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.
Brainstem Upper Motor Neurons
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Medical Neuroscience
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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.
Rencontrer les enseignants
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
Welcome back to this tutorial, on upper motor neuronal control of movement.
now I'd like to turn our attention away from the cortex, and into the brain stem.
My learning objective for you here, is that I want you to be able to discuss the
neural centers, that give rise to medial descending projections.
From upper motor neurons in the brainstem, to lower motor neurons
associated with eventual horn of the spinal cord and even in the brain stem
itself, in the motor nuclei of the cranial nerves.
So our brainstem upper motor neurons include, primarily two sorts of
populations of cells. There's a population of neurons
associated with the vestibular nuclei. And we've talked some about these, in a
previous tutorial. And then there's a population of neurons,
that emanates form the central core region of the brainstem.
That is the core of the tegmentum of brainstem, we call this core the
reticular formation. And the reticular formation really
extends throughout the entire brainstem, but the portion of it that gives rise to
projections to lower motor neurons, is primarily found in the pons and the
medulla. Okay.
So, let me just take a, a minute or two to remind you, of the projections that
arise from the vestibular nuclei and descent and terminate in the spinal cord.
you'll recall that there are vestibulospinal connections, that are
concerned with mediating reflexes that are sensed by our vestibular apparatus.
And the sensory signal is going to be some kind of an acceleration, or movement
of the head. So this movement could be a rotational
acceleration of the head, which would be sensed by our semicircular canals.
It could be a linear acceleration or a tilt of the head, sensed by the otolithic
organs. and these signals might very well
necessitate an adjustment of posture. And in postural adjustments imply that
there must be some change in the output of the spinal cord involving the axial
muscles, the proximal limb muscles, and perhaps even the distal muscles of the
limbs. And these reflexes connecting head
movement to postural adjustment, are mediated near these vestibulospinal
connections. So there is a lateral vestibulospinal
tract, which arises from our lateral vestibular nucleus.
One division of this vestibular nuclei complex.
And this projection is primarily an ipsilateral projection, that runs down
the spinal cord and terminates among the cell groups, that are concerned with
extensor tone, primarily in the lower extremities.
So one example that I've given you before would be, standing up on a bus or a
train. That is unexpectedly encountering bumps
or jolts along the path. And so these are sensations, that are
being detected by the vestibular system. And in order to stay upright, to not fall
over as the floor is moving under you feet, there needs to be a moment to
moment adjustment in posture. And that's what's mediated via this
lateral vestibular spinal tract. Now, there are other divisions of this
vestibular nuclear complex, that give rise to a medial vestibular spinal tract.
And this projection is much more bilateral, than our lateral
vestibulospinal tract and it's terminating among medial lower motor
neuronal columns. And these columns are mainly found in the
cervical region of the spinal cord, so one example of the operation of this
medial vestibulospinal tract. would be the reflexes that are elicited
with a sudden forward rotation of the head.
So what I'm thinking of is, tripping and falling.
So in order to brace ourselves for the impact with the ground, we want to get
our face out of the way, so we dorsiflex our neck, and we often extend our arms to
brace ourselves against the fall. So that's a reflex.
So the forward acceleration of the head, triggers this reflexive response that's
mediated via the cervical levels of the spinal cord.
That's our medial vestibulospinal pathway.
Now, I want to make just a couple of points and then we'll be ready to move
on. I don't want you to be confused about the
nomenclature of these vestibulospinal pathways, especially the lateral
vestibulospinal pathway. This pathway, is somewhat lateral in the
anterior and medial white matter of the spinal cord, but we still consider it
part of our descending medial systems, for upper motor neuronal control of
posture. And the more proximal muscles of the
limbs. So, consider together these
vestibulospinal projections, part of that medial descending system.
That adjusts posture and sets the gain of local segmental reflexes.
One additional point I would make about these vestibulospinal connections, is
that they're operating on sensory feedback.
So there is some kind of action that's being sensed, and then an adjustment is
made subsequent to that action. So this is a feedback operation,
adjusting posture in response to a sensory signal.
Okay, let's move on and talk about the reticulospinal system.
So, the reticular formation is really a complicated network of structures, that
we find throughout the tegmentum of the brainstem.
So indeed the reticular formation is a complicated network of nerves, that spans
the the cord of the brainstem. one can even argue that its continuous
with the intermediate gray matter of the spinal cord inferiorly.
And continuous with the lateral portions of the hypothalamus, on the superior side
of the brainstem. Well, within this network of cells, we
now know that there are actually discreet circuits.
And many of these circuits, have specific functions with respect the visceral motor
control and somatic motor control. There are elements of the reticular
formation, that do in fact project broadly and diffusely throughout the
nervous system. And those are projections that have an
important role, in modulating brain function in various regions.
Broadly speaking, we recognize the rostral part of the reticular formation,
coming from the mesencephalon. And from the rostral part of the pons, as
having those kinds of projections that modulate activity in the forebrain.
the more inferior part of the reticular formation.
And the caudal pons and in the medulla. That's where we tend to find these
circuits, that have a kind of pre-motor function.
That is, they help to coordinate the output of lower motor neurons.
