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PROFESSOR: OK, we have to finish talking about axon
growth today.
Mostly about regeneration of axons.
But first we have a little bit--
we were talking about sprouting, and the factors
affecting collateral sprouting after damage in the brain.
And I wanted to talk about the ability of axons to compete.
Their growth vigor can be affected by various factors.
For example, chemical factors we've already mentioned.
Growth factors like NGF.
And remember this figure that shows NGF effects on a dorsal
root ganglion, or a synthetic ganglion
would be very similar.
But growth figure can be affected by activity as well.
So for example, when axons representing the right eye and
the left eye, parts of the retina that represent a
similar part of the visual field, grow
into adjacent areas.
If one eye is shut for a while, the more active eye
will take over more space.
That's because the axons are more able to compete for
terminal space there.
And that happens in the cortex, it's also been
observed in the tectum of some animals.
This illustrates a growing axon early in development
where all its endings have a lot of ability to compete.
But when they extend, and the arbor reaches a certain size,
the growth figure goes down, and now the axon
won't compete anymore.
But we can alter that also by damaging the growing axon.
So for example, here I show an axon that's reached an arbor
of full size, but now we do partial damage.
The best evidence for this has been obtained in
the olfactory system.
When you do that, the arbor will grow in other places,
further than it normally grows.
So this part of the arbor will sprout additional terminals.
But it might also develop terminals more proximally.
This has been demonstrated both for the optic tract and
the lateral olfactory tract.
So it appears that axons will grow until they've reached a
certain size, and then they lose that ability to compete
for terminal space.
So keep in mind the two kinds of factors, extrinsic factors,
like chemical factors from the outside, or activity.
And intrinsic factors, which is this conservation of
terminal quantity.
That is, it has to be due to a factor inside the cell.
The cell is programmed to form a certain quantity of arbor.
Once it's formed that, it won't try to
grow additional arbors.
You observe it in the scan, for example.
You observe it in innovation of muscle.
That's, in fact, where some of these factors were first
discovered, but then now it's been discovered in central
nervous system in a number of different systems as well.
So now we can think of this term, plasticity, very broadly
in terms of these different developmental stages, and of
course, learning.
We now know that there's some of these developmental stages,
like proliferation of cells, cells undergoing mitosis,
migration of cells, differentiation of the cells
grow to their axons.
And then all of these things happen to some degree even in
the mature brain.
It's only in recent years that we found out about that
continuation in the mature brain.
In fact, for a long time we doubted whether any structural
changes happened in the mature brain at all.
But now, it's been seen as a correlate, at least at the
level of synaptic terminals.
And [INAUDIBLE] arbors as a correlate of
many types of learning.
And this factor, I'd like to say a little bit more about
now, and we'll return to some of those other things later,
the phenomena of neural depth.
I've mentioned what a trophic factor is.
That many cells have to obtain a certain amount of trophic
factor from their terminals in order to survive.
If they don't get enough, they will kill themselves.
It's a form of cell suicide.
We call it apoptosis.
Cells early in development, of course, aren't in contact with
their terminal, so how do they survive?
They apparently survive because there are intrinsic
factors that are present when they're growing.
The cell with the growing axon that disappear later.
And that has been found in the visual system, for example,
and there's evidence now for these two different factors
that play such a role.
Let's look at some evidence for trophic effects that keep
cells alive.
These are experiments on the motor neurons that innovate
the limb muscles.
And I've brought pictures from the Purves and Lichtman
chapter, which some of you have read.
Look at these pictures at the bottom.
Here you see a chick with two legs on one side.
Here's a frog with three legs on one side, one real leg on
the other side.
His front legs haven't been tampered with in development.
But simply by transplant techniques, you can produce
animals like this.
Why would they do that?
Because of the phenomenon of cell death during development.
When motor neurons develop in the ventral horn of the spinal
cord, and similarly for brain stem motor neurons, there is a
super normal number of neurons produced initially, and then
some of them die.
And they die after the axons have grown out and they've
established terminations.
They've innovated their target.
So the question was why?
