide the receptor surface for taste.
Interwoven around the bodies of the taste cells is a branching terminal
network of taste nerve fibers ,stimulated by the taste receptor cells.
Some of these fibers invaginate into folds of the taste cell membranes.
Vesicles form beneath the cell membrane near the fibers.
contain a NT substance that is released
through the cell membrane
to excite the nerve fiber endings
in response to taste stimulation.

13. Generation of Nerve Impulses by the Taste Bud

16. Taste Reflexes Are Integrated in the Brain Stem.
From the tractus solitarius, many taste signals are transmitted
within the brain stem itself directly into the superior and inferior
salivatory nuclei,
And these areas transmit signals to the submandibular,
sublingual, and parotid glands to help control the secretion of
saliva during the ingestion and digestion of food.

17. Taste Adaptation
Rapid Adaptation of Taste
Almost completely within a minute of continuous stimulation.
Studies of taste nerve fibers, adaptation of the taste buds
themselves usually accounts for no more than about half of this
rapid taste adaptation.
Final extreme degree of adaptation that occurs in the sensation
of taste CNS, mechanisms unknown.

18. TASTE PREFERENCE AND CONTROL OF THE DIET
Taste preference : An animal will choose certain types of food in
preference to others, and the animal automatically uses this
preference to help control the diet it eats.
Its taste preferences often change in accord with the body’s
need for certain specific substances.
EX: if a person becomes sick soon after eating a particular type
of food, the person generally develops a negative taste
preference/ taste aversion for that particular food thereafter.

19. 1:Adrenalectomized, salt-depleted animals automatically select
drinking water with a high concentration of NaCl in preference to
pure water, and the amount of NaCl in the water is often
sufficient to supply the needs of the body and prevent death due
to salt depletion.
2:Animal given injections of excessive amounts of insulin
develops a depleted blood sugar level, and the animal
automatically chooses the sweetest food from among many
samples.
3:calcium-depleted parathyroidectomized animals automatically
choose drinking water with a high concentration of Cacl2.

20. Taste abnormalities
•Hypoguesia: seen in smoker , nerve lesion,
aging after 45 years
•Aguseia: In vitamin B12 and Zinc deficiency.
•Dysguseia: Drugs: Metronidazole
•Taste blindness :

21. Olfaction : The least understood of senses.
Because
1:the sense of smell is a subjective phenomenon that cannot be
studied with ease in lower animals.
2:The sense of smell is poorly developed in human beings
compared with the sense of smell in many lower animals.

22. OLFACTORY MEMBRANE: Lies in the superior part of each
nostril.
Medially: The olfactory membrane folds downward along the
surface of the superior septum.
Laterally :It folds over the superior turbinate and even over a
small portion of the upper surface of the middle turbinate.
Has a surface area of about 2.4 square centimeters.

24. Olfactory Cells :Receptor Cells for olfaction.
About 100 million of these cells in the olfactory epithelium
interspersed among sustentacular cells.
Olfactory cells are Bipolar nerve cells derived originally from the
CNS.
The mucosal end forms a knob from which 4 to 25 olfactory hairs
also called olfactory cilia, measuring 0.3 micrometer in diameter
and up to 200 micrometers in length, project into the mucus on
the inner surface of the nasal cavity.
Projecting olfactory cilia form a dense mat in the mucus,
These cilia react to odors in the air and stimulate the olfactory
cells.
Bowman glands :In the olfactory membrane are many small,
secrete mucus onto the surface of the olfactory membrane.

25. Mechanism of Excitation of the Olfactory Cells.
The odorant substance, come in contact with the olfactory
membrane surface.
First diffuses into the mucus that covers the cilia and then it
binds with receptor proteins in the membrane of each cilium.
Receptor protein , a long molecule threads its way through the
membrane about seven times, folding inward and outward.

26. The Na+ ions increase the electrical potential in the positive
direction inside the cell membrane,
Thus exciting the olfactory neuron and transmitting action
potentials into the CNS by way of the olfactory nerve.
This mechanism helps greatly multiply the excitatory effect of
even the weakest odorant.

27. Physical factors affecting degree of stimulation
•Volatile substances sniffed into the nostrils can
only be smelled.
• The stimulating substance must be at least
slightly water soluble so as to pass through
the mucus to reach the olfactory cilia.
•It is helpful for the substance to be at least
slightly lipid soluble, lipid constituents of the
cilium are a weak barrier to non–lipid-soluble
odorants.

28. Membrane Potentials & Action Potentials
The membrane potential inside unstimulated olfactory cells
about –55mv.
•At this potential, most of the cells generate continuous APs at a
very slow rate, varying from 1/ 20 seconds up to 2- 3 / second.
•Odorants cause depolarization of the olfactory cell membrane,
from −55 millivolts to −30 millivolts .
•NO. of APs increases to 20 to 30 /second,
•The olfactory receptors obey principles of transduction.

