Medical Neuroscience explores the functional organization and neurophysiology of the human central nervous system, while providing a neurobiological framework for understanding human behavior. In this course, you will discover the organization of the neural systems in the brain and spinal cord that mediate sensation, motivate bodily action, and integrate sensorimotor signals with memory, emotion and related faculties of cognition. The overall goal of this course is to provide the foundation for understanding the impairments of sensation, action and cognition that accompany injury, disease or dysfunction in the central nervous system. The course will build upon knowledge acquired through prior studies of cell and molecular biology, general physiology and human anatomy, as we focus primarily on the central nervous system. This online course is designed to include all of the core concepts in neurophysiology and clinical neuroanatomy that would be presented in most first-year neuroscience courses in schools of medicine. However, there are some topics (e.g., biological psychiatry) and several learning experiences (e.g., hands-on brain dissection) that we provide in the corresponding course offered in the Duke University School of Medicine on campus that we are not attempting to reproduce in Medical Neuroscience online. Nevertheless, our aim is to faithfully present in scope and rigor a medical school caliber course experience. This course comprises six units of content organized into 12 weeks, with an additional week for a comprehensive final exam: - Unit 1 Neuroanatomy (weeks 1-2). This unit covers the surface anatomy of the human brain, its internal structure, and the overall organization of sensory and motor systems in the brainstem and spinal cord. - Unit 2 Neural signaling (weeks 3-4). This unit addresses the fundamental mechanisms of neuronal excitability, signal generation and propagation, synaptic transmission, post synaptic mechanisms of signal integration, and neural plasticity. - Unit 3 Sensory systems (weeks 5-7). Here, you will learn the overall organization and function of the sensory systems that contribute to our sense of self relative to the world around us: somatic sensory systems, proprioception, vision, audition, and balance senses. - Unit 4 Motor systems (weeks 8-9). In this unit, we will examine the organization and function of the brain and spinal mechanisms that govern bodily movement. - Unit 5 Brain Development (week 10). Next, we turn our attention to the neurobiological mechanisms for building the nervous system in embryonic development and in early postnatal life; we will also consider how the brain changes across the lifespan. - Unit 6 Cognition (weeks 11-12). The course concludes with a survey of the association systems of the cerebral hemispheres, with an emphasis on cortical networks that integrate perception, memory and emotion in organizing behavior and planning for the future; we will also consider brain systems for maintaining homeostasis and regulating brain state.
Functional Microanatomy of Neurons
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Medical Neuroscience
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From the lesson
Neuroanatomy: Introducing the Human Brain
Your introduction to Medical Neuroscience continues as you experience in this module a brief introduction to the human brain, its component cells, and some basic anatomical conventions for finding your way around the human central nervous system.
Meet the Instructors
Leonard E. White, Ph.D.Associate Professor
Department of Neurology, Department of Neurobiology, Duke University School of Medicine; Department of Psychology & Neuroscience, Trinity College of Arts & Sciences; Director of Education, Duke Institute for Brain Sciences; Duke University
Hello, everyone. Welcome to medical neuroscience and
welcome to our first tutorial on the Functional Micronanatomy of Neurons.
As we think together, I have three learning objectives for us to consider.
First, I want you to be able to describe in very basic terms the classes of cells
found in the central nervous system. I want you to be able to focus on one
particular class that we'll spend much of our time in unit one of this course
considering, the neurons, and I want you to think about the functional microanatomy
of neurons. Now, I'll spend this tutorial explaining
what I mean by that, but essentially I want you to be able to differentiate
between the different parts of a typical neuron in terms of their structure and
their function. And then lastly, I want you to be able to
describe how that microanatomical structure of a typical neuron is
compounded many thousands of times into the structure of neural tissue.
And when we think of neural tissue, we're basically talking about grey matter, and
white matter. Okay.
Well let's begin by looking deep into the brain.
And to do that, I'm going to show you a slide that I prepared some years ago.
Through the motor cortex of human brain. So we're looking at just a few hundred
square microns of tissue that has been stained with a particular dye called
Thionine to reveal the presence of what we called Nissl substance, which is
basically the. the rough endoplasmic reticulum or the
machinery that's making proteins within cells.
So it's just a great way to look at the cells that are present in tissue and to
appreciate something about the composition of this tissue.
So we're looking here and we can see a variety of different types of cells.
And, indeed, the brain is a very complicated place even from a histological
perspective. So, there are a variety of types of cells
here and I think we can roughly categorize them into three types.
There are neurons, which are the primary processors of neural signals.
