olfaction Flashcards
Rats vs humans vs dogs smelling
- individual receptors for all of these animals are equally sensitive : The difference lies in the number of receptors they each have
– Humans have 10 million and dogs have 1 billion olfactory receptors
ORNs replaced about every 30 days in rodent models and 40-60 days in humans
Rats are 8 to 50 times more sensitive to odors than humans
Dogs are 300 to 10,000 times more sensitive
Dogs can smell cancer in humans… not that they are used much for diagnostic purposes
are humans bad at olfaction
Yes-ish, but now we mostly think no
Across mammals, olfactory genes constitute 3-5% of the genome.
Humans: 350 functional olfactory receptor genes and 70% of the genes encoding for olfactory receptors are pseudogenes
It is suggested that this is indicative that we de-valued olfaction at some point during our evolution
Rodents: 1100 functional olfactory receptor genes and only 5% of the genes encoding for olfactory receptors are pseudogenes
Other primates also have a low percentage of pseudogenes
“Reduced repertoire of genes for olfactory receptors is compensated by the great capacity of human brain processing.”
olfactory cilia
Where odorant receptors are located; where olfactory neurons appear to be damaged in SARS-CoV-2 infection
Olfactory receptors are similar in structure to _____
Olfactory receptors are similar in structure to muscarinic acetylcholine receptors
advantages of olfaction *** (4)
• Locate & track food/prey at a distance (Helping predators)
• Long-distance warning beyond what you can see/hear and in the dark (When cannot see)
• Sexual attraction : Pheromones- except in humans – Vomeronasal system: a sensory system dedicated to pheromone sensation
• Hedonic reward system: Appreciation for fine foods and wine and fancy pants stuff
-Comparing humans vs dogs- To dogs: it is just a bag of food
T/F both dogs and humans have a hedonic system but only dogs have a vomeronsasal system
FALSE ( i think)
Vomeronasal system: a sensory system dedicated to pheromone sensation - humans do not have pheromones but they do have the Hedonic reward system
Hedonic reward system: Appreciation for fine foods and wine and fancy pants stuff
are olfactory receptor molecules G-protein coupled?
YES
olfaction, like the other special sensory information goes directly to the thalamus: True or Fase
FALSE
-Does not directly go to thalamus, unlike other special sensory information
where does the pyriform cortex send its efferents?
where does it send efferents directly?
Sends efferents to the thalamus, hypothalamus, hippocampus, amygdala, and orbitofrontal cortex
-Both indirect and direct to the orbitofrontal cortex
which target of the olfactory bulb is responsible for which reward processing in the brain + is Involved in motivated behaviors, including eating.
olfactory tubercule
odorant
Molecules capable of eliciting responses from receptors in the olfactory mucosa.
olfactory epithelium**
- Pseudostratified epithelium that contains olfactory receptor cells, supporting cells, and mucus-secreting glands.
- The olfactory epithelium is a specialized epithelial tissue inside the nasal cavity that is involved in smell. In humans, it measures 9 cm2 (3 centimetres by 3 centimetres) and lies on the roof of the nasal cavity about 7 cm above and behind the nostrils.
- Lines the nasal cavity
- consists of 3 cell types: basal, supporting, and olfactory receptor cells.
