Chemical Senses Flashcards
How does olfaction differ from other sensory modalities in stimulus dimensionality?
Olfaction is multidimensional
Many different molecules can occur so cannot be described in terms of dimensions
How do olfactory sensory neurons covert sensory stimuli into neural signals?
Olfactory sensory transduction
Olfactory sensory transduction
- Odorant binds to receptor on cilia of olfactory neuron, activating GPCR
- Alpha subunit dissociates from G protein to activate adenylyl cyclase to convert ATP to cAMP
- cAMP opens cation channels to allow entry of Na+ and Ca2+ into neuron
- Ca2+ opens Cl- channels to be released from the neuron leading to depolarisation
Each olfactory receptor responds to a unique profile of odorants, T of F?
T
What happens as sensory neurons mature?
They narrow down to express a single olfactory receptor each
What happens if olfactory sensory neurons express the same receptor?
They converge onto the same glomerulus
What does the antennal lobe in the Drosophila equate to in humans?
The olfactory bulb
What is the pathway from the olfactory receptor neurons to the brain in Drosophila?
- Olfactory receptor neurons
- Glomeruli
- Local neurons
- Projection neurons
- Brain
What is the pathway from the olfactory sensory neurons to the brain in mammals?
- Olfactory sensory neurons
- Glomeruli
- Granule cells, periglomerular cells
- Mitral cells, tufted cells
- Brain
How is odour specificity carried through to the brain?
Receptor-specific matching of sensory neurons to second order neurons
What is the role of the first relay synapse?
It transforms the odour code
How does the first relay synapse transform the odour code?
- Synaptic adaptation emphasises the start of the odour
2. Converging sensory neurons onto second order neurons to reduce noise and strengthen weak responses
What are the functions of lateral cross talk?
- Gain control - allows them to be sensitive to both very weak and very strong odours
- De-correlation - makes responses as different as possible
What are the key brain areas that are involved in processing smell?
Human: Piriform complex (learned behaviour) Amygdala (innate behaviour) Insect: Mushroom body (learned behaviour) Lateral horn (innate behaviour)
What is innate behaviour?
Trying to catogerise smells
What is learned behaviour?
Discriminating odours
How can you show a brain region is required for a certain behaviour?
Silence that region
How do the codes differ for innate and learned behaviour?
Dense for innate
Sparse for learned
Describe the biased random walk olfactory search behaviour in bacteria
Bacteria can swim straight or tumble
If things are getting better (increased concentration of nutrient) they will run more and tumble less
Describe the biased random walk olfactory search behaviour in C.elegans
If the odour increases there will be increased firing from the cilia to the sensory neuron, increased firing to the inter-neurons which suppresses the motor program that leads to turn behaviour
Describe the olfactory search strategy in Drosophila
- Good odour, fly upwind
- If the odour is lost, wait before turning around to deal with turbulent odour pumes
- When source is reached, other sensory cues are used to reach the target
Describe the active sensing olfactory search strategy
Move head around to sample a larger space and generate fast changes of detected odour concentration
How do gustatory sensory neurons convert sensory stimuli into neural signals?
Taste transduction using metabotropic and ionotropic receptors
How are sweet/bitter/umami tastants detected?
Tastant stimulates GPCR
G protein stimulates PLCB2 which converts PIP2 into DAG and IP3
IP3 releases calcium from stores through the IP3R3 channel which opens ATP and Na+ channels
How are salt tastants detected?
NaCl dissociates into Na+ and Cl- causing the Na+ concentration outside the cell to increase
Na+ moves into the cell through ENaC
Membrane is depolarised
How are sour tastants detected?
H.Ac moves across the membrane and dissociates to form H+
H+ blocks the proton-sensitive channel (K+) that leads to depolarisation of the membrane
Outline the taste circuit
Cranial nerves VII, IX, X
Solitary nucleus of the brainstem
VPM of thalamus
Insula and parietal cortex
Describe lateral inhibition in taste detection
Bitter compounds stimulate bitter sensory neurons and inhibit sweet
Sweet sensory neurons synapse onto feeding second-order neurons
Bitter sensory neurons synapse on to avoidance second-order neurons
GABAergic interneurons reinforce
Which areas of the brain process taste?
Sweet or bitter in insula
Which experiment shows that mice avoid bitter stimuli
Bitter areas of the insula can be optogenetically activated which will make mice avoid it
There is channelrhodopsin in bitter neurons - use blue light to stimulate and cause avoidance