And as I mentioned we're going to spend a lot of time together thinking about
neurons. There are neuroglial cells, which are the
subject of the next tutorial. And neuroglial cells, or just glia for
short, perform a rich variety of functions.
And just to summarize those functions I would say they support the electrical And
the chemical function of neurons. And then of course, the brain being a
high, highly vascular structure requiring a constant supply of blood and that blood
comes nutrients, such as glucose, oxygen and the ability to eliminate waste
substances like carbon dioxide. So with that rich supply of blood that we
find throughout the central nervous system we'll find vascular endothelium cells in
brain tissue as well. So let's focus now on a neuron, and I'll
make a few general remarks first and then we'll get on with our survey of the
microanatomy of a typical neuron. So neurons are the fundamental unit of
function in the central nervous system. This is a very important point and this is
the reason why we'll spend so much of our time and energy in unit one thinking about
how neurons actually, actually function. Now, for those of you who have a bit more
background in the field of neuroscience or are more in tune with contemporary
thinking in the field of neuroscience you'll realize that some might challenge
this dogma that's been with us since the debate of Ramon y Cajal and Camillo Golgi
more than a century ago. So with the more modern discussions for
the aficionados acknowledged, I would continue to assert that the fundamental
unit of function in the central nervous system is indeed the neuron.
Now, neurons possess all the metabolic machinery that are common to other somatic
cells so there's a region of the neuron that contains the nucleus and many of the
organelles that are necessary. For cellular life so in that respect
neurons are very much like other somatic cells.
But in several important respect neurons are quite distinct.
Neurons come in a rich diversity of form. And we call that form morphology, so when
one sees diversity of morphology you should be thinking that there might also
be diversity of function or physiology. And indeed, that's the case as form and
function and structure are intimately connected within the central nervous
system, even at the level of individual brain cells.
Neurons have unique bioelectrical properties that distinguish them from
most, but not all other somatic cells. For example, the machinery that's
necessary to generate electrical signals in muscle cells is quite similar to what
we see in most nerve cells. And finally, there are specializations for
intercellular communication that are pretty unique to the nervous system.
And for many neurons, most neurons in the mature central nervous system.
This involves the secretion of special chemical molecules called
neurotransmitters. Now again neurons have not evolved this
capacity, independently of other kinds of cells in the body, there are other
secretory cells that are well known in other kinds of tissues.
But the challenge of inter-cellular communication demands some pretty
interesting and some pretty unique specializations that we'll come to
shortly. All right.
Now lets survey the different parts that make up a typical neuron such as the one
illustrated here. And this will give us the chance to think
about what I consider to be the functional microanatomy of neurons.
And let's begin with the cell body. So neurons have cell body which as I
mentioned contains all the essential organelles importantly including the
nucleus that are common to other somatic cells.
And for most neurons, the cell body is actually quite rich in organelles because
of the incredible synthetic capacities that are required for neurons to maintain
their morphology as well as to support their function.
So we find an abundance of endoplasmic reticulum, both smooth and rough, for
example, for the synthesis of lipid molecules and proteins, respectively.
We find an abundance of mitochondria for the generated the enormous energy supply
that's necessary to keep neurons functions within their homeostatic states.
So, for a variety of reasons, the cell body is a very important component of the
neuron. Now from that cell body grow out numerous
different sorts of protoplasmic extensions, and I would like to draw your
attention to one set of extensions which are typically short, about 100 microns in
length, and they emanate in all different directions from the cell body.
These are called dendrites. Now, dendrites are very important for the
microanatomy of a neuron, because what they do is that they extend the surface
area that allows the cell to receive inputs from other neurons.
So wherever we see dendrites we think of this as an input zone because dendrites
are the means by which cells receive synaptic contacts from other neurons.
Now some synaptic contacts are made directly into cell bodies, but for the
most part we find them on dendrites. Now, dendrites also have some very
peculiar, very fine microstructural features and these called spines.
So, if I can illustrate just the length of a dendrite for you.
I'll show you what I mean. Imagine that's a dendrite, and if we were
to look at very high power, we might see under a microscope that there are these
tiny, mushroom-shaped protrusions. Out the side of that dendrite, these are
called spines. Sometimes they're not quite so mushroom
like in they're shape, they're more of a filament that extends some short distance
away. And sometimes there seems to be a little
stalks and a protrusion at the tip of that stalk.
So collectively, these yet increase the surface area of even a single dendrite,
and provide a place for synaptic contact from other neurons.
So wherever we see a spine, we can be quite confident that there is a synapse in
one state of formation or another from the axons that are surrounding that dendrite.
