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PROFESSOR: So these are the topic outlines you've just
reviewed for your midterm.
We started talking about specific systems by talking
about motor systems before the exam.
Now we're going to do sensory systems starting with taste
and olfaction.
Originally scheduled for two sessions, but I'll get through
as much of it as I can today.
And then we're going to have three sessions
on the visual system.
And then talk a little bit about audition before we talk
about the forebrain.
And that will include the limbic system and
hypothalamus.
Five classes, one or two of then on the corpus striatum.
I've been doing two but I'll decide on that
during spring break.
And then finally the neocortex at the end of the term.
And I would like you to memorize the cranial nerves.
I've given a little mnemonic there, on old Olympus's
towering tops a Finn and German viewed some hops.
You're welcome to make up your own limerick.
But if you memorize that phrase, which is very easy,
the first letters correspond to the cranial nerves as
listed above there.
And you should also know which are primarily sensory,
providing input into the CNS.
Which are the motor, these are the motor ones, oculomotor,
cochlear, abducens, and the hypoglossal.
And the purely sensory, olfactory, I've taken most
species as mostly sensory, of course, but there are a few
efferent fibers, but we don't consider it motor.
So the olfactory and the optic are sensory.
Trigeminal is mostly sensory but there is a separate motor
component to the trigeminal.
So that's a mixed nerve.
Facial is mixed.
Auditory distributor is primarily sensory again.
I'll take out that because it's mixed, but auditory
distributor, it contains efferent fibers that go to
cochlea so they do have a kind of motor function, but it's
really to modulate the input.
So that's different from the trigeminal where the motor
component of the trigeminal is controlling jaw movement.
And the spinal accessory here I should include in the motor.
There may be a few sensory fibers there too but it's
mostly motor.
The others are all mixed nerves.
All right, taste and olfaction.
The neural basis and evolution of taste and olfaction and
then a couple of special topics.
One is the birth of new neurons in the olfactory
system in the adult.
It's going on in our brains all the time.
And then some findings about plasticity after lesions that
were found in the olfactory tract.
Studies of both taste and olfaction give really good
examples of what we call mosaic evolution.
That is, structures in those systems get a lot bigger in
some species than in others.
So even if the whole brain is getting bigger it doesn't mean
these structures are always getting
proportionately bigger.
I'm interested in both of the systems, but especially
olfactory because of their importance in evolution.
We're going to talk about today.
Well before we get to the chordates there was a lot of
chemoreception.
And taste and olfaction are both chemoreceptive systems.
But even if you go back to sponges you have
chemoreception.
It's a kind of taste, you could say.
They're tasting the water.
Or maybe it's a little more olfaction.
But they're responding to the water near those pores and
that affects the way they respond.
You see them in hydra as well.
We also have non-taste and olfactory chemoreceptors in
vertebrates, including ourselves.
For example, we detect carbon dioxide levels of the blood
and we detect blood pressure also.
But carbon dioxide levels or chemoreceptors affects heart
rate and breathing.
We detect glucose levels in the blood like chemoreceptors
responsiveness to glucose levels.
And those detectors are in the brain in the hypothalamus.
And also we can respond to toxins in the blood.
The scenario called the area postrema in the hindbrain that
when you get certain toxins in your blood it induces
vomiting, vomiting reflexes.
And there's a few other examples as well.
Olfaction and taste are different in the kind of
molecule that they respond to.
And, of course, where in the body we're responding.
So we're going to start with taste.
And taste comes into what we call the visceral sensory
column in the hindbrain.
These are secondary sensory cells that not very large in
the cross section but they get this visceral sensory input
from the abdominal cavity, from the stomach and
intestines.
But the rostral part of the nucleus is called nucleus
gustatorious.
It gets taste input.
And the taste inputs come in through three cranial nerves.
We think of the seventh nerve as the major taste nerve, but
they also come in from back of the mouth, very far back in
the tongue, and even the sides of the throat through cranial
nerves nine and ten.
