Well, the answer is that the brain has an architecture which is what’s called post- mitotic.  There are only a few restricted areas in the brain and central nervous system where there are new nerve cells being born.  For the most part, you rely on the compliment of nerve cells you are born with and which continue to divide for a very short window after you were born and then stopped.  So basically, what you're born with is what you have to make last a lifetime.  And there’s a reason for that because if brain cells were dividing all over the place, remember that brain cells have long connections that they make from one cell to the other.  And those connections are crucial to you being able to do the right thing, say the right thing have memories and for your brain to be able to work properly.  If those cells were dividing all over the place and making aberrant connections, then it will be very, very difficult to preserve that architecture.  So there’s kind of method in the madness. 

The problem is that as that is a fixed structure, it’s very hard to repair it by getting the cells to re-divide because basically, if you have an injury, evolutionary speaking, that’s bad enough to destroy a part of your brain or your nervous system.  The chances are you’d be dead anyway.  So, we haven’t really evolved the ability to repair the brain and spinal cord.  In some animals though, that can happen and things like gold fish, lampreys, and also even salamanders can restore whole limbs, bits of their nervous system.  If you take the eye out of a frog, turn it around and put it back in again, it will rewire itself back into the brain, only because the eyes now are upside down.  The animal see upside down and it does the wrong thing.  If you hold a fly in front of it, instead of jumping forward at the fly, it jumps backwards and takes a bite out of the deck and that won a Nobel Prize for Roger Sperry a few years ago and proves that some animals can regenerate their nervous system, but certainly, not us unfortunately. 

The reason for this is that the nervous system is wired up, so that you balance out movements. so that if a muscle gets made longer than it thinks it should be, you have an organ called a spindle, which is inside the muscle, which signals length of the muscle and if that gets stretched it feeds back on to the mototr nerve supplying the muscle and increases the firing of the motor nerve-making the muscle get a bit shorter. So if you have 2 antagonistic muscles, one hand and the other hand, pulling against the other then one's going to win a bit and that's going to make the muscle on the other arm get a bit stonger then that will pull back and that one will win a bit and make the muscle on that side pull harder and so on. And it will just do a tug-of-war so you won't go anywhere because the brain is set up so you're finger's in the middle.

We posed this question to Julie Williams from the University of Cardiff...

Julie -   Alright.  I think it probably is a general problem.  Learning, you probably need to have well activated nerve cells to create the synapses to learn new information.  So I think this probably could be affected by stress.  So I would say it’s a general problem.

Chris -   Because one other thing that people have realized is that the hormones that make our stress cortisol, there are receptors for those chemicals in the brain, and they do seem to cause damage if they're present chronically to those parts of the brain, particularly the hippocampus which is concerned with making memories.  So maybe that’s part of the manifestation, Julie.

Julie -   We also know that people who have had major depression throughout their lifetime or bipolar disorder tend to have a highly – a slightly higher risk of developing Alzheimer's disease at later life.  This could be due to the medication they take or it could be due to the actual process of depression.  So there are some areas of support for that.

We posed this question to Ed Wild from University college London...

Well it’s a big question and the answer is that there’s a lot of promise in stem cells, but we’re probably several years away from being able to see the benefits of the research that’s going on.  You mentioned at the beginning of the program that brain cells, once they're dead, they're gone and they can't be replaced from within the brain because brain cells don't divide.  And the hope of stem cell research in neurodegenerative diseases is that you can take these stem cells which are capable still of dividing and becoming any kind of cell they like, put them into the brain and they’ll then re-grow, and replace the cells that have died.  But as you also mentioned earlier on, the brain is a phenomenally complex thing and performing its functions normally, depends not just on the cells being there but on the connections, the billions and billions of connections that there are between the brain cells.  And even if you could get the brain cells, the stem cells to differentiate into neurons that behave completely normally, you’d probably never be able to get them to make all the right connections.  So there’s certainly be a lot of work, training the stem cells to make the right connections and behave the same way as the cells around them.

Absolutely. People talk about this blood-brain barrier. This notional structure which in some way keeps the brain isolated, cocooned inside you biochemically and physically-away from what's happenign in you blood stream and it;s absolutely true. The history of the blood brain barrier goes back a hundred and something years to a guy called Paul Ehrlich, who was a German scientist, he was interested in dyes initially. He used to inject dyes inot animals and then see which bits of the body got stained. andhe was intrigued to see that when he put dyes into the blood stream, much of the time the dye did not get into the brain. And so he realised there must be some kind of barrier separating what goes round in the blood stream from the delicate issue inside the brain. We not understand more about what this blood-brain barrier is. It's a bit contrived, what what's going on is that you basically have special junctions between ceels that line the surfaces of the brain, that separate the brain tissue from blood vessels and these cells make very tight junctions, that's what they're called, and this effecttively means that there's a barrier which is the membranes of those cells separating what's in the blood stream from what's in the blood tissue. 

And what this means is that certain substances can move very easily into the brain, especially if they're substances that can dissolve well in fat, because of course the membranes of cells are made of fat. So lipid-solube drugs like heroin, cigarettes-nictone, cocaine, they're very oily molecules. they go into the brain beautifully and that's why they tend to be addictive. Because they move preferentially into fatty tissue like the brain. Other substances which don't dissolve in fat very well, don't get into the brain very well. But there are some exceptions. Those exceptions are things that the brain needs. So sometimes if it needs a certain chemical that wouldn't be able to diffuse in very easily, it has special transporters which can scrutinise what is going past in the blood, grab goodies that it wants and move those into the brain. This is what people found when they were giving the drug L-Dopa for Parkinson's Disease. -Dop is an amino acid, dissolves in water, doesn't dissolve in brain tissue very well but it gets into the brain much betetr than it should do and the reaon is theer are these special transporters that get hold of it and shove it into the brain.

What he’s referring to is a concept called plasticity and this is where nerve cells can change the connections they make from one set of nerve cells to the other set of nerve cells and in this way, there’s a limited capacity of a bit of a brain that’s damage so that signals can no longer go through.  Then other networks or other weaker connections can be strengthened to help to bypass that blockage.  There’s no degradation that goes on.  There’s merely a strengthening, a reinforcement or a bypass via other bits of the brain to get around an obstruction or a problem which was holding things up.