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Lecture 14 - Excitable Cells (15/03/2018)
Sensory Pathways II
Olfactory System - Smell
The periphery olfactory NS is similar across species:
The structure of the peripheral olfactory system is very similar in vertebrates and in insects.
The example given is of the drosophila and the mouse.
Olfactory sensory neurons of a fly are located on the fly's antennae and maxillary palps, in a mouse they are on its nose.
Each olfactory sensory neurone expresses only one type of olfactory receptor, but there are many different types.
Receptors are located in the neuron's dendrites, and are exposed to the outside world,
where odorants (smelly molecules) come from. Olfactory receptor determines what molecules (odorants) will be detected by that particular one.
Neurons that express the same receptor type will converge onto the same place in the brain called the glomerulus.
The fly has about 50 glomeruli, whilst a mouse has about 2000.
Projection neurons are the second-order olfactory neurons and they connect the olfactory bulb with the cortex in mice and the antennal lobe with the higher order brain centres in flies.
in human's odorants pass through the nostrils and are bound by the olfactory receptors,
located in the olfactory cilia (aka dendrites of the osns). GPCRs:
Human olfactory receptors are GPCR's, with the odorants as the ligands.
The receptors are associated with a specific type of olfactory G-proteins.
The odorant binding leads to the opening of a cyclic nucleotide-gated channels and the depolarisation of olfactory receptor neurons.
The G-protein is activated.
Its alpha-subunit in turn activates adenylate cyclase ACIII, which leads to an increase in the concentration of cAMP.
cAMP acts as an intracellular ligand and opens the cAMP-gated cation channels, allowing
Ca2+ and Na+ into the cells.
This leads to the depolarisation of the olfactory receptor neurons in response to an odorant.
The elements of this pathways were figured out by doing experiments in genetically modified (mutant or transgenic) mice.
Insect olfactory receptors are not GPCR's, but are instead ion channels, which are directly opened upon binding of an odorant.
Odorants have a diverse chemical structure, and we cannot predict how something smells purely based on the structure of the molecule.
Each odorant binds to various receptors and activates many neurons, and in turn each neurone is activated by many odorants, acting as a combinatorial code.
The image shows responses of 60 individual frog neurons to a panel of odorants. The size of the red circles indicates the strength of responses.
Each olfactory receptor can bind and detect a lot of different molecules, but with a varying affinity.
In turn, each chemical (odorant) can bind to many different types of receptors.
Thus, the identity of the chemical must be somehow encoded in the combination of receptor neurons and glomeruli that this chemical activates.
This is similar to how colour information is extracted by comparing the activities of your 3 cones types, however here there are not 3, but thousands of different receptor types that need to be compared.
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