The chemo receptive inputs from the body come in through
the vagus nerve.
That's nerve 10.
Let's talk a little bit about the innervation of the tongue.
What nerves innervate the tongue?
There's actually three of them.
The movements of the tongue are hypoglossal movements.
The sensory input from the time is touch, so when you
touch you're not tasting.
You might taste too, but we also touch.
We have very good touch sensations on the tongue.
That's the fifth cranial nerve, same as
the rest of the face.
And then the seventh for the taste inputs.
So that's what I just talked about.
This is a parasagittal section of the head of a hamster.
This is the position equivalent to our taste buds
on our tongue.
These are the fungiform papillae in hamster.
But all the blue letters there indicate taste receptors.
And you can see that there's many of them, in the back of
the throat, in the soft palate, and even further back
in the throat and I've labeled the structures there.
Many animals have this.
We have more taste receptors in the back of our throat too
early in development, but most adults get all their taste
input just through the tongue.
Of course some of what we call taste is actual olfaction.
But that's coming in through the nasal epithelium not
through any of these types of receptors.
I think I put the Brodal reading this year
for covering taste.
But anyway, I took this out of the Zigmond book.
And I'll give you another picture of it where I've
changed it slightly.
But I've outlined here in this parasagittal section of the
entire brain the positions of the tween-brain here and the
midbrain here.
You can see the superior and inferior collicula there.
Inferior colliculus here, superior colliculus here in
the rough of the midbrain.
There's hypothalamus below the and thalamus above in the
diencephalon.
Now what I've done here is I've made in blue there, the
major pathways that lead up to neocortext.
There is a taste region in the gustatory neocortex.
In humans it's part of the insula, the cortex buried in
the Sylvian fissure.
But it comes from a nucleus in the thalamus that's part of
the ventral nucleus.
The ventral posteromedial nucleus.
The VMP gets trigeminal input too.
But the medial-most part is called the parvocellular part.
It looks a little different and this stain, that's the
part that's getting taste input.
And the taste input, some of that, at least in primates is
coming directly from the gustatory nucleus.
Now it's labeled NST here because its anatomical name is
the nucleus of the solitary tract.
That's why it gets that name.
We can avoid some of these almost synonymous terms in
neuroanatomy.
You should know it's the nucleus in the solitary tract
that gets both visceral inputs and taste inputs.
Those are the secondary sensory cells.
They're coming in there through these seventh, ninth,
10th cranial nerves.
And there are some axons that go all the way to the thalamus
from that nucleus.
But a lot more of them, just like in the auditory system,
they stop in a nucleus in the midbrain.
It's called the parabrachial nucleus.
It sounds like a strange name but it means it's next to,
para, the brachium.
And there's several brachia in the brain.
This brachium refers to the biggest one, the brachium
conjunctivum, coming from the cerebellum carrying fibers
forward into the thalamus.
So the parabrachial nucleus is the major
relay for taste pathway.
But that doesn't end of the study of the pathways.
The parabrachial nucleus has other projections into the
forebrain, mainly hypothalamus and one right to the amygdala.
So it's like the auditory system in that way.
It has a projection into the amygdala.
It bypasses the thalamus altogether.
In the case of taste it also goes into the hypothalamus.
And I've added one that I think-- there's always minor
projections that people don't talk much about.
And there's a projection that you can find in the rat and
I'm sure in many other animals.
I've not studied that in hamster although
I would like to.
It goes from the nucleus of the solitary tract directly to
the ventral tegmental areas.
Which is an interesting one because that's where the
dopamine cells are located that project
into the limbic system.
And it's an area which when stimulated electrically
results in pleasurable sensations.
Animals will work in order to turn that stimulus on.
And we talk about the reward pathways in the brain
originating in that ventral tegmental area.
And of course taste very early in evolution played an
important role in rewarding animals for
[? approachment. ?]
The other things here are simply, we remember there's
always other pathways, usually reflex pathways.
These are the ones going from the nucleus to the solitary
tract secondary sensory cells right into the reticulum
formation that leads directly to the motor neurons.
