1. The olfactory membrane lies in the superior part of each nostril and contains olfactory cells that are receptor cells for smell. These cells have cilia that project into nasal mucus and react to odors in the air. 2. When an odorant binds to receptor proteins in the cilia, it stimulates the olfactory cells. This causes sodium ions to enter the cells, generating an electrical signal that is transmitted to the olfactory cortex via the olfactory bulb and tract. 3. The olfactory system adapts rapidly to smells. It has a high sensitivity, detecting some odors in concentrations of trillionths of grams. Pathways transmit smell signals to areas involved in basic behaviors and conscious

1. Sense of smell
Dr. Sai Sailesh Kumar G
Associate Professor
Department of Physiology
NRIIMS
Email: dr.goothy@gmail.com
Phone: 7799585858

2. Introduction
Smell is the least understood sense
Why?
Smell is a subjective phenomenon
Can not be studied with ease in lower animals
Smell is poorly developed in human beings when compared with
lower animals

3. Olfactory membrane
Lies in the superior part of each nostril
Each nostril it has surface area of 2.4 square cm
Olfactory cells are the receptor cells of smell sensation

4. Olfactory cells
Bipolar nerve cells
100 million of these cells are present in olfactory epithelium
Mucosal end of the olfactory cell forms a knob from which 4-25
olfactory hairs / olfactory cilia arise
These cilia project into mucus that coats the inner surface of
nasal cavity

6. Olfactory cells
Olfactory cilia form dense mat in the mucus
It is the cilia that react to the odour in the air
Then it stimulates the olfactory cells
Small bowman glands are present between the olfactory cells
Bowman glands secrete mucus

7. Stimulation of olfactory cells
1. The portion of olfactory cells that respond to olfactory chemical
stimuli is olfactory cilia
2. Odorant substance comes in contact with olfactory membrane
surface
3. Diffuses into the mucus that covers the cilia
4. Binds with receptor proteins in the membrane of cilium

8. Stimulation of olfactory cells
Receptor protein is long molecule
It threads its way through the membrane about seven times
Odorant binds with the portion of the receptor protein that folds
to outside
Inside of folding protein is coupled to G protein
G protein is combination of 3 sub units

9. Stimulation of olfactory cells
Upon activation of receptor protein
Alpha sub unit breaks away from the G protein
Activates adenylyl cyclase
Converts ATP to CAMP
CAMP activates sodium ion channel
Opens its gate
Allows large number of sodium ions into receptor cytoplasm

10. Stimulation of olfactory cells
Electrical potential moves towards positive direction
Inside the cell membrane
Excites olfactory neuron
Transmits the action potentials to olfactory cortex

12. Special features
This mechanism multiplies the excitatory effect even the weakest odorant
Only volatile substances that can be sniffed into the nostrils can be smelled
Stimulating substance must be at least water soluble so that it can pass
through the mucus to reach the cilia
The substance should also be lipid soluble (slightly)
Lipid constituents of the cilium are weak barriers to non-lipid soluble
odorants

13. Membrane potentials
Membrane potentials inside unstimulated olfactory cells is -55mv
Most odorants reduce the potential to -30mv
Changes voltage to a positive direction
 The number of action potentials increases

14. Rapid adaptation of olfactory sensation
Adapt 50 % in the first second
There after they adapt very little and very slowly
Smell sensation adapt within a minute after entering strong
odorous atmosphere
Psychological adaptation is far greater than degree of adaptation
of receptors

15. Rapid adaptation of olfactory sensation
Large number of centrifugal nerve fibers pass from olfactory
regions of the brain backward
 Along the olfactory tract
 and terminates on special inhibitory cells in the olfactory bulb
 Feedback inhibition
Suppress the relay of smell signals

16. Primary smell sensations
Campherous
Musky
Floral
Pepperminty
Ethereal
Pungent
Putrid

17. Primary smell sensations
However, this list does not represent true primary sensations of
smell
Studies suggest the existence of at least 100 primary sensations
of smell
Some studies reported that there are at least 1000 types of
odorant receptors

18. Affective nature of smell
Smell sensation has an affective quality
Pleasantness or unpleasantness
Smell is more imp than taste, especially in the selection of food
A person who has previously eaten food and disagreed with him
Often nauseated by the smell of the same food a second time
Perfume of the right quantity can be powerful stimulation of human
emotions

19. Threshold for smell
Methyl mercaptan –can be smelled when only one 25 trillionth of a
gram is present in each ml of air
Very low threshold
This substance is mixed with the natural gas to give the gas an odour
That can be detected when even small amounts of gas leak from the
pipeline

20. Pathway of smell
Olfactory portions of the brain were the first portions developed
in primitive animals
The olfactory nerve fibers leading backward from the bulb are
called cranial nerve I or olfactory tract
Both the tract and bulb are the anterior overgrowth of the brain
tissue from the base of the brain

21. Pathway of smell
Olfactory bulb lies over the cribriform plate
Short axons from olfactory cells terminates in multiple globular
structures with in the olfactory bulb – glomeruli
Each bulb has several thousands of such glomeruli
Around 25,000 axons terminates in one glomeruli
 Mitral cells and tufted cells dendrites also terminates in glomeruli

23. Pathway of smell
These dendrites receive synapse from olfactory neurons
Mitral and tufted cells send axons through the olfactory tract
Transmit olfactory signals to higher levels in CNS
Some research has suggested that different glomeruli respond to
different odours

24. Primitive and newer olfactory pathway
Olfactory tract enters the brain at the anterior junction between the
mesencephalon and cerebrum
There the tract divides into two pathways
One passing to medial olfactory area of brain stem
Other passing to lateral olfactory area
Medial olfactory area- primitive olfactory system
Lateral olfactory area – less old and newer system

25. Medial olfactory area
Consists of group of nuclei located immediately anterior to
hypothalamus
Septal nucleus
Concerned with basic behavior

26. Lateral olfactory area
Prepyriform and pyriform cortex
Cortical portions of amgydala
From these areas signals pass to all portions of limbic system
Esoecially hippocampus
Important to learn to like or dislike certain foods
Help to develop aversion to certain foods that have caused nausea and
vomiting

27. Lateral olfactory area
Some signals also pass to paleo cortex
Anteromedial portion of temporal lobe
This is the only area in the cortes where sensory signals pass
directly to cortex with out passing through thalamus

28. Newer pathway
Newer pathway pass through thalamus
Passing through dorsomedial thalamic nucleus
Then to latero-posterior quadrant of orbitofrontal cortex
Conscious analysis of odor

29. Disorders
Anosmia – loss of ability to smell
Hyposmia – reduced ability to smell
Dysosmia or parosmia – distorted sense of smell

30. Vomeronasal organ
In addition to the olfactory mucosa, the nose contains another
sense organ, the vomeronasal organ (VNO),
 which is common in mammals but until recently was thought nonexistent in
humans
The VNO is located about half an inch inside the human nose next to the vomer
bone, hence its name.
It detects pheromones, nonvolatile chemical signals passed subconsciously
between individuals of the same species

31. THANK YOU