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.
Micturition
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Duke University의 강좌에서
Medical Neuroscience
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Movement and Motor Control: Visceral Motor Control
We conclude our survey of movement and motor control by considering the visceral motor system, perhaps better known as the autonomic nervous system. As you study this lesson, consider how the disparate physiology of the viscera has impact not only on the internal state of the body, but also on implicit processing in the forebrain. We will return to this matter when we consider the neurobiology of emotions near the conclusion of Medical Neuroscience
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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
Well, welcome back to this final part of our tutorial on the visceral motor
system. And, I feel much better now that I've
taken care of my full bladder. I hope you don't mind us talking frankly
about this rather base physiological system but it is an important system
nevertheless. It's one that has been very important in
the evolution of, of mammals that have used chemical signals in urine as a means
for social communication, and it remains an important aspect of human behavior.
in fact, so much so that I think there's some really fascinating neuroscience
behind the governance of our bladder and social cognition.
So I'm going to reflect upon that just a little bit as we go along.
But really, my primary goal is to use the governance of the bladder and the control
of the motor acts that are necessary to void urine as a way to illustrate the
integrated action of the visceral motor system and the somatic motor system.
Well, okay, so let's get into this session.
And as we do we will, of course, immediately once again appreciate the
complexity of the brain and its extension into the spinal cord as the body's most
complex organ system. And we will again be highlighting
circuitry that provide for the foundation of human behavior.
In this case a very particular human behavior that I hope you will appreciate
in a new way as a result of this segment of our tutorial.
So, my learning objective for you in this part, is that I want you to be able to
discuss the interplay among the sympathetic and parasympathetic divisions
of the visceral motor system. Together with the volitional somatic
motor system in the control of micturition.
Well, just to be clear, what we're talking about is urination.
It's voiding the bladder expelling urine from our bodies.
Well, as I said, this isn't perhaps the most pleasant of topics to spend time
together, especially given the diversity of life histories and social values and
social norms that I'm sure are represented in our learning community.
But again, it provides a wonderful opportunity to illustrate the operations
of the central nervous system, controlling human behavior.
Well, let's identify the key principles that are involved in coordinating the
activities of the bladder. And then we'll talk about how they are
regulated, in order to fill the bladder with urine, and then how to expel that
urine from the bladder. So here's an illustration using the male
form as our model illustrating the various neural elements that are
conveying sensory signals from the periphery and motor signals out to
peripheral structures that govern this process of filling the bladder as well as
voiding the bladder. Well let's begin on the sensory side.
So as we can see, there are sensory afferents that grow out from dorsal root
ganglion cells, that innervate the walls of the bladder and these sensory signals
are mechano-sensory receptors that are sensitive to the stretch that is imposed
upon the bladder as it fills with urine. There are also chemo-sensory elements
associated with free nerve endings, or poly modal c afferents for example, are
part of this sensory innervation to the bladder wall.
So we're sensitive not just to the distension of the bladder, but also to
the chemical environment within the lumen of that bladder.
Well these sensory signals then are conveyed to the appropriate sensory
systems. Here illustrated here would be the
anterolateral pathway. That's conveying the sensory information
up to integrative centers in the brain stem.
Well, there are important motor elements that I want to identify for you.
and let's begin with the sympathetic innervation.
That sympathetic innervation is coming down from integrative centers in the
brain stem and in the hypothalamus. And it reaches this intermediolateral
cell column and for control of the bladder we're talking about the lower
part of that cell column in the lower thoracic, upper lumbar segments and the
preganglionic neuron grows an axon, that reaches one of these pre vertebral
ganglia associated with the inferior part of the pelvis.
And there we find the ganglionic neuron that then supplies innervation to the
bladder wall. Not shown in this illustration, is
additional innervation from the sympathetic ganglia to an internal
sphincter muscle that is under the control of the sympathetic system.
And this provides for a way to ensure bladder filling rather than bladder
voiding by governing the contraction of smooth muscle at the base of the urethra
that drains the bladder. Well finally the parasympathetic division
of the visceral motor system is important has an important role to play in
governing the bladder. the parasympathetic outflow comes from
the sacral segments of the spinal cord. It is receiving input from higher brain
centers, which we'll talk about in just a moment.
