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.
Primary Motor Cortex, part 3
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Medical Neuroscience
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From the lesson
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.
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
So what is represented in the primary motor cortex?
it doesn't seem to be a strict representation of the body and its
muscles. So, I think what we can conclude then,
based on the evidence to date, is that what's represented in the cortex is
movement, not muscle. In fact, we might go so far as to
hypothesize that truly whats represented is movement intention and I'll give you
some examples of what I mean by that. Now we do know from a long history of
investigation in this part of the brain, that there various dimensions of movement
that are represented in the motor cortex. For example, the amount of force that we
generate with muscle contraction. the directionality of movement if we're
talking about moving an articulated muscular skeletal structure like the
upper extremity. the amplitude of the movement.
whether it's a small movement or a large movement.
These features, these dimensions of movement seem to be encoded in our
cortical network. but we also seem to have representation
of movements that are ethologically meaningful.
For example, the primary motor cortex seems to be concerned with coordinating
movements of the hand and the lower part of the face.
for example, moving the hand to the mouth.
That's one aspect of movement that seems to be organized in the primary motor
cortex. And perhaps that's one advantage of
having the representation of the face adjacent to the representation of the
hand in the body map. We also see evidence that organized in
the motor cortex is representation of skilled manual behavior in the space
that's right in, right in front of us. What we call central personal space.
this might be very important for bringing objects in to our near visual field so
that we can inspect and examine them. imagine the importance for example of
having a careful look at a piece of fruit to make a judgement about whether it's
truly palatable or not. Well, with all of this being represented
in the primary motor cortex, we can make some predictions as to what we might find
in a patience that has had damage to this part of the brain, or to the pathway that
emanates from it. Generally speaking, what we find is that
lesions of the primary motor cortex impair the fractionated or the skilled
movements of our distal extremities in the lower part of our face.
So, for example, some kind of fractionation of the fingers of the hand
would be impaired with damage to the primary motor cortex.
So I want to come back just for a minute and say more about my first point here,
that what's encoded in the primary motor cortex may very well be the intention of
the movement. So, I think a really interesting series
of studies were performed by Michael Graziano and his colleagues where they
were probing the movements that could be generated in the brain of a rhesus monkey
when the motor cortex was stimulated. One of the interesting features of this
study that I think it is, make it especially noteworthy, is that the
duration of stimulation was longer than is typically done in the kinds of body
mapping experiments that were done throughout the 20th century.
rather than stimulating for just a very brief moment in time Dr.
Graziano and his colleagues decided to extend the period of stimulation to a
period a few seconds. Which was thought to mimic the more
natural evolution of activity that emerges in the motor cortex during the
performance of motor behavior. And what was found, was micro stimulation
of certain sites in the motor cortex produced behaviors.
Not just movements, but behaviors and here's one example.
what we're looking at here, is a record of movement.
With stimulation of the hand region, of the pre-central gyrus in this rhesus
monkey. And the the blue the blue plus signs
indicate the starting position of the hand, at the onset of stimulation.
And then the red circles indicate the end position of the hand at the termination
of the movement that was elicited with micro stimulation.
And I think what's obvious here, is that at the particular site of stimulation,
what was observed is, movement of the hand from peripheral regions.
Be they to the left, or to the right of the mid line, it didn't seem to matter.
And what we found here, is hand to mouth movement.
So, where ever the starting position of the hand happened to be at the time of
stimulation, it was brought towards the mouth.
And it seems as though in this particular location, what's really important are the
kinds of movements that would be needed for feeding.
When Graziano and his colleagues moved the micro electrode in a slightly
different direction in the primary motor cortex what was discovered is a site.
Where the hand was moved not to the mouth, but more towards this central
personal space. Where an animal could inspect through
vision and presumably other such special sensory systems just what was being held
in the hand. So these kinds of studies suggests that
what's represented in the primary motor cortex is movement intention.
Bringing the hand to the mouth or bringing the hand to central mid line for
visual examination. So how might such movement be encoded in
the firing patterns of neurons in the primary motor cortex?
Well one idea might be, that for each particular movement that is illustrated
here with the black line, there may be a neuron that is specifically tuned for the
expression of that movement. Well that would be one idea, and it turns
out that that is highly unlikely. Rather, what we find is encoding not by 1
neuron at a time, but by populations of neurons.
And encoding for the various dimensions of movement that I alluded to earlier.
The direction of movement. The, force that's generated.
the amplitude of the movement. And I'd like to show you an additional
experiment done some years earlier. that really made this point, that
movement direction is encoded in the population activity of the motor cortex,
not by any single neuron. So here's the simple experiment that's
being illustrated here. There is a monkey that's trained to move
a central joystick in the direction of some kind of target.
And that target might be a green light that would eliminate at a particular
location in this apparatus. So there's a particular directionality to
the movement. And then question then is, well how does
the motor cortex encode this directionality of movement towards a
visual target. And the answer seems to be that the
encoding is done at the level of the population.
So what we're looking at here are series of raster plots that report the action
potentials that were recorded during particular trials.
So each row that we see here, we have one, two, three, four, five rows.
These are five trials. And then each little tick mark that we
see represents the occurrence of an action potential.
Now, as the monkey moved the joy stick towards various visual targets and these
various directions, what was generally observed in the firing of individual
cells in the motor cortex is that they tended to respond very broadly.
With movements directed towards the left and in the upward direction.
So these movements that are represented by this yellow wedge of this circle here
are those that led to an increase in the firing of action potentials around the
time of the movement. And movement is represented by this red
line here, which indicates at time 0, when the movement actually occurred.
The firing of this cell seems to increase a few hundred milliseconds before the
movement was made and continues during the course of the movement.
Now notice what happened as the animal moved in the opposite direction.
So as the animal moved broadly to the right, what we find is a suppression of
activity. Just before and during the course of the
movement. So there seems to be a very broad measure
of tuning that can be reported at the individual neuronal level.
In fact, if we look at the peak responses as the animal moves in different
directions, what we see is a, is a very broad tuning function.
And this is noteworthy, because it suggests that no one neuron encodes
precisely the direction of movement. the responses, the contributions of any
one neuron are simply too broadly tuned to achieve the functional goal.
So, how must this happen? Well, the functional goal is achieved
through encoding of direction in a population of neurons.
So what was found as many neurons were recorded in the pre-essential [UNKNOWN]
during directional movement, is that well no one neuron seem to be sufficiently
tuned to produce the moment that a vector could be defined by the active
population. That aligned quite precisely with the
movement that was actually performed. So for example, as the monkey moved the
joystick up into the right, we see this big red arrow that indicates the
direction of the movement that was performed.
there is a population of cells that are active.
And if one then computes the vector average of their tuning functions during
the execution of the movement, we find a vector average that aligns very nicely
with the direction of movement. So, for each movement that was executed,
there's a population of neurons that are contributing to the directionality of the
movement that's actually expressed. It's as if there is some kind of ensemble
code for the direction of movement, with many neurons contributing information
from which the precise movement is governed.
So this then, is an example of what we call a population code in the cerebral
cortex. We find similar codes, we think, all over
the cerebral mantle. with the idea being that for the most
part single neurons are not sufficiently tuned to express behavior.
But rather, the behavior is the product of an ensemble code reflecting the
activities of the entire population of neurons.
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