If it's that some of them simply can't get enough growth
factor, well then if we provide them additional places
to terminate where they can get that growth factor, like
additional muscles, as in these cases, put an
additional leg there.
Then many of those motor neurons shouldn't die.
And that's exactly what happened.
So if you look at the spinal cord of these animals and
count the number of motor neurons on the side with the
additional legs, on the side with the normal single leg,
you'll find different numbers of motor neurons-- there are
many more motor neurons where there's the additional muscle
tissue being innovated.
Well of course, that means that population size matching
could be a major reason why this mechanism exists.
You don't have to determine in the genome the exact number of
neurons needed in a population.
You can produce more than what's needed, and then when
the axons form their terminals, they occupy all the
terminal sites, the ones that don't find a place to
terminate will simply die off.
So we get a matching source of, the source of a cell, cell
number, and the terminal size.
But another thing it does possibly is correct for
errors, like when we get a topographic map.
But I've already mentioned that we know about other
factors that determine topography.
But what I didn't mention is that when a topographic map
forms, like the map of the retina in its growth to the
midbrain tectum, it forms a topographic distribution, so
one part of the retina's represented one place in the
tectum, another part of the retina's represented in
another place.
The map often has mistakes in it.
It's not a perfect system.
And there is a disappearance of most of those mistakes
during the period of cell death in the retina.
Because in that system, just like with motor neurons
innovating muscles, there's a super normal number of neurons
in the retina, and then some of them die.
And during this period of cell death, there's some error
correction.
So we think there is evidence for both of these kinds of
purposes of naturally occurring cell death in the
nervous system.
Neurotrophins are believed to be a major trophic factor that
explain some of these phenomena.
We know that, I mentioned that activity
increases growth bigger.
Activity also is associated with release of growth factors
in the neurotrophin family.
It's also been associated with certain forms of learning, and
it's been found more recently when new neurons are produced
in the adult brain.
So some of these phenomenon may come up
later in the class.
What I want to say now is a little bit about another form
of plasticity, the regrowth of axons after damage.
It's an area that I've worked on for a number of years.
The general problem of axon regeneration is this.
When axons--
axons, of course, when they're growing seem very plastic, but
can they regenerate if you cut them?
The answer is yes, if you cut them really early when they're
still elongating.
But after they enter their terminal area and they start
to arborize, they go through a transition to a different
state where very few of them can regenerate anymore.
In fact, many systems we can't get any of them to regenerate.
And the ones that do, don't grow very far, or they aren't
able to establish permanent connections.
So something changes in the adults.
And the question is what is it?
How much of it is intrinsic, the intrinsic ability of the
neuron changes, but simply can't elongate anymore?
Or could it be there's just inhibitory factors in the
environment?
The dominant theory of people working on the problem of
regeneration has been that inhibitory factors appear, and
there's a lot of evidence that growing axons are inhibited in
the adult tissue environment when they're not inhibited
from growing in the embryonic tissue in [UNINTELLIGIBLE]
Now, when axons can't--
but there's additional evidence that intrinsic
factors also play an important role.
Maybe not for all neurons, but many neurons, probably the
majority in most systems, can't regrow after the animal
reaches a certain age.
And then we know that if an axon's cut off from its
termination site, it's not going to get those growth
factors from the termination site anymore.
So what's going to happen?
Well the cells will gradually die.
In fact, when the animal's pretty young,
they die pretty fast.
But let's say a person suffers damage here in the occipital
cortex, the visual cortex.
It gets its visual input from a cell group in the thalamus
called the lateral geniculate body.
Many of you know about, and if you don't, you learn about it
later in the class.
So a person who suffers a damage as an adult to the
visual cortex, over time--
and this happens fairly slowly in the human--
the cells in the genicular body will shrivel--
they look like they're losing weight--
and then they'll start to die.
Well as they die, now what happens to the axons coming
from the retina that are terminating on
those thalamic cells?
Some of them die, but not all of them.
Some of the cells in the retina start dying too.
Why don't all of them die?
Well, because those same axons that terminate in the
genicular body, some of them at least have terminals in
other places too.