Now, not all dendrites grow out spines. Some dendrites are smooth.
And where we see cells that grow out smooth dendrites we know that these cells
tend to be those that have an inhibitory effect on the cells that they make
synaptic connections to. So the presence or absence of spines on
dendrites is one means by which we can classify neurons.
And we know that, that classification based on the morphology of the dendrite.
Gives us some insight into the functional properties of those cells.
The morphology of dendrites is one way that we can differentiate various classes
of neurons. Even among those that do in fact grow out
spines. One of the very interesting types of cells
that we find in the cerebral cortex is call the pyramidal neuron, and it's called
that because the cell body is shaped something like a little pyramid.
And from that cell body grow out a series of dendrites, and from the apex of the
pyramid is one rather thick, stout dendrite that we call the apical dendrite
because it grows out from the apex towards the surface of the brain.
At the base of the pyramid are a variety of other dendrites called basal dendrites.
And this special apical dendrite might have some unique ...properties, with
respect to its role in receiving signals from other neurons, and those might be
somewhat distinguished from those in the basal dendritic tree.
So the distribution of dendrites is, again, another important way that we have
of classifying different kinds of neurons. Some neurons are multipolar.
And these neurons give rise to dendrites that seem to just emanate in all different
directions without necessarily growing out a single apical dendrite.
So again, another example of how dendritic morphology can give us some insight in to
the structure and physiology of different cells in the brain.
All right, so let's continue our survey. Now growing out from the cell body is a
very particular protoplasmic extension, that has unique properties that allow it
to generate electrical signals. And this process is call the axon.
Now, whats illustrated here is a fairly simple picture of what a typical axon
might look like. Its a single process often that grows out
from the cell body and can grow for some distance some axons are short.
Being less than 100 microns and others are very long.
In fact, some are incredibly long. Consider for example a cell that might
have it's cell body in the motor cortex right about here inside your head and that
cell has to grow an axon all the way down through the brain, through the brainstem
and into the spinal cord. Perhaps some length in the spinal cord,
maybe even to the lumbar or sacral segments of the spinal cord.
So that could be maybe a half a meter or more, depending upon your height.
Or consider the motor neuron in the spinal cord itself that's going to send an axon
out in a spinal nerve to innervate a muscle.
If you're a very tall person that axon might be close to a meter in length,
really an amazing dimension considering the fact that the cell body itself is
probably close to about 50 microns in diameter.
So that gives you the sense of how much bio synthetic capacity must be built in to
these neurons as well as some insight as to their Need for, the energy.
And the raw materials that's necessary to grow out and maintain the function of that
axon. Now, on the function of that axon.This
axon has the ability to generate an electrical signal.
It's a special signal that we call an action potential.
And we'll talk more about that in a later tutorial.
That action potential is regenerated along the length of the axon and in that way it
propagates from cell body towards the terminal ending of that axon.
So for that reason, we think of the tissue that contains the axons as a conducting
zone and allows for the flow of information from cell body towards axon
terminal. Now, at the end of these axons, that's
where we find our synaptic terminals, or synapses.
And these are specialized contacts that allow one neuron to transfer a signal to
another. They come basically in two varieties.
We'll say more about these in a later tutorial.
There are electrical synapses that have specialized junctions that allow charged
molecules to pass directly from one neuron to another, and with it is conveyed the
electrical signal. Other kinds of synapses are chemical,
which is that there is a space or a gap or a cleft from one neuron to the next.
In that cleft is breached by a chemical that synthesize and released by one
neuron, diffuses across that space, and then interacts with receptors on another.
So, there's a chemical message that mediates the electrical signal from one
neuron to another. We'll have a lot to say about synapses,
how they work what are the important regulatory mechanisms that control
synaptic activity as we progress through unit one.
So much more on synapses as we go. Now, what I'd like for us to do is to
consider how this brief survey of the micro anatomy of a single neuron could be
compounded many tens of thousands times in the structure of neural tissue.
Now, imagine tens of thousands of neurons arraigned in parallel, very much like the
one we're looking at here. And what I want you to appreciate about
this parallel organization is that where we find the cell bodies and the dendrites,
that is, the input zone of one neuron, we'll find the input zone of many
thousands of neurons that are arrayed in parallel In tissues such as the cerebral
cortex, and we have a name for that. We call this gray matter.
So as we begin to look into the inside of the human brain I'll be highlighting for
you various divisions of gray matter. Those are distinguished from white matter.
And now, we can think about the cellular components that help us define what
exactly do we mean by grey matter and white matter.
