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.
General Principles of Sensory Systems, part 1
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Aus dem Kurs von Duke University
Medical Neuroscience
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Aus der Unterrichtseinheit
Sensory Systems: General Principles and Somatic Sensation
We have reached a significant juncture in Medical Neuroscience as we turn our attention to the organization and function of the sensory systems. We will begin our studies with the somatic sensory systems, which includes subsystems for mechanical sensation and pain/temperature sensation. But before we get there, it is worth considering first some organizing principles that will set the stage for studies of somatic sensation and all the other sensory systems of the body.
Treffen Sie die Kursleiter
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
Hi everyone. Welcome to this tutorial on the general
principles of sensory systems. We are at a bit of a pivot point in the
course where our focus has been on gross brain anatomy.
As well as the principles of neurophysiology that underline neural
signaling and even a discussion of neuroplasticity.
Now we're going to begin to bring these concepts together.
On the one hand, the discussion of anatomy, in the other, the discussion of
cellular physiology and we want to bring them together.
As we discuss the organization and function of our sensory motor systems.
So this is the beginning of that pivot and what I'd like for you to be able to
focus on in this tutorial are some of the general principles that will help us
understand the organization of our sensory systems which is what we'll
discuss first and then we'll move on and discuss the motor systems.
So this topic pertains once again to our key concept or foundational core concept
in the field of neuroscience pertaining to the complexity of the brain.
And this tutorial I trust will help break down some of that complexity for you as
we consider how sensory systems are organized, and how they function.
And this will allow us to maybe reflect for just a moment on this incredible
capacity we have to explore our world. Even the world with inside our own brain.
You know, I'm reminded of a student I once had some years ago who I will not
name in case you're out there somewhere but he showed up one day to the lab and
he said, you know I just don't think the brain should be allowed to study itself.
And I thought, what a profound observation you know, the kidney doesn't
get to study itself nor does the liver, but somehow the brain has the credentials
that allow it to study itself. And I think that is an interesting
thought to consider. Well let's move on and consider what we
have to learn from this tutorial today. So I have several learning objectives for
you, I want you to be able to account for the generation of action potentials in
peripheral axons in response to somatic sensory information.
Now, the somatic sensory system will be the first of the sensory systems that we
consider, so I thinks it's useful to see how this general problem of action
potential generation in a sensory system is realized in this one domain of
sensation, that of somatic sensation. I want you to be able to account for
differences in mechanosensory discrimination across the surface of the
body. Again, we're going to take somatic
sensation as our model paradigm, but the basic principles will apply to our other
sensory systems as well. And I want to consider in fairly general
and broad terms, how information is coded in the nervous system.
So, I'd hope that you'd be able to have a discussion of the important factors that
influence information coding. And one very important concept in sensory
physiology is that of the receptive field.
So I want you to have a thorough understanding of what the receptive field
is. And one way to ensure yourself of that is
to have a discussion with someone about it.
Okay, what I'd like to do next is to consider some general organizational
principles that will apply to our consideration of our sensory systems.
And we'll see how they played out specifically in different systems over
the next several tutorials, but today I'd like to keep it fairly general.
So let's begin with one principal, and that is the principle of neuronal
pathways. So let me illustrate for you, in very
simple terms, what I mean by a neuronal pathway.
Let's imagine that there's some kind of sensory surface out here perhaps this is
the skin. And the skin is innervated by nerve
fibers and those nerve fibers are grown by cells in this case cells that reside
in the dorsal root ganglion so an axon extends out and there's some kind of
specialized receptor structure as the axons reaches the skin surface.
So this is what we call the peripheral axon of this neuron.
So this neuron can be considered a first order neuron in this pathway.
Well there is a central process that enters the nervous system.
So let's say this is now into the central nervous system.
And that first order neuron synapses on a second order neuron.
And that second order neuron projects some distance through the central nervous
system.to typically a third neuron. We'll call that a third order neuron.
And then, that third order neuron provides input to a vast network of
interconnected cells. And some processing station within the
brain, perhaps this processing station is the cerebral cortex.
So, we might have some, some complicated network that allows all this to function.
So we might call this now the cortex. So the neural pathway would be the first
order neuron, the second order neuron, the third order neuron and then once we
get into our complex network that is being modulating by this ascending input,
we sort of give up on our numbering scheme.
So, we'll see how that works? But the pathway, in essence, is a series
of neurons that are connected. But the pathway is essentially a set of
neurons that are connected in a serial fashion.
And this allows neurosignals to be generated at some receptor surface and
then be transmitted into the nervous system ultimately to be processed at the
level of the cerebral cortex. Now, I drew this pathway in the
horizontal dimension across the screen just to make use of the space that I
have, but I want to make an important point about how we talk about our
pathways in the brain. So, let me clear that and redraw this in
the vertical dimension. So often we have some receptor connected
to our first order neuron, which projects to a second order neuron.
But now we often think about this as ascending into the nervous system and
here's our complicated network again representing the cerebral cortex.
So here's our first order neuron, our second order neuron, our third order
neuron and then the cortex. Now, this we might consider to be a
sensory pathway and the information is essentially flowing in the ascending
direction. Well, this cortical network also has a
motor component to it where these sensory signals are eventually integrated within
some kind of a network that coordinates and produces an output.
And that output eventually reaches our motor neurons and behaviors result.
So we can imagine that there's some output neuron here that is going to grow
an axon which will probably synapse on some kind of local circuit cell.