So they survive because of what we call sustaining
collaterals.
So when we want to induce regeneration, our first
problem is just to preserve the damaged cells.
We have to keep them from dying, because they're not
going to be able to grow axons if they die off.
Another problem is to provide a permissive environment.
We know they don't like to grow in adult tissue.
We have to give them a substrate to grow on.
So you might have to provide a bridge.
They don't like to grow through scar tissue that forms
in the brain due to both connective tissue invasion,
and glial cells.
We call that a glial scar.
In the spinal cord, it's both a glial scar and connective
tissue innovation from the meninges.
So we have to provide a permissive environment.
Now, if the cells have lost their growth figure when
they're older, we might have to use chemical factors to
promote that growth.
That's a separate factor.
And then when they grow their terminals, they might reach
terminal regions that have been invaded by other axons.
So now what are we going to do?
After all that, we get regeneration, other terminals
have formed.
And that happens too, due to collateral
sprouting, for example.
So that might require various forms of plasticity of the
regenerative connections.
So sometimes even after regeneration takes place, the
functional recovery does not occur, or
it occurs very slowly.
Rutledge Ellis-Behnke who's now a professor at University
of Hong Kong wrote a paper on this.
He called it "The Four P's--" because of each of those words
starts with P--
"Four P's of Regeneration." To correct a notion in the
literature that there was just one factor we
had to worry about.
Cells lose their ability to regenerate in other animals.
And there's actually multiple factors.
I'm going to tell you a little bit about using peripheral
nerve as bridges.
This is based on the discovery that axons in the brain that
won't grow in the adult brain environment, will grow into a
peripheral nerve that's artificially
transplanted into the brain.
Like you can take a nerve from the leg or the eye.
From the same animal so you don't get an immune response.
And you put that in the brain and axons will grow into it,
whereas they won't grow in the brain.
Then we've developed some new materials that can used to
form bridges.
And for this we've collaborated with bioengineers
with some nanotechnology applications.
And then in addition, there's chemical methods for
inhibiting scar formation or for breaking up scars, and
also the possibility of genetic transfections to alter
the ability of axons to grow, cells to grow their axons.
This has been much more difficult because the
techniques for genetic transfection don't always work
very well, so we've focused on the first two here.
Now in this picture, you're looking down on the brain stem
of a hamster.
This is the coddle into the midbrain, rostral into the
midbrain, thalamus.
So the hemispheres I've taken away just to show you the top
of the midbrain.
Spinal cord's back that way.
Here comes the optic tract fibers.
Most of these came from the other eye.
Most of them crossed over.
What I'm showing here is the position where I
made a knife cut.
Cut all the axons entering the tectum.
And then across--
here's a side view now where I'm showing the position of
the cut there.
And I'm showing how I can take a little piece of peripheral
nerve from the leg of the animal.
And it's a very tricky procedure because I have to--
I can't just plunk the endings in on either side.
It doesn't work.
I have to de-sheath little bundles of axons.
They're all in these little bundles inside separate little
packets of axons.
I get them out and I do some
preliminary work on the endings.
And I get all those little bundles coming out.
And then I take a tiny glass pipette, wire that nerve
across and poke those endings just below the surface in the
superior colliculus or optic tectum here.
And right into the surface of the optic tract in
front of the cut.
And I can fit up to three of those pretty easily across a
single cut in the optic tract.
So I'm going to show you--
I hope I have it here.
Here we are.
Well, if that doesn't work--
that worked.
This just shows you behavioral test
where we show the animal--
the reason he doesn't actually look up, even though the
stimulus is placed up here, he's not looking up because
the projections that we've generated
isn't totally accurate.
But he does turn.
Let's just show through the labeling of retinal fibers,
you see some of them coming through a bridge here.
It's one of three bridges, and these are
terminals in the tectum.
They've all regenerated.
The tectum had no terminals before a procedure.
This is just a close-up.
This animal is 5 and 1/2 months old.
So he's a pretty old hamster.
Hamsters only live around a year.
Sometimes a year and a half or two years.
