PSYC2274 Chapter Notes - Chapter 14: Odor Detection Threshold, Olfactory Receptor Neuron, Nostril
1
- Olfaction is the sense of smell, while gustation is the sense of taste.
- Our sense of smell is critically involved in our personal experience of food, as we will
see more of later.
- There are two particular ways in which we can perceive odors.
- Orthonasal olfaction is sniffing in and perceiving odors through our nostrils. It
occurs when we smell something outside of us. The molecules travel through our
nostrils and up our nose onto the olfactory epithelium.
- Retronasal olfaction is perceiving odors through your mouth while breathing and
chewing and occurs when we are smelling something that is inside our mouth and
is what gives us the experience of flavor. Molecules travel through our mouth and
up the back of our mouths into our upper nasal cavity and onto the olfactory
epithelium.
- An odor is an olfactory sensation.
- The stimulation for odors are chemical compounds called odorants which are airborne
molecules that produce an olfactory sensation.
- In order to be smelled, an odorant molecule must be volatile (able to float through
air), small, and hydrophobic (repellent to water).
- However, some molecules that meet these requirements still don’t smell to us.
- Essentially all odorants are organic compounds (molecules built from a carbon
backbone).
- Chemical senses are the only truly direct ones.
- Olfaction is the least informative sense about the larger environment. This is because it
evolved to provide information about the very local environment.
- In addition, there is no direct WHERE component to olfaction. Even dogs can only detect
gradients!
- Olfaction is primitive and tightly linked to memory/emotion (there are bad smells/good
smells, unlike other senses).
- Also, the distinction between smell and taste is pretty blurry. Olfaction is for substances
carries by the air, though!
- Some smells are produced by single odorants (like banana), while others are complex
combinations of odorants (perfumes, wines, etc.)
- Many animals are macrosmatic, meaning they have a keen sense of smell critical to
survival. They use it to find food, mates, homes, and avoid danger and more.
- Humans are microsmatic, meaning our smell sense isn't crucial to survival.
- There appears to be a trade-off between brain area devoted to vision and that to olfaction.
- The detection threshold is often much lower than the recognition threshold. For
instance, it is easy to notice a smell, but naming it can be quite difficult sometimes.
- Thus, odors can be much more easily discriminated than identified, and
identification is greatly enhanced by verbal cuing.
What is the human olfactory apparatus?
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2
- The primary function of the nose is to filter, warm, and humidify the air we breathe.
- The breath must rise past a narrow space called the olfactory cleft, which is in the back
of the nose where air flows and where the olfactory epithelium is located. Then, the air
settles on the mucous membrane of the olfactory epithelium, whose primary function is
to detect odorants in the inhaled air. It is also located on both sides of the upper portion of
the nasal cavity and the olfactory clefts and contains 3 types of cells.
- The epithelium are dime sized patches (1 on each side) 3 inches up the nose.
- Interestingly, our 2 nostrils take in different amounts of air, which is called nasal
dominance. This alternates between the two nostrils throughout the day, meaning that the
two nostrils continually vary in their sensitivities to odorants as a function of the amount
of air inhaled.
- The 3 types of cells in the olfactory epithelium are:
- Basal cells are the precursor cells to the next two.
- Supporting cells which provide metabolic/physical support for the olfactory
sensory neurons.
- Olfactory sensory neurons (OSNs) are the main cell type. These small neurons
are located under a mucous layer in the epithelium. The cilia on the OSN
dendrites contain the receptor sites for odorant molecules.
- Cilia are the OSNs dendrites and have odorant receptors (ORs) on their tips.
- The OSNs end in cilia embedded in mucosa.
- Odorant molecules that are dissolved in muosa bind to receptor proteins on cilia,
which causes depolarization.
- Each OSN produces a single type of receptor protein, and all OSNs expressing
the same type of receptor project to the same glomerulus.
- The interaction between an odorant and the OR ultimately produces an AP that is
transmitted to the olfactory bulb.
- In order to start an AP, about 7 or 8 odorant molecules must bind to a receptor.
- It takes about 40 nerve impulses for a smell sensation to be reported.
- We can detect an INFINITE number of smells.
- Axons on the ends of OSNs opposite the cilia (dendrites) pass through the tiny holes of
the cribriform plate, which separates the nose from the brain.
- (general)Anosmia refers to the total inability to smell, which is often the result of a sinus
illness or head trauma.
- Specific anosmia is the inability to detect a specific odorant type, from genetic absence
of a specific receptor type.
- All of our OSNs die and regenerate about once every 28 days. They are replaced from the
basal cells.
Looking into the neurophysiology of olfaction
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Document Summary
Olfaction is the sense of smell, while gustation is the sense of taste. Our sense of smell is critically involved in our personal experience of food, as we will see more of later. There are two particular ways in which we can perceive odors. Orthonasal olfaction is sniffing in and perceiving odors through our nostrils. It occurs when we smell something outside of us. The molecules travel through our nostrils and up our nose onto the olfactory epithelium. Retronasal olfaction is perceiving odors through your mouth while breathing and chewing and occurs when we are smelling something that is inside our mouth and is what gives us the experience of flavor. Molecules travel through our mouth and up the back of our mouths into our upper nasal cavity and onto the olfactory epithelium. The stimulation for odors are chemical compounds called odorants which are airborne molecules that produce an olfactory sensation.
