Olfaction Flashcards
Role of the Olfactory System
Processes info on the identity, quality and concentration of odourants
Define Odourants
Airborne volatile chemical stimuli
What linings make up the nasal cavity?
The olfactory epithelium and the respiratory epithelium
Define the Olfactory Epithelium
A sheet of neurons & supporting cells that line 1/2 of the nasal cavity
How is the olfactory system unique amongst the other sensory systems
The path of the info from the peripheral receptors to the olfactory cortex does not include a thalamic relay (although later olfactory processing does)
What types of cells are found in the olfactory epithelium?
Basal cells, sensory receptor cells and supporting cells
Describe the olfactory receptor neurons
- Bipolar cells
- Send dendrites into the mucosa layer
- Interact with odorants via olfactory cilia
- Their axons pass through cribriform plate connect with
neurons in the olfactory bulb
Role of mucus in olfactory epithelium
- Protects the exposed receptors and supporting cells
- Controls ionic milieu of cilia
- Is primary site of odor transduction
Role of Supporting Cells in the olfactory epithelium
Detoxify potentially harmful chemicals before they reach the sensory neurons
Role of the Basal Cells in the olfactory epithelium
- Continually divide to replenish sensory neurons as they only have a lifespan of aprox. 60 days
- Continuous cycle of degeneration & regeneration allows us to regain sense of smell after having a cold
Describe the cilia in the olfactory epithelium
- Project into mucus
- Contain receptor proteins that recognise odorants as well as all the necessary transduction machinery needed to amplify the sensory signals and transduce them into electrical ones
Describe the olfactory signal transduction cascade
- Odorant binds to receptor protein
- Activates G-protein (alpha subunit dissociates)
- Subunit activates adenylyl cyclase III enzyme
- Enzyme catalyses conversion of ATP to cyclic AMP
- Increase in cAMP causes cyclic-gated cation channels in membrane to open
- Influx of Na (+) and Ca (2+) into cell, depolarising
- Ca (2+) acts as 2dry messenger molecule, causing Ca (2+)-gated Cl (-) channels to open
- Cl (-) leaves cell, amplifying depolarisation
- The depolarisation is conducted passively from cilia to axon hillock
- AP is then generated via voltage-gated Na (+) channels & transmitted to olfactory bulb
How does the olfactory receptor current change with increasing odorant concentration?
It increases in proportion with the changing concentration.
How is the olfactory receptor repolarised after olfactory transduction?
The excess calcium is removed via the sodium / calcium exchanger as sodium is transported INTO the cell.
How does the olfactory receptor recover after olfactory transduction?
Calumodulin kinase II-mediated mechanisms restore heterotrimeric G protein and decrease cAMP levels via activation of phosphodiesterases.
What kind of responses to prolonged odors evoke and why?
Receptor current oscillates - achieved by reducing cation channel sensitivity to cAMP.
Describe the olfactory receptor molecules.
Receptors are protein molecules embedded in the olfactory cilia membrane.
How do olfactory receptor axons reach the neurons in the olfactory bulb?
They pass through the cribRIform plate.
What do olfactory receptor axons do in the olfactory bulb?
They excite mitral and tufted relay neurons in the olfactory glomeruli. Afferents from olfactory receptor cells expressing a particular receptor molecule selectively converge on just two glomeruli in each bulb.
What do periglomerular cells and granule cells do in the olfactory bulb?
They make reciprocal dendro-dentritic synapses with mitral cells, mediating lateral inhibition which sharpens mitral cell odour tuning.
How does olfactory information leave the olfactory bulb?
Mitral cell axons leave the bulb in the lateral olfactory tract. The anterior olfactory nuclei mediates inhibition between the two bulbs via the anterior commissure.
Which 5 regions does the lateral olfactory tract connect to?
- Anterior olfactory nucleus
- Olfactory tubercle
- Pyriform cortex
- Amygdaloid complex
- Entorhinal cortex
What does the orbitofrontal cortex mediate?
Conscious perception of odor.
What do the amygdala and entorhinal cortex form?
Part of the limbic system.
What is the limbic system involved in?
The affective component of odour perception - mediates emotional and autonomic responses.
