Olfaction is one the major sense. In the following presentation, a brief description of the olfactory system is given. In this following topics are discussed: olfactory membrane, olfactory bulb, odor pathway, anosmia, directional smelling and plasticity. By the end of it, you will be able to describe the olfactory pathway of the nervous system.

1. OLFACTO
RY
SYSTEM
USRA HASAN
A13156217001
M.SC. MEDICAL PHYSIOLOGY
3RD SEMESTER

2. CONTENTS
Introduction
Olfactory
Membrane
Olfactory
Bulb
Odor
Transduction
Odor
Pathway
Anosmia
Directional
Smelling
Plasticity References

3. INTRODUCTION
Olfactory system is the
oldest and one of the most
vital part of our brain.
It is a part of chemosensory
system, senses and
processes the smell or
odor.
The ability to smell is done
by specialized sensory cells,
called olfactory sensory
neurons.

4. OLFACTORY MEMBRANE
• The 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.
• In each nostril, the olfactory membrane has a surface area of
about 2.4 square centimeters.

6. • The Olfactory Cells are the bipolar nerve cells derived originally from
the CNS, about 100 million.
• The human olfactory epithelium contains about 50 million bipolar
olfactory sensory neurons.
• The olfactory epithelium consists of three kinds of cells: olfactory
receptors, supporting cells, and basal cells.
• New olfactory sensory neurons are generated by basal stem cells as
needed to replace those damaged by exposure to the environment.

7. • Sustentacular (supporting) cells mucosal end forms a knob from
which 4 to 25 olfactory hairs, or olfactory cilia.
• Olfactory Cilia measures 0.3 micrometer in diameter and up to 200
micrometers in length.
• They project into the mucus which coats the inner surface of the
nasal cavity, and form a dense mat in the mucus.
• The olfactory epithelium is covered by a thin layer of mucus secreted
by the supporting cells and Bowman glands, which lie beneath the
epithelium.

8. • Olfactory receptors are the first-order neurons of the olfactory
pathway.
• Each olfactory receptor is a bipolar neuron with an exposed
knob-shaped dendrite and an axon projecting through the
cribriform plate and ending in the olfactory bulb.
• The parts of the olfactory receptors that respond to inhaled
chemicals are the olfactory cilia that project from the dendrite.

9. OLFACTO
RY BULB
Olfactory bulb, structure located in
the forebrain of vertebrates that receives neural input about
odors detected by cells in the nasal cavity.
The axons of olfactory receptor (smell receptor) cells extend
directly into the highly organized olfactory bulb, where
information about odors is processed.
The olfactory bulbs are paired evaginations protruding from
the rostroventral telencephalon, receiving the afferent input
from the three sets of receptors; the olfactory sensory
neurons, septal and vomeronasal organs.

10. • Macroscopically, the bulb is a flattened, oval, grey-reddish,
mass which lies superior to the medial edge of the orbital plate
of the frontal bone near the lateral margin of the horizontal
plate of the ethmoid bone.
• It is inferior to the anterior end of the olfactory sulcus, on the
orbital surface of the frontal lobe of the cerebral hemisphere;
the gyrus rectus is directly superior to the cribriform plate of
the ethmoid bone.

11. • Out of several cell types present in it, the two crucial types of
cells in it are tufted cells and mitral cells.
• They give rise to the fibers of the olfactory
tract, granule and periglomerular cells, consisting of dendrites
which are confined within the olfactory bulb.
• The arrangement of its cell layers is into six chief layers
(superficial to deep).

12. i. The most superficial layer, contains fibers of olfactory nerves.
ii. Glomerular layer, contains the synaptic glomeruli, where terminals of
olfactory nerve fibers divide and synapse with the dendrites of mitral
cells, tufted cells and periglomerular cells.
iii. The external plexiform layer, composed of a plexus of dendrites of mitral
and tufted cells.
iv. A layer of mitral cells containing somata (or soma) of mitral cells and few
granule cell bodies, which send dendrites to glomerulus and external
plexiform layer and axons to olfactory tract.
v. The internal plexiform layer composed of the axons of the mitral, tufted
cells and granule cells.
vi. The granular layer containing clusters of granule cells, their superficial
and deep processes.

13. • The olfactory bulb receives fibers of the olfactory nerves, which
synapse with neurons within the bulb, and fibers arising from
this bulb in turn, form the olfactory tract.
• It contains several neuropeptides or neurotransmitters,
including GABA, dopamine, glutamate, aspartate, enkephalin,
luteinizing hormone releasing hormone (LHRH) and substance
P.
• These neurotransmitters play a very important role in
modulation of smell perception.

15. STIMULATION OF OLFACTORY CELLS
• When the odorant substance comes in contact with the olfactory
membrane surface, it diffuses into the mucus that covers the cilia.
• The odorant then binds with receptor proteins (outside fold only) in
the membrane of each cilium.
• Each receptor protein is actually a long molecule that threads its way
through the membrane about seven times, folding inward and
outward.
• The inside of the folding protein is coupled to a G protein, itself a
combination of three (3) subunits.

16. OLFACTORY
TRANSDUCTION
• Transduction is the conversion of stimulus energy into a graded
potential in a sensory receptor.
• Olfactory receptors react to odorant molecules in the same way that
most sensory receptors react to their specific stimuli.
• A generator potential (depolarization) develops and triggers one or
more nerve impulses. In some cases, an odorant binds to an olfactory
receptor protein in the plasma membrane of an olfactory hair.
• The olfactory receptor protein is coupled to a membrane protein
called a G protein, which in turn activates the enzyme adenylate
cyclase.