Many of these lower motor neurons are concerned with visceral motor functions,
including cardiac rhythms and breathing rhythms, and perhaps even digestive
rhythms. Other, other networks are concerned with
somatic motor function. And here's where we imagine our
paramedian pontine reticular formation coming in, and helping to organize
horizontal gaze movements. But there are other circuits, within this
reticular formation that are concerned with modulating the gain of segmental
reflexes in the spinal cord. As well as, making adjustments of
posture. So, to go back to the previous slide for
a moment, I would highlight that there are.
Indeed projections from this reticular formation, down through the anterior
medial white matter of the spinal chord. And then near their level or termination,
they tend to bifurcate and supply bilaterally, the medial columns of lower
motor neurons that are found in the spinal chord.
Now, one key way to differentiate the contributions of the reticulospinal
connections from those of the vestibulospinal connections, has to do
with time. You'll recall that I made the point that
the vestibulospinal projections, are operating as a feedback circuit.
taking a sensory signal, the movement of the head and implementing a motor
adjustment. So in this sense, the vestibulospinal
system, is operating based on the information acquired in the past.
The reticulospinal system, is concerned primarily with making anticipatory
adjustments of posture. That is, making adjustments of posture
that anticipate, the actions that are about to happen.
So, here's an example that helps me understand differentiation of these two
systems. Imagine a sprinter, and that sprinter is
in the blocks awaiting the sound of the starter's pistol.
So that sprinter has just heard the words, ready, set, and now there's an
anticipation. There is a time of preparation of the
body, especially the lower body, for this explosive motor action, of sprinting out
of the starter's blocks. And engaging in this pattern of
locomotion, that is essential for competing in this activity, this, this
sprint. That moment of ready and set, is where
our reticulospinal system, will be engaged in setting the tone of the
segmental circuits. So that an explosive motor action can be
executed with maximum energy, and performance, and skill.
So this reticulospinal connection, is going to be especially active during this
time of ready and set. It will be preparing the circuitry of
lower motor neurons, for this explosive action that's about to happen.
Okay, now you're that sprinter the gun's going off.
You've exploded out of blocks, and now you're running down the lane in the
course of this race. Imagine that you stumble, that stumble is
not something that you planned and its not something that you necessarily
anticipated. It just happened.
Well, that stumble is going to evolve some unexpected acceleration of your
head. That will engage the distributive system
and hopefully. You'll be able to catch yourself, and
maybe break stride a little bit. But you'll make some adjustment, I trust,
and continue on in the race. Well, that stumble, and the adjustment
that allowed you to continue in the race, is a function of our vestibulospinal
system. Vestibular systems sensed the unexpected
acceleration of the head, and an adjustment in posture, and in the
mechanics of locomotion were implemented via the vestibulospinal system.
Well, I hope this image of starting and then running a race has been helpful and
helps you to differentiate the contributions of the reticulospinal
system from the vestibulospinal system. Well, if the reticulospinal system is
going to serve to prepare you for action. To anticipate the activity that is being
planned for, then it implies that it must know what that planned activity is all
about. The implication anatomically is that
there ought to be connections from our pre-motor cortex, down to the reticular
formation, and indeed there are. there are projections that emanate from
our motor cortex, that are informing these neurons in the reticular formation,
probably bilaterally about the activity that's about to be performed.
And then the reticulospinal connections, are those that actually make the
appropriate adjustments of gain and muscle tone for the actions that are
necessary. Now, there's one other structure that is
often considered to be a collection of upper motor neurons, in the brainstem
that I want to mention. And it's found right here, in the dorsal
part of the midbrain. And what I'm attempting to identify for
you here is the superior colliculus. Recall that the superior colliculus, is
one of the little hills that forms the tectum or the roof of the midbrain, just
above the cerebral aqueduct which we see here.
So there's the inferior colliculus. That's an auditory relay and then the
superior colliculus which is the structure that I'd like to just mention
very briefly here. So the superior colliculus, is a motor
structure that integrates sensory information from vision, from audition,
from the canal sensation. And probably also from pain and
temperature sensation. So these sensory signals are integrated
in the networks of the superior colliculus, and a motor signal emanates
out of the deeper layers of the structure.
The superior colliculus is concerned with making reflexive adjustments of gaze and
posture. Towards a sensory stimulus.
So I already gave you the example of a bolt of lightening perhaps there could be
a unexpected pinch of the skin. Or maybe a sound that comes from some
lateral position in our auditory environment that captures our attention.
Either one of those three examples will often induce a reflexive shift of our
gaze and our attention, towards a particular stimulus.
Well how does the superior colliculus do this?
Well we've talked elsewhere about the role of the superior colliculus.
In coordinating saccadic eye movements, through a portion of the reticular
formation, that coordinates movements in the horizontal plane or in the vertical
plane. But there are also adjustments of the
cervical spinal cord, that are concerned with the turning of our head and the
adjustments and the stabilization of our posture.
Those adjustments that are mediated in the spinal cord, are carried out by
projections from the superior colliculus into the reticular formation, which then
projects to the spinal cord. There may also be direct projections in
the human nervous system from the superior colliculus, to the lower motor
circuits of the cervical cord themselves. But we know that at least there is a role
for the reticular formation in mediating the signals that are coming out of the
superior colliculus. So, it's fair enough to include the
superior colliculus in our conversation about upper motor neurons in the
brainstem okay. We'll shift gears again and talk about
the neural circuits for expressing emotional behavior.
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