Olfactory and Gustatory Systems Flashcards
Is olfaction processed by labelled line or combinatorial code?
- Mostly combinatorial
- Labelled line coding used for specialised chemicals e.g. pheromones
What makes olfactory sensory neurons so sensitive to stimuli?
The signal transduction mechanism involved G protein coupled receptors which allows for signal amplification
What kind of receptors are olfactory receptors in insects?
Ion channels
How many types of olfactory receptor are there per olfactory sensory neuron?
One
How many molecules does each type of olfactory receptor respond to?
Each olfactory receptor responds to a different subset of chemical cues
How are olfactory sensory neurons so specific?
As olfactory neurons mature the narrow down to express one type of olfactory receptor each
What is a glomerulus?
Olfactory neurons expressing the same olfactory receptor converge onto the same place in the olfactory bulb in a glomerulus structure
What is the equivalent of the olfactory bulb in drosophila?
Antennal lobe
What is the equivalent of olfactory sensory neurons in drosophila?
Olfactory receptor neurons
Which cells form the connections between glomeruli in mammals? (2)
- Granule cells
- Periglomerular cells
Which cells form the connections between glomeruli in drosophila?
Local neurons
Which cells carry information from a glomerulus to the brain in mammals? (2)
- Mitral cells
- Tufted cells
Which cells carry information from a glomerulus to the brain in drosophila?
Projection neurons
How is specificity maintained at the second level of the olfactory system? (2)
- Olfactory sensory neurons expressing the same receptor converge onto the same glomerulus
- The projection neuron/mitral cell/tufted cell carries information from only this glomerulus to the brain
What is the point of having a synapse between the sensory neurons and the 2nd order (projection) neurons? (2)
- Adaptation in the synapse allows response to changing olfactory stimulation
- Convergence onto 2nd order neurons reduces noise and strengthens weak responses
What is the point of having inhibitory interneurons between the glomeruli? (2)
- Gain control: be able to sense changes in the stimulus even at high odour intensities by suppressing the output of the sensory neurons
- De-correlation: strongest signals inhibit the weaker signals in order to make different odours more distinct from each other
Where in the human brain is learned olfactory behaviour processed?
Piriform complex
What is the equivalent of the piriform complex in drosophila?
Mushroom body
What cells are in the mushroom body?
Kenyon cells
Where in the drosophila brain is learned olfactory behaviour processed?
Mushroom body
Where in the human brain is innate olfactory behaviour processed?
Amygdala
What is the equivalent of the amygdala in drosophila?
Lateral horn
Where in the drosophila brain is innate olfactory behaviour processed?
Lateral horn
How might a brain region be silenced in an experiment?
Optogenetic inhibition e.g. using halorhodopsin
What is a biased random walk? (2)
- Olfactory search behaviour
- If things are getting better, go straight (run), if things are getting worse/staying the same, turn (tumble)
Which organisms show biased random walk behaviour? (2)
- Bacteria
- C. elegans
How does the biased random walk behaviour work in C. elegans? (2)
- Increasing odour: sensory neuron signals to inhibitory interneuron which suppresses turning
- Decreasing odour: sensory neuron is quiet so no signalling to the inhibitory interneuron so the worm turns around
How do flies search for olfactory stimuli? (3)
- Flies upwind in the odour plume when it smells something attractive
- If it loses the odour, waits a while before turning
- Uses other senses to identify the source when it gets closer
How does active sensing behaviour work? (2)
- Organisms move their heads from side to side creates changes in odour intensity which are well processed by the olfactory system
- Can sync the sniff cycle with the head movements
How do humans sense the direction of an odour?
Humans use the slight differences in odour intensity between the 2 nostrils to determine the direction of an odour
Which molecule causes a sour flavour?
Protons (H+)
Which molecule causes an umami flavour?
Glutamate
Which tastes trigger ion channels? (2)
- Salty
- Sour
Which tastes trigger G protein coupled receptors? (3)
- Sweet
- Bitter
- Umami
What is the pathway of gustatory information from the tongue to the brain? (3)
- Cranial nerves carry gustatory signals from the tongue to the solitary nucleus of the brainstem
- Some signals are passed from there to the hypothalamus and amygdala
- The rest of the signals are sent to the VPM of the thalamus and then to the insula and parietal cortex
What is the VPM?
Ventral Posterior Medial Nucleus of the thalamus
How does lateral inhibition work in the gustatory system? (2)
- Bitter sensing neurons activate GABAergic interneurons which inhibit signal release from sweet sensing neurons
- Mechanism to stop you eating poison by emphasising the bitter taste over the sweet taste
How does the circuitry for innate behaviour and learning behaviour differ? (3)
- Innate behaviour needs to categorise odours but learning behaviour needs to discriminate between new odours
- Innate behaviour odour coding is dense (lots of neurons active) but needs to be sparse for learning
- Connectivity is stereotypic for innate behaviour so the same neurons are active for the same odour every time but for learning the connectivity is random and not developmentally pre-specified