It's sort of a middle-aged hamster.
Now, then we went to another procedure initially on young
animals, but now we've done it on older animals, where we
make a deep cut in the midbrain, cutting all the
fibers going into the coddle tectum.
So where else, we make the cut up there.
But now we're not going to use the peripheral
nerve bridge at all.
We're not going to mess his leg up by taking a large nerve
out of the leg.
Instead we're going to inject that site with a
self-assembling peptide solution.
It's an engineered material that's
not available naturally.
It looks like a little water in the vile.
It's a very dilute solution.
But it has a wonderful property.
As soon as it contacts a salt solution, for example, the
cerebrospinal fluid will work very well.
It forms, it assembles, it self-assembles into this
microfiber mesh work.
And we got interested in trying that.
It was developed at MIT by Shuguang Zhang in biological
engineering, and fellow from this department he was working
from, Todd Holmes.
He was working with Dick Wurtman.
And the initial discovery was if we look at that cut, if we
don't put the material in there's a gap.
Axons can't grow across a fluid gap.
No axons can regrow in these brains.
But when we did that injection, most of
the gap closed up.
And that was very consistent.
So here's the model now.
I'm showing the eye connecting mostly to the opposite side of
the midbrain, and then controlling by means of this
cross tectospinal pathway that we've mentioned in the class,
and caused the animal to turn right for something, something
shown on the right eye.
I should say in the right visual field.
So now here's the brain.
In this case, it's a photograph, and this is the
superior colliculus here, and this is where
I'm making my cut.
Sorry, I don't have the journal file.
So there's the lesion again.
So if we make that kind of a lesion, we discover the gap
has axons that grow into it.
There is regeneration.
In fact, I should go to the journal here so I can--
there's a picture of where the hemispheres would be
if I left them in.
I drew them in on the picture here.
And this is what I did here in this animal.
I made this kind of cut.
Cutting all the axons.
The axons are coming in like that over the thalamus into
the colliculus behind.
Now, if I do this biology on the brain and label all the
retinal accents, so I get them to pluress.
Here is the position where my cut was, but you can see, the
axons have crossed and they've innovated the
tectum behind them.
So we're getting a lot of regeneration in these cases.
Much more, in fact, than we had expected.
So let me now show you first of all, a blind animal.
Look in here.
If I can get the video to go on.
He's blind on the right side.
This just proves it.
He's showing the stimulus on one side, no response at all.
Unless you touch the whiskers, of course.
But as soon as you move into the other
visual field, he turns.
So now this animal has the same lesion as the other one.
The only difference was the first one, the control, just
had saline injected.
This one had the
self-assembling peptide injected.
It doesn't turn quite as fast as normal,
but it's pretty good.
He's got vision and he can find his reward.
He won't start for lack of vision.
So that'll give you just a little bit of flavor of some
of the research in that area.
It's still going on at the University of Hong Kong, and
I'm often asked by students whether I'm doing it here
because they'd like to work on that.
But getting support proved to be much easier in Hong Kong,
so we moved that project over there.
Let's talk about, just give an introduction to the motor
system now.
This chapter's been posted.
I'm sorry I didn't post the list of readings and questions
until today.
I just forgot about it.
But I did post the chapter.
So if you haven't read it yet, please read it.
And I'll get the next one up by tomorrow.
So now we've looked at all levels of the central nervous
system in an introductory way.
And now we're going to begin looking at specific functional
systems, and we're going to start with motor systems.
I'm not going to start with a motor cortex, which you often
hear about in discussions in the department
here, are motor systems.
It came relatively late, invertebrate evolution.
I want to consider first, the evolution of motor control,
the major functional demands, which we've mentioned before.
They certainly preceded the vertebrates and the neocortex.
We'll look at what structures for motor control are present
in all of the vertebrae.
Then we'll look at what are the elaborations of that
system into the mammals.
And when we start talking about specific organization,
that'll be tomorrow, we'll start with motor neurons and
see how they're organized.
So these are the three major types of movements that we
find represented in membrane, remember, we've
talked about before.
Locomotion for approach and avoidance movements.
