Smell and taste, and anatomy of the olfactory system
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True or false: olfaction relays through the thalamus?
False - this is the only special sense not to go through the thalamus
What is located on the rostral/caudal side of the cribiform plate?
- Rostral - olfactory epithelium
- Caudal - olfactory bulb
Where is the vomeronasal organ located and what is its function?
It is located between the nasal and oral cavities. Its function is mainly to detect pheromones.
What type of response involves the vomeronasal organ and which species do this?
Flehemen response - draw air into the vomeronasal organ, usually over faeces or urine, to try and draw pheromones into here. Cats and horses use this.
Why must odourants be partially lipid soluble?
As the cilia in the olfactory epithelium project into lipid rich mucous. Therefore odourants have to be partially lipid solve to be solubilised in the mucous layer and get to the receptors on the cilia.
How does an odourant eventually generate an AP?
If the odourant strikes the receptor you get activation of adenyl cyclase which leaded to ATP activating cAMP which opens ion channels. Calcium and sodium enter the cell, calcium opens chloride channels allowing chloride to leave the cell. This eventually leads to depolarisation and an AP.
Describe the structure of the olfactory bulb, with regards to receptors
In the olfactory bulb primary receptors from the olfactory epithelium synapse with mitral cells (secondary neurones) at the glomeruli.
Why are mitral cells very sensitive but poor at localising smells?
There are lots of primary neurones but few secondary neurones meaning the receptor field for each mitral cell is very large and this gives them great sensitivity. However, having large receptor fields means there is poor localisation (although this is slightly improved by lateral inhibition)
Where do axons from the mitral cells project into?
The limbic system
True or false: olfaction is highly sensitive?
True - about one odourant molecule per 1000 receptors generates a response
Olfaction shows complete/incomplete adaption?
Complete adaption - don't detect persistent odours.
List some of the reasons dogs have a better sense of smell than humans
- Larger olfactory epithelium so more receptors
- Twice as many active receptor proteins
- Olfactory bulb is larger
- Vomeronasal organ is inactive in humans
Which sense is closely linked to olfaction?
Taste - over 75% of flavour comes from sense of smell
Where are sensory cells in the tongue located?
Sensory cells are located within taste buds, which are located within papillae in the tongue.
What are the different types of tastants?
Salty, sour, sweet, bitter, umami
What does each tastant detect?
- Salty - effectively Na and Cl ions
- Sour - measure of acidity
- Bitter - measure of poisons
- Sweet - from sugars e.g. glucose, fructose
- Umami - from monosodium glutamate
What are the central pathways for taste?
From the tongue stimulation of the sensory neurones is passed up through the facial nerve, glossopharyngeal nerve or vagus nerve to the medulla. From the medulla it goes to the thalamus and then sensory cortex.
Describe the pathway for the other sensation of taste that comes from the nose
Sensation passes from the nasal epithelium, to the limbic system and then sensory cortex. (Some tracts pass straight from the olfactory bulb to the sensory cortex)
True or false: you do not need both pathways to keep your sense of taste?
False - the two pathways must be active to have a sense of taste, if you lose your sense of smell you largely lose your sense of taste
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