exam 3 Flashcards
(43 cards)
What are odorant receptors (e.g., ionotropic vs metabotropic receptors or ion channels or
GPCRs?
Odorant receptors are G-protein-coupled receptors (GPCRs), not ionotropic receptors.
They do not form ion channels themselves but initiate a metabotropic cascade leading to ion channel opening.
Where are odorant receptors expressed specifically?
On what part of the ORN? For example, are they
densely expressed in the axonal terminals or in another part of the cell?
They are densely expressed on the cilia of ORNs.
The cilia project from dendritic knobs at the apical end of the ORNs, facing the nasal cavity.
Not expressed on axonal terminals.
Are ORNs inhibitory or excitatory? What NT do they utilize?
ORNs are excitatory and use glutamate as their primary neurotransmitter.
Do ORNs regenerate?
Where are the cell bodies of ORNs located?
Yes! ORNs regenerate throughout life, approximately every 30–60 days.
2ND: In the olfactory epithelium, a specialized region in the nasal cavity.
Do odorant receptors have broad or narrow tuning curves?
What does it mean
in terms of ligand to receptor binding?
Odorant receptors have broad tuning curves,
A single receptor can bind multiple odorants (with varying affinities).
A single odorant can bind multiple receptors. This leads to a combinatorial code for odor detection.
2ND: Broad tuning means:
Low specificity but high sensitivity to a range of similar molecules
Each odorant activates a combinatorial pattern of receptors → This pattern is decoded by the brain to identify the odor
Does the size of the olfactory epithelium
vary amongst species?
Yes!
In species with a strong sense of smell (e.g., dogs), the epithelium is larger and more densely packed with ORNs.
In humans, it’s relatively small.
Each olfactory receptor neuron expresses the products
(proteins) of how many odorant receptor genes?
Why can a single ORN be activated/depolarized by
multiple odorant molecules?
Does it express multiple
kinds of odorant receptors on its cilia membrane?
1ST: Only ONE functional odorant receptor gene is expressed per ORN
2ND: Because the receptor it expresses has a broad tuning curve.
However, each ORN expresses only one receptor type on its cilia.
3RD: However, each ORN expresses only one receptor type on its cilia.
On what part of the neuron should I apply an odorant if
I wanted to stimulate that neuron in a laboratory under
experimental conditions?
Apply odorants to the cilia (on the apical surface of the ORN).
That’s where receptors and signal transduction machinery are localized.
Are all odorant receptor genes in the
human genome functional?
No.
Humans have ~400 functional odorant receptor genes.
But also ~600 pseudogenes (nonfunctional due to mutations).
Olfactory signal transduction in the cilia:
opening of what types of membrane
channels causes membrane
depolarization?
What does the G protein activate (what is
the enzyme)?
1/2: Odorant binds → GPCR (odorant receptor)
G protein (Golf) activates adenylyl cyclase
cAMP is produced → opens cyclic nucleotide-gated (CNG) channels
Na⁺ and Ca²⁺ influx → depolarization
Ca²⁺ also activates Cl⁻ channels, causing Cl⁻ efflux, further depolarizing the cell
3RD: The G protein (Golf) activates adenylyl cyclase III (AC3) → increases cAMP.
Olfactory bulbs:
what are glomeruli?
Do the axons from olf. receptor neurons end in the glomeruli?
Do glomeruli receive input from RANDOM ORNs or from ORNs expressing the
products of the SAME odorant receptor genes?
Glomeruli are spherical structures in the olfactory bulb where:
Axons of ORNs (that express the same receptor gene) converge
They synapse onto mitral and tufted cells
3RD: Each glomerulus receives input from only one type of odorant receptor (not random).
how complex are olfactory bulbs
Yes—very complex:
Multiple cell layers: glomerular layer, external plexiform layer, mitral cell layer, granule cell layer
Many cell types: mitral cells, tufted cells, periglomerular cells, granule cells
Contains both excitatory and inhibitory circuits
DO not forget that olfactory bulbs
project to olf. cortex directly via the olfactory tract!