Escape from predators so you can stay alive.
And of course, all the things we need to approach things
about-- foraging for food, exploring, seeking a goal
object of various sorts.
It could be a nest, it could be a mate, and so forth.
These are very basic for all the different
motivational systems.
The next one is orienting of the head and body.
You need to be able to do that to accomplish the goals of
these drives that we mentioned already.
And then finally, to finish the action, acquiring the
object in some way, you need to grasp it in some way in
most cases.
You can grasp it with the mouth directly, or grasp it
with the lips.
We call these general purpose movements because they're
needed for many different actions patterns.
So just to review, the control of these three types of
movement from the midbrain, we know they're descending
pathways from the midbrain locomotor motor area.
And depending on how you simulate that region of the
coddle midbrain, you can get what looks like escape
behavior, or a simple locomotion that could be used
for approach.
We find that tectospinal tract, which we just
mentioned--
we saw it on my diagram in the previous slides there--
controlling orienting by turning movements.
And then the rubrospinal tract controlling limb movements,
reaching and grasping.
And for oral grasping, you don't need the limb
movements at all.
There are connections from the tectum also into the
hindbrain for that.
This is a picture we saw before.
The origin of the tectospinal tract from neurons here, like
this one, crossing to the opposite side and descending
into the medial midbrain and hindbrain.
And then this neuron in the rubrospinal tract, important
for controlling distal muscles of the limbs.
Remember the midbrain was the connecting link between
membrane structures, or forebrain structures in
general, not just the endbrain, but the
tweenbrain, as well.
The only way those forebrain mechanisms could control
movement was through the membrane, when in the earlier
period of forebrain evolution, which covers most of the
period of evolution of animals.
Midbrain also controls the visceral nervous system.
And remember, in our discussions, we were using the
blue color for these movements.
We were using the red color for the visceral nervous
system, and control of motivational states.
And in addition, the midbrain's involved in various
kinds of fixed action patterns,
like predatory attack.
We usually associate these things with the hypothalamus,
but in fact, they can be elicited from
the midbrain directly.
The hypothalamus is simply a slightly higher controller of
those same movement patterns.
So a little more about approach and avoidance first
involving locomotion.
It was a major importance in the
evolution of head receptors.
These functions played crucial roles in the evolution of
olfaction and vision.
And I'm just summarizing for you here the role of olfaction
in enticing approach or avoidance.
And I summarize here the output pathway from the
olfactory system, how they got from the olfactory input to
the output.
Going through the striatum and hypothalamus
to reach the midbrain.
Going through the medial pallium, which we now call
hippocampal formation, again, to reach these midbrain
mechanisms.
Similarly, for vision, these are the two
forebrain sensory systems.
They didn't exist without forebrain.
You have various links to motor control.
The importance of the striatum and medial pallium, for vision
was that it allowed those links to be plastic.
And there were also inputs that reached the midbrain from
below, taste, somatosensory inputs and auditory inputs.
And all of those, of course, also went up to the striatum
as it was mentioned here.
I also want to mention something that's often
neglected in these kinds of discussions that we don't
initiate locomotion only by inputs from the outside, and
initiate it from inside.
For example, from hypothalamic neurons, these are initiation
of activity by an intrinsic motivational state.
In this state that builds up we call a drive.
And Konrad Lorenz gave it a more specific, theoretical
term, his actions specific potential.
And all of his discussions were based on the model of
motivational control that was not a neural model, but that
has been bodied now in more formal models involving
computer simulation.
Some of that has been done right here at the media lab in
Bruce Blumberg's lab.
And he was explicitly inspired by the kind of model that
Konrad Lorenz developed.
Now, you've had some readings from Larry Swanson.
This is a picture on page 111 of Swanson's book.
Just talk about his motor system hierarchy.
Here at the bottom of the hierarchy we have the pools of
motor neurons mostly in the spinal
cord, also in the hindbrain.
Above them we have the pattern generators.
Now, those are in the spinal cord or in the hindbrain.
That's where the activity originates that's driving the
motor neurons.
