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Molecular Signals for Axon Guidance, part 2

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Medical Neuroscience

Duke University

4.9 (1,279 ratings) | 120K Students Enrolled

Lehrplan anzeigen

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.

Skills You'll Learn

Brain, Neurological Disorders, Neurobiology, Neurology

Bewertungen

4.9 (1,279 ratings)

5 stars
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RR

Nov 25, 2016

While I greatly respect Dr. White's obvious immense knowledge of the neural anatomy, I feel taking this course did very little beyond showing me that perhaps medicine and anatomy wasn't for me.

SJ

Jun 27, 2017

I've always wanted to attend a course like this which offers such a detailed description of the fundamentals of neuroscience. Glad I found it and sure as hell recommended it to all my friends.

Aus der Unterrichtseinheit

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

Unterrichtet von

Leonard E. White, Ph.D.
Associate Professor

Okay, so I introduced two categories of molecules that help mediate the

signalling that's required to grow circuitry in the developing nervous

system. We talked about Cell-associated

molecules, now let's talk about the Diffusible molecules.

And there are essentially molecules that are secreted by target tissues and

diffuse out into the extracellular fluids.

These molecules can affect the growth and the survival of neurons and their

processes whether they be dendritic branches or axons.

There are two categories of diffusible molecules, based on their mechanisms of

action and how they affect the development of the neuron and its

processes. I'd like to talk first about tropic

molecules. These are molecules that guide the growth

of axons toward or away from the source of this particular molecule.

So, the directed growth is what we mean when we talk about tropism and so this is

a tropic molecule that directs the growth of an axon.

They comes essentially in two functional valences, they can be chemo attracted

molecules. Or, chemo-repellent molecules.

So, I'd like to give you some examples of each.

There are also trophic molecules. So, while tropism refers to directed

growth, trophism refers to nourishment. So, these are molecules that provide some

kind of nourishment. Now, not nourishment in the form of

glucose to be burnt up or ATP to be hydrolyzed and release energy from the

chemical bonds. But nourishment in a way that promotes

the survival and the growth of neurons and their processes.

In the peripheral nervous system these trophic molecules become relly important.

In matching the axonal innervation with the target tissue that's present.

And, we'll also see the importance of trophic molecules for development of

appropriate circuits within the central nervous system as well.

Now one, very important and very interesting aspect Of trophic molecule

production is that this is activity dependent.

So this will provide the basis for competitive interactions in developing

circuits. So very interesting concept, we'll say

more about that in our next tutorial. So let's consider some examples of our

diffusible molecules. So again the general mode.

Of operation here is that these are molecules that are secreted by target

tissues. And these molecules interact with

receptors that are present on the surface of growth cone.

These diffusible molecules interacting with their receptors can have widespread

effects on the neuron. that transduces these signals.

There can be local effects that can affect the growth patterns at the level

of the growth cone and the individual axonal branch in question.

But there might also be signals that go back to the cell body and affect gene

transcription in the entire cell. So there are a variety of consequences of

the interaction of these diffusable molecules with their receptors.

So, in this figure, we see an illustration of several, different kinds

of diffusable molecules, and their effect.

we see some source of, diffusable chemo attractant signals here.

That begin to divert the growth of an axon towards that source of that

chemoattractant. We also see contact-mediated

chemoattractant signals that are important for helping to establish the

growth of axons in parallel. and this is critical for the formation of

axonal pathways, where there are many hundreds of thousands of axons,

typically, growing together in parallel. Now, there are also chemorepellant

signals that can be produced at some local source.

And you'll see that as a growth cone begins to approach and explore, it may

interact with one of these signals and then be diverted away to grow in the

opposite direction from that source. There may also be a contact mediated

chemo repulsion signals That help to make sure that the growth cones at the leading

edge of a developing pathway, begin to explore, the entire environment.

Giving these growth cones an opportunity to seek out appropriate sources of chemo

attractant. And avoid the appropriate.

Sources of chemo repellents. Now also in this figure, we see the

presence of trophic molecules that are expressed and they don't necessarily

direct the growth of an axon into the source of that trophic support.

Rather what they do is they validate that growth.

So the growth is directed by the Tropic molecules but the survival and the

ability of that axon and the neuron that gave rise to that axon to thrive, is

dependant in some measure on the acquisition of tropic support.