Which are the only cells which project out of the bulbs and into primary olf. cortex (2 cell
types)?
Which are some main inhibitory cells that are thought to mediate lateral inhibition in
bulbs?
Mitral cells
Tufted cells
→ These are the main projection neurons of the bulb. They send axons via the olfactory tract to the primary olfactory cortex.
2ND: Periglomerular cells: Inhibit mitral/tufted cells within glomeruli.
Granule cells: Inhibit mitral/tufted cells via dendrodendritic synapses in deeper layers (external plexiform layer).
What are granule cells? Why did we say they are important?
What phenomenon seen in sensory systems are they thought to mediate in the olf. bulbs?
They are axless GABAergic interneurons.
They mediate dendrodendritic inhibition onto mitral cells.
2ND: Functionally, they are thought to contribute to:
Lateral inhibition (sharpening odor representations)
Pattern separation
Odor discrimination
Olfactory learning & memory
Is olfactory adaptation calcium-mediated (is calcium required for both proposed intracellular pathways of adaptation)?
What are the two proposed signaling pathways that ultimately lead to a reduced response to the odorant? (Hint: Ca2+
mediated reduction of CNG channel activity and/or Ca2+ mediated recuction of AC activity).
Yes—both proposed intracellular pathways for adaptation require Ca²⁺.
2ND: Ca²⁺ → binds calmodulin → inhibits CNG channels → ↓ Na⁺/Ca²⁺ influx → ↓ depolarization
Ca²⁺ → activates calmodulin → inhibits adenylyl cyclase (ACIII) → ↓ cAMP → ↓ CNG channel opening
→ Net effect: reduced sensitivity to sustained odorants (adaptation).
What are the direct targets of the bulbar projection neurons (meaning, what brain areas do the projection neurons of the
bulbs send their axons to?
Piriform cortex (primary olfactory cortex)
Amygdala
Entorhinal cortex
Olfactory tubercle
These areas mediate odor perception, emotion, and memory.
Does olfactory information require thalamic relay just like every other sensory modality (meaning
does the information from the bulbs have to go first to thalamus before it goes to primary olfactory cortex)?
No. Olfactory information is unique:
→ It bypasses the thalamus and goes directly to primary olfactory cortex.
(Thalamus is involved later in higher-order olfactory processing, not initial relay.)
Where exactly is plasticity
heavily studied in hippocampus
that we discussed in class?
What synaptic region?
What are Schaffer collaterals?
Are
CA3 neurons glutamatergic?
1ST/2ND: In the CA1 region, particularly at synapses between CA3 and CA1 neurons.
3RD: Axons from CA3 pyramidal neurons that synapse onto CA1 pyramidal neurons.
4TH: Yes—CA3 pyramidal neurons release glutamate.
What postsynaptic receptors mediate
plasticity?
AMPA receptors (AMPARs): fast excitatory response
NMDA receptors (NMDARs): critical for synaptic plasticity
Which postsynaptic receptors
are considered coincidence sensors and
why?
NMDARs are coincidence detectors because they:
Require glutamate binding
And postsynaptic depolarization (to relieve Mg²⁺ block) → They only allow ion flow when both conditions are met.
What is the ion of importance that
they are permeable to which participates
in signaling intracellular pathways?
Ca²⁺ (calcium): enters via NMDARs and triggers intracellular pathways
What are NMDARs?
ARE THEY GPCRS?
N-methyl-D-aspartate receptors
Ligand-gated ion channels
Permeable to Na⁺, K⁺, and crucially Ca²⁺
2ND: No—they are ionotropic receptors (ligand-gated channels), not GPCRs.
Does activity of postsynaptic
NMDARs correlate with changes in
synaptic strength in general?
Yes—more NMDAR activation → more Ca²⁺ influx → stronger synaptic changes (LTP/LTD).
Does
activity of NMDARs correlate with
levels of intracellular calcium?
Yes—NMDAR opening is a major source of Ca²⁺ in dendritic spines.
