- [Voiceover] Smell perception in mammals involves the
interactions of airborne odorant molecules
from the environment
with receptor proteins on the olfactory neurons
in the nasal cavity.
The binding of odorant molecules
to the receptor proteins
triggers action potentials in the olfactory
neurons and results in
transmission of information to the brain.
Mammalian genomes typically have approximately
one thousand functional
odorant-receptor genes,
each encoding a unique odorant receptor.
Alright, part a,
describe how the signal is transmitted across
the synapse from an activated olfractory
sensory neuron to the interneuron
that transmits the information to the brain.
Alright, so let's draw this out.
So, this is part a, so let's, let me draw the
activated olfactory sensory neuron,
so, it might, so those are its dendrites, there,
so, I'm not gonna draw a perfect,
so those are its dendrites right over there,
and this is its axon,
it's axon,
and then this is the axon terminals.
The axon terminals, just like that.
And I could draw other details if I want,
I don't think you'd have to draw all of these details
on the actual test.
But, just to get a sense of things,
we could also draw the
well, we could do it in a different color,
we could also draw
the nucleus right over there.
So this is the activated olfactory sensory neuron,
so activated
olfactory, olfactory sensory neuron.
sensory neuron.
And let me draw, let me draw the interneuron.
So, the interneuron, so I'll draw one of its
dendrites right over there.
So this is the interneuron,
drawing its dendrites,
and then its axon.
I'll draw it a little bit better than that,
its axon just like that and it goes to the brain.
And if we zoom in on the synapse,
where we have to transmit the signal between the two,
so if we zoom in,
if we zoom in,
right over there,
we would see the axon terminal of the
sensory neuron.
So, maybe it looks something like that.
And then the, the, the interneuron
right over that.
Just like this.
And this is the synapse.
So, this is the zoomed in of the synapse.
And the activated olfactory sensory neuron,
you have this activation potential
that is going to go down the axon,
and then when it gets to
when it gets to the axon terminal right over here,
it will trigger the release of neurotransmitters.
So, these neurotransmitters are typically
hanging out in, in these, these vessels
right over here, but then when the action potential comes,
they will get released into the synaptic cleft.
And then, the interneuron is going to have,
is going to have proteins that sense, that sense
those neurotransmitters,
and that activates that neuron.
So, once again this is the interneuron,
interneuron, and we could say that interneuron,
interneuron
gets activated, actually it could be inhibited
as well, but activated by, by neurotransmitters
neurotransmitters
released into synapse,
released into synapse
by the sensory neuron,
by sensory neuron due to action potential,
due to action,
action potential.
And this is probably enough, but I
went through a few more pains to draw it out
a little bit, and you could draw other details.
You could draw the myelin sheath and all of that,
you could draw the nuclei of the different cells,
but the basic idea is that the signal goes from
one neuron to another with the release of these,
these chemicals, these neurotransmitters,
which are actually fairly, fairly small molecules,
but then they trigger the next neuron,
and then after that signal goes they get all
they get metabolized by enzymes and things
and there might be a few that just always stick around
but for the most part they signal
from one neuron to another.
And they could activate the next neuron,
or they actually could inhibit it as well,
but in this case, they would probably activate.
Alright, let's do part b.
Let's do part b.
Explain how the expression of a limited number
of odorant receptor genes, you see there's thousands,
receptor genes, can lead to the perception of
thousands of odors.
Use the evidence about the number of odorant
receptor genes to support your answer.
So, one way to think about it is, and this is
this is kind of a theory, here, is that
you don't necessarily have a one-to-one
mapping between odorant, odorant receptors
and odorant molecules, so don't,
don't necessarily,
necessarily have one-to-one
relationship
between, between odorant molecules
odorant molecules and receptor proteins.
And receptor proteins.
Maybe, maybe one protein can,
can detect multiple molecules,
maybe one, maybe a given
protein can
detect multiple molecules.
Or vice versa, or vice versa,
whoops I keep having trouble
with my little pen here.
or vice versa,
or vice, vice versa.
And so then, you could have this
could lead to
many combinations,
combinations being detected.
So even though you could have, maybe these one
thousand odorant genes,
they code for one thousand receptor proteins,
and so you might say well,
well those'll only be able to detect one thousand
different molecules, well, no,
each of those could detect more than one,
and then, and when they come in different combinations
they might trigger the brain in different ways.
And then, many, I mean
I could even add that,
this could lead to many combinations being detected
which would be different,
different smells in the brain,
different smells
as perceived by brain,
perceived by brain.
Now, there is some possibility that each of these genes,
maybe they could be coded into proteins
in different ways, but they do tell us each encoding
a unique odorant-receptor, so I like,
I like going with this one,
you don't necessarily have a one-to-one relationship,
each receptor could recognize multiple molecules,
or one molecule could be bound by multiple receptors,
and then the different combinations
of all of the above means that you could have
much more than one thousand molecules
being detected, and especially different molecules
in different combinations, you could have thousands
upon thousands of smells quote, unquote,
being detected.