Olfaction Flashcards
Where is our sense of smell based?
In the olfactory epithelium.
What is the olfactory epithelium (OE)?
A thin sheet of cells high up in the nasal cavity.
Briefly summarise the process of olfaction.
The chemicals of a smell travel into the nose.
They dissolve in the mucus layer.
This activates neuronal receptors embedded in the mucus layer.
The neuronal signals are forwarded via the olfactory bulbs, along the olfactory tracts which project up to the olfactory cortex in the brain.
What is the orthonasal pathway?
A direct route that activates receptors in the OE when we breathe in through our nose.
What is the retronasal pathway?
A route that allows us to perceive the odours coming from food or drink in our mouth via the back of the throat as we exhale.
Which olfactory pathway contributes to our detection of the flavour of food?
Retronasal pathway.
Name the three main cells in the OE.
Olfactory receptor cells (ORC).
Supporting cells.
Basal cells.
Describe the function of ORCs.
The site of transduction in the olfactory system.
Where do the axons of the ORCs project to?
Directly to the cortex.
Describe the four main roles of the supporting cells in the OE.
Maintain ionic balance.
Synthesise neuromodulatory substances.
Isolate neighbouring sensory cells.
Produce the mucus layer with the help of Bowman’s glands.
What is the function of basal cells in the OE?
Source of new receptor cells.
Describe the structure of ORCs.
Bipolar.
Soma, apical dendrite and basal axon.
Dendrites end in an olfactory head which carries cilia.
What is uncommon about ORCs?
They are one of the few neurons that have a life-death-regeneration cycle.
What is the mean life expectancy of ORCs?
1-2 months.
In which part of the ORCs are odours detected and the transduction process activated?
The cilia.
What are expressed by ORCs?
GPCRs.
Name the special form of G-protein that is found only in ORCs.
G-olf.
Odorant molecules bind to the olfactory receptor protein. What does this receptor-odorant interaction lead to?
Stimulation of the G-olf protein.
What does stimulation of the G-olf protein lead to?
Activation of adenylyl-cyclase.
What does activation of adenylyl-cyclase lead to?
Synthesis of second messenger cAMP.
What does synthesis of cAMP lead to?
Opening of cyclic nucleotide-gated channels.
What does the opening of cyclic nucleotide-gated channels lead to?
Influx of calcium and sodium ions from mucus layer into cilium of ORCs.
What does the influx of calcium ions into the cilium of ORCs lead to?
Opens chloride channels, so chloride flows out from the cell and depolarises the membrane.
It is thought that the calcium-activated chloride current might amplify the transduction signal. How would this be possible?
It is thought that the internal chloride concentration must be extremely high, so that efflux of chloride from the cell causes depolarisation instead of hyperpolarisation.
Odour stimulation evokes a slow receptor potential in the cilia. Where does this then travel to?
It propagates down the dendrite to the soma.
A train of action potentials are generated which propagate continuously down the olfactory nerve axon to the olfactory bulb.
Why are active ion transporters expressed in the membrane of the cilia?
To maintain the electrochemical gradients necessary for signal transduction.
How does active ion transport maintain the electrochemical gradients necessary for signal transduction?
Sodium, potassium and chloride co-transporters, and chloride/bicarbonate ion exchangers transport chloride ions back into the cell.
Sodium/calcium exchanger transports calcium ions back out of the cilia lumen.
What are the two unusual features of the olfactory signalling pathway?
Receptor binding proteins at the beginning.
cAMP-gated channels near the end.
There are over 1000 odorant receptor genes, each of which has a slightly different structure. What are the implications of this?
Each receptor protein encoded by these genes differs in its ability to bind odorants.
Each ORC expresses only one of the 1000 types of receptor genes. What does this tell us?
There are approximately 1000 types of receptor cells.
How is the olfactory receptor sheet within the OE organised?
Into large maps or zones of receptor cells that express a different subset of receptor genes.
How do we know that modulation of the response to odours begins in the OE?
Microelectrode recordings show us that each individual receptor cell not only has a specific preference but also responds to several different odours as well.
Name the protein that plays a central role in adaptation to a continuous smell stimulus.
Calmodulin.
How does diffusion contribute to adaptation?
The chemical molecules of an odour can diffuse. Levels become too low to activate a receptor and an action potential.
How does scavenger enzymes in the mucus layer contribute to adaptation?
They break down the chemical molecules. This prevents sufficient detection.
How does activating cAMP in the ORC contribute to adaptation?
It may activate other signalling pathways present in the OE that prevent transduction.
What forms the olfactory nerve?
Small clusters of unmyelinated axons of the ORCs that penetrate through the Cribriform plate and end in the olfactory bulb.
What is the Cribriform plate?
A thin sheet of bone that extends over the OE.
What is the role of glomeruli in the olfactory bulbs?
It is the input layer for primary olfactory axons as they synapse onto mitral cells.
What are mitral cells?
Second-order olfactory neurons contained within the glomeruli.
Why are odorants recognised via a complex activity of several glomeruli?
Single odorants can activate multiple glomeruli.
Each glomerulus is activated by multiple odorants.
How is olfactory input modified within glomeruli and between the two olfactory bulbs?
By inhibitory GABAergic and dopaminergic interneurons.
Through excitatory interaction.
What is the role of inhibitory interneurons in modifying olfactory input?
They enhance the contrast of the olfactory signal by limiting the amount of sensory information before it is projected further along the signalling pathway.
The axons of which cells form the olfactory tract?
Mitral cells.
What is the role of the olfactory tract?
To transmit the olfactory information to the higher olfactory centres.
Are the fibres of the olfactory tract contralateral or ipsilateral?
Ipsilateral.
Only a few fibres of the olfactory tract connect the olfactory bulbs of both hemispheres via which two structures?
Anterior olfactory nucleus.
Anterior commissure.
Where does the olfactory tract project to?
The cerebral cortex and then to the thalamus.
How does the olfactory pathway differ from other sensory systems?
In most sensory systems, information passes through the thalamus first and then projects up to the cerebral cortex, not the other way around.
Areas that receive direct input from the olfactory bulb make up which brain region?
Primary olfactory cortex.
Name the three main components of the primary olfactory cortex.
Piriform cortex located at the junction of the frontal and temporal lobes.
Amygdala and rostral entorhinal cortex in the temporal lobe.
What is the primary role of the piriform complex, amygdala and rostral entorhinal cortex in olfaction?
They are part of the limbic system with roles in emotion, motivation and memory. Therefore, this is why certain odours can evoke strong emotions and memories.
Where does the olfactory pathway project to from the primary olfactory cortex?
Secondary olfactory cortex.
Name the seven main regions that make up the secondary olfactory cortex.
Regions and nuclei of:
Prefrontal cortex.
Insular cortex.
Amygdala.
Hypothalamus.
Thalamus.
Hippocampus.
Basal ganglia.
What is the overall role of the secondary olfactory cortex?
Underlies olfactory-controlled behaviour, such as controlling food intake, feelings, memory and hormonal balance.
What enables the olfactory system to distinguish many more odorants than it has ORCs?
The combination of spatial and temporal coding.
Why is it suggested that the brain uses temporal coding to encode the quality of odours?
Odours are a relatively slow stimulus, which suggests that rapid timing of action potentials isn’t used by the brain to encode the timing of odours.