That modulates the output of a series of neurons that ultimately supply some kind
of defector such as a skeletal muscle. So this now is an output pathway which
can be conceptualized as descending. From a higher level from the cerebral
cortex for example, down to a lower level maybe in the spinal cord where we have a
motor neuron leading to the contraction of a skeletal muscle.
Okay, so pathways can be either ascending or descending.
Ascending typically implies a sensory function, descending implies a motor
function. In our nervous systems that are
bilaterally symmetrical there's a very interesting principle of organization in
play, and that that is a principle of contralateral representation.
So, let me illustrate that for you. Imagine that we have some body part in
question here, maybe it's an upper extremity, and that upper extremity is
going to be represented in the contralateral, or the opposite side, of
the cerebral hemisphere. So this is the cortex, let's say this is
the post central gyrus, which is where we find our somatic sensory cortex.
So this body part will find its representation in the somatic sensory
cortex, something like this. Now, contralateral representation implies
that there must be some sort of crossing of the midline.
Somehow this information has to get from one side of the body to the other in
order for the ascending sensory pathway to relay it to the appropriate processing
station at the level of the cortex. So as we work through our studies of our
sensory pathways, please pay attention to this issue of decussations.
So decussations simply means midline crossing.
So we're talking about where in the nervous system does this pathway cross
the mid-line? So that the principle of contralateral
representation can be understood. Now this is not just an academic exercise
for future clinicians, this is a critical point.
One of your challenges in the examination of your patients will be to understand,
to have a mental model of what's wrong with the nervous system.
There may be a focal lesion somewhere in this nervous system.
And understanding where sensory and motor pathways cross the midline, will be
critical for your proper understanding of what level of the nervous system has been
impaired. Now this issues of decussation, it's not
just for sensory systems, it's also for motor systems.
Now if we get just in front of the post central gyrus, we may be in the pre
central gyrus which is where we have our motor control networks and there may be a
neuron there in the pre central gyrus that gives rise to a descending axon.
And that descending axon has to confront the same challenge of contralateral
representation. At some point, there has to be a crossing
of the midline, a decussation and as it turns out the decussations.
For sensory and motor pathways, are not in exactly the same place.
Even within our somatic sensory systems, different pathways cross the midline at
different locations. And that will be critical for
understanding the neurological signs and symptoms of injury and disease, that you
may observe in your patients. Now let's turn to a new topic, the topic
of sensory transduction. By sensory transduction what I mean is
the process that allows for the energy that's present in the environment.
Either in physical or chemical form, to be converted into electrical signals
within nerve cells. And sensory transduction is a problem for
each of the sensory systems that must be solved in some ingenious way.
And in fact it often is and many times there are highly conserved mechanisms
that are employed across animals with nervous systems, including us.
And sometimes those methods seem quite surprising.
so I hope you have some fun discovering how sensory transduction works in each of
the sensory systems that we will consider in this course.
Let's consider one example from the somatic sensory system.
Well, we know that the somatic sensory system is concerned with Touch with
physical contact is one modality of stimulation.
And it must happen that physical contact leads to the generation of eletrical
signals. So, sensory transduction would be part of
that story in addressing the question, well.
How does this mechanical contact lead to the generation of electrical signals?
Here's how this works for the receptors that are sensitive to mechanical
deformation of the body surfaces. Imagine that we have some kind of
receptor ending at the end of our first order neuron This happens to be a
beautifully laminated structure called a Pacinian corpuscle.
And this Pacinian corpuscle contains a nerve ending that has cations selected
channels. And these ion channels are sensitive to
stretching forces that are applied to the membrane.
So when there's some kind of mechanical interaction with the structures
containing these pacinian corpuscles, these ion channels will open.
Not because they're voltage gated or because they are activated upon by
neurotransmitters as we saw in our previous tutorials on neuro signalling.
But rather, because the mechanical forces open these channels, so here's what we
observe. When a physical force is applied to this
tissue, these ion channels will open, this will allow our cations to enter.
In this particular case, the principal permiant ion is a sodium ion.
And so, as I think should now be familiar to you, as sodium rushes in to a neuronal
cell it carries with it its positive charge.
And that leads to the depolarization of the sensory ending of this axon.
Let's have a physiological perspective on this process now.
Imagine that a fairly weak stimulus is applied.
And that weak stimulus will lead to the influx of relatively small number of
sodium ions. So one can imagine a fairly weak
depolarization of this receptor. Now, we have a term that we use to apply
this kind of depolarization. We call this a receptor potential,
because we're talking about a change in membrane potential, in a receptor ending,
of a primary sensory axon. Now, if the strength of this stimulus is
increased, what we would expect to see has a greater response and indeed, that's
what we see. So with the stronger stimulus, we see an
increase in the amplitude of this receptor potential.
But notice that, we don't yet generate an electrical signal that can propagate, as
long as this potential stays below threshold, for generating an action
potential. But with a stronger stimulus, it very
well may be that this receptor potential is now sufficient magnitude to achieve
the threshold for generating an action potential.
Once that threshold is achieved, an action potential fires.
Now notice what's going on here, the generator potential is a function of the
stretch activated ion channels. But once that membrane is depolarized to
achieve the threshold for generating an action potential, now our friends the
voltage gated sodium and potassium channels are activated and an action
potential is generated. So different kinds of ion channels
contribute in different phases to the transmission of information along the
axon of the primary sensory neuron. Now in the case of our sub-threshold
potential receptor potential that fail to elicit an action potential.
We're never going to know about that contact through this sensory ending
because there has been no action potential.
There's been no signal transmitted into the central nervous system that requires
a receptor potential of significant amplitude to achieve threshold.
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