So grey matter is where we find the cell bodies, the dendrites and the synaptic
connections of neurons. White matter is the conducting zone,
that's where we find axons and the special
insulation forming cells that surround those axons called glial cells.
Specifically in the brain the oligodendrocyte.
So that is another component of white matter.
And then of course, both grey matter and white matter require nutrients in the form
of vascular supply. So there are a vascular endothelial cells
in both kinds of tissue. Both grey matter and white matter.
And then, finally, let me again emphasize that, where we find the output zone of one
neuron, we find the input zone of many others.
So, grey matter is both input zone and output zone, depending upon which neuron
we have in mind. Okay.
Well, enough illustrations, let's look at some actual brain cells.
So, here are some pyramidal neurons in the cerebral cortex that I prepared myself.
And what we're looking at is a stain that shows us beautifully.
A pyramidal shaped neuron here. There's it's cell body, sort of a pyramid
looking, something like that, and a thick, stout apical dendrite progressing off
towards the surface of the cortex, which is out here somewhere.
Here is another pyramidal neuron and it's thick apical dendrite.
We also see some basil dendrites extending out away from the cell bodies here.
Now, we also have an axon. At least one that I can recognize.
And it is this process right here. It's extending away some distance away
from that cell body. So that's the means by which this
particular cell is going to generate electrical signals and then propagate
those signals to its neighbors, some quite near.
And then some at a great distance away. Here's another pyramidal cell in the
visual cortex, but we don't see it's apical dendrite because we're looking down
from the top, so it's apical dendrite is actually right about there.
And then here's it's cell body. And from it's cell body are all these
beautiful, branching basal dendrites. Now, if we look very closely at these
dendrites. And I know this image is not quite as
clear as we would want it to be. We can see some spines on these dendrites.
In that region and here again down there. So, if we have a careful look at one such
area I think you'll be able to appreciate that there are, in fact, some small little
mushroom shapes, some filaments that are extending away from those dendrites.
And so this would be an example of a typical pyramidal neuron that grows
numerous spines along. It's, basal dendrites as well as on an
apical dendrite, but typically, a little bit further away from the cell body.
Okay, here's another view. Had a, a large collection of pyramidal
neurons. And we can see, again, their cell bodies.
There are little pyramidal shapes here, apical dendrites heading towards the
surface of the brain and then lots and lots and lots of multipolar basal
dendrites going off in all different directions.
Now, we also can detect the present of axons that are coming out and extending
down into this white matter. That's what all these thin filaments are
that we can recognize heading down through this relatively unstained background
tissue. So, here we see all the various components
of the functional anatomy of a neuron, the dendrites, the cell bodies, the axons.
Well, we can't quite see everything because we haven't yet seen the synaptic
terminals. And to give you a sense of that, I want to
show you, you've got a different view of brain tissue.
So this is tissue that's stained with a green dye that has labeled axons as they
make their way through brain tissue, specifically the gray matter of the
cerebral cortex. And I would just draw your attention to
these little points of light. These little spots that we can see along
many of these axons. So, these little points of light are
actually places of synaptic contact. Now, some axons, they would grow out a
terminal ending, and then there'd be synaptic junction at that terminal ending.
Others tend to make this beaded type of structure and that's what we see here
along these green axons. Synaptic boutons and they're making what
we say en passant or in-passage synaptic connections.
So along these boutons there may be synaptic contacts with dendrites out along
the sides here. I know I drew them as if they look like
they perhaps are spines but they're really boutons because this is an axon not a
dendrite. So this is what we see here in these
little points of light, synaptic boutons. Here's one last view of neurons and their
axons, so we see cell bodies of retinal ganglion cells and axons that are growing
out away from them towards the optic nerve head.
And these are axons that are going to form the optic nerves that allow the retina to
connect up with the brain and send visual signals into visual processing centers in
the brain. Now, let's return to some illustrations of
different kinds of neurons and recognize some of the various classes of cells that
are present in the nervous system. We've talked a fair amount about the
cortical pyramidal cell already and this is a great example of what we call a
projection neuron. And the reason why we call it that is
because, as I've alluded to, it grows out an axon.
That can then project for quite some distance within the central nervous
system. Maybe hundreds of microns, maybe even
millimeters, tens of millimeters, maybe even tens of centimeters depending upon
whether the cell is projecting within the brain or whether it's growing out an axon
down through the brain stem and into the spinal cord.
So projection neurons are called that because they grow axons that project
outward away from the cell body over a considerable distance.
And these cells often are excitatory, meaning that they have a excitatory effect
on their synaptic partners. They make their synaptic partners more
likely to generate their own electrical signals.
