Olfaction

Question 1

Advantages of olfaction

Answer 1

• locate and track food/prey at a distance
• long-distance warning beyond line-of-sight and in the dark
• sexual attraction
• hedonic reward system

Question 2

Physiological and behavioural responses to odours

Answer 2

• 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

Question 3

Role of the turbinates

Answer 3

• increase surface area of the epithelium
• covered in thin olfactory neuroepitheloum

Question 4

The neuroepithelium contains…

Answer 4

• olfactory sensory neurons (OSNs)
• supporting/sustentacular cells
• a stem-cell population from which new OSNs are generated

Question 5

Targets of the VSN axons

Answer 5

in glomeruli in the accessory olfactory bulb (AOB)

Question 6

What is the cribiform plate

Answer 6

• bony structure with tiny holes, separating the nose from the brain
• OSN axons pass through these holes to enter the brain

Question 7

Location of the apical dendrite of a sensory neuron

Answer 7

• projects through support cells to the nasal cavity
• capped by dendritic cilia

Question 8

Why can cilia be considered structures for olfactory signal transduction

Answer 8

they have receptor sites for odorant molecules

Question 9

How is an action potential generated

Answer 9

• odors are detected by the receptors
• odors are transduced into an electrical signal
• generation of AP

Question 10

Structure of ORs

Answer 10

GPCRs

Question 11

OR activation of Adenyl cyclase type-III

Answer 11

via Golf (G protein)

Question 12

Role of cAMP in olfactory signal transduction

Answer 12

opens a nonselective cyclic-nucleotide gated (CNG) cation channel, depolarising ciliary membrane

Question 13

Effect of Ca2+ influx

Answer 13

opens Ca2+-activated chloride channels

Question 14

Effect of Cl- leaving ORN

Answer 14

• down conc gradient
• further depolarises cell
• provides amplification

Question 15

Effect of AP generation in ORNs

Answer 15

release of glutamate

Question 16

Intracellular Cl- concentration of OSNs

Answer 16

• unusually high
• due to membrane pump NKCC1
• maintains its own Cl- battery in case the Na+ gradient is insufficient to support threshold current

Question 17

Define amplifiable response

Answer 17

• 1 odor molecule activates many G proteins
• one AC many cAMP molecules

Question 18

Why is adaptation important

Answer 18

allows for sensitivity and extends range of concentration of odor stimulation

Question 19

Role of calcium entry via CNG channels in adaptation

Answer 19

• 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

Question 20

Why is calcium entry important in adaptation response

Answer 20

they have very steep concentration-response relations, so the cells are particularly sensitive to small changes in concentration

Question 21

How is an odor encoded

Answer 21

by the specific combination of responding neurons

Question 22

Sensory neuron responses

Answer 22

• 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

Question 23

Define the glomerulus

Answer 23

spherical structures containing the incoming axons of the OSNs, the first processing station in the brain

Question 24

Role of the periglomerular cell

Answer 24

(inhibitory) connects one glomerulus to another

Question 25

Role of the granule cell

Answer 25

(inhibitory) connects one mitral cell to another

Question 26

Role of mitral cells

Answer 26

(excitatory) recieve odor information from a receptor/sensory neuron, refine the signal and amplify it an

Question 27

Projection of impulses

Answer 27

• activation of mitral cells
• impulses flow from olfactory bulbs
• through lateral olfactory tracts
• to thalamus, hypothalamus, amygdala, and other regions of limbic system

Question 28

Projection of impulses in thalamus

Answer 28

sent to piriform lobe of olfactory cortex and part of frontal lobe (where smells are interpreted and identified)

Question 29

Projection of impulses in hypothalamus, amygdala and other regions of limbic system

Answer 29

smell innervates emotional responses

Question 30

How is olfaction tied to memory

Answer 30

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

Question 31

How is olfaction unique

Answer 31

• direct and intimate connection to the limbic system
• explains why scents tend to have such a strong emotional associations

Question 32

Anatomical peculiarities

Answer 32

• 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