This document discusses the anatomy and physiology of the human olfactory system. It describes: 1. The main structures involved in smell including the olfactory epithelium, olfactory bulb, and olfactory cortex. 2. The cell types within the olfactory epithelium including bipolar receptor neurons, supporting cells, and basal stem cells. 3. How odorant molecules bind to receptors on cilia and trigger signal transduction pathways to activate olfactory neurons. 4. The pathways from the olfactory bulb to various parts of the limbic system involved in emotional and memory processing of smells.

1. Lakhan.M.S

2.  1.Respiration
 2.Air-conditioning of inspired air.
 3.Protection of lower airway.
 4.Vocal resonance.
 5.Nasal reflex functions
 6.Olfaction.

3.  Highly devoloped in Rabbits & Dogs
Macrosmatic.
 Humans- not highly devoloped-
Microsmatic

4. Olfaction or Olfactory perception
- the sense of smell mediated by a group
of specialised sensory cells in nasal cavity.
Odour
-the property of a substance which gives it
a particular smell.

5. Olfactory neuroepithelium- well developed
11 weeks of gestation
complete differentiation of olfactory cells.
 Human fetus has well developed
Vomeronasal organ on each side of nasal
septum-regresses in late fetal life

6. Olfactory Mucus Membrane -Nervous
System closest to external environment
 From Supporting cells,Bowman`s gland in
lamina propria & respiratory mucosa.
 Moist & protective enviroment.
 Disperses odourants to olfactory receptors.

7. From mucus
Odourant chemicals
Diffuse Transported by
Odourant Binding Proteins
to receptor.

8. 1.Main Olfactory System (Olfactory Nerve)
2.Trigeminal Nerve
3.Vomeronasal System
4.Cranial Nerve 0

9. Olfactory Nerve
Common odour sensations
Trigeminal Nerve
-Chemical & Nonchemical Stimuli
Somatosensory sensations
- Reflexive responses
Mucus Secretion
Halting of inhalation

10. Vomeronasal system
Non functional in humans
Rudimentary vomeronasal tube
 Cranial Nerve 0
Loose plexus of ganglionated nerves
Close proximity to Vomeronasal Organ.
Not connected to olfactory bulb
Vestige of an ancient nerve- function lost.

11.  Only 10-15% air reaches the olfactory
epithelium
Polyps in superior meatus,excessive turbinate removal can affect olfaction

12.  Bipolar sensory receptor neurons
 Supporting/Subtentacular cells
 Duct cells of Bowman’s gland.
 Microvillar cell.
 Basal cells

13.  Bipolar
 Club shaped peripheral knob that bears cilia.
 Extends odourant receptor-containing cilia
into mucus.
 Olfactory nerve 15 -20 foramina in
cribriform plate synapse in olfactory blub.
 6 millions olfactory axons bilaterally

14.  Insulates Bipolar cells
 Mucus prodution.
Duct cells of Bowman’s gland
Mucus production.

15.  Located at surface of epithelium
 Sends tufts of microvilli into nasal mucus.
Basal cells
-Sit along lamina propria,
-serve as stem cell population that can
differentiate to replace olfactory receptors
lost in turnover.

16. Olfactory Receptor Axons
coalesce
30-50 fascicles( Olf.Filaments)
cribriform plate & pia matter
Synapse with Second order Neuron
in glomeruli of Olfactory Bulb.
Glutamate-Main Neurotransmitter
Dopamine- modulation of Olfactory nerve input.
Chemical energy of receptor binding Neural Signal
Signal transduction ,by activation of G protein

17. Olfactory Bulb- Complex structures on ventral
surface of frontal lobe directly over cribriform plate

18. Olfactory bulb
Second order Neurons
Mitral cells Tufted cells
 Axons of Mitral cells form Olfactory tract

20.  Anterior Olfactory Nucleus-Pyriform Cortex-
Olfactory tubercle-Entorhinal area-Amygdaloid Cortex-
Corticomedial nuclear group of amygdala.

