The exact physiological processes that take place in Alzheimer's disease are still in dispute, but in general terms we can say that there are random disruptions of the connections in various networks in the brain. The tangles of neural axons and dendrites that are found upon post-mortem dissection of the brains of Alzheimer's patients are like tangles of wires in an electrical appliance. Just as an appliance cannot function properly if its wires are not laid out properly, so the brain cannot function if its neurons are not connected properly.
But this analogy is only partial. As we have seen throughout this book, the networks in the brain do not just send information from one place to another. All the information there is in the brain - not just knowledge about the world, but all our capacities to do the things that require a brain, from walking upright to going shopping to cooking meals to discussing philosophy over dinner - is located within the networks themselves. An accumulation of random disruptions of the connