Principles of Neuroscience Lecture 10, Gustation & Olfaction Flashcards
What is flavour?
Flavour is the sensory experience of food and drink
What is the different between olfaction and taste?
How do they inter-relate?
Olfaction: volatile chemicals entering nose and landing on olfactory receptor cells
Gustation: chemicals taken into mouth and eliciting response in taste cells
There are separate centres in the brain that deal with each.
Flavour sensation is a combination of both taste and smell.
What factors influence flavour perception?
Taste Smell Appearance Temperature Texture Chilli Fat
What is the epiglottis?
Flap of cartilage over the trachea, preventing food entering during swallowing
Outline the different taste regions of the tongue.
Sweet
Salty
Sour
Bitter
Why are humans adapted to taste these 5 flavour groups and not others?
These flavours are important because the compound are energy rich (sugar), required for bodily functions (salt), indicate food going bad (sour, acid) or toxicity (bitterness)
Describe the sensory structures in the tongue
The tongue has papillae which are visible to the naked eye, and contain many taste cells.
Taste cells are the sensory cells of the gustatory system.
How many taste cells does a normal tongue have?
2000-5000
Can taste cells be replaced?
Yes, they are turned over every two weeks
Do individual taste cells is a papillae detect the same chemicals, or different ones?
Taste cells within a single taste bud have different sensitivities.
However, the final output from a taste cell is integrated centrally so as to create the regional sensitivities of the tongue.
Describe the generalised process of gustatory transduction
Chemicals from food enter the taste pore and interact with receptors on the taste cell.
Depolarisation of the taste cell due to transduction
Fusion of vesicles containing neurotransmitters onto the gustatory afferents
Describe the path from the taste cell to the brain
Taste cell - Gustatory afferent - medulla, Gustatory nucleus in the solitary nucleus tract - Ventral Posterior Medial Nucleus in the thalamus - Primary gustatory cortex - Hypothalamus - Amygdala
Which region of the thalamus does gustatory information pass through?
The Ventral Posterior Medial Nucleus
Which area of the medulla do gustatory neurons pass through?
The Gustatory nucleus of the solitary tract
Describe Salty transduction in taste cells
Na+ is detected by the amiloride- sensitive Na+ channels.
These sodium channels open, and the cell is depolarised.
Describe sour sensitivity in taste cells
Protons are detected by a H+ channel
The channel opens and protons rush in, depolarising the cell
By which receptor is sugar is recognised in taste cells
Sugar acts a ligand on the T1R2/T1R3 heterodimer.
By which receptor is umami recognised in taste cells?
T1R1/T1R3 heterodimer
Describe transduction after the heterodimer receptor of sugar and bitter tastes in the taste cells
- Heterodimer activated
- Alpha-gustducin activated (G-protein)
- Phospholipase C activated, which makes IP3
- TRPM3 channel opened by IP3
- Cell depolarised by membrane channel as well as Ca2+ from ER
- Vesicles release neurotransmitters onto gustatory afferents
By which receptor is bitter recognised in taste cells?
How does the transduction pathway proceed?
T2R receptor.
This is similar to T1R3 and the pathway is the same after the receptor.
What is the generalised transduction pathway in taste cells?
- Ligand binds to receptor, causing opening of Na, K and Ca ion channels
- The cell is depolarised by sodium, potassium and calcium
- Calcium dependent release of neurotransmitters from vesicles.
Which molecule is umami?
Glutamate
What is special about transduction of glutamate signals?
There are metabotropic receptors on the membranes of vesicles that are activated by glutamate
Which neurotransmitter is released by taste cells?
This is controversial. There are five. It could be any one or a combination of them
- Acetylcholine
- Serotonin
- Glutamate
- Noradrenaline
- GABA
Where do the gustatory afferents synapse?
In the gustatory nucleus of the solitary tract of the Medulla
Are the projections from the medulla bilateral or contralateral?
Neither, Ipsilateral
Where is the primary gustatory cortex?
Frontal and temporal lobes
What are the two regions of the primary gustatory cortex?
Insula
Operculum
Where are the olfactory receptor cells?
Roof of the nasal cavity
What type of cells are the O.R.C.s?
The olfactory receptor cells are neurons
However, they are embedded in the nasal epithelial cells
Describe the process of transduction in O.R.C.s
- A receptor of the basal end of the O.R.C. detects a odourant molecule
- G-olf (olfactory G-protein) is activated by the coupled receptor
- Adenyl cyclase is activated by the G-olf
- Adenyl cyclase converts ATP - cAMP
- cAMP interacts with ion channels, causing them to open
- Ca2+ rushes in, depolarising the olfactory receptor cell.
- Calcium dependent neurotransmitter release from vesicles
How many types of scents can the human nose detect?
2000
What is the pathway from the O.R.C.s to the brain?
O.R.C (axon moves through Cribiform plate) Glomerulus in Olfactory bulb, Second order neurons / mitral cells & granule cells Primary Olfactory cortex Thalamus Amygdala
Which neurons form the olfactory nerve?
The second order neurons
What is special about the central pathway for olfaction?
Information from the olfactory bulb goes first to the:
- Pyriform cortex
- Amygdala etc.
Then continues on to:
- Thalamus
- Hypothalamus
- Hippocampus
- Orbitofrontal PFC
Describe the structure of the glomeruli. Where are they?
The glomeruli are behind the cribiform plate.
The are the axon terminals of the O.R.C.s and the dendrite trees of the second order neurons
What is important about the sensitivity in the glomeruli?
Each glomerulus has information about only one type of smell.
If a O.R.C.s has receptors for multiple molecules, the axon will branch into two glomeruli.
What are the “tuning interneurons”?
These are the granule cells.
These cells interact with mitral cells, ensuring only one odour per 2nd order neuron
Describe the very specific mapping of odours onto the olfactory bulb
O.R.C.s synapsing with a particular glomerulus have the same receptive field.
This information is taken by one second order neuron to that specific olfactory bulb region
How do we perceive flavour?
Information from the gustatory and olfactory sensory cells get integrated in the brain
What is the role of Granule cells?
They are tuning interneurons. They ensure that each mitral cells or other second order neuron is sending information about one odourant
What is a mitral cell?
They are the second order neurons
They connect to the olfactory receptor neurons in the glomeruli
What determines the receptive field of olfactory receptor cells?
The odorant receptors that they to have sets up their ‘sensitivity profile’
–> they are sensitive to different arrays of molecules
Describe how the glomeruli relate to the receptive field
Each glomerulus receives information about only one odourant, they they have the same receptive field
