Lecture 1 - Olfaction

Question 1

What is the chemical sense of smell?

Answer 1

detects volatile chemicals such as food danger and other animals at a distance

Question 2

What is the chemical sense of taste?

Answer 2

ingested chemicals that discriminate nutritious from toxic

Question 3

What is the taste system mediated by?

Answer 3

taste buds and the palette epithelium

Question 4

What does the vomeronasal detect?

Answer 4

pheromones; a sensory organ

Question 5

What is important to know about odorants and how many can we detect?

Answer 5

10,000 different odorants is the classical estimate; the new estimate is one trillion
-there is great diversity in the structures of odorants and we range in the concentration amount we need of a particular odorant to detect
-we can detect enantiomers of odorants

Question 6

Why are there no odor primaries?

Answer 6

we have so many odor receptors so we can detect so many different types of odors so there are no odor primaries

Question 7

What do the olfactory sensory neurons project an axon to?

Answer 7

the olfactory bulb

Question 8

Where do the olfactory bulb axons then pass through?

Answer 8

cribriform plate; is a bony plate and the axons coalesce to form a nerve bundle

Question 9

Where are odorants detected?

Answer 9

on sensory cilia that project to the olfactory mucosa; the cilia increase the SA on which we can detect odorants

Question 10

What kind of sensory neurons does the olfactory epithelium have?

Answer 10

-has unipolar sensory neurons which has a process that goes to the brain in the olfactory bulb

Question 11

What is the overall anatomy of the olfactory system?

Answer 11

odorants detected on the sensory cilia that project onto olfactory mucosa —-> olfactory sensory neurons on olfactory epithelium —-> cribriform plate —> olfactory bulb —-> coalesce intro nerve bundles

Question 12

How is the human olfactory system impoverished?

Answer 12

-humans have fewer olfactory neurons than rats and dogs
-they have fewer receptor genes
-rodents have a larger olfactory bulb than humans and more olfactory receptor cells and dogs also have more than humans

Question 13

Where is sensory response initiated in the olfactory system?

Answer 13

-in the cilia; in response to an odorant there is an inward current and firing of an AP when the odorant is exposed to the cilia but not when the odorant is exposed to the cell body which proved the cilia is implicated in initiating the sensory response; there are no receptor cells present at the cell body

Question 14

How does sensory transduction occur?

Answer 14

1. receptor cells on the cilia bind the ligand
2. G protein (7 transmebrane domains) exchanged GDP for GTP
3. adenylate cyclase makes cAMP from ATP
4. cAMP open the CNG (or cyclic nucleotide gated channel)
5. Na+ and Ca2+ enter the cell causing depolarization

Question 15

How was the mechanism for sensory transduction proven?

Answer 15

-identified and cloned G protein adenylyl cyclase and CNG channels in sensory neurons at the epithelial surface of the cilia
-found that each are expressed in olfactory sensory neurons by immunocytochemistry and in situ hybridization
-knockout mice for each of the olfactory transduction components show no electrical response to odorants

Question 16

When identifying odor receptors what were three things to consider in terms of the type of receptor, how many receptors, and where they should be expressed?

Answer 16

-they are G-protein coupled receptors
-there are a lot of different receptors
-should be expressed in the olfactory epithelium

Question 17

How were the odorant receptors identified?

Answer 17

Linda Buck knew they would be similar to other GPCRs in amino acid sequence and used PCR and made primers to identify all GPCRs to amplify them in PCR via mRNA made from cDNA

Question 18

What are odorant receptors like?

Answer 18

-the receptors are GPCRs and have 7 transmembrane domains
-there are hundred of odor receptor genes
-they are encoded in intronless genes scattered throughout the genome (and it was expected they they might be light immunoglobulins which use VDJ recombination to generate diversity which is not true

Question 19

How many types of receptors does one sensory odorant neuron detect?

Answer 19

one type of receptor

Question 20

How are the olfactory sensory neurons in the olfactory epithelium distributed?

Answer 20

randomly

Question 21

How did they discover the scattering of olfactory sensory neurons in the olfactory epithelium?

Answer 21

-Linked the M71 gene in mice with GFP and used a protein called olfactory marker protein to show what regions light up green and showed and showed the random distribution

Question 22

How many zones of expression are present for olfactory sensory neurons in the olfactory epithelium and what is the distribution of receptor cells in each zone?

Answer 22

-4 zones
-random distribution in each zone

Question 23

How many subsets of odorants does a neuron respond to?

Answer 23

one subset

Question 24

How many receptors can a odorant activate?

Answer 24

several receptors

Question 25

How was it proven that each neuron responds to a specific subset of odorants?

Answer 25

-different cocktails of odorants was given to a cell that has the M71 odorant receptor and found that it responds most strongly to mixture F

Question 26

Combinatorial Coding in Olfaction

Answer 26

important to note that receptor 5 can respond to multiple different odorants and receptor 14 only responds to one

Question 27

Describe the convergence in the olfactory bulb of axons from olfactory sensory neurons that express the same receptor.

Answer 27

-there are no odor primaries since there are so many odor receptors with a lot of overlap between them; while the odorant cells are randomly distributed these cells find each other and coalesce and target the same area of the olfactory bulb which is the glomerulus

Question 28

How was the convergence of sensory neurons in the olfactory bulb discovered?

Answer 28

-they put the lacZ enzyme next to an odor receptor to show all the axons of one receptor find each other and form a glomerulus in the olfactory bulb

Question 29

What is the glomerulus?

Answer 29

-is a collection of axons from olfactory neurons and dendrites of a single or two mitral/tufted cells

Question 30

Is there spatial coding in the olfactory epithelium? What about the olfactory bulb?

Answer 30

no spatial coding in the olfactory epithelium but there is in the olfactory bulb since if you give a different odorant specific glomeruli will light up in response to that particular odorant

Question 31

What are the central projections of the olfactory bulb?

Answer 31

-many targets like the cortical projection which is the piriform cortex and the amygdala in the hypothalamus which is why odors can be linked to emotions

Question 32

What to mitral/tufted cells do?

Answer 32

connect to a single glomerulus and project to several areas of the olfactory cortex
-dye is injected into a single glomeruli and it was seen that the mitral/ tufted cell projected to the cortex and amygdala; also showed plasticity in the olfactory system too