Now, in addition to the long axon that projection neurons send off in a great
distance away, I'll just point out as it's illustrated here that the axon can
actually give rise to more local branches.
That can activate cells that are sitting right next to the cell of origin of that
axon. So there are local connections as well as
a single axon that projects over a great distance.
Now let's look at these cortical pyramidal cells in context.
Here to the left we have our illustration of the individual pyramidal cell.
And now we can see the pyramidal cell that is present within the cortex itself.
So a body, apical dendrite, basal dendrite, and then a long axon.
And here are two other cortical pyramidal cells.
So this is a typical view of what a projection neuron looks like and where it
might be found in the brain. So the pyramidal cell is a projection
neuron, it sends axons over considerable distances that have the effect of exciting
their synaptic partners. They may also give rise to local
collaterals that might just excite their near neighbors as well.
So they sort of have a dual function of activation local cells but also cells at a
distance. Now in addition to these projection cells,
we can also recognize some neurons that send axons over a much shorter distance.
Only perhaps 100 microns. Or a few hundred microns.
One example is found here. And this cortical stellate cell.
So the cell has a cell body, it's typically multipolar, there are dendrites
that go off in different directions, and then there is an axon.
And that axon may branch over the nearby territory within the dendritic tree of
that cell or just beyond it. So this is called an interneuron, because
it doesn't project very far at all. So within the same bit of gray matter we
can have projection neurons and we can have interneurons.
Now the interneurons come basically in two physiological classes.
There are those that excite their neighbors.
And then there are those that inhibit their neighbors.
And they're roughly equally divided with about as many excitatory interneurons as
there are inhibitory interneurons. If we look into the spinal cord we can
also see examples of projection neurons as well as interneurons.
Here's an illustration of a very simple circuit that we have throughout our spinal
cord and in the sensory and motor elements of our brain stem as well, and this is an
illustration of the knee jerk reflex also known as the myotactic reflex.
So we have two kind of projection neurons that are involved here.
We have a sensory neuron with its cell body, in the dorsal root ganglia.
Associated with our spinal nerves very close to the spinal cord itself.
And it has a long axon that runs out through a spinal nerve.
And in this case, innervating a sensory structure embedded within the skeletal
muscle itself. This dorsal root ganglion cell grows a
essential process of it's axon then enters the spinal cord.
Now, it's worth pointing out this particular type of neuron to make the
following point. This sensory axon is sending signals into
the spinal cord. For that reason we call it an afferent
neuron sending information centrally. There's another type of projection neuron
here in the spinal cord and it's called the motor neuron.
It has a multi-polar cell body in the spinal cord and it grows an axon out to
the muscle. And the motor axon is sending signals away
from the central nervous system to the muscle and for that reason we call it an
efferent axon. So get comfortable with this difference
between afferent and efferent. Afferent means signals coming in to the
central nervous system. An efferent, meaning signals going out of
the central nervous system. Sometimes we use these terms to talk about
the flow of information with respect to the cell body itself.
We can think of the dendrites as being afferent because information is coming in
towards the cell body. And we can think of the axon as efferent
because signals are going away. Now, we also have within the spinal cord a
short axon interneuron. And that's found right here.
So this interneuron has the effect of inhibiting the antagonistic motor
neuron. So that when our hammer tap hits our
patella tendon, the quadriceps can contract.
While the antagonistic muscle, the hamstring muscle, is going to be more
relaxed. Because the suppressive or inhibitory
effect of this interneuron on the motor neuron that supplies that hamstring
muscle. Okay, well this concludes our survey of
the microanatomy of neurons, as well as a brief discussion of different classes of
neurons. I hope it's helped you understand the
different parts of neurons, what they look, how they function.
So to help you consolidate your understanding of all of this, I'll leave
you with just a couple of study questions. Concerning what we've been talking about
as the functional micro-anatomy of a neuron, I want you to talk about how does
information flow through a neuron. And think about the various parts that are
listed out in these response options and go ahead and select your choice.
And then lastly, let's take this concept and compound it.
And apply it to understanding the structure of neural tissue.
Specifically in this case, white matter. I want you to think about what are the
cellular components that we find in grey matter and white matter?
And in this question consider those components found in white matter, and
choose your appropriate response option. Okay, well that concludes our first
tutorial. I'm sorry it went on as long as I did.
I will try to be more concise in the future, but I hope you enjoy thinking with
me about these topics. I know I enjoyed sharing them with you.
So on to our next tutorial, considering the non-neural cells that make up the
central nervous system. See you next time.
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