physiology

1. CHEMICAL SENSES
Olfaction & Gustation
Dr. Misbah-ul-Qamar

2. Olfaction
• Poorly developed in
humans.
• Human olfaction is capable
of distinguishing between
roughly 10,000 unique
odors.
• Sense of smell can trigger
memory as the smell
analyzing region of brain is
closely connected to
amygdala & hippocampus
that handle memory &
emotion.
Dr. Misbah-ul-Qamar

3. Importance of smell
Dr. Misbah-ul-Qamar

4. To be smelled
A substance must be:
1. Sufficiently volatile (easily vaporized) for
entry in nose with inspired air
2. Sufficiently water soluble to be dissolved in
mucus
3. atleast slightly lipid soluble .
Dr. Misbah-ul-Qamar

5. OLFACTORY MEMBRANE/ MUCOSA
• Location: Located in the upper part of the nasal
cavity.
• Area: 2.4-3 sq.cm.
• Cell population: Inhabited by 3 CELL TYPES
– Olfactory receptor cells– replaced every 2 months,
decline with age; approximately 1% of these sensory
receptor cells are not replaced each year
– Supporting cells—secrete mucous
– Basal cells– precursors for new receptor cells.
• Mucous is present on top of membrane.
Dr. Misbah-ul-Qamar

6. Reception for Olfaction
• Olfactory receptor cells are bipolar neurons
derived from CNS.
• About 100 million of 1000 different types in each
individual. A given receptor can respond a
particular odor component.
• Receptor cells are interspersed by much smaller
number of sustentacular cells
• Basal cells along the basement membrane of the
olfactory epithelium regularly divide & yeild
differentiated cells that replace lost neurons.
Dr. Misbah-ul-Qamar

7. Olfactory Receptor Cell
• Apical surface of receptor cell exhibits a knob
that emits 4-25 olfactory hair or cilia.
• Cilia contain the receptors which provide
binding sites and project into the mucus.
• Axons of olfactory receptor cells collectively
form the olfactory nerve.
Dr. Misbah-ul-Qamar

8. Glands of Bowman
• Spaced among the receptor cells.
• Secrete mucous onto the epithelial surface of
olfactory membrane.
Dr. Misbah-ul-Qamar

9. Olfactory Bulb
• It is a neural structure of forebrain involved in
olfaction.
• It lies over the cribriform plate of the ethmoid
bone that separates the cranial and nasal
cavities.
Dr. Misbah-ul-Qamar

10. Olfactory Bulb
Dr. Misbah-ul-Qamar

11. Stimulation of Olfactory Cells
• Odorant molecules diffuse into the mucus
• Binding to the receptor protein that is linked
to a cytoplasmic G-protein
• Α-subunit of the G-protein separates away
Dr. Misbah-ul-Qamar

12. Stimulation of olfactory cells
Separated unit activates adenyl cyclase
Formation of Cyclic AMP
sodium channels are activated
sodium ions enter the receptor cell &
depolarize it
Dr. Misbah-ul-Qamar

13. Upon stimulation of the olfactory cells
• The depolarization of receptor cell leads to
production of action potential in the olfactory
sensory fibers.
• Membrane potential of un-stimulated
olfactory cell is -55mV with baseline activity of
of AP(once every 20 sec to 2-3 per sec)
• Depolarization brings membrane potential to -
30mV with 20-30AP/sec
Dr. Misbah-ul-Qamar

14. Cascading Effect
• A pattern to enhance the effect of a weak
odorant molecule.
• A single dissolved molecule can activate many
receptors and so on.
• This process multiplies the excitatory effect
and greatly enhances the sensitivity of the
system.
Dr. Misbah-ul-Qamar

15. Initial olfactory sensation
• The intensity of initial olfactory stimulation is
proportional to the logarithm of the stimulus
strength.
Dr. Misbah-ul-Qamar