29. Rapid Adaptation of Olfactory Sensations
Olfactory receptors adapt = 50 % in the first second after stimulation.
Then adapt very little and very slowly.
Postulation: Neuronal mechanism : Large numbers of centrifugal
nerve fibers pass from the olfactory regions of the brain backward
along the olfactory tract and terminate on special inhibitory cells in
the olfactory bulb, the granule cells.
The onset of an olfactory stimulus, CNS quickly develops strong
feedback inhibition to suppress relay of the smell signals through the
olfactory bulb.

30. Classification : >100 primary sensations of smell.
1. Camphoraceous
2. Musky
3. Floral
4. Peppermint
5. Ethereal
6. Pungent
7. Putrid .
Odor blindness
Discrete odor blindness identified >50 different substances.
May be due to lack of the appropriate receptor protein in
olfactory cells for that particular substance.

31. Affective Nature of Smell
Pleasantness or unpleasantness,
A person has eaten food in past that disagreed with him/her is
often nauseated by the smell of that same food on a second
occasion.
Perfume of the right quality can be a powerful stimulant of
human emotions.
In some animals, odors are the primary excitant of sexual
drive.

32. Threshold for Smell
The minute quantity of stimulating agent in the air can elicit
sensation.
•Methylmercaptan 1/ 25 trillionth of a gram is present in each
ml of air.
•Very low threshold, can be used to detect natural gas leak by
mixing with natural gas to give the gas an odor.
Gradations of Smell Intensities
•Concentrations 10 - 50 times above the threshold evoke
maximum intensity of smell.
•Thus detects the presence or absence of odors rather than
with quantitative detection of their intensities.

36. SMELL SIGNALS TRANSMISSION
•part of the brain that originally sub served olfaction
later evolved into the basal brain structures limbic
system,
•The olfactory nerve fibers leading backward from the
bulb , olfactory tract.
•Both the tract and the bulb are an anterior outgrowth
of brain tissue from the base of the brain;
•Small nerves pass upward through cribriform plate
from the olfactory membrane in the nasal cavity to
enter the olfactory bulb in the cranial cavity.

37. Olfactory bulb
Each bulb has >1000 glomeruli, each of which is the terminus
for > 25,000 short axons from olfactory cells.
Each glomerulus also is the terminus for dendrites from about 25
large mitral cells and about 60 smaller tufted cells.
These dendrites receive synapses from the olfactory cell neurons, and
the mitral and tufted cells send axons through the olfactory tract to
transmit olfactory signals to higher levels in the CNS.

38. Centrifugal Control of Activity in the Olfactory Bulb by
the CNS.
•Many nerve fibers that originate in the olfactory portions of
the brain pass from the brain in the outward direction into the
olfactory tract to the olfactory bulb (i.e., “centrifugally” from
the brain to the periphery).
•These nerve fibers terminate on a large number of small
granule cells located among the mitral and tufted cells in the
olfactory bulb.
•The granule cells send inhibitory signals to the mitral and
tufted cells.
• It is believed that this inhibitory feedback might be a means
for sharpening one’s specific ability to distinguish one odor
from another

39. Olfactory Pathways In CNS
•The olfactory tract enters the brain at the anterior junction
between the mesencephalon and cerebrum; there, the tract
divides into two pathways,
• One passing medially into the medial olfactory area of the
brain stem. Represents a very primitive olfactory system.
•Other passing laterally into the lateral olfactory area. It is the
input to (1) a less old olfactory system and (2) a newer
system.

40. The Primitive Olfactory System
•The Medial Olfactory Area consists of a group of nuclei
located in the mid-basal portions of the brain immediately
anterior to the hypothalamus.
•Most conspicuous are the septal nuclei, which are midline
nuclei that feed into the hypothalamus and other primitive
portions of the brain’s limbic system.
•This is the brain area most concerned with basic behavior.

41. Less Old Olfactory System
The Lateral Olfactory Area: composed mainly of the prepyriform
and pyriform cortex plus the cortical portion of the amygdaloid
nuclei.
Signal pathways from this area pass into almost all portions of
the limbic system, especially the hippocampus, important for
learning to like or dislike certain foods depending on one’s
experience.
The lateral olfactory area and its many connections with the
limbic behavioral system cause a person to develop an absolute
aversion to foods that have caused nausea and vomiting.
Removal of the lateral areas abolishes the more complicated
olfactory conditioned reflexes.

42. Many signal pathways from this area also feed directly into an
older part of the cerebral cortex, paleocortex in the
anteromedial portion of the temporal lobe.
This area is the only area of the entire cerebral cortex where
sensory signals pass directly to the cortex without passing first
through the thalamus.

43. •The Newer Pathway. A newer olfactory
pathway that passes through the thalamus,
passing to the dorsomedial thalamic nucleus
and then to the lateroposterior quadrant of
the orbitofrontale cortex, has been found.
• On the basis of studies in monkeys, this
newer system probably helps in the conscious
analysis of odor

44. Clinical aspect of smell
•Hyposmia
•Parosmia
•Anosmia