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.
Premotor Cortex
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Aus dem Kurs von Duke University
Medical Neuroscience
1230 Bewertungen
Aus der Unterrichtseinheit
Movement and Motor Control: Lower and Upper Motor Neurons
We come now to another pivot in Medical Neuroscience where our focus shifts from sensation to action. Or to borrow a phrase made famous by C.S. Sherrington more than a century ago (the title of his classic text), we will now consider the “integrative action of the nervous system”. We will do so in this module by learning the basic mechanisms by which neural circuits in the brain and spinal cord motivate bodily movement.
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
Well welcome back, I think we're ready to continue to discuss the organization of
upper motor neurons and focus on the premotor cortex.
So my learning objective for you, is that I want you to be able to discuss the
organization of the premotor cortex, and its contributions to the control of
volitional movement. So, I'll remind you again that the
premotor cortex is a region of the posterior frontal lobe, that sits just in
front of the pre-central gyrus. it might include a bit of the anterior
bank of the pre-central gyrus but it extends into the posterior part of the
superior, middle and inferior frontal gyri.
And it also extends onto the midline of the hemisphere.
here for example is our paracentral lobule, so this is where we would find
our primary motor cortex and then just anterior to it.
In the medial bank of the superior frontal sulcus, and then in the banks of
the cingulate sulcus, including the cingulate gyrus itself we find a medial
extension of that pre-motor cortex. Illustrated here in this slide is the
distribution of the premotor cortex. And a rhesus macaque brain, where the
details of organization are much more clear.
At least at this stage in our understanding.
So where we find this more orangish color, that's our premotor cortex.
Now there has been a fairly radical change in our thinking about the premotor
cortex in recent years. some time ago we imagined some kind of
hierarchical organization where the premotor cortex was primarily involved in
planning movement. But the actual execution of the movement
required signals that were derived from the primary motor cortex.
So it was thought that the premotor cortex at a higher plane in some kind of
hierarchy for motor control. Well, those concepts are, are beginning
to unravel as we have the greater appreciation for what's actually encoded
in the motor cortex. But we also, understand much better now
that the premotor cortex itself, gives rise to descending projections to lower
motor neurons. So the argument for premotor cortex,
being conceptualized as a higher plane for motor control seems to be losing some
of its attraction. I think the concept that is becoming much
more viable, and frankly much more interesting and, and productive for
understanding upper motor systems. Is the notion that this premotor
cortex actually comprises of a mosaic of areas.
That have some kind of modular organization, reflecting the encoding of
ethologically important motor behaviors. Now it is helpful to find some way to
simplify our understanding of this mosaic.
And I would suggest a means to simplify would be to recognize a medial part of
our premotor cortical mosaic in the lateral part.
Where exactly one would make that division is not so clear.
But perhaps one would, would roughly subdivide this premotor cortex at about
the location of the superior frontal sulcus.
So, our medial division of the premotor cortex, corresponds to what in the older
literature was, described as a supplementary motor area.
So for those of you that are looking at other resources its support of your
learning, you, you may run into that phrase supplementary motor cortex.
It's certainly is still used in both clinical and scientific discussion.
But for our purposes, we'll simply consider that supplementary motor cortex
or SMA for short, a division that is localized here to the medial part of the
premotor cortex. Now included in that medial part of the
premotor cortex, is a very interesting set of regions, here in the banks of the
cingulate sulcus that will spend just little bit of time talking about.
They seem to be involved in the expression of emotional behavior.
Out on the dorsal lateral convexity of the hemisphere, we see other regions of
this regional medial premotor cortex that are concerned with organizing bi-manual
activities. And this implies that they have a rich
set of colossal activities, as they do so bimanual coordination seems to be a
function of this premotor cortex. There is also a division of this medial
premotor cortex, that is especially concerned with governing voluntary
saccadic eye movements, we call this region the frontal eye field.
And this is a part of the premotor cortex that helps to orient our attention, and
therefore what we're looking across the midline to some location in the
contralateral visual hemifield. Now generally speaking, these medial
parts of the premotor cortex seem to be especially involved with organizing
movements that are self-initiated. Movements that are not necessarily
triggered or directed by sensory cues. One example of what I mean by this, in
the frontal eye field, would be turning our gaze to some object in the
contralateral part of the visual field that we intend to look at.
Perhaps there is a clock, over to your right and you want to look towards your
right, to see what time it is. so, there wasn't necessarily an emerging
stimulus that just grabbed our attention, rather there was an intentionality, about
the shift of gaze and that intentionality was self initiated.
Now contrast that if you will recall to the role of the superior colliculus in
organizing eye movements. If you're outside and a bolt of lightning
happened to strike somewhere off in the distance to one side.