And the preganglionic neuron, as you know, will grow a long distance, in this
case it will supply a ganglion, that is very close to the bladder itself.
And, that, ganglion will then provide innervation to the detrusor muscle, which
is the principle muscle of the bladder wall.
Now, let's say just a bit about the functions of these different sources of
innervation. So, the sympathetic innervation to the
bladder is going to promote bladder filling, so it's going to relax the
bladder wall. It's going to contract the internal
sphincter muscle, and together that will allow the bladder to fill.
The parasympathetic system on the other hand is going to promote the emptying of
the bladder. Activation of parasympathetic inputs to
the detrusor muscle will cause that muscle to contract and the bladder to
void. Well in order to actually expel the urine
we have to engage not just the visceral motor system but the somatic motor system
and the reason is that there is a somatic motor efferent that comes from the sacral
segments of the spinal cord and innervates an external sphincter muscle
that's in the floor of the pelvis. And that external sphincter muscle helps
to ensure the appropriate and timely release of urine from the bladder.
So this external sphincter helps to ensure that urine only flows out through
the urethra at the appropriate time. So in order to actually void the bladder,
one needs to coordinate not just the parasympathetic outflow to the bladder
wall, but also the somatic motor outflow to this external sphincter muscle.
Okay, so let's see how all this works. Let's consider first the context of
bladder filling. So bladder filling is going to
essentially involve an increase in sympathetic activity that will dominate
parasympathetic activity. So we're sort of shifting the balance
here in favor of sympathetic activity. So in order to fill the bladder, we have
to relax the muscle of the bladder wall, that detrusor muscle, and so this is a
beta adrenergic effect. And it allows that muscle to relax when
norepinephrine is released via these ganglionic sympathetic neurons.
Now, meanwhile, sympathetic activity to the internal sphincter is going to cause
that internal sphincter around the base of the urethra to contract.
This is an alpha adrenergic effect. So the detrusor muscle's relaxing but the
internal sphincter's contracting. So by increasing the sympathetic activity
we promote bladder filling. So what that means is that we need to
relax this detrusor muscle, and we need to activate this internal sphincter,
holding that urine in place within the bladder.
Now during the process of bladder filling we also want to coordinate the activity
of this somatic motor neuron that's supplying input to this external
sphincter exciting that striated muscle causing it to contract.
The more those sensory signals signal the increase in filling of the bladder the
more tone must be generated in that external sphincter in order to remain
continent, in order to retain that urine within the bladder.
But what about bladder voiding? Well, voiding the bladder means that we
have to shift this balance. So now there needs to be an increase in
parasympathetic activity. Where parasympathetic activity now
dominates sympathetic activity. So for that to happen, we need to
increase the tone of the detrusor muscle. And this is mediated via the muscarinic
cholinergic activation of that detrusor muscle.
Meanwhile, we want to decrease the tone of that internal sphincter.
And this seems to be mediated via a parasympathetic activation of the nitric
oxide system. And this causes that internal sphincter
muscle now to relax. But, what about our somatic motor system?
Well, we are not going to void urine successfully, unless we have a way of
decreasing the output of this somatic motor neuron to that external sphincter
muscle. And indeed this is where descending
inputs come into play. There is an inhibitory interneuron that
is in a position to be governed by higher brain centers and inhibit the tone of
that external sphincter muscle. that muscle needs to relax in order for
us to actually void urine from the bladder.
Well, how is all of this coordinated and governed by higher brain centers?
Well I want to walk you through this. this is a illustration of the neural
systems that are engaged when we actually void urine from the bladder.
So, as I just outlined for you, this involves activation of the
parasympathetic system. Our preganglionic and then our
postganglionic neurons causing the contraction of the bladder and the
relaxation of the internal sphincter muscle.
But there's also this inhibition of the somatic motor neurons which will allow
that external sphincter muscle to relax and the bladder to contract and expel
urine out through the urethra. Well this is all coordinated by some
really fascinating networks that involve the ventral medial forebrain and key
integrative centers in the brain stem. One key center is found in the pons, and
it goes by the catchy title, the pontine micturition center.
So, this center needs to be activated in order to coordinate the lower motor
activities that result in successfully voiding urine from the bladder.
