Why is the nervous system crossed over?

28 June 2010
Presented by Diana O'Carroll


PET image of a human brain


Why does one side of the brain control the other side of the body? Surely it would make more sense if the left half of the brain told the left side of the body what to do. We find out what the current thinking on this conundrum is. Plus, we ask how an electric toothbrush can disturb vision.

In this episode

The optic chiasma - where the optic nerve fibres cross.

00:00 - Why is the right side of our brain in control of the left side of our body?

Why is our brain the wrong way round? Why is the right side of our brain in control of the left side of our body, and vice versa?

Why is the right side of our brain in control of the left side of our body?

We put this question to Roger Keynes, Professor of Neuroscience at the University of Cambridge...

Roger - Well that's a tough question. I mean, it's a "why" question and it's an evolutionary question, and answering "why" questions in biology is very difficult.

In this case, I think we have to go back several hundred million years to see the very first evolving animals with complicated nervous systems that are getting more and more sophisticated.

At some point, vision evolved and probably started becoming more important when mammals developed binocular vision, with their two eyes looking forwards from the front of the animal.

This arrangement means that when and animal is looking at something, each of the two retinas - one in each eye - are seeing the same thing, providing binocular 3D vision.

But the point there is that the lens of the eye inverts the image that forms on the retina, so in binocular animals like us, things seen to our left are sensed by the right half of our left eye, and by the left half of our right eye, and that's a product of the physics of the situation.

If each eye were to send all its nerves to just one half of the brain, the picture of the world on one side of the body would be split between the two halves of the brain. Instead, it makes much more sense for this picture to be fused in just one half of the brain by the crossing over of some of the nerves of the two eyes.

But more recently, people have been thinking theoretically about how can you wire up a clever brain just on first principles and they decided that, probably, it's useful to wire things crossed over simply because - for reasons we don't need to go into - it prevents or reduces wiring errors compared with if you wire things up just on the same side of the brain.

Diana - So wiring might be better off crossed, and binocular vision could be the cause of our brain alignments; and some theoretical work has indicated that a brain is actually more likely to wire itself up correctly during development if one half controls the opposite side of the body. On the forum, RD pointed out that the crossover is still present in eyeless animals and insects, while Diver John said that it could just be a quirk that all subsequent life forms have adapted to.


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