** Bowman’s gland:
Produce mucus
Found in the epithelium
***Supporting cells
Supporting cells
make mucus
might be where SARS-CoV-2 is disrupting olfaction
- *basal cells
- type of cells
Are the source of new ORNs
Stem cells within the olfactory epithelium
(see location) - are going to regenerate the olfactory receptor neurons, refill our nasal epithelium
Basal cells are stem cells that give rise to the olfactory receptor cells
Olfactory receptor neurons: where are the receptors, how are odorants transduced (please know details of “Odorant Signal Transduction” slide with the GPCR on it), where are the axons sent
- Odorant (ligand) binds GPCR
- Golf becomes active
- Activates adenylyl cyclase III
ACIII: is a membrane bound enzyme that can be activated by G-proteins to catalyze the synthesis of cAMP from ATP - AC activates cAMP, which acts as a second messenger
- cAMP opens cAMP-gated ion channel to let in calcium
- Calcium opens calcium-gated Cl- channels so Cl- leaves cell
- Voltage-gated sodium channels open =>Depolarization and action potentials
Action potentials are sent to the olfactory bulb (via the olfactory cell axons)
Golf protein activates the adenylyl cyclase III, which converts ATP to cAMP, activating cyclic nucleotide gated channels – causes influx of calcium
T/F Dogs can smell cancer in humans
TRUE: Dogs can smell cancer in humans… not that they are used much for diagnostic purposes
Hedonic reward system
Hedonic reward system: Appreciation for fine foods and wine and fancy pants stuff
-Comparing humans vs dogs- To dogs: it is just a bag of food
Pyriform Cortex
-function
Component of cerebral cortex in the temporal lobe pertinent to olfaction; so named because of its pearlike shape
- Does much of the processing of olfactory information and identifying odorants
- Has been shown to participate in odor discrimination, association, and learning (Bekkers and Suzuki, 2013)
• Lateral olfactory tract
major target:
• Lateral olfactory tract: The projection from the olfactory bulb to higher olfactory centers.
- the pyriform is a major target of lateral olfactory tract, especially in humans
- combinatoral coding
pyriform cortex : structure
- Unique in that it only has 3-layers instead of 6 like the rest of the cortex
- Believed that this is due to it being phylogenetically much older than the rest of the cortex
- Allocortex: 3 layers – older – looks more like hippocampus
- The allocortex (also known as heterogenetic cortex) is one of the two types of cerebral cortex, the other being the neocortex. It is characterized by having just three or four cell layers, in contrast with the six layers of the neocortex, and takes up a much smaller area than the neocortex.
• As information comes in from the olfactory bulb, information about unique odorants synapses on overlapping targets in the _____
- As information comes in from the olfactory bulb, information about unique odorants synapses on overlapping targets in the pyriform cortex
- The synapses are plastic- can change due to experience
- This likely helps with odor detection and learning odorant combinations of different things (combinatorial coding)
• Olfactory tubercule
- an Olfactory bulb target
- AWARD PROCESSING
- Involved in motivated behaviors, including eating.
- Learned odor-induced attractive and aversive behaviors
Damage to orbitofrontal cortex
can lead to a central loss of smell detection (nose works, but brain does not perceive the smells)
Glomeruli: (glomerulus)
Characteristic collections of neuropil in the olfactory bulb; formed by dendrites of mitral cells and terminals of olfactory receptor cells, as well as processes from local interneurons.
Cell types of the olfactory bulb
Mitral cell, Tufted cell, Periglomerular cell, Granule cell
Tufted cell
(a cell type of the olfactory bulb)
local circuit neuron; may play a role in odorant sensitivity
where does smell begin?
with intake of odorants in the nose; odorants are dissolved within the mucus of the nasal cavity
what is beneath the cribriform plate and what interaction takes place there?
the olfactory epithelium is beneath; cilia reach down into the nasal cavity, and odorants interact with receptors at the cilia – activate the receptors
olfactory receptors –> olfactory bulb travel via
olfactory nerve (cranial nerve I)
olfactory receptor’s axons
The axon sent to the glomerulus → and then the mitral/ Tufted cell
mitral cell
the one receiving the olfactory information from the olfactory receptor neurons
which cell play a role in odorant sensitivity
Tufted cell and Periglomerular cell
granule cell
make dendrodendritic synapses on mitral cells; believed to play a role in local inhibition and synaptic plasticity
believed to play a role in local inhibition and synaptic plasticity
granule cell
make dendrodendritic synapses on mitral cells
granule cell
how many mitral cell for each glomerus ?
one mitral cell for each glomerus
explain olfactory coding
- All the OR axons ending in one glomerulus contain are from receptors expressing same olfactory binding protein.
