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  • Jody B. Miller

When and How the Brain Develops - Part One

The following post is primarily an excerpt of an explanation about development of the brain by Dr. Susan Book Heimer from UCLA.

As I think of my three children, who are completely different...I often wonder about all the elements that came into play as their brains were developing.

Whether it was my first born who was born a couple weeks late, developed her intellect quickly and is now at the top B-school in the world. Or my eldest son who is more than half his life behind with autistic spectrum behaviors, even though he is very social and determined. My youngest excelled at all sports and is very intuitive. Like I said, all very different.

I can see how much of Dr. Book Heimer's discussion relates to my own experiences as a parent. It may for you too.

Here is the excerpt from Dr. Book Heimer.

Hello welcome to today's, UCLA health webinar. My name is dr. Susan Book Heimer. I am a professor of psychiatry and bio behavioral sciences here at UCLA, and I do research in brain imaging today's. Today's topic is going to be.

How does a child's brain develop? What I'm going to be focusing on, are first how brain development is designed to form connections which, together, we call the human connectome. I'll also talk about how these connections relate to brain function, and I'll talk about the adolescent, brain, the teen brain and why that teen brain is so let's say difficult.

I'll also talk about a few things that can go wrong in brain development because of these abnormal connecting connections that can develop, and then I'll mention some studies that are going on at UCLA.

Now one of the first things that we need to know about brain development is that it's really not about size. The brain in fact does get bigger, but that's not really what's important. What's important is what's happening inside brain development - it's all about forming connections.

We are actually born with a lot more brain cells than we're going to use and in two critical periods of development - at about age two and then again at adolescence.

We actually lose a lot of neurons during this period of time. However, our brains are forming connections, the good ones, the ones that we are using and are going to be maintained, and the rest are going to be pruned away.

Just like we would cut off an old branch that is no longer useful, so we use the phrase, use it or lose it when we think about brain development. In this way, there's a very important relationship between our environments and our brains.

I had Strep "B" during the last trimester of my pregnancy with my cognitively delayed son. I was treated with antibiotics and was told that they would not pass through to my child. The risk of having Strep B is that a child can be born retarded if it is not treated fully. This infection can lead to retardation among other potential complications. I would attribute my infection to biological factors that affect the brain.

(cont. Dr. Book Heimer)

What our brains are exposed to, what we do, what we see and experience will all help determine which of those connections are going to stay and which ones are going to be cut away. The brain starts very, very primitive and very quickly develops into having the general brain shape. But the main thing that changes is the folds in the brain. The folds in the brain which give rise to this characteristic brain pattern are there because they increase the surface area by having folds. We can get more neurons into that tissue and the more neurons we have the more we are able to use that information to do complex tasks that humans do, so humans have the most folded brains that there are.

When and How the Brain Develops - Part One

There are three major changes that take place during brain development. The first one is what we call the proliferation of dendritic branches, just as a tree grows with a long central stalk, which we would call in the brain and axon.

It also develops all of these branches and brain cells sprout these branches, which connect to other brain cells, so that's one thing that occurs.

Another one is this process called myelination, which is a covering of the axons.

These are the major branches that I was talking about, and this myelin is made of a very fatty substance that actually comes from certain kind of neurons - and it's like an insulation material that allows information to process and to be sent very quickly from one brain cell to the next.

And finally, there's that process of pruning, dying off of brain cells and brain connections.

The little branches and the cells themselves are quite small during development. These branches really start growing quite a bit, although the actual number of neurons is not changing by nine months, you can start to see many of these connections forming and becoming more complex,

At the age of two years, the connections are incredibly dense. They're the same number of neurons, but everything is practically connected to everything else.

However, if you contrast that with a young adult brain, you can see that a lot of those connections have started to prune away and the cells themselves are larger and the connections between cells are thicker. They've become stronger with age, but they are fewer of them.

This is the general process. What we talk about is a transition between brain plasticity, that is the potential for the brain to do a lot of different things, and brain efficiency. That is just keeping those connections that we need so that we can use our brain energy as efficiently as possible.

Although different areas of the brain thin at different rates over this period of time - and in fact the frontal lobes - are the last areas of the brain to thin, they don't even complete their maturation until at least the mid to late 20s.

Now I'm, going to talk a little bit about myelination. From the age of one week through the age of ten years, the brain gets a lot bigger, but one of the other changes is that the brain looks very dark. When the brains look much lighter it's because we have no myelin none of that fatty substance in the infant brain. It slowly starts to develop and then accelerates incredibly rapidly, so it's the myelin that makes the white matter.

These connection tissues, look very, very white. I'll explain a little bit more about myelin and how that works.

When and How the Brain Develops - Part One

This is a picture of a brain cell called a neuron, and the axon - that long central branch that will lead this brain cell to another brain cell.

In order to make connections at birth. There is no insulation around any of these connections, but what happens is a certain kind of brain cell comes in and forms a little sheath and wraps itself around that axon and a number of these different neurons wrap themselves around the axon.

It's like the kind of insulation that you would see on an electrical wire. We don't actually see the wires. We just see the plastic surroundings that insulates the wire. It allows that electricity to move safely within that wire, but here, in this case of myelin, it also allows something special to happen since the brain communicates with electrical signals.

This fatty substance allows those signals to skip over from these different nodes all the way down. It allows sort of a speed pathway of connection, and so the more myelin there is the faster these connections can form and therefore the better.

There really are only two areas of the brain that have this myelin that allow for this rapid connection. One is the visual cortex - the eyes through the central part of the brain into the visual system, and the other is the mouth motor cortex, and you can guess why an infant doesn't have to do very much except find what it wants to suck and have the motor system to suck it. That's all an infant wants to do, but as the infant starts to develop more and more systems come online. This myelin starts to get formed and it allows that child to develop faster and better cognitive skills.

So the more that brain is in use, the more that this white matter starts to develop and we see that within one year of life we go from almost no myelin into a well myelinated brain.

When and How the Brain Develops - Part One

Another picture of connections is called the corpus callosum. It's a connection of those fibers as connecting fibers between neurons that connects the left side of the brain to the right side of the brain.

Our two hemispheres have learned how to talk to one another and to do it very, very quickly and efficiently. So why do we have all of this connection? Well, different parts of the brain have different roles.

Each part of the brain does something different but every part of the brain has to talk to other parts of the brain. It sends electrical signals throughout the brain through those white matter tracts to form these connections so that different areas can talk to each other and work together to perform complex tasks like talking. This is all about connectivity or what we would call functional connectivity that is not just the structure of those pathways, but what those pathways are doing and the cognitive skills that can emerge, because the brain is using those pathways to talk to other areas of the brain.

There are important connections in the brain, major fiber pathways. One, for example, goes from the motor system down through the spinal cord and the corpus close and connecting the two hemispheres of the brain. On the outside, there is very little happening. Those fine connections have not been formed. Compare that to the adult brain which has elaborate and deep connections. So development is really all about forming connections and forming them the right way together.


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