21.  Anterior Olfactory Nucleus
Coordination of inputs from contalateral
olfactory cortex
Transfer of Olfactory memories from one side to
other
 Pyriform Cortex
Olfactory discrimination
 Amygdala
Emotional response to olfactory stimuli
 Entorhinal Cortex
Olfactory Memories

22. Olfactory tract enters brain
2 pathways
Medial Lateral
Olfactory stria Olfactory stria
.

23. Axons of tufted cells- Medial olfactory stria -
cross midline-synapse with opposite olfactory
bulb
Axons of Mitral cells- Lateral olfactory stria -run
to olfactory cortex of same side.
Other fibres- Intermediate Olfactory Stria -
olfactory tubercle.

24.  Consists groups of nuclei located anterior
to hypothalamus.
 Septal nuclei - hypothalamus & other
primitive portions of brain’s limbic system.

25.  Composed of
prepyriform
pyriform cortex
cortical portion of amygdaloid nuclei.
 signal pathways- almost all portions of
limbic system , especially hippocampus-
important for learning like or dislike for
food stuffs.

26. 26
Olfactory Tract
Medial Olfactory area Lateral Olfactory area
Septal Nuclei
Hypothalamus
Limbic system
(primitive parts)
Prepyriform cortex
Pyriform Cortex
Amygdala
Limbic system
(hippocampus)
Thalamus
Orbitofrontal
Cortex
Olfactory receptor cell Olfactory nerve
Olfactory bulb
(Very Old
Olfactory System)
(Less Old Olfactory
System)
(Newer System)

27.  Very Old Olfactory System
More primitive responses to olfaction
salivation, primitive emotional drives to smell
 Less Old Olfactory System
Learned control of food intake
Aversion to food that have caused nausea & vomiting
 Newer System
Odour discrimintation & analysis of odour

28. Swallowing and Deglutition
Retronasal Airflow
Production of flavour from swallowed food
Adds Smell to Taste

29. Olfactory Fatigue Adaptation to Smell of
Odoriferous Substance
Odour which was first appreciable
gradually becomes inappreciable
Mediated by Calcium ions
Degree of adaptation measured by rise in
threshold concentration to excite the
sense of smell.

30.  Anosmia
 Hyperosmia
 Cacosmia
 Hyposmia
 Phantosmia
 Olfactory Agnosia
 Heterosmia
 Presbyosmia
 Osmophobia

31.  Molecular structure – Moncrieff 1967
 Electrochemical Reactions
 Stereospatial patterns
 Molecular Properties
 Olfactory mucus morphology

32.  Odour mol + Receptor  Photochemical reaction 
SIGNAL TRANSDUCTION
 Briggs and Duncan, 1962
 Receptors containing carotenoids

33.  Mozell, 1970
 Certain receptors could have a
stereospatial, lock and key form, and
receptor cells fire when the surface
membrane is altered.

34. 
LOCK KEY
Confirmational change
in receptor+mol
complex
SIGNAL
TRANSDUCTION

35.  Laffort, Patte and Etcmeto,1974
 Molecular properties depends on
-molecular volume at boiling point
-proton affinity and donation,
-local polarization within the molecule..

36.  Holley and Doving,1977
Nature of smell - Pattern of stimulus within
mucosal configuration of receptor cells
This theory is based on
specific receptor sites &
on their position within olfactory mucosa

37.  Vibration Theory
 Olfactory Pigment Theory
 Enzyme Theory
 Penetrating and Puncturing theory

38.  Randerbrock 1968
Olfactory perceptors are peptide chains vibrating
in an alpha helix.
Odourant molecules forms a band with peptide
chain modulating the vibration-transmitted to
nerves.
Pigment Theory
Rosenberg 1968
odourant molecule + olfactory pigment- increased
electrical conductivity

39.  Davies
 Odourant molecules penetrate membrane
of olfactory receptor cell- diffuse-leaving a
hole.
 Leakage of Sodium & pottasium occurs-
nerve impulse