- So each mitral cell codes for one kind of odorant molecule. This is the primary basis of olfactory coding.
explain olfactory coding
- All the OR axons ending in one glomerulus contain are from receptors expressing same olfactory binding protein.
- So each mitral cell codes for one kind of odorant molecule. This is the primary basis of olfactory coding.
what is the olfactory nerve?
what is the olfactory nerve?
the axon that projects up to the olfactory bulb; cranial nerve 1
how is the olfactory epithelium structured?
how is the olfactory epithelium structured?
stratified, with 3 main cell types; olfactory receptor neurons at the business end, with cilia projecting down – these are bipolar cells that continually regenerate; supporting cells with Bowman glands, produce mucus; basal cells that support the regeneration of new cells – regeeneration is unusual for neurons, occurs about every 2 weeks
what is mucus?
water base with mucopolysaccharides, proteins, antibodies, enzymes, odor binding proteins and salts
how are olfactory signals transduced?
how are olfactory signals transduced?
G protein coupled receptor, similar to taste; the G protein is Golf
what happens when there is an influx of calcium?
activation of calcium gated chloride channels, causing outward flux of chloride due to higher concentration in olfactory cells
how is the signal propagated, post-cascade?
propagated to soma, where you get the action potential that propagates down the olfactory nerve
what is observed within the olfactory epithelium?
what is observed within the olfactory epithelium?
different (in diagram, colored) cells express different genes for the receptors; the receptors are broadly tunes, respond to many odorants but show preference; precise mapping of axons of cells expressing specific receptor genes to glomeruli
how are olfactory receptors ordered?
how are olfactory receptors ordered?
each glomerulus receives input from cells expressing the same olfactory receptor – some receptor cells respond very specifically, some very broadly
where are glomeruli found?
where are glomeruli found?
in the olfactory bulb
is there convergence in the glomeruli?
is there convergence in the glomeruli?
yes, within a specific glomerulus there is roughly 25000 primary neurons synapsing onto 100 second order cells – mitral and tufted cells
what do the primary neurons do at each glomerulus?
release glutamate onto mitral cells, which project to the lateral olfactory tract
lateral olfactory tract
of a single olfactory glomerulus via the lateral olfactory tract (LOT) to multiple olfactory bulb targets, including the pyriform cortex (PIR), entorhinal cortex (ENT), amygdala (AMG), olfactory tubercle (OT), and accessory olfactory nucleus (AON).
Lateral Olfactory tract- most stays ipsilateral, but some axons do crossover. Backup mechanism?
single olfactory glomerulus via the ______ projects to multiple _______targets, including the (5)
most stays of these projections travel ipsilateral or cross over?
-why?
of a single olfactory glomerulus via the lateral olfactory tract (LOT) to multiple olfactory bulb targets, including the pyriform cortex (PIR), entorhinal cortex (ENT), amygdala (AMG), olfactory tubercle (OT), and accessory olfactory nucleus (AON).
Lateral Olfactory tract- most stays ipsilateral, but some axons do crossover. Backup mechanism?
what is the function of other cells besides the mitral cells? function of other cells considered to be important for sharpening of the signal to the brain
what is the function of other cells besides the mitral cells? function of other cells considered to be important for sharpening of the signal to the brain
what areas of the cortex do the mitral cells project to?
what areas of the cortex do the mitral cells project to?
accessory olfactory nucleus, piriform cortex, amygdala, olfactory tubercle, entorhinal cortical area
what is considered to be the olfactory cortex?