16. Rapid Adaptation of olfactory
sensations
• The receptors adapt about 50% during the first second
and thereafter adapt very little and very slowly.
• Olfactory adaptation is a central mechanism.
• Neuronal mechanism for adaptation: centrifugal fibers
from brain backward to granule cells (inhibitory cells in
olfactory bulb).
• This feedback inhibition suppress relay of smell signals
providing adaptation
• This is not a physiological process which takes place at
the level of receptors but rather a mechanism altering
perception.
Dr. Misbah-ul-Qamar

17. Primary Olfactory Sensations
• As many as 100
• Narrowed down to 7 which are:
• Camphoraceous
• Musky
• Floral
• Peppermint
• Ethereal
• Pungent
• putrid
Dr. Misbah-ul-Qamar

18. Affective Qualities
• Smell sensation either could be pleasant or
unpleasant.
• Threshold of some odorant molecules is
extremely low( 1/25 billionth of a mg)
Dr. Misbah-ul-Qamar

19. Transmission of Signals into CNS
• The olfactory fibers( axons of receptor cells)
collect into bundles of 20 or more pass
through perforations in the cribriform plate of
ethmoid  enter the olfactory bulb.
• Olfactory bulb is a complex neural structure
containing several different layers of cells
• Each olfactory bulb is lined by small ball like
neural junctions(glomeruli)
• Fibres terminate in relation to glomeruli.
Dr. Misbah-ul-Qamar

20. Olfactory Glomerulus–
1st relay station
• This is a tangled knot of mitral and tufted cell
dendrites and olfactory nerve fibres.
• Each of the glomeruli receives synaptic input
from only one type of olfactory receptor (which
in turn responds to only one discrete component
of an odorant)
• Glomeruli sort & file various components of
odoriferous molecule before relaying signal to
higher levels.
• Mitral cells in glomeruli refine the smell signals.
Dr. Misbah-ul-Qamar

21. Olfactory Tract
• It is formed by the axons of mitral and tufted
cells.
• It leaves the olfactory bulb after receiving
signals and enter specialized regions of the
cortex.
• Both olfactory tract & bulb are an anterior
outgrowth of brain tissue from the base of the
brain
Dr. Misbah-ul-Qamar

22. Dr. Misbah-ul-Qamar

23. Unique Feature of Olfactory Tract
• Main olfactory tract does not first pass
through the thalamus before reaching cortex.
Dr. Misbah-ul-Qamar

24. Cortical Areas of Olfaction
• Medial olfactory area
• Lateral olfactory area
Dr. Misbah-ul-Qamar

25. Medial Olfactory Area
• It is represented by septal nuclei
• It exerts primitive behavioral aspects of
olfactory signals e.g: licking, salivation & other
feeding responses caused by smell of food or
by emotional drive associated with smell.
• Signals from this area project to hypothalamus
and other regions for controlling same aspects
of olfaction
Dr. Misbah-ul-Qamar

26. Lateral Olfactory Area
• This area is concerned with specific behavioral
responses related to odors i.e: learned control of food
intake
• Example: aversion to food that have caused nausea &
vomiting
• The area is composed of following regions:
Prepiriform area
Piriform area
Cortical amygdaloid region
• From here, the signals are directed to less primitive
limbic structures e.g: Hippocampus
Dr. Misbah-ul-Qamar

27. Newer Olfactory Pathway
• Signals from primary cortical olfactory area
are projected to dorsomedial thalamic nucleus
and then to orbitofrontal cortex.
• It is a phylogenetically newer pathway
• Involved in conscious perception+ analysis of
odor and also odor discrimination
Dr. Misbah-ul-Qamar

28. Olfactory pathway
Dr. Misbah-ul-Qamar

29. Main olfactory destinations
• Primary olfactory cortex piriform cortex
• Amygdala
• Entorhinal cortex
Dr. Misbah-ul-Qamar