This is nucleus ambiguous which is the nucleus
controlling swallowing and vocalization both.
Seventh nerve nucleus controlling facial gestures.
Fifth nerve controlling jaw movements and so forth.
I've just given you a simplified network diagram
here of that gustatory nucleus.
I've omitted the reflex connections that have just
shown the ascending pathways to midbrain and forebrain that
you saw in the previous diagram.
So it makes it much simpler.
The pathway going up to neocortex.
Note here that I've shown that this pathway that goes
directly into the thalamus does have branches to other
parts of the thalamus.
These are what we often call the older thalamus,
paleothalamus.
Parts of the thalamus that project not only to neocortex,
but they have other projections too including some
descending projections.
They tend to be very widespread projections, not as
specific as these groups like the ventral posterior nucleus.
And it's important in evolution because it was these
pathways that projected into the corpus striatum which I
don't show there.
But that's how taste input could reach the corpus
striatum without going through neocortex.
And that was probably a very primitive connection.
Here I've just done the same diagram.
I put them on the Shmoo brain for you.
And the only trouble is that way of representing the brain
it's hard to show where the amygdala is because it's off
in the hemisphere.
So I've just shown part of the limbic system there.
Otherwise it's really the same pathways I've showing you.
You wouldn't expect taste to have the same constraints as
something like vision and audition that have important
spatial localization functions.
And that's what we find.
There are actually projections to both sides of the brain if
you follow the taste pathways from the gustatory nucleus.
But most of them are ipsilateral.
So they're like the older somatosensory pathways, spinal
reticular pathways.
But studies have shown projections to both
sides of the brain.
So from the midbrain on I show them going on both sides here
from the gustatory nucleus on either side.
OK, so this is just what we said.
We've already talked about the specialized taste systems.
We don't have time to do that again.
These are those pictures of the Buffalo fish with this
huge gustatory nucleus.
That's the part that's enlarged.
His gustatory nucleus is really the more caudal part of
the nucleus because it's coming from far back in the
mouth not the tongue.
The catfish has both seventh and 10th nerve input that
causes an enlargement of that nucleus to the solitary tract.
And the catfish, remember, can taste throughout much of his
body's surface because of the large distribution of the
seventh nerve.
Just a little on the taste reflexes to the hindbrain.
On the motor side many of them go to the salivatory nuclei.
The nuclei that elicit saliva secretions.
They're really preganglionic motor neurons of the
parasympathetic system.
Parasympathetic has a number of functions and one of them
is secretion of substances likes saliva.
It's part of the visceral motor column that we've talked
about before.
The salivatory nuclei.
And most of them have outputs through the
seventh cranial nerve.
That's all I'm going to talk about taste.
Because I want to spend a little more time on olfaction
and on taste.
I'm going to start by talking about talking about the
olfactory system in primitive vertebrates.
Because of the importance of olfaction in evolution, and
there's some interesting arguments that have been going
on ever since the Noda methods were first applied to studying
olfactory pathways in amphibian and reptiles.
Briefly it was this.
The old view was that olfaction totally dominated
the endbrain, it was in the primitive condition.
And that then other sensory systems, as I told you before,
took over large parts of the forebrain.
So we'll review that.
And now I'll talk specifically about olfactory pathways in
vertebrates, especially mammals.
And we'll talk a little bit about the receptor
organization and how that's represented in the central
nervous system before we get to the plasticity issues.
All right, this is a picture that shows a representative
cross section from an amphibian, a
reptile, and two mammals--
a primitive one, the opossum, and human.
And let's just look at where the olfactory input.
If you're tracing axons from the olfactory bulb.
you're going to find that in this animal it's going to
terminate mainly out there.
That's the piriform area of the brain.
Now in fact, amphibians if you look at other sections of the
levels of the forebrain you'll see other projections in these
ones in the lateral.
In general terms it's the lateral pallium for that area.
And similarly in reptiles it's out here.
Tends to go right to the surface.
