For SSC-II & HSSC-II

1. SENSE OF SMELL
EXPERT VISION ACADEMY

2. Smell

3. Chemical Senses: Taste and Smell
 Both senses use chemoreceptors
 Stimulated by chemicals
 Taste has four types of receptors
 Smell can differentiate a large range of chemicals
 Both senses complement each other and
respond to many of the same stimuli
 Olfaction (smell) and gustation (taste)
 Both project to cerebral cortex & limbic system
 Evokes strong emotional reactions

4. Olfactory organs
•Contain olfactory receptors and supporting epithelial cells
•Cover parts of nasal cavity and a portion of the nasal
septum
Olfactory receptors
•Chemoreceptors
•Respond to chemicals dissolved in liquids
Olfactory Epithelium
•Located on roof of nasal cavity
•Contain olfactory receptor cells with columnar supporting
cells
•Covered by mucous to trap airborne molecules
Sense of Smell

5. Olfactory Epithelium

6. 6
Cells of the Olfactory Membrane
 Olfactory receptors
 bipolar neurons with cilia
or olfactory hairs
 Supporting cells
 columnar epithelium
 Basal cells = stem cells
 replace receptors
monthly
 Olfactory glands
 produce mucus

7. Sense of Smell
 Air entering the nasal cavity must make a hairpin turn to
stimulate the olfactory receptors before entering the
respiratory passageway below
 Sniffing
 also brings the air superiorly across the olfactory
epithelium
 intensifies the smell

8. Olfactory tract
Olfactory bulb
(a)
Nasal
conchae
Route of
inhaled air
Olfactory
epithelium

9. Mitral cell (output cell)
Olfactory
gland
Olfactory
tract
Olfactory
epithelium
Filaments of olfactory nerve
Cribriform plate of ethmoid bone
Lamina propria connective tissue
Basal cell
Supporting cell
Dendrite
Olfactory cilia
Olfactory bulb
Glomeruli
Axon
Olfactory receptorcell
Mucus
Route of inhaled air
containing odormolecules(b)

10. Olfactory Cells
 Dendrites of each olfactory cell
 called olfactory cilia
 extend into the nasal cavity
 Olfactory axons
 project upward through the foramina in the cribriform
plate of the ethmoid bone of the skull
 synapse on neurons within the olfactory nerve

11. Physiology of Smell
 Series of events
 the chemical must be volatile
 it must be in the gaseous state as it enters the nasal cavity

the chemical must be water soluble
 so that it can dissolve in the fluid containing the olfactory epithelium
 the dissolved chemicals stimulate the olfactory receptors by
binding to protein receptors in olfactory cilium membranes
 the generation of APs in the olfactory cells
 an impulse travels through the olfactory cell axons to the
olfactory nerve where the smell sensation is transmitted to the
brain

12. OLFACTORY PATHWAY
 As air enters the nasal passageway the level of odor-
producing chemicals dissolve in the mucus membrane
 With in the mucus membrane the olfactory cilia capture the
chemicals
 After reaching a threshold level it is passed to the olfactory
nerves located in the olfactory bulb
 The impulse passes through the olfactory tract and into the
thalamic and olfactory centers of the brain for interpretation,
integration, and memory storage.
 The sense of smell can create powerful and long-lasting
memories. That often persist from early childhood to death

13. Olfactory Nerve Pathways
 Olfactory receptor fibers synapse with neurons in the
olfactory bulbs (cranial nerve I).
 Impulses travel along the olfactory tracts
 Impulses are interpreted in olfactory cortex.
 Olfactory receptor neurons are in direct contact with the
environment and can be replaced if damaged.
 Olfactory receptors undergo sensory adaptation rapidly
 Sense of smell drops by 50% within a second after
stimulation

14. Transmission of smell signals to CNS

15. Abnormalities
 Anosmia – absence of sense of smell
 Hyposmia – diminished olfactory sensitivity
 Dysosmia – distorted sense of smell
 More than 75% of humans over the age of 80
have an impaired ability to identify smells

16. Anosmia
Absence of the sense of smell
 Trauma
 Colds or allergies producing excessive mucus
 Polyps causing blockage
 1/3 are from zinc deficiency