BMS248 Lecture 4 - Olfactory and Gustatory systems

Question 1

What is labelled line code?

Answer 1

Each receptor input is processed by an individual neuron in the CNS and has its own individual output e.g. sweet
Used for specific things like pheromones

Question 2

What is combinatorial/pattern code?

Answer 2

A receptor input is processed by many different neurons in the CNS, these combine to give an output e.g. sweet
Main method of olfaction

Question 3

What receptor is involved in olfactory sensory transduction in mammals?

Answer 3

G-protein-coupled-receptor (Golf) - Adenylyl cyclase, cAMP, Ca2+ activated Cl- channel
Insects use ion channels

Question 4

Why does the method of transduction allow us to detect faint odours?

Answer 4

Second messengers amplify the sensory signal at every step which increases the sensitivity of the neuron

Question 5

Why does each olfactory receptor only respond to a unique profile of odorants?

Answer 5

Due to the shape of the receptor and therefore what odorant molecule (ligand) can bind to the Golf

Question 6

As olfactory sensory neurons mature what happens to the type of receptors they express?

Answer 6

Narrow down to express a single olfactory receptor each
Each neuron expresses a single type of receptor

Question 7

Axons of olfactory sensory neurons expressing the same receptor converge on the same what?

Answer 7

Glomerulus (in the olfactory bulb)

Question 8

What is the olfactory bulb known as in drosophila?

Answer 8

Antennal lobe - receives inputs from olfactory receptor neurons

Question 9

Sensory neurons transfer information to what kind of neurons at glomeruli

Answer 9

Second order (relay) - as they are the second neuron in a pathway

Question 10

Where do they second order neurons in the glomeruli project to?

Answer 10

Mammals: Mitral cells + Tufted cells (in olfactory bulb) - then to brain
Drosophila: Projection neurons (in antennal lobe) - then to brain
(These receive input from 1 second order neuron)

Question 11

Receptor specific matching of sensory neurons to second-order neurons in the glomeruli ensures what?

Answer 11

That odour specificity is carried through
I.e. the same receptor type converges on the same glomerulus

Question 12

What is the first relay synapse - location + function?

Answer 12

The synapse between sensory neurons and second-order neurons (in glomeruli)
It transforms the odour code

Question 13

What emphasises the start of the odour?

Answer 13

Synaptic adaptation - decrease in firing rate (of the receptor/sensory neuron)

Question 14

Converging sensory neurons onto second order neurons results in what? (2)

Answer 14

1. Reduces noise (unwanted firing)
2. Strengthens weak responses

Question 15

What do the local neurons do between the glomeruli?

Answer 15

Lateral inter-glomerular inhibition

Question 16

What are the two functions of this lateral inhibition?

Answer 16

1. Gain control: sensitivity to both very weak and very strong odours
2. De-correlation: make responses of neuronal population to different odours as different as possible

Question 17

What areas of the brain are responsible for learned olfactory behaviour? (Associating a stimulus) - human and drosophila

Answer 17

Human - Piriform cortex
Drosophila - Mushroom body (Kenyon cells)

Question 18

What areas of the brain are responsible for innate olfactory behaviour? (Genetically hardwired) - human and drosophila
How is this known?

Answer 18

Human - Cortical amygdala
Drosophila - Lateral horn
Silencing of these brain regions change an animals normal response to an odour

Question 19

Innate Odour Cicuitry:
1. Purpose?
2. Activity?
3. What odours?
4. Connectivity?

Answer 19

1. Purpose - categorise
2. Activity - dense (odour coding) - respond to many different odours
3. What odours - certain preferred
4. Connectivity - stereotyped

Question 20

Learned Odour Circuitry:
1. Purpose?
2. Activity?
3. What odours?
4. Connectivity?

Answer 20

1. Purpose - discriminate
2. Activity - sparse (odour coding) - respond to fewer odours
3. What odours - arbitrary
4. Connectivity - random

Question 21

What is the Bacterial and C.elegans olfactory search behaviour (biased random walk)

Answer 21

If the concentration of nutrient increases then they run (straight) more and tumble (turn) less.
Circuit active when odour increases - this suppresses turning behaviour

Question 22

What are two other olfactory search strategies? - drosophila and mouse

Answer 22

What are two other olfactory search strategies? - drosophila and mouse

Drosophila - casting behaviour
Mouse - coordinate sniff cycle with head movement

Question 23

What receptor detects umami, sweet and bitter tastants (ligands)?

Answer 23

Metabotropic GPCR - PLC, PIP2, IP3, ^Ca2+
Signal amplification via second messengers

Question 24

What receptor detects salt?

Answer 24

Ion channel - ENaC

Question 25

What receptor detects sour?

Answer 25

Ion channel - proton sensitive

Question 26

What is the pathway of the taste circuit?

Answer 26

1. Taste buds
2. Solitary nucleus of brainstem > hypothalamus + amygdala
3. VPM of thalamus
4. Insula + Parietal cortex

Question 27

What receptor is involved in lateral inhibition in taste?

Answer 27

GABA B receptor - at sensory neuron synapses with second-order neurons - GABAergic interneurons

Question 28

Different tastes activate different parts of the brain (insula), therefore optogentically activating specific areas of the insula in mice leads to what?

Answer 28

Makes mice either approach or avoid a stimulus e.g. activating the bitter area would make mice avoid a stimulus vs sweet which would make them approach