PSYC2274 Chapter Notes - Chapter 14: Olfactory Receptor Neuron, Anosmia, Olfactory Bulb
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What type of environment does olfaction provide information for?
- Olfactory senses evolved to provide information for the immediate, local environment.
They are least informative about the larger environment.
What is olfaction linked to?
- Olfaction is highly primitive and linked to emotions and memory. You remember certain
smells as either good or bad due to your prior experience with them.
What are odorants?
- These are airborne molecules that produce an olfactory sensation. They must be able to
dissolve in the air, they must be small, and they must by hydrorepellent. Anything that
dissolves in the air but does not produce a smell can't be classified as an odorant, such as
water vapor.
What are organic compounds?
- These are molecules with carbon backbones, and essentially make up every odorant.
Are most odorants simple or complex?
- Some familiar smells are produced by one odorant, such as banana, while others are more
complex combinations of odorants, like perfume or wine. Most things we smell are the
result of complex combinations.
Most animals are macrosomatic. What does this mean?
- This means that they rely on their olfactory sensations for survival. They use it to find
food, mates, shelter, etc., and they would not survive without their sense of smell.
Humans are microsomatic. What does this mean?
- This means that our olfactory sense is not crucial for survival. There also appears to be an
evolutionary trade-off between the brain area devoted to vision and olfaction. More of
our brain area is devoted to vision, whereas in most animals, it is devoted more to smell.
What does threshold have to do with an odorant?
- The higher the threshold, the less sensitive you are to an odorant. It does not smell as
strong to you.
Compare detection threshold to recognition threshold:
- The detection threshold is much lower than the recognition threshold. You can detect an
odorant without being able to label it. This is shown by the tip-of-the-nose phenomenon.
Odors can be much more easily discriminated than identified, and identification is
enhanced by verbal cuing. For instance, if you can not place a certain smell, someone else
might label it, and then you will have an “aHA!” moment.
What is the olfactory epithelium?
- This is where smell begins. These are 2 dime-sized patches (one on each side) about 3
inches up your nose. It contains 3 cell types: basal cells (precursor to the next to),
supporting cells, and OSNs (olfactory sensory neurons). OSNs are fully replaced roughly
every 28 days by the basal cells.
Tell me more about the OSNs:
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- The OSNs end in cilia, which are embedded in mucosa. The odorant molecules that are
dissolved in the mucosa bind with the receptor proteins on the cilia, which causes
depolarization. Each OSN produces a single type of receptor protein. The axons of the
OSNs pass through the cribriform plate (part of the skull) and synapse on mitral cells in
the olfactory bulb. The synapses then occur in the glomeruli.
- The connections between OSNs and mitral cells are highly specific. Each mitral cell
receives input from only a few OSN types.
What is a recognition profile?
- This is how each odorant produces a unique pattern of activity across a receptor.
Molecules with similar structures will often produce similar profiles, but not always!
They are very prone to adaptation effects, and they fade quickly!
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What is olfaction?
- Olfaction is the sense of smell, versus gustation, which is the sense of taste. The two are
usually combined and can easily be mixed up and confused. Both are critically involved
in our experience of food.
What is orthonasal olfaction?
- This is when we sniff in and perceive odors through our nostrils. It also occurs when we
smell odors that are outside of us. The odorant travels up our nose and onto the olfactory
epithelium.
What is retronasal olfaction?
- This is when we perceive odors through our mouth while breathing or chewing. It also
occurs when we smell odors that are inside of us. It is the odorants that we are smelling in
our mouth and that give us the experience of flavor. The odorant travels through our
mouth and up into the upper nasal cavity until finally reaching the olfactory epithelium.
What are odors, and what are their stimuli?
- Odors are olfactory sensations, and odorants are their stimuli. Odorants are chemical
compounds, but not all chemical compounds are odorants. In order to be smelled, a
molecule must be volatile (be able to float through the air), small, and hydrophobic
(repellent to water). However, some compounds that meet these requirements are still not
odorants, like water vapor, as they do not produce smells. Evolutionarily, we had no
reason to detect these substances, as they posed no threat to us in the concentrations
found in nature.
What is the primary function of the nose?
- To warm, filter, and humidify the air we breathe.
What is the primary function of the olfactory epithelium?
- To detect odorants in the air. There are 3 cell types: basal cells, supporting cells, and
OSNs. They are located on both sides of the upper portion of the nasal cavity.
- The olfactory epithelium is the “retina of the nose.”
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Document Summary
Olfactory senses evolved to provide information for the immediate, local environment. They are least informative about the larger environment. Olfaction is highly primitive and linked to emotions and memory. You remember certain smells as either good or bad due to your prior experience with them. These are airborne molecules that produce an olfactory sensation. They must be able to dissolve in the air, they must be small, and they must by hydrorepellent. Anything that dissolves in the air but does not produce a smell can"t be classified as an odorant, such as water vapor. These are molecules with carbon backbones, and essentially make up every odorant. Some familiar smells are produced by one odorant, such as banana, while others are more complex combinations of odorants, like perfume or wine. Most things we smell are the result of complex combinations. This means that they rely on their olfactory sensations for survival.