You may very well make a reflex movement of eyes and head posture in order to see
that bolt of lightning while it persists. Well, that is a reflex of saccade that's
coordinated at the subcortical level, through circuitry that involves the
retina sending signals to the superior colliculus.
And then the colliculus sending output that governs the movements of the eyes
and the muscles that orient our head and our neck.
We call that a reflex saccade. that wasn't self initiated in the same
sense, as the desire to see what time it is.
So now lets turn our attention to the lateral parts of the premotor cortex.
Here what we find is a mosaic [UNKNOWN] that are involved in organizing
movements, that are often guided by sensory information.
Or at least with the interactions that we might have with the world around us, and
that would include social interactions as well.
For example, we find areas that exist here in the inferior part of this
pre-motor cortex, that are especially concerned with social communication.
Now for us and the human brain we can recognize one region that is of great
interest, and it's found in the posterior part of the inferior frontal gyrus.
And this is a region that goes by the name Broca's area.
So, Broca's area is part of this premotor cortex, that organizes the vocal motor
apparatus for the production of speech. They are very likely nearby regions also
part of this premotor mosaic, that is involved in the production of language in
written form. Either through the act of writing with a
writing implement, or, of course now, we can touch on keyboards and touchscreens.
And that is a motor act, that is involved in communicating thought.
And semantic content through symbolic representation, and that seems to be one
function of these more lateral and inferior divisions of this premotor
mosaic. Also in this same region of the pre-motor
cortex, we have some really fascinating neurons that have captured world wide
attention in recent years. These are neurons that are concerned not
just with the expression of movement, that we make with our own body, but also
encoding the intentions of movements that we see other individuals perform.
And because of this mirror like capacity of encoding our own movements, and
observe movements, such neurons with this behavior are called mirror motor neurons.
Mirror motor neurons were discovered quite accidentally, when recordings were
being made in this lateral premotor cortex, while monkeys were engaged in
visually guided reaching behaviors to pick up a small object.
And what was observed, is that when the monkey simply sat passively while the
experimenters reloaded the apparatus by executing a very similar type of hand
movement. The neurons in the premotor cortex in the
monkey's brain, responded at the sight of the human hand performing the very
similar act that the monkey himself was about to execute.
This is illustrated here, in this figure from our textbook.
what we find is a recording from one such neuron in the lateral part of the
premotor cortex. And we're looking at raster plots, such
as what we saw previously, so each row is an individual trial.
And each little tick mark represents an action potential.
What was really surprising at the time was that the very same neuron, was seen
to increase its firing rate when a very similar act was observed in a different
agent. In this case the hand of the
experimenter. Well as you might imagine this was a
really shocking discovery that neurons in the motor cortex might represent movement
observed in another person. So, quite a lot of work has gone on in
the last two decades or so to really explore, the encoding of movement and
tension, that is observed here in this lateral premotor cortex.
what is discovered is that these neurons seem to care about the agent itself.
if a tool is used for example, to preform a particular movement goal, then this
neuron might not be concerned with that particular behavior.
In this example, if a food pellet is retrieved with a pair of pliers this
neuron doesn't seem to fire. whereas the natural movement that the
monkey makes, or is observed by a human hand will elicit a robust set of
discharges in this premotor neuron. very interestingly, if the monkey is
prevented from actually observing the contact of the hand with the food pellet,
that is the consummation of the motor act. by occluding that view with some kind of
a screen, we still see activation of this so-called mirror motor neuron in the
lateral premotor cortex. It's as if what this neuron is encoding,
is the goal of the behavior, or the intention of the behavior, which in this
case, is executing your precision grip to retrieve a pellet.
Even when that act is not observed, there's still activity in this premotor
neuron. So the discovery of these mirror motor
neurons has really led to, I think, a really remarkable hypothesis.
And that is that, the lateral part of the premotor cortex is involved in encoding
the intention of movement. And that movement intention might provide
a foundation for social cognition, because these neurons are concerned not
just with my intentions, but also with your intentions.
When we observe the activities of others we might actually represent that behavior
within our motor system. And that might give us a sort of
knowledge or understanding of the intention of that motor act.
And this is a very active and frankly quite controversial domain of cognitive
neuroscience at the moment, so we don't quite know how this is going to all play
out. But, I do suspect that this is an
important discovery, that is going to inform our understanding of social
cognition. Perhaps even the basis for disorders that
lead to impairments in social cognition such as some developmental disorders that
result in the autism spectrum of disorders.
This also provides some intriguing possibilities for intervention.
If, in such neurodevelopmental disorders of cognition, there may be an underlying
problem with the motor system. Then perhaps there is a movement-based
intervention, that might help to restructure connections in this part of
the brain that could produce some functional benefit to the patient.
Well, that's total speculation on my part at this point.
But I think it's an interesting idea that's worthy of some study.
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