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Getting to Grips with the Brain

Introduction

Note: This article is a supplement to Acupuncture and the Limbic System , which contains a number of anatomical references. It describes an approach to understanding the brain that I find useful but it is deliberately limited in scope and is not intended as even an elementary account of brain anatomy or embryology.

Why I wrote this article

When I started to study human anatomy as a medical student I wasn't given any overall view of the subject. Instead, we were simply told to begin dissecting the abdomen, starting with the abdominal wall, where there were various nerves we were expected to identify. We had to understand their course in the region and how they were related to the various layers of muscle in the abdominal wall, but we saw only those portions of the nerves that were located in the area we were dissecting. I had no idea where they came from or how they were related to the nervous system as a whole.

As time went on, of course, I began to make sense of what I was seeing and to relate the nerves I was dissecting to the segmental arrangement of the spinal cord. But it took me some time to feel reasonably confident that I understood what I was doing.

When it comes to the brain similar problems arise. Brain anatomy is intimidating for most of us and it's all too easy to get lost in the welter of details. What you want at the outset is just a basic idea of how the brain is organised and where its main components are located.

There are plenty of books and internet resources which supply this kind of information, but even these can seem rather daunting the first time you encounter them. I've come to believe that it's best to start, not with the mature brain, but by going back literally to the beginning and seeing how the brain develops over time to take the form it does.

That means starting with embryology. You might think that this would be an 'advanced' subject, and of course the embryology of the nervous system is indeed very complex, but the essentials are easy to grasp. This approach works for me and I hope it may work for you too. If it doesn't, forget it and take another route to understanding.


The very beginning (Figure 1)

Figure 1

In its simplest state the central nervous system is just a closed tube (the neural tube). The front end will become the brain and the rest will form the spinal cord (Figure 1a). This is true of the embryo, and it also describes how the nervous system has developed in the course of evolution. You probably began your biological studies by dissecting 'lower' animals such as worms and the dogfish, so you will be familiar with these more 'primitive' forms of the nervous system. To a certain extent embryology 'recapitulates' evolution.

Quite early on the front of the neural tube acquires three bulges (the primary brain vesicles). These are destined to become the forebrain, midbrain and hindbrain (Figure 1b). The neural tube now looks like those segmented party balloons that you may have blown up as a child. So far, of course, there is no resemblance to the adult human brain, but have a little patience.

Next, the forebrain acquires some side bulges (secondary brain vesicles). The two at the front are the future cerebral hemispheres - the biggest and most obvious parts of the adult brain as we look at it from the outside (Figure1c). (There are two smaller cup-shaped bulges just behind these, which will become the eyes, but we don't need to bother about those here.)


The embryo at 4.5 weeks (Figure 2)

Figure 2

At this point we can see several things.

(a) The neural tube has now been folded over, which brings the floor of the forebrain into contact with the floor of the hindbrain (Figure 2). This will cause various structures in the mature brain to lie closer together than they would be if the doubling-over of the neural tube hadn't happened.

(b) The hindbrain has acquired some new bulges, which will become the cerebellum.

(c) The hollow tube is the forerunner of the ventricles of the brain.


The embryo at 5 weeks (Figure 3)

Figure 3

The folding already apparent in Figure 2 has now increased. Notice also that the developing cerebral hemispheres are beginning to curl round, making them appear rather like the horns of a ram (Figure 4).

A ram's horns (Figure 4)

Figure 4


The embryo at the beginning of the fourth month (Figure 5)

Figure 5

By now the cerebral hemispheres have expanded and curled further and are beginning to look more like a brain as we usually think of it. The two lobes of the cerebellum can also be seen although they are still quite small.


Side view of embryo (Figure 6)

This shows the right cerebral hemisphere. It has been formed by the continued curling process.


Figure 6

In shape, this reminds me of a boxing glove (Figure 7).

One boxing glove (Figure 7)

Figure 7

Of course, there are two cerebral hemispheres, so two boxing gloves, side by side (Figure 8).


Two boxing gloves (Figure 8)

Figure 8


Relation to the adult brain (Figures 9 & 10)

Figure 9 Figure 10

In terms of the boxing glove analogy, the fingertips correspond to the frontal lobe, the back of the glove is the parietal lobe, the wrist is the occipital lobe, and the thumb is the temporal lobe. The hollow interior of the glove corresponds to the ventricles and the leather on the outside to the cerebral cortex.





Location of some limbic system structures (Figures 11 & 12)

(a) The amygdala is a group of nuclei (collections of nerve cells) situated at the tip of the temporal lobe, i.e. at the end of the 'thumb'.

(b) The hippocampus is formed by a folding up of the inner margin of the temporal lobe. This gives it a 'Swiss roll' appearance in cross-section. Note that it is, therefore, part of the cerebral cortex (Figure 11).

Figure 11

(c) The cingulate gyrus is a layer of cortex on the inside of the glove, adjacent to the little finger (Figure 12).


Figure 12

Conclusion

This concludes my brief foray into the development of the brain. It is, I repeat, very limited in scope; it isn't intended to constitute even a basic primer in brain embryology. It's intended only to help you locate the main structures referred to in Acupuncture and the Limbic System. But if, like me, you find that this way of thinking about the brain prompts you to explore this fascinating subject further, so much the better,

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