They are the interneurons in the cord, in the networks of
neurons connecting to those interneurons as well that
generate patterns.
How do we know that?
Well we can eliminate all the higher inputs to the spinal
cord, and we can still get, for example, a stepping
pattern, alternate stepping, if we stimulate the feet in
the right way.
Can an animal walk if he doesn't have the brain
connected to the spinal cord?
What would he be missing, besides vision, and audition,
and olfaction?
He doesn't need those things to walk, right?
But he would have some problems because he would lack
his vestibular input.
He'd have a little trouble standing up.
But actually, do you need vestibular input?
Am I using only vestibular inputs to keep
from falling over?
Now, I'm rather top heavy here.
Why don't I just fall over?
If we eliminated my vestibular system, I would still get
input from my feet.
And so I would have some ability to balance just with
spinal mechanisms.
Although actually, I almost don't do too well with just
spinal mechanisms.
Above that, above these pattern generators, and
Swanson shows that to be a lot of neural apparatus.
But in fact, it's all in spinal cord and hindbrain.
Above that, you have the pattern, central pattern
initiators.
And above that, central pattern control.
The initiators are cell groups like those in the midbrain
that we call the midbrain locomotor area.
When you stimulate them, it initiates locomotion.
And when certain lesions are made in the midbrain that cut
off higher mechanisms from the midbrain locomotor area, you
can get an animal that just persists and walk and walk and
walking because there's nothing
to shut it off anymore.
And that's been seen with the neurological syndrome.
And then he calls even higher mechanisms, for example, those
in the hypothalamus, central pattern controller.
So let's look at that just for locomotion.
And I've doctored his slide a bit here.
This is the same model on the right side in black there, but
now it's specifically applied to locomotion.
And he shows his little schema there of various orders of
neurons in the locomotor pattern generator just to
indicate that that contains some hierarchy itself.
So here would be a neuron which when stimulated would
trigger a whole pattern.
What do you call it in a--
we're studying a little animal and we find a single neuron
that we can stimulate and initiate a
whole pattern of movement?
What do you call it?
What do you call that cell?
You call it a command cell.
A command neuron.
Do we have command neurons in the mammal?
Actually, probably not single cells, but there are groups of
cells that when you stimulate will cause a pattern to be
generated like that.
And then here above it, like the locomotor pattern
initiator of the midbrain locomotor region.
That would be this region.
And then above that, locomotor pattern controllers.
He puts here hypothalamic locomotor region, and I'm
indicating that that's true for any of the drives that can
initiate locomotion.
They would be represented here in the hypothalamus.
So there you have that hierarchy.
But now what controls that?
Why does the animal starts to walk well.
For example, very early in the evolution of the forebrain,
olfactory input reached the midbrain and the hypothalamus
through this, what we now call the ventral striatum
structures.
And just behind the olfactory bulb in the basal forebrain.
So I'm showing that here, and I made it darker because that
was really an evolution.
They reached the hypothalamus.
In fact, there were some olfactory fibers that went
right to the hypothalamus in, say, the hagfish or the sea
lamprey, which are pretty similar to
those ancient animals.
And then some of the fibers go from corpus striatum to the
midbrain, the midbrain locomotor area.
But then other inputs came into the corpus striatum,
remember, through these, what we call the old thalamus--
those are still there.
Striatum is still getting such input from the visual system,
the auditory system, and the somatosensory system.
They're coming in, reaching that same means of controlling
locomotion.
Well we have pathways reaching neocortex
that do the same thing.
Again, getting into corpus striatum, controlling
locomotion, it's input that comes into the thalamus
reaching the neocortex.
But here, something new has happened.
Once you get neocortex, and the neocortex gets very large,
and neocortex starts to bypass the striatum, it can now
project directly into the midbrain, and to some degree,
even directly to the lower motor mechanisms we'll call
them, the pattern generators.
And in the animals with the largest neocortex, including
us, all the way to the motor neuron.
So that gives you an introduction of what we'll be
talking about.
We'll say a little more at the beginning about these
locomotor controllers and then we'll go on with the
introduction to the motor system.