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
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
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
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