That comes through the inteaction of these neuro [UNKNOWN] molecules and their

reeceptors. We'll say more about that in just a

moment. First I'd like to gove you some examples

of some of the tropic molecules and then talk about one very important pathway,

that illustrates the operation of these milecules in development.

So, there are several representatives of these Trophic molecules that we now know

about. On the left, we see examples of the

Netrin and Slit Family of trophic molecules.

Netrin. Is a soluble protein that is released and

interacts with its receptors DCC and an associated protein called Unc5.

So these interactions can result in the assembly of Actin filaments, leading to

the directed growth. Of that growth cone, and its axon being

elongated in it wake towards the source of netrin.

Now, there's a related, tropic molecule called slit that interacts with the

receptor called robo. And slit antagonizes the activity of

netrin. So.

Netrin and slit together provide sort of a push, pull system, or a go, stop kind

of system that allows for the direction of growth.

And this is particularly important at those forks in the road, as I'll show you

in just a moment. Now another defusable trophic molecule

illustrated here. Are the semaphorins.

So semaphorins are cell associated molecules that can be released and

diffuse away from their source. And the semaphorins can interact with

receptors. two of them illustrated here, plexin and

neuropilin. These receptors can likewise interact

with transduction machinery that can affect the assembly of [UNKNOWN] skeleton

and the patterened growth of that growth column.

However, what the Semaphorins end to do is, promote the disassembly.

Of these neuronpillaments and that results in the retraction or the collapse

of the phalapaudia and the lamelapaudia/g.

So the semaphorins are important signals that mediate the repulsive signals that

are encountered as axons explore their environment.

So, let's consider one pathway. And indeed this is the pathway that I had

in mind in our study question a little while ago.

this pathway is the spinothalamic tract. Now recall the second order axon of the

spinothalamic pathway. It has to do something which I find

completely remarkable. This axon is grown out by a neuron in the

dorsal horn of the spinal cord. And this happens throughout the entire

length of the spinal cord. The axon will grow out very near its

level of origin. And it will begin to grow in the anterior

medial direction. So that axon encounters the mid line of

the spinal cord and the ventral while commissure, the interior white commissure

of the spinal cord. And it makes an important growth choice.

It grows across that mid line and enters the anterolateral white matter on the

contralateral side of the spinal cord. So, there is a decision to cross a

mid-line here in the outgrowth of this second order axon.

But there's also one more remarkable aspect to the growth pattern.

Once across the mid-line and into the anterolateral part of the white matter,

this axon now makes a hard turn in the superior direction.

And grows to the ventral posterior complex of the thalamus.

So the implication is that there must be some chemo attracted signal that draws

this axon towards the anterior midline of the spinal cord.

And then once across the midline, there must be some signal that directs the

growth. away from that segment of origin, and

towards the superior axis. So we imagine there may be some

combination of chemoattractant, and chemorepellant molecules that achieve the

necessary pattern of growth for this second order axon to establish the

spinothalamic tract. And indeed, what we imagine happening

with the growth of this pathway Is these second order axons, these commissural

axons are attracted towards the region of the floorplate.

And they're attracted by the presence of netrin, which is secreted by this special

region of the neurepithelium called the floorplate.

So these axons begin to grow in towards that floorplate.

And then once grow across that floorplate, now slit seems to be produced

just lateral to the location of that floorplate.

And slit now begins to negate the influence of netrin.

And there is likely some other kind of signal, some other repulsive signal, that

is present here in the anterolateral part of the developing neural tube.

Perhaps a semaphorin, which will collapse the growth cone from growing in this

direction, and rather promote growth away from the source of that chemorepellant.

there very likely is also some kind of chemo-attractant that is in some

concentration gradient drawing this axon towards the superior aspect of the

developing hindbrain, even into the forebrain, reaching its ultimate target

in the diencephalon. So this is a wonderful example of how the

presence of chemo retractant and chemo repellent molecules can mediate what are

really some spectacular patterns of growth that we've learned about in our

survey of the structure of the sensory motor systems.

I'll refer you to a beautiful box in chapter 23, that talks about another

famous fork in the road. That would be the optic chiasm.

Where similar functions, seem to direct the nasal retinal axons across the optic

chiasm, while repelling the temporal retinal axons which remain on the

ipsilateral side of the brain, progressing through the ipsilateral optic

tract. So, turn to the box there in chapter 23

to learn more about that particular pathway.

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