30. Detection of pheromones by VNO
• Pheromones: non-volatile, odorless chemical
signals passed subconsciously from one individual
to another.
• VNO: vomeronasal organ located half an inch
inside nose next to the vomer bone.
• Binding of a pheromone to its receptor on surface
of neuron in VNO triggers AP that travels through
non-olfactory pathways to the limbic system,
governing emotional response.
• Messages conveyed by VNO bypass cortical
consciousness
Dr. Misbah-ul-Qamar

31. Abnormalities of Olfactory Sensation
• Anosmia total loss for all odors
• Temporary permanent
• Hyposmia reduced ability
• Hyperosmia exaggerated sensation.
Perceptual disorder.
• Phantosmia olfactory hallucination, could
be central or peripheral
Dr. Misbah-ul-Qamar

32. Gustation
Sense of
Taste
Dr. Misbah-ul-Qamar

33. Main function of sensation of taste???
• Taste is a relative crude sense that serves
primarily as gatekeeper to GIT.
• Used to separate undesirable foods from the
pleasant ones
• To avoid lethal foods
Dr. Misbah-ul-Qamar

34. Functional unit of taste:
A Taste Bud(3000-10,000 in number)
Composition of the taste bud
Receptor cells Sustentacular cells
(modified epitelial (supporting cells)
cells) about 50 in number few only
Sensory nerve fibers are intertwined among the cell bodies of
receptors
Dr. Misbah-ul-Qamar

35. Physiologic structure of taste bud
• Taste bud is a bundle of taste receptor cells
• Its supporting cells are embedded in the
covering of papillae
• Cells of taste buds undergo constant cycle of
growth , apoptosis & regeneration.
Dr. Misbah-ul-Qamar

36. Taste bud
The taste buds are ovoid
bodies with a diameter of
50-70 microns.
In adults about 10,000
taste buds are present------
the number is more in
children
In old age, many taste buds
degenerate & the
sensitivity of taste
becomes weak
Dr. Misbah-ul-Qamar

37. Taste bud are found in relation to
tongue papillae
Taste pore:
• Formed by apical surfaces
of taste cells
• Taste hair protrude from the
pore
• Surface for taste molecules
provided by taste hair/
microvillus
Location of Tongue papillae
• Fungiform : on ant. 2/3 of
tongue
• Circumvallate: forming a V-
shape, on post. 1/3 of
tongue
• Foliate: along lateral
margins of tongue
• Filiform: no taste buds
Dr. Misbah-ul-Qamar

38. Tongue Papillae
Dr. Misbah-ul-Qamar

39. Filiform papillae
• These are small & conical shaped papillae
• Situated over the dorsum of tongue
Dr. Misbah-ul-Qamar

40. Fungiform papillae
• These are round in shape
• number of taste buds in each is moderate (up
to 10)
Dr. Misbah-ul-Qamar

41. Circumvallate papillae
• These are large structures
• Each papilla contains many taste buds (up to
100)
Dr. Misbah-ul-Qamar

42. Other locations for few taste buds
• Palate
• pharynx
• Tonsils
• Epiglottis
• Proximal esophagus
Dr. Misbah-ul-Qamar

43. Primary taste sensations
• Salty
• Sweet
• Umami
• Bitter
• sour
Dr. Misbah-ul-Qamar

44. 5 Primary taste sensations
• Sour---caused by acidic substances
• Salty---cations of ionized salts.
• Sweet---
sugars,glycols,alcohols,aldehydes,organic
chemicals. The inorganic substances which
produce sweet taste are lead & beryllium.
• Umami---L-glutamate(e.g: meat extract, aging
cheese)
Dr. Misbah-ul-Qamar

45. • Bitter---alkaloids,long chain nitrogen containing
items(e.g: quinine, caffein, strychnine, nicotine)
• Many plants, fungi, & some animals produce
toxins as a natural defense mechanism.
• Most bitter tastants are detected by GPCRs.
• Quinine is bitter tasting toxin with antimalarial
properties extracted from a tree bark. It blocks
most classes of K channels & causes nonspecific
memb. depolarization
Dr. Misbah-ul-Qamar

46. Other taste like sensations
• Taste of fat constitute a sixth basic taste but
the transduction mechanisms are not fully
delineated.
• Chemical sensations that mimic hot (e.g: the
burning sensation associated with chilli
pepper) & cold (e.g: menthol) are not tastes
but rather are mediated by somatosensory
pathways located in oral cavity or nasal
passage.
Dr. Misbah-ul-Qamar

47. How difference in taste is appreciated
• Each taste bud typically responds to only one
of the five primary taste substances
except
When an item is present in very high
concentration
Dr. Misbah-ul-Qamar

48. 13 taste receptors
• Sodium receptors(2)
• Potassium receptors(2)
• Chloride receptors(1)
• Adenosine receptors(1)
• Hydrogen ion receptors(1)
• Inosine receptors(1)
• Sweet receptors(2)
• Bitter receptors(2)
• Glutamate receptor(1)
Dr. Misbah-ul-Qamar

49. Receptor potential
application of substance to be tasted
depolarization of receptor cell
(by opening ion-specific channels)
response in associated nerve fibres
Dr. Misbah-ul-Qamar

50. Afferent taste signals
• From ant.2/3 of tongue:
signals travel in branches of trigeminal
nerve joins the chorda tympani( branch of
facial nerve)
• From post.1/3 of tongue:
signals carried by fibres in glossopharyngeal
nerve
• From epiglottis + other areas
signals carried within branches of vagus
Dr. Misbah-ul-Qamar

51. Transmission of signals into CNS
• Afferent signals through axons of 1st order neurons
enter into the nucleus solitarius located in brain stem(
precisely medulla oblongata) through solitary tract.
• Neurons of tractus solitarius are 2nd order neurons.
• Axons of these run through medial leminiscus.
• Next, the third order neurons are in the posteroventral
nucleus of thalamus so axons pass rostrally to
ventromedial nucleus of thalamus.
• Then, axons from 3rd order neurons project into
parietal lobe & signals reach the cerebral cortex.
Dr. Misbah-ul-Qamar

52. Dr. Misbah-ul-Qamar

53. Final taste perception
• In ventral region of postcentral gyrus which
curls into lateral fissure of cerebral cortex.
• Taste center: center for taste sensation is in
the opercular insular cortex i.e: in lower part
of postcentral gyrus which receives cutaneous
sensations from face.
• The taste fibers do not have an independent
cortical projection.
Dr. Misbah-ul-Qamar

54. Activation of saliva secretion
• It is a taste reflex.
• Mediated by preganglionic parasympathetic
fibres to sup., inf. salivatory nuclei and then
postganglionic fibres to submandibular,
sublingual and parotid glands.
• 30 ounces of saliva every 24 hours
• Sympathetic activation causes dry mouth
Dr. Misbah-ul-Qamar

55. Abnormalities of Taste Sensations
• Ageusia: total loss of taste.
– Lesion in facial nerve, chorda tympani or mandibular
division of trigeminal nerve causes loss of taste in
anterior 2/3 of tongue
– Lesion in glossopharyngeal nerve leads to loss of taste
in post. 1/3 of tongue
– Ageusia is uncommon except in patients with Sjogren
syndrome.
– Sjogren patients suffer from an autoimmune disease
that impairs exocrine gland function, including salivary
glands. Saliva is required to carry tastants in dissolved
form through taste bud pore.
Dr. Misbah-ul-Qamar

56. Taste abnormalities
• Hypogeusia: decrease in taste sensation. It is due
to increase in threshold for different taste
sensations.
• Dysgeusia disturbance in taste sensation, in
temporal lobe syndrome
• Metallic dysgeusia ( a persistent metallic taste) is
a common side effect of many antibiotics (e.g:
tetracycline & metronidazole) & antifungals.
• Taste blindness rare genetic disorder, inability
to recognize substances by taste
Dr. Misbah-ul-Qamar