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About how many olfactory receptors do we have?
We have 10-100 million olfactory receptors in our noses.
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Where are olfactory receptors located?
- They are present in a specialized olfactory epithelium.
- Olfactory epithelium covers the inferior surface of the cribiform plate. Extends along superior nasal concha.
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What cells are present in the olfactory epithelium?
- Olfactory receptors
- Supporting cells
- Basal cells
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Function: Olfactory Receptors
- - Bipolar neurons- the first order neurons of the olfactory pathway.
- - Axon project through cribiform plate (at base of ethmoid bone) and ends at olfactory bulb.
- - Olfactory haris are cilia that project from the dendrites and are stimulated by odorants (chemicals with smell).
- - The odorant chemical produces a generator potential which initiated the olfactory response.
*Part of the olfactory epithelium*
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Function: Supporting Cells
- - Columnar epithelial cells in mucous membrane of nose.
- - Provide nutrients, electrical insulation and support to the olfactory receptors.
- - Provide a role in detoxification of chemicals.
*Part of the olfactory epithelium*
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Function: Basal Cells
- - Basal cells are stem cells- cells with a capacity to divide and differentiate.
- - Basal cells replace the olfactory receptors that have a lifespan of only about a month.
(This is unusual as mature neurons are usually not replaced)
*Part of the olfactory epithelium*
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What is the purpose of Bowman's Glands?
- Bowman's Glands = Olfactory Glands
- Provide mucous to cover the surface of the olfactory epithelium.
- Provide lubrication and helps dissolve some odorants.
- Innervated by facial nerve VII - some chemicals cause stimulation of both nasal mucous glands and lacrimal glands. (Ex: pepper)
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Physiology of Olfaction
- We can recognize about 10,000 different odors.
- Odorants activate the enzyme adenylate cyclase. This results in a change of events that produces the messenger molecule cAMP, which in turn opens sodium channels resulting in membrane depolarization.
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Olfactory Transduction
(Page 601 Figure 17.2)
- 1. Odorant molecule binds to receptor
- 2. Binding activates a G Protein that activates adenylate cyclase resulting in production of cAMP
- 3. cAMP opens sodium ion channels
- 4. Sodium ions enter resulting in a depolarization that may activate an action potential
- 5. The action potential propagates along the axon of the olfactory receptors.
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What is meant by odor threshold and adaptation?
- Olfactory sensation has a low threshold: Only a few molecules of certain substance need be present in air to be perceived as an odor. (Example: methyl mercaptan this is an odorant added to natural gas to make it smell)
- Adaptation: Decreasing sensitivity to odors occurs rapidly. Olfactory receptors adapt by about 50% in the first second or so after stimulation but adapt very slowly thereafter.
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Whats the olfactory pathway?
*Know the olfactory pathway from the olfactory receptors to higher brain centers*
- Each side of the nose bundles of the slender, unmylinated axons of olfactory receptors extend through about 20 olfactory foramina in the cribriform plate of the ethmoid bone.
- 40 or so bundles of axons collectively form the right and left olfactory nerve (I) that terminate in the brain in the olfactory bulbs.
- Axons extend along olfactory tract to the limbic system and olfactory area of the frontal lobe.
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What different tastes can be detected?
- Sour
- Sweet
- Bitter
- Salty
- Umami ("Meaty or Savory" by Japanese scientists, Stimulated by monosodium glutamate, MSG)
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