18. PHYSICAL FACTORS AFFECTING DEGREE OF
STIMULATION
Only volatile substances that can be sniffed into the nostrils, can be smelled.
Stimulating substance must be atleast slightly water soluble so that it can pass
through the mucus to reach the olfactory cilia.
Lipid constituents of the cilium are a weak barrier to non–lipid-soluble odorants,
so it is helpful for the substances to be atleast slightly lipid soluble.

19. MEMBRANE POTENTIALS AND ACTION
POTENTIALS IN OLFACTORY CELLS
• The membrane potential inside unstimulated olfactory cells, averages about
–55 millivolts, where most of the cells generate continuous action potentials
at a very slow rate, varying from once every 20 seconds up to 2 or 3 per sec.
• Most odorants cause depolarization of the olfactory cell membrane,
changing the voltage in the positive direction.
• Also, number of action potentials increases to 20 to 30 per second, which is
a high rate for the minute olfactory nerve fibers

20. • The rate of olfactory nerve impulse changes approx. in proportion to
logarithm of the stimulus strength.
• So, olfactory receptors obey principles of transduction similar to the
sensory receptors.

21. OLFACTORY PATHWAY
• Odorant molecules (chemicals) dissolve in the mucus and bind
to odorant receptors on the cilia of olfactory sensory neurons.
• The mucus provides the appropriate molecular and ionic
environment for odor detection.
• The axons of the olfactory sensory neurons (first cranial nerve)
pass through the cribriform plate of the ethmoid bone and
enter the olfactory bulbs.

22. • In the olfactory bulbs, the axons of the olfactory sensory
neurons contact the primary dendrites of the mitral cells and
tufted cells to form anatomically discrete synaptic units called
olfactory glomeruli.
• The olfactory bulbs also contain periglomerular cells, which are
inhibitory neurons connecting one glomerulus to another, and
granule cells, which have no axons and make reciprocal
synapses with the lateral dendrites of the mitral and tufted
cells.

23. • Axons of mitral cells leave the olfactory bulb and form olfactory
tract, which runs backward and ends in olfactory cortex,
through the intermediate and lateral olfactory stria.
• Olfactory cortex includes the structures, which form a part of
limbic system, anterior olfactory nucleus, prepyriform cortex,
olfactory tubercle and amygdala.

24. ANOSMIA
Blunt force trauma to the face can lead to the loss of
the olfactory nerve, and subsequently, loss of the
sense of smell. This condition is known as anosmia.
When the frontal lobe of the brain
moves relative to the ethmoid
bone, the olfactory tract axons
may be sheared apart.
Certain pharmaceuticals (e.g.
antibiotics), can cause anosmia
by killing all the olfactory
neurons at once.

25. • If no axons are in place within the olfactory nerve, then the
axons from newly formed olfactory neurons have no guide to
lead them to their connections within the olfactory bulb.
• There are temporary causes of anosmia, as well, such as those
caused by inflammatory responses related to respiratory
infections or allergies.
• Loss of the sense of smell can result in food tasting bland.

26. • A person with an impaired sense of smell may require
additional spice and seasoning levels for food to be tasted.
• It may also be related to some presentations of mild
depression.
• The ability of olfactory neurons to replace themselves
decreases with age, leading to age-related anosmia. That’s why
some elderly people salt their food more than younger people
do, which increases the risk of cardiovascular diseases in the
elderly.

27. DIRECTION
AL
SMELLING
• Directional smelling is the ability of humans to
discriminate between odorous stimuli perceived
either from the right or from the left side.
• It can only be assumed, when the olfactory
stimulants simultaneously excite the trigeminal
somatosensory system.
• The trigeminal nerve conveys information about
the location of a source of smell.
• Directional smelling exclusively mediated by
the olfactory nerve does not exist.

28. PLASTICITY
The capability for change associated with learning is termed plasticity.
Neural plasticity underlies our ability to change our behavior in response to stimuli from
the external and internal environments.
It involves changes in individual neurons— for example, synthesis of different proteins or
sprouting of new dendrites—as well as changes in the strengths of synaptic connections
among neurons.

29. • It includes mechanisms ranging from changes in membrane
excitability to changes in synaptic efficacy to neurogenesis and
apoptosis.
• The nervous system exhibits neural plasticity, however, it has
very limited regenerating powers.
• It is expressed nearly ubiquitously in the olfactory pathway,
from olfactory receptor neurons to the higher-order cortex.

30. • The olfactory pathway is heavily innervated by modulatory
systems known to regulate plasticity and memory in the
olfactory system.
• In response to learning and odor experience, olfactory circuits
display short- and long-term synaptic plasticity, non-synaptic
plasticity of membrane biophysics, morphological changes in
dendritic complexity, and experience-dependent neurogenesis
and apoptosis.

31. REFERENCES
Kobal, G., Van Toller, S., &
Hummel, T. (1989). Is there
directional
smelling?. Experientia, 45(2),
130-132.
Wilson, D. A., Best, A. R., &
Sullivan, R. M. (2004). Plasticity
in the olfactory system:
lessons for the neurobiology
of memory. The
Neuroscientist, 10(6), 513-
524.
https://www.kenhub.com/en/l
ibrary/anatomy/the-olfactory-
pathway#
Principles Of Anatomy And
Physiology 13th Ed (2010)
Guyton And Hall Textbook Of
Medical Physiology 13th Ed
(2015)

32. THANK YOU