The document summarizes the anatomy and physiology of the olfactory system. It describes the main parts including the olfactory epithelium containing olfactory receptor cells that detect odors, as well as supporting and basal cells. It explains how odors bind to receptors and trigger signals to the olfactory bulb and various parts of the brain involved in smell perception and response, like the limbic system and orbitofrontal cortex. The olfactory system is divided into very old, less old, and newer parts associated with more primitive and learned odor responses.

1. O F
G Y
L O
S IO
Y L
H L
P E
S M
KASHMEERA.N.A
II SEM MSc ZOOLOGY
ROLL NO:37

2. Important for enjoyment &
selection of food .
Flavours are combinations of
taste and smell (smell contribution
is about 80 %)
Gives warning of harmful
substances or places

3. Primary Sensations of Smell
Based on psychological studies, one attempt to classify
these sensations is the following:
• 1. Camphoraceous
• 2. Musky
• 3. Floral
• 4. Pepperminty
• 5. Ethereal
• 6. Pungent
• 7. Putrid
In recent years, specific studies of the genes that encode
for the receptor proteins, suggest the existence of at least
100 primary sensations of smell

4. Anatomy of Olfactory
Receptors

5. • The nose contains 10–100 million olfactory receptors
contained within an area called the olfactory epithelium.
• olfactory epithelium lies in the superior part of each
nostril.
• In each nostril, the olfactory membrane has a surface
area of about 2.4 square centimeters.
• The olfactory epithelium consists of three kinds of cells:
 olfactory receptors,
 supporting cells / sustentacular cells
 basal cells

6. Olfactory receptors
• The receptor cells for the smell sensation are the olfactory cells .
• They are actually bipolar nerve cells derived from the CNS .
• There are about 100 million of these cells in the olfactory epithelium.
• The mucosal end of the olfactory cell forms a knob .
• From knob 4 to 25 olfactory hairs (olfactory cilia), project into the
mucus that coats the inner surface of the nasal cavity.
• These projecting olfactory cilia form a dense mat in the mucus.
• These cilia react to odours in the air and stimulate the olfactory
cells

8. Supporting cells/sustentacular cells
The receptor cells in the olfactory epithelium are interspersed among
sustentacular cells or supporting cells.
Supporting cells are columnar epithelial cells.
 They provide physical support, nourishment and electrical insulation for
the olfactory receptors,
They help detoxify chemicals that come in contact with the olfactory
epithelium.

9. Basal cells
• Basal cells are stem cells located between
the bases of the supporting cells.
• They continually undergo cell division to
produce new olfactory receptors, which live for
only a month or so before being replaced.
• This process is remarkable - olfactory
receptors are neurons, and mature neurons
are generally not replaced.
• The olfactory renewal process is carefully
regulated - a bone morphogenic protein
(BMP) exerts an inhibitory effect.
• [ BMPs are a large family of growth factors
originally described as promoters of bone
growth]

10. Spaced among the olfactory cells in the olfactory membrane are many small
Bowman’s glands that secrete mucus onto the surface of the olfactory membrane
mucus is carried to the surface of the epithelium by ducts.
The secretion moistens the surface of the olfactory epithelium and dissolves
odourants so that transduction can occur.

11. Physiology of Olfaction

12. MECHANISM OF EXCITATION OF OLFACTORY CELLS.
Cilium is the portion which respond to the olfactory chemical stimuli.
The odourant substance on coming in contact with olfactory surface
first diffuse in to the mucus which covers the cilia.
 Then binds with a receptor protein that protrudes through the ciliary
membrane.
This receptor is a long molecule, it threads its way through the
membrane 7 times, folding inward and outward.
Odourant binds with portion of receptor and coupled to G-PROTEIN.
G-PROTEIN –a combination of 3 subunits.

13. Odourant receptor
heptahelical receptor/
serpentine receptor/
G protein-linked receptor (GPLR)

15. On excitation of receptor,an alpha subunit breaks away
from G-PROTEIN and activates adenylcyclase.
Activated cyclase converts many molecules of intracellular
adenosine-tri-phosphate into cyclic-adenosine
monophosphate(cAMP).
This cAMP activates another near by membrane protein,a
gated sodium ion channel.
Allows large number of sodium ions to pour into receptor
cell cytoplasm.
Sodium ions helps in exciting the olfactory neuron and
transmitting action potential in to the CNS through an
olfactory nerve.

17. Mechanism of olfactory cell stimulation
Odourant + receptor protein
↓
Activation of G protein
↓
Activation of adenylate
cyclase
↓
ATP → cAMP
↓
Opening of Na+ channels
↓
Na+ influx
↓

18. Physical factors affect the degree of
stimulation.
 Only volatile substances that can be sniffed into
the nostrils can be smelled.
 The stimulating substance must be at least
slightly water soluble so that it can pass through
the mucus to reach the olfactory cilia.
 The substance should be at least slightly lipid
soluble, because lipid constituents of the cilium
soluble
itself are a weak barrier to non-lipid-soluble
odourants.

20. Cribiform plate separates nasal cavity and cranial cavity.
Olfactory bulb lies above cribiform plate.
Small nerves from olfactory membrane in nasalcavity pass through the small
perforations in the cribiform plate to enter olfactory bulb in the cranial cavity.
Olfactory nerve fibers leading from olf.bulb are called Cranial nerve I or olf.
Tract.

21. Short axons from the olfactory cells terminate in multiple globular structures
within the olfactory bulb called glomeruli.
Each bulb has several thousand such glomeruli,each of which is the terminus for
about 25,000 axons from olfactory cells.
Each glomerulus also 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.
These dendrites receive synapses from the olfactory cell neurons,
 the mitral and tufted cells send axons through the olfactory tract to transmit
olfactory signals to higher levels in the central nervous system.
Some research has suggested that different glomeruli respond to different
odours.

23. Olfactory pathways into the
Central Nervous System

24. 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, and the other passing laterally into the lateral
olfactory area.
The medial olfactory area represents a very old olfactory system, whereas the
lateral olfactory area is the input to
(1) A less old olfactory system and (2) a newer system

25. Olfactory pathway
Olfactory receptor cell Olfactory nerve
Olfactory bulb
Olfactory Tract
Medial Olfactory area Lateral Olfactory area
Prepyriform cortex Thalamus
Septal Nuclei Pyriform Cortex
Amygdala
Hypothalamus
Limbic system Limbic system Orbitofrontal
(primitive parts) (hippocampus) Cortex
(Very Old (Less Old Olfactory (Newer System)
Olfactory System) System) 25

26. LIMBIC SYSTEM

27. Very Old Olfactory System
More primitive responses to olfaction
salivation, liking lips and primitive emotional drives
to smell
Less Old Olfactory System
Learned control of food intake
Aversion to food that have caused nausea and
vomiting.
Newer System
Conscious perception & analysis of odour
Odour discrimination

28. Thank You
Take time to smell the flowers….