Neurulation

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En provenance du cours de Duke University

Medical Neuroscience

1238 notes

Duke University

Medical Neuroscience

1238 notes

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.

À partir de la leçon

The Changing Brain: The Brain Across the Lifespan

This module represents another turning point in Medical Neuroscience. Now that we have surveyed human neuroanatomy and our sensory and motor systems, we are ready to take a step back and consider how this magnificent central nervous system came to be the way that it is. We will also learn how the brain re-wires itself across the lifespan as genetic specification, experience-dependent plasticity and self-organization continue to interact, re-shaping the structure and function of neural circuits throughout the central nervous system.

Embryological Subdivisions of the Human CNS8:47

Major Forces In Early Brain Development12:36

Neurulation11:17

Formation of the Early CNS12:35

Inductive Signaling In CNS Formation15:10

Proliferation and Migration, part 117:23

Proliferation and Migration, part 210:59

Growth Cones11:42

Molecular Signals for Axon Guidance, part 113:22

Molecular Signals for Axon Guidance, part 212:36

Neurotrophins13:08

Neurotrophin Receptors and Synapse Formation11:43

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

Well the story of the developing nervous system really begins with this process

that we call gastrulation. So gastrulation refers to an invagination

of the developing embryo at a stage that we call the blastula.

And this process of invagination in the blastula produces three principle germ

layers. And those germ layers are the ectoderm,

the most outer layer, the mesoderm, or the mesoderm, which is the middle layer,

and then the endoderm, which is the inner of these three layers.

This is a fundamental event in the developing embryo, and it sets the stage

for the formation of the nervous system. Now one very important structure that is

formed in this stage of development is something called the notochord.

Now, the notochord is a long, rod-like structure that is derived from mesoderm.

And, here if we make a cross section through this developing embryo, will see

the notochord here in this central location cutting cross section.

So, what's special about the notochord? Well, I'll say more about that as we

progress and talk about inductive signalling.

But the notochord is responsible for sending out chemical signals that

interact with this ectoderm that overlies this rod-like structure that runs along

the length of the embryo. And these chemical signals induce a

change and the fate of the ectoderm that overlies this notochord.

That region of this ectoderm differentiates into a colon epithelium

that we call a neural plate. And this neural plate, or sometimes we

call it neuroectoterm will begin the process of forming the entire central

nervous system. So the notochord is responsible for

reducing the differentiation from the neural plate.

I should also mention that the notochord helps to establish our primary axis of

orientation in the developing embryo. The notochord forms in a more dorsal

position in the developing embryo. Now we'll call it a gastrula.

And so the presence of the notochord helps us to find what is dorsal versus

what is ventral. the length of the notochord establishes

the anterior to posterior axis of the developing embryo.

And the fact that it's a mid-line structure helps to define the axis from

medial out to lateral, obviously, in both directions.

So, the presence of the notochord helps to define the bilateral axis of symmetry

in the developing nervous system. Well following gastrulation and the

development of the notochord and the onset of inductive signaling that forms

the neural plate, the next major phase of embryo genesis is called neurulation.

Neurulation is defined by the initial formation of the nervous system.

And what happens in neurulation is that the neural plate begins to rise up in

it's lateral margins. And begins to close off in a process that

will form a tube out of that neural plate.

So this begins with the formation of a groove that runs the anterior to

posterior length of the embryo just above the notochord.

And as the walls of this neural tube begin to grow out, they begin to come

together along the dorsal midline. And we can see that process transpiring

over, weeks, three into week four in, human embryo genesis.

Here we proceed a little bit further, and we can see that now the neural tube has

actually closed off underneath this over lying ectoderm.

And there is a progressive closure of this neural tube that begins near center

of this developing neural tube and then extends out into the interior and

posterior directions. So this process continues and now we're

into, well into the fourth week of embryonic life.

And we have the anterior end and the posterior end of this developing neural

tube progressively closing. The anterior end of this developing

neural tube is going to form the brain and the rest of its length, all the way

down to the posterior tip is going to form the spinal cord.