But it doesn't go to most of the dorsal cortex.
And this medial pallium, medial-most cortex, it's
equivalent to the hippocampus in mammals.
Not very much of it gets direct olfactory input.
Here in the opossum we see a little fissure in an otherwise
smooth brain animal.
This is all neocortex.
Then there's a little fissure there.
We call it the rhinal fissure.
And the olfactory input always goes in below that fissure.
And there's some going into that area of
the amygdala as well.
This is hippocampus in the medial part.
And here's human, where the hippocampus seems to be moved
even further eventually.
And where's the olfactory system there?
Well it's right there?
Why?
It seems to become relatively smaller.
And it is relatively smaller compared to the neocortex.
But what's happened of course is the neocortex has grown so
large whereas the olfactory cortex has not grown in the
equivalent way.
And I've shown some of the other major features there of
those brains.
Now if you look, that term piriform cortex.
What does it mean?
Piriform means pear shaped.
It's not piriform.
Some people write it P-Y-R and they're quite mistaken.
That would mean the fire brain.
This is piriform.
This is the pear shaped brain.
And it's named because in many mammals, if you look at the
brain from the ventricle side you see this pear shape of
this structure that the olfactory bulb projects to.
This is actually neocortex up here and
that's the rhinal fissure.
It can be traced.
Perhaps here it might even go out this way, this position of
the hypothalamus and the optic chiasm.
there's a structure in here, at least the lateral part of
it gets direct olfactory input.
It's really the base of the striatum.
Olfactory tubercle is really part of the striatum and it
gets direct olfactory input.
And this is all piriform cortex.
The caudal most part of it here does get indirect
olfactory input.
But it's getting a lot of other sensory input too.
That's the entorhinal cortex.
It's become very important in humans and other primates
because of its major it has in sending input into the
hippocampal formation.
And this structure that I've outlined there is the position
of the amygdala.
This is actually a ventral view of a rabbit brain.
It's traced from this picture, which we'll look at again in a
little while from Alf Brodal's textbook.
But let's look first at projections in primitive
vertebrates like the lamprey and the hag.
We've mentioned them before.
This is one version of a cladogram that shows primitive
ancestor here the line to amphibians branched off from
much later than hagfish.
Hagfish branched the earliest, that group of animals from the
primitive ancestor.
Lampreys a little bit later.
It made those animals particularly interesting
because they appeared to be similar to animals that were
more primitive than other living animals.
It's not that there weren't a lot of other animals, but we
can't study anatomically the ones that are extinct.
So we have to pick the ones that are still surviving.
When people first studied olfactory projections trying
to test, let me show you why were
these studies so important.
This is my summary of the hypothesis
that people were testing.
The idea is, and I talked about this before, that the
early forebrain was dominated by olfaction.
It originated as an olfactory structure.
So I show the olfactory bulbs there at the top projecting
into most of the forebrain.
And then the ideal is that through projections from the
mid-brain, other modalities like vision, audition, and
somatic sensation developed into the forebrain as well.
And I'm suggesting they initially simply overlapped.
And then as they became more dense they segregated.
I've seen this exact same process happening in the
visual system where I can get the two eyes to project into
the same coliculas.
And if I follow it in development they'll grow in--
this is an abnormal connection, I've made the
connections cross the midline abnormally by a lesion--
they will mix like this.
Just like this.
But then as the projections from each of the sides
actually become denser and denser, they segregate.
They compete with each other.
And this is what happens.
So one eye occupies one part, the other eye occupies the
other part.
I'm suggesting the same kind of thing was happening here.
So they've parceled out the forebrain.
So now what evidence is there for that?
First study was of the sea lamprey by these couple of
Russians published in 1993.
And they looked at projections of the olfactory bulb and of
the thalamus.
The thalamic projections in this section of the, it's
mostly forebrain here, sorry mostly endbrain.
But some of this is diencephalon there.
But the thalamic projections are shown by the plus signs
and the olfactory bulb projections in the dots.