Pyriform cortex
what is the entorhinal cortex?
an area of the medial temporal cortex that is a major source of neural signals to the hippocampus; related to spatial memory and spatial organization
entorhinal cortex
Receives olfactory information from the olfactory bulb and from the pyriform cortex
Sends projections back to both structures
In rodents, this area is almost exclusively dedicated to olfaction
Vs humans
May play a role in smell anticipation
Area becomes active in rodents when they expect a certain smell to show up
Seems to also be important for odor discrimination memory
what does the piriform cortex activate?
orbitofrontal cortex
what does the piriform cortex, olfactory tubercle, amygdala, and entorhinal cortex activate?
what does the piriform cortex, olfactory tubercle, amygdala, and entorhinal cortex activate?
orbitofrontal cortex, thalamus, hypothalamus
hat does the entorhinal cortex activate?
hat does the entorhinal cortex activate?
the hippocampal formation
what is the vomeronasal organ?
what is the vomeronasal organ?
a collection of specialized receptor cells, near to but separate from the olfactory epithelium, that detect pheromones and send electrical signals to the accessory olfactory bulb in the brain; prominent in cats, dogs, and rodents, less so in primates
olfactory receptor neurons
- bipolar neurons that span the epithelium
- extend short dendrites to epithelial surface, terminates in olfactory knob –> olfactory cilia - Cilia extend into a thick layer of mucus produced by Bowman’s glands - long central process (unmyelinated axon), bundle form olfactory n, pass thru skull base, synapse in olfactory bulb - turn over 30-60 days (exposed location) - direct access to odorant, but exposed to pollutants, allergens, etc
Primary sensory neurons from which of the following systems has the smallest axon (which is alos unmyelinated)?
a. Olfactory
b. Vestibular
c. Gustatory
d. Visual
e. Auditory
Primary sensory neurons from which of the following systems has the smallest axon (which is alos unmyelinated)?
Olfactory
Horizontal cells contribute to horizontal integration in the retina. The analagous cell contributing to horizontal integration in the olfactory bulb is the
a. Amacrine cell
b. Bipolar cell
c. Ganglion cell
d. Pyramidal cell
e. Periglomerular cell
f. Mitral cell
Horizontal cells contribute to horizontal integration in the retina. The analagous cell contributing to horizontal integration in the olfactory bulb is the
Periglomerular cell
Odorant receptors are
a. Located in the olfactory bulb
b. Located on dendrites of mitral and tufted cells
c. Located on neurons that project directly to the olfactory cortex
d. Located on neurons in the olfactory epithelium that project to mitral cells and from there directly to the olfactory cortex
e. Located on sustentacular cells that project to the olfactory bulb
Located on neurons in the olfactory epithelium that project to mitral cells and from there directly to the olfactory cortex
Which of the following does NOT increase the ability to discriminate many different odors?
a. Many different receptors
b. Pattern of olfactory receptors activated by a given odorant
c. Projection of different mitral cell axons to different parts of the brain
d. Neural processing in the amygdala
e. Lateral inhibition
Which of the following does NOT increase the ability to discriminate many different odors?
Neural processing in the amygdala
The Olfactory System: Central Stem Cells vs Peripheral Stem Cells
- central stem cells give rise to granule cells: stem cells in the subventricular zone of the lateral ventricles generate new neurons that migrate into the olfactory bulb where they mature into granule cells.
- peripheral stem cells give rise to olfactory receptor neurons, assisted by ensheathing cells (a glial cell)
stem cells in the olfaction system
basal cells
true / false The golmerulus holds the cell bodies for olfaction
FALSE The golmerulus has NO cell bodies – just where the axons are synapsing
face pain:
First pain/sensory discriminative pain and second pain/affective-motivational pain kept separate like in body pain systems
auditory info and face information of the trigeminothalamic path are both reaching the VPM. The auditory info is going from the cochlea to the _____ lemniscus, face pain is going to the ______ lemnisucs.
auditory info and face information of the trigeminothalamic path are both reaching the VPM. The auditory info is going from the cochlea to the lateral lemniscus, face pain is going to the medial lemnisucs.
cell bodies for pain info
• Cell bodies of the nerves reside in the dorsal root ganglia or trigeminal ganglion, like those of other sensory receptors