So, stimulation of this structure in experimental animals will produce
urination. And it's on that kind of evidence, as
well as, an understating of the anatomy, that we recognize this as a key center
that must be engaged in order to successfully coordinate this
parasympathetic outflow together with the inhibitory signals that relax the
external sphincter muscle. Well the pontine micturation center is
under the influence of neurons in the peri-aqueductal grey matter.
This is a fascinating set of cells that surround the cerebral aqueduct in the
midbrain. you'll recall that there are networks of
cells in this structure that are involved in producing analgesia, that is, a
top-down, means by which we can control the transmission of pain signals in the
spinal cord. There's also a center in the
periaqueductal gray that can produce locomotion, so different kinds of
networks are in this structure. And here's one that seems to be important
in integrating inputs from yet higher brain centers in governing the output of
this pontine micturition center. Well the periaqueductal gray receives
input from the hypothalamus, hopefully no surprise to you now, as well as the
amygdala, and together these structures are governed by parts of the orbital and
medial pre frontal cortex. Well this is where the story of
micturition I think gets really fascinating because there remains in
human cultures still some a social dimension to voiding urine.
In some parts of the world, it's perfectly fine to void urine in public
and to do so, without embarrassment, without shame.
In other cultures, in other societies, one would not think of voiding urine in
public. So, one can appreciate I think that there
is a social dimension to governing the output of this peri-aqueductal brain, the
pontine micturition center. So, it's not surprising then, that this
network in the brain stem should be controlled by parts of the limbic
forebrain that are involved in social cognition.
Indeed, one of our great neuroanatomists of our time, a Dutch neuroanatomist by
the name of Gert Holstege, was bold enough to publish a paper in the
scientific literature that he entitled, Micturition and the Soul.
And I think what Professor Holstege understands quite well is that there is a
certain soulishness about the orbital and medial prefrontal cortex.
And this same part of the brain that helps to define who we are as a person,
as a human being, within a social context, within a social structure, is
also engaged in governing some of our most basal physiological functions
including voiding urine from our bladder. So, I for one, certainly appreciate
Professor Holstege's bold proclamation of relating the governance of micturition to
the neurobiological basis of the human soul.
Well, maybe that's another great topic to kick to the discussion forum if you wish.
So, I look forward to seeing your thoughts about that.
Well, to wrap up our discussion of the visceral motor system, I hope you'll take
away from this an appreciation that the visceral motor system operates in
coordination with our somatic motor system.
It's receiving sensory input, it's receiving top down signals, even from the
highest integrative centers of the forebrain.
Well, I think we're ready to bring a conclusion to unit four and our
consideration now of our means by which the brain can control movement and
mediate various dynamic aspects of human behavior.
Where we're going next in the course is to step back actually quite a bit back
and consider the development of the brain and the important mechanisms that unfold
really across the lifespan that account for the changes that our brains undergo
from conception all the way through advanced age.
Well, finally, we're going to end up medical neuroscience by talking about
human cognition. That will be the final unit of the
course. Well, the good news is that, I think
you've had all of the anatomy that you're going to get from me.
And, now that we've completed our consideration of our sensory motor
systems, what remains in the course is to actually understand that anatomy in a
functional context, in an even deeper way, by considering its embryo genesis,
its change across the life span and how the sensory motor systems are integrated
at the highest levels of human cognition. Well, we will along the way consider some
clinical cases that will help reinforce your understanding of this functional
anatomy. And how neurological signs and symptoms
can help you build a neuro-anatomical model of the brain of the patient that
might be in front of you, or the spinal cord of that patient and allow you to
localize lesions within the central nervous system based on your holistic
appreciation of the function and the behavior and the experiences of that
patient. Well that's a lofty goal.
We'll get there by the end of this course and I just thought I would pause for a
moment here and give you a sense of perspective as to where we are in that
progression. Well, I do want to leave you, in this
part of our tutorial, with a final study question.
And hopefully, that will allow you to integrate a little bit of what we've been
talking about in this session, and maybe prepare you for the unit quiz that
hopefully you are about to embark on. And I'll see you on the other side of the
quiz, when we consider the changing brain in unit five.
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