And you can see that there's large areas of overlap.
The whole structure is getting both kinds of input.
Even here in this primitive hemisphere there is some
thalamic input and a lot of olfactory input.
So it's exactly what this middle picture would predict
existed in early evolution of the systems.
And then the hagfish, which is an even more primitive animal,
was studied almost the same time as the other study by
Northcutt and the student Wicht.
Northcutt was a comparative anatomist who had, on the
basis of studies about olfactory projections in
reptiles and a few amphibia claimed that the old
hypothesis was wrong.
So then he undertook a study of this very primitive animal.
This is a top view of the brain.
You see the telencephalon there isn't much bigger than
the whole, just the olfactory bulbs out in front there.
And here the diancephalon is only slightly smaller.
And there within the telencephalon.
TM means the tectum mesencephali.
So that's the midbrain tectum.
And then you have the large hindbrain behind it.
Now what we're going to look at is cross sections made at
these levels that you see here.
So the first one was done right through there.
And it's showing the lesion they made.
And then we'll look at a few sections behind showing where
the projections went from a huge injection they made into
one olfactory bulb.
Like you see there.
All they're doing is putting the section in a microscope
and tracing where they can see the label.
They were labeling the axons right to their terminals.
They were also labeling cells that projected to the bulb,
but we're interested in these axons.
And you get a little bit behind the bulb and you start
seeing these axons radiating out over the forebrain.
And here they are a little further back.
And we see the entire surface of the forebrain is covered
with olfactory projections.
They're also going deeper and they end up
going to both sides.
They didn't stick to one side.
If we go back, this is the transition now to tween brain.
There's still a lot of olfactory projections here at
the surface and some deeper and the diencephalon.
Even further back in the diencephalon there's still
some olfactory projections.
The only dyanstolic area to get olfactory projections in
any mammals is a tube going into the hypothalamus and
there are not many.
So this was their summary.
They show the cladogram down here and picked out what they
thought was a representative amphibian that had limited
olfactory projections to this lateral pallium.
So that's like piriform cortex of a mammal.
This medial area, so the olfactory tract divides into
medial component and lateral component.
But important point is there's quite a bit of endbrain there
that's not getting olfactory projections.
But in the lamprey a much more widespread projection.
In the hagfish it seems to be a complete dominance, even
parts of the diencephalon getting heavy inputs of the
olfactory system.
So basically supporting this kind of stage in those animals
or even perhaps the first picture since we're not sure
about all of the other sensory representations in the hagfish
like we are in the lamprey.
So now what about mammals?
First of all, the quantitative data of Heinz Stephan.
He collected a lot of data on the relative sizes of brains
and different components of the brain.
I've shown some of it before.
I just wanted to point out these extremes.
We talk about mammals that are macrosmatic having a big
olfactory system like the dog.
Animals that are microsmatic having very
small olfactory systems.
I want you to look at the upper left and the lower right
here because they're comparing the size of the medulla
oblongata on the abscissa here and then on the ordinate of
this diagram is the size of the hippocampus.
And just note that as one increases so does the other.
That's what we call concerted evolution.
You would get a similar result if you just took the size of
the whole brain against the size of the hippocampus.
But look at the upper left now where they've done the same
plot of medulla on the abscissa, but on the ordinate
they plotted the size of the olfactory bulb.
And you see it's much more variable.
Some animals have a relatively large olfactory bulb like
these animals.
Other animals relatively small olfactory bulb.
Human relatively small.
We want to compare animals along a line like that so ones
with a relatively small olfactory system are down
here, ones with a relatively large factory
system are up there.
So I want to look at some connections now.
Let's start with this diagram because I want to show you
that the olfactory system actually has two major
components.
We talked mostly about the main olfactory bulb which is
outlined in blue there.
And those are the projections of that structure, the main
ones we talk about.
But there was a discovery not all that long ago though it's
been some time now, that pheromones that when they're
detected by mammals use pheromones to identify the
members of their species and important in mating.
The input goes through the vomeronasal organ and the roof
of the mouth to part of the olfactory bulb called the
accessory olfactory bulb.
And it's shown there in red.
We don't have that but it's very prominent in ungulates,
for example.
And that part of the olfactory bulb has a projection that
differs from the rest.
It projects quite specifically to the cortical nucleus of the
amygdala there in the temporal part of the brain and a little
nucleus right in front of it, the nucleus of the olfactory
tract that's really like an anterior part of the amygdala.
There's the medial nucleus the amygdala gets a little bit of
the projection.
And functional studies have shown the importance of that
part of the amygdala in controlling mating behavior in
a number of animals including the hamster, not just the
ungulates, but a number of different mammals where
pheromones are important.
Now here in humans, in a section through the head, this
is the position of the olfactory epithelium.
The bone here that separates the nasal cavities from the
brain cavity is called the cribriform plate.
The olfactory bulbs sit just above it.
And they're showing their little channels through the
cribriform plate where the olfactory filaments they call
them, the axons of the primary olfactory neurons that are
located there in the epithelium are going into the
olfactory bulb.
Now this is a picture of the human brain.
There's the rabbit brain again.
And really all of this they show is a little bit
restricted, but it's all of this cortex.
That's the endorhinal in the back.
Just a little bit of that is getting olfactory input.
That's the position of the hypothalamus
which is why it's white.
But in human now, this is the side view.
Here's the olfactory bulb and I will
indicate that in the red.
So there's the bulb.
And this is the tract going back.
And now we see a little bit of the projection there at the
base the brain.
And then we can follow it right in to the front of
temporal lobe cortex there.
Very little of it goes into the enter rhinal.
I'll just draw a few there.
The smaller dots there are entorhinal cortex.
I'm sorry I kept this picture.
There's a nicer picture of ventral view of the human
brain in the [? Hamer ?]
book that I found, but I haven't had
time to scan it yet.
Maybe I can do that and show it to you later.
Just a little easier to compare
with this rabbit brain.
And this is Brodal's picture which then I flip on its side.
And he under-represents the piriform cortex projections so
I've added there, right in here, a few more projections
because this is actually a pretty extensive projection
before you ever get to the amygdala back
here which he's showing.
Amygdala and periamygdaloid cortex and the cortex near it
in the temporal lobe near the hippocampus.
He shows a few of the projections here which are not
large the septum and hypothalamus.
And notice there's a projection
that crosses the midline.
Comes out of the bulb and crosses--
actually he's showing one coming from the opposite side,
but they go in both directions.
And we'll see that now.
First, let's look at the cells a little more carefully.
This is a picture of receptor cells in the olfactory
epithelium with their dendritic end sticking up in
the olfactory mucosa.
And that's where molecules from the air bind to receptor
molecules there and the dendritic portions of those
receptor cells.
To activate the cells their axon then goes into the
olfactory bulb through this cribriform plate just shown by
the line there.
And there they're contacting dendrites of the mitral cells,
these large cells in the olfactory bulbs.
There's also processes of the so-called periglomerular cells
that are a kind of interneuron that you find inside the bulb.
You have interneurons up here contacting mitral cells and
interneurons here that to do so also but at the level of
glomerulus.
And note here the axon of the mitral cell goes out into the
olfactory tract that's projecting to all that
olfactory cortex.
But then there's axons.
A number of them are shown in the diagram that contact these
interneurons in the bulb.
We call them centrifugal because they're coming from
the center, from the brain someplace, and going out
towards the periphery.
In this case to the olfactory bulb.
And they're contacting these cells.
So now let's look at some of Cahal's pictures.
Here he was using a kitten olfactory bulb.
In this section he's drawn several mitral cells and he
shows the dendrites extending out towards the surface and
then ending in this bush-like arborization.
He also shows axons coming from the olfactory epithelial
coming in and in a bush-like fashion too.
So of course actually these correspond to those.
Remember he's just showing a few cells.
So axons come in and they form these bushes that end on the
bushes of the dendrites.
That's what makes up the glomerulus.
And there you see some of the periglomerular cells there
also and they also end in those little bushes.
Because they're ending on the same glomerulus.
So here's a lower [UNINTELLIGIBLE] picture.
He likes to draw these so you get a general orientation.
And here the section is a sagittal lung or parasagittal.
Then here he shows the glomerular eye in the bulb.
The mitral cells.
Their axons go back and there's the
lateral olfactory tract.
And these are the dense terminal arbors of the lateral
olfactory tract that extend throughout the
whole olfactory cortex.
So that's in general the way the olfactory axons travel.
They travel on the surface of the brain, the ventral part of
the brain, ventral and ventrolateral.
And then they arborize here.
And their arbors are a form of collaterals.
It's not that each axons just finds its proper place in the
olfactory cortex and terminates just there.
That's not how it works.
They're very widely branched axons.
Almost all of these terminals are from collaterals.
I think he shows that on-- yeah.
This is the lateral olfactory tract up here and
there you see axons.
You can actually see the little tea junctions.
They branch and they're terminating on the dendrites
of these cells in the cortex.
Note that the deeper lying cells are pyramidal in shape
just like a neocortex.
This is a more primitive cortex.
There's only a couple of cell layers.
And you can see the dendrite going up towards the surface
and contacting the olfactory axons.
Now here's another low power view where he's made a section
through the bulbs and then back through the
brain behind it.
And you see axons going between the olfactory bulbs
there from one bulb to the other crossing through a
commissure that's called the anterior commissure that's
just next to the corpus callosum that
connects the neocortex.
Notice that same commissure has fibers that go into the
hemisphere including neocortex.
That's a different limb of the same commissure.
Now in a similar picture, here's Cahal's more
diagrammatic picture showing information
flowing through the bulb.
Here he shows, you can see his arrow there, coming from the
primary olfactory neurons that end in these glomeruli contact
the mitral cells.
The mitral cell axons, you can see the mitral cell axon going
back into the tract.
And then there it starts to arborize the cortex.
And there's also axons that crossover, contact
interneurons in the opposite bulb.
We don't know as not much as we'd like to about two
efferent, those axons, the centrifugal axons that contact
the bulb in that way.
We only have time to just introduce this.
A little bit now about the organization of that initial
part of the projection, from epithelium primary sensory
neurons to the secondary sensory neurons in the bulb.
If you take a little bit of retrograde tracer like HRP and
you put it one small place in the bulb, you find out you get
a whole region.
Or if you can't keep it confined to one glomerulus you
get a small region of bulb.
You find you get this longitudinal stripe in the
nasal epithelium.
If you look at, this is a more realistic picture here in B,
it will follow a stripe like that.
Well if you have a marker for receptors that are located in
the membranes of the primary sensory neurons, they're
located in these longitudinal stripes too.
So for example, here's a picture of different gene
expression zones corresponding to these receptors and you'll
see that different molecules are distributed in different
places but always in these longitudinal zones.
When they labeled green, yellow, blue and red there to
show these positions.
And each of those segments of the olfactory epithelium
corresponds to a zone in your olfactory bulb.
And studies have shown, here's a olfactory receptor axons
corresponds to one particular receptor that shows they've
marked the whole cell.
You can see where it originates in the primary
sensory neurons.
You can see the axon going in.
Those converge on particular glomeruli.
Usually there's more than one.
Talking about a single glomerulus.
There's usually several of them.
But they're very specific.
And that's given rise to speculations about the
receptor code for different orders.
Here's the diagram, we'll end with this.
Here's one of those longitudinal zones in the
olfactory epithelium.
You see they just show two kinds of receptors.
There's actually many different kinds.
In here the one they show in black the axons are all
converging on just two glomeruli out there.
So all these glomeruli in that zone of the wall are receiving
input like that from receptors of cells located in that one
zone of the olfactory epithelium.
That's the kind of topography that's been discovered.
So we'll take up with the slide next time
after spring break.
Usually I finish all