Olfaction Flashcards
Advantages of olfaction
- locate and track food/prey at a distance
- long-distance warning beyond line-of-sight and in the dark
- sexual attraction
- hedonic reward system
Physiological and behavioural responses to odours
- visceral responses: smell food (salivation and gastric motility), noxious smell (gag)
- infants recognise mothers by scent
- mothers can recognise baby by scent
- women housed together synchronise menstrual cycles
Role of the turbinates
- increase surface area of the epithelium
- covered in thin olfactory neuroepitheloum
The neuroepithelium contains…
- olfactory sensory neurons (OSNs)
- supporting/sustentacular cells
- a stem-cell population from which new OSNs are generated
Targets of the VSN axons
in glomeruli in the accessory olfactory bulb (AOB)
What is the cribiform plate
- bony structure with tiny holes, separating the nose from the brain
- OSN axons pass through these holes to enter the brain
Location of the apical dendrite of a sensory neuron
- projects through support cells to the nasal cavity
- capped by dendritic cilia
Why can cilia be considered structures for olfactory signal transduction
they have receptor sites for odorant molecules
How is an action potential generated
- odors are detected by the receptors
- odors are transduced into an electrical signal
- generation of AP
Structure of ORs
GPCRs
OR activation of Adenyl cyclase type-III
via Golf (G protein)
Role of cAMP in olfactory signal transduction
opens a nonselective cyclic-nucleotide gated (CNG) cation channel, depolarising ciliary membrane
Effect of Ca2+ influx
opens Ca2+-activated chloride channels
Effect of Cl- leaving ORN
- down conc gradient
- further depolarises cell
- provides amplification
Effect of AP generation in ORNs
release of glutamate
Intracellular Cl- concentration of OSNs
- unusually high
- due to membrane pump NKCC1
- maintains its own Cl- battery in case the Na+ gradient is insufficient to support threshold current
Define amplifiable response
- 1 odor molecule activates many G proteins
- one AC many cAMP molecules
Why is adaptation important
allows for sensitivity and extends range of concentration of odor stimulation
Role of calcium entry via CNG channels in adaptation
- negative feedback pathway
- increase of intracellular calcium acts on channel to decrease sensitivity to cAMP, thereby requiring a stronger odor stimulus to produce sufficient cAMP
Why is calcium entry important in adaptation response
they have very steep concentration-response relations, so the cells are particularly sensitive to small changes in concentration
How is an odor encoded
by the specific combination of responding neurons
Sensory neuron responses
- respond to a single odorant or a specific repertoire of chemically related odorants
- respond to odorant by inward current flow, which depolarises neuron
- relationship between odorant concentration and size/duration of inward current
- sufficient depolarisation triggers AP
Define the glomerulus
spherical structures containing the incoming axons of the OSNs, the first processing station in the brain
Role of the periglomerular cell
(inhibitory) connects one glomerulus to another
Role of the granule cell
(inhibitory) connects one mitral cell to another
Role of mitral cells
(excitatory) recieve odor information from a receptor/sensory neuron, refine the signal and amplify it an
Projection of impulses
- activation of mitral cells
- impulses flow from olfactory bulbs
- through lateral olfactory tracts
- to thalamus, hypothalamus, amygdala, and other regions of limbic system
Projection of impulses in thalamus
sent to piriform lobe of olfactory cortex and part of frontal lobe (where smells are interpreted and identified)
Projection of impulses in hypothalamus, amygdala and other regions of limbic system
smell innervates emotional responses
How is olfaction tied to memory
it skips the thalamus and the main olfacotry bulb is directly connected to the amygdala, which is involved in the formation of memories/emotional experiences
How is olfaction unique
- direct and intimate connection to the limbic system
- explains why scents tend to have such a strong emotional associations
Anatomical peculiarities
- each sensory neuron expresses only a single receptors
- cells expressing the same receptor converge on only 1 or few glomeruli
- there are 10,000,000 neurons and 1000 receptors
- each mitral cell innervates a single glomerulus via its apical dendrites
- cells for detecting an odor are dispersed in epithelium
- all detection of the odor is gathered and summated into a specific cluster of olfactory bulb neurons