Now, I'd like to return to this concept of inductive signaling, and say a little

bit more about it. Because it's at this stage in embryo

genesis, neurulation, that the inductive signals derived from the notochord are so

important. For defining many of the important

regions of that developing neural tube that will set the stage for further

differentiation into the brain and to the spinal cord.

So if we back up just a little bit in neurulation, so over here to the left we

have the folds of the neural plate beginning to come together to form the

neural tube. And the notochord is giving rise to these

chemical signals that are inducing these morphogenic events in this overlying

neural plate. But in addition to simply inducing the

folding of that neural, well I should not say simply, it's really an amazing feat

in development. That such a thing would happen, but in

addition to inducing the formation of this tube, the notochord is also giving

rise to signals. That are specifying some particular

regions and one particular region is that portion of the neural plate that directly

overlays the notochord. This becomes a region that we call the

floor plate. And this becomes quite important, because

it in turn gives rise to inductive signals that will influence the

surrounding portion of this neural tube. Now there's another special region, that

responds to inductive signals, and that is found on the dorsal side of this

developing neural tube. We call that the roof plate, and the roof

plate will give rise to signals that will influence the dorsal aspect of this

epithelium that is forming the walls of the neural tube.

And one additional region I want to highlight for you is that in the ridge of

this folding neural tube that begins to come together, there's a special

population of cells here. That we call the neural crest.

And the neural crest will actually pinch off from the margins of this neural plate

as the neural tube is forming. And the neural crest ends up sitting just

along the dorsal and lateral margins of the neural tube once that tube has

formed. Now the neural crest is a source of many

different types of cells that are derived from it.

cells begin to differentiate and they begin to migrate away from this region of

the neural crest. And as they migrate away, they are

subject to chemical signals from the nearby structures in the mesenchyme and

in the mesodermal derivatives, such as these somites that we see here, sitting

just lateral to the neural tube. So, as these cells migrate away from that

dorsal lateral neural crest. they are exposed to all kinds of signals

that generate a rich variety of progeny of these neural crests.

And these progeny include both neurons as well as non-neural structures.

so, here's just a, a brief example of some of the derivatives of this neural

crest. So the neural crest pro-generative cell

can be influenced by various factors to form our sensory nerves that we find in

structures like the dorsal root ganglia. Other signals can induce formation of the

peripheral cells of the visceral motor system.

And so that would include our ganglionic neurons that are are intervening our

visceral tissues. so, essentially the entire peripheral

nervous system, including the enteric nervous system, is derived from neural

crust. And, as I mentioned, also non-neural

structures who derive from neural crests, such as the chromaffin cells and the

melanocytes and others that are not depicted here.

Now, how is all this accomplished, how is this possible?

Well, it is possible because of inductive signalling.

If I can just back up to this slide again and just highlight the means by which

these signaling interactions occur. So, I mentioned the notochord is a very

important source of inductive signaling. the floorplate and the roofplate become

important sources of inductive signaling. As do the somites and these mesenchyme

cells, through which the neural crest derivatives must migrate.

So, just to be clear, I've highlighted for you in the handout what I mean by

inductive signalings, so let me just draw your attention to that.

So, inductive signalling is the ability of a cell or tissue to influence the fate

of nearby cells during development by the synthesis in secretion of chemical

signals. Now these chemical signals are either

steroid hormones or peptide hormones, so they have an impact on the transcription

of genes in the cells. That they interact with.

The precise timing of the expression of these inductive signals becomes crucial

for the proper formation of the develping brain.

So what we will see is that in order to form the nervous system correctly.

These inductive signals need to be turned on and turned off in a very precise

pattern in both space within the embryo. But also in terms of developmental time.

And all of this is regulated in complex way that we're just now beginning to

understand. And again, it's subject to probation that

can produce various forms of defects in the developing brain, as inductive

signals might be either blocked.

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