The sense of smell is the least understood of the human senses. It is mediated by olfactory receptor cells in the nasal cavity that detect odors and transmit signals along the olfactory nerve to the olfactory bulb and various areas of the brain involved in smell perception. While humans have a poorly developed sense of smell compared to many animals, smell plays an important role in pleasure, taste perception, and danger detection. The sense of smell provides insights into sexual function and can strongly trigger memories.

1. - Dr. Chintan
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2. - Smell is the least understood of our senses.
- This results partly from the fact that the sense of smell
is a subjective phenomenon that cannot be studied
with ease in lower animals.
- Another complicating problem is that the sense of smell
is poorly developed in human beings (Microsmatic) in
comparison with the sense of smell in many lower
animals (Macrosmatic).
- Important for pleasure and for enjoying the taste of
food.
- alert us to potential dangers, e.g. smoke
*

3. - lies in the superior part of each nostril
- In each nostril, the olfactory membrane has a
surface area of about 2.4 square centimeters
- Olfactory Cells - The receptor cells for the smell
sensation - bipolar nerve cells derived originally
from the CNS
- about 100 million of these cells in the olfactory
epithelium interspersed among sustentacular
cells
*

4. - The mucosal end of the olfactory cell forms a knob
from which 4 to 25 olfactory hairs (also called
olfactory cilia), project into the mucus that coats
the inner surface of the nasal cavity
- cilia react to odors in the air and stimulate the
olfactory cells
- Spaced among the olfactory cells - Bowman’s
glands that secrete mucus onto the surface of the
olfactory membrane.
- Olfactory cells are constantly being replaced with
a half-time of a few weeks
*

7. - The portion of each olfactory cell that responds to the
olfactory chemical stimuli is the olfactory cilia.
- The odorant substance, on coming in contact with the
olfactory membrane surface, first diffuses into the
mucus that covers the cilia.
- Then it binds with receptor proteins in the membrane
of each cilium - OBP
- Receptor protein – G protein – cAMP pathway – opening
of Na channels – action potential – exciting the olfactory
neuron - olfactory nerve – CNS
*

8. - Only volatile substances that can be sniffed into
the nostrils can be smelled
- Substance must be at least slightly water soluble
so that it can pass through the mucus to reach the
olfactory cilia.
- substance to be at least slightly lipid soluble,
presumably because lipid constituents of the cilium
itself are a weak barrier to non-lipid-soluble
odorants.
- Sniffing is a semi-reflex response that usually
occurs when a new odor attracts attention.
*

9. - The olfactory receptors adapt about 50
per cent in the first second or so after
stimulation.
- Thereafter, they adapt very little and
very slowly.
- our own experience that smell sensations
adapt almost to extinction within a
minute or so after entering a strongly
odorous atmosphere.
*

10. - Because this psychological adaptation is far
greater than the degree of adaptation of the
receptors themselves
- most of the additional adaptation occurs
within the CNS
- Large numbers of 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.
*

11. - 1. Camphoraceous
- 2. Musky
- 3. Floral
- 4. Pepperminty
- 5. Ethereal
- 6. Pungent
- 7. Putrid
- 100 primary sensations of smell
*

12. - affective quality of either pleasantness or
unpleasantness.
- smell is probably even more important
than taste for the selection of food.
- a person who has previously eaten food
that disagreed with him or her is often
nauseated by the smell of that same
food on a second occasion.
*

13. - the minute quantity of stimulating agent in the
air can elicit a smell sensation.
- the substance methylmercaptan can be
smelled when only one 25 trillionth of a gram is
present in each milliliter of air.
- Because of this very low threshold, this
substance is mixed with natural gas to give the
gas an odor that can be detected when even
small amounts of gas leak from a cylinder.
*

14. - Olfactory bulb - olfactory tract – olfactory nerve – 1st
cranial nerve
- both the tract and the bulb are an anterior outgrowth
of brain tissue from the base of the brain
- olfactory bulb lies over the cribriform plate, separating
the brain cavity from the upper nasal cavity
- The cribriform plate has multiple small perforations
through which an equal number of small nerves pass
upward from the olfactory membrane in the nasal
cavity to enter the olfactory bulb in the cranial cavity
*

16. - short axons from the olfactory cells terminating in multiple
globular structures within the olfactory bulb called glomeruli
- Each glomerulus is the terminus for dendrites from about 25
large mitral cells and about 60 smaller tufted cells, the cell
bodies of which lie in the olfactory bulb superior to the
glomeruli – granule cells - Periglomerular cells
- mitral and tufted cells send axons through the olfactory
tract to transmit olfactory signals to higher levels in the CNS
- Mucus – cilia - Axons of olfactory cells – glomeruli in bulb –
dendrites of mitral, tufted cells in bulb – axons of mitral,
tufted cells in tract - CNS
*

18. - Olfactory tract divides into
- medially into the medial olfactory area (stria) of the
brain stem – very old olfactory system
- other passing laterally into the lateral olfactory area
(stria) - a newer & less old system
- The Medial Olfactory Area (very old) – septal nuclei –
hypothalamus – limbic system – removal – not much
effect
- The Less Old Lateral Olfactory Area - prepyriform and
pyriform cortex plus portion of the amygdaloid nuclei –
limbic system (hippocampus) – learning & aversion
*

19. - lateral olfactory area - anteromedial portion of the
temporal lobe (cerebral cortex)
- *This is the only area of the entire cerebral cortex where
sensory signals pass directly to the cortex without
passing first through the thalamus
- The Newer Pathway - passes through the thalamus,
passing to the dorsomedial thalamic nucleus -
orbitofrontal cortex
- conscious analysis of odor
- Granule cells, Periglomerular cells – lateral inhibition
*

22. - In rodents and various other mammals, the nasal
cavity contains another patch of olfactory mucous
membrane located along the nasal septum in a well-
developed vomeronasal organ.
- This structure is concerned with the perception of
odors that act as pheromones.
- Its receptors project to the accessory olfactory
bulb and from there primarily to areas in the
amygdala and hypothalamus that are concerned
with reproduction
- The organ is not well developed in humans.
*

23. - Evidence for the existence of pheromones in
humans ???
- close relationship between smell and sexual
function - The perfume ads
- The sense of smell is said to be more acute in
women than in men, and in women it is most
acute at the time of ovulation.
- Smell and, to a lesser extent, taste have a
unique ability to trigger long-term memories
*

24. - Pain Fibers in the Nose - Naked endings of many trigeminal
pain fibers are found in the olfactory mucous membrane -
stimulated by irritating substances
- peppermint, menthol and chlorine - sneezing, lacrimation,
respiratory inhibition
- anosmia (absence of the sense of smell), hyposmia
(diminished olfactory sensitivity), and dysosmia (distorted
sense of smell).
- Olfactory thresholds increase with advancing age, and
more than 75% of humans over the age of 80 have an
impaired ability to identify smells.
- Anosmia + Hypogonadism (Kallmann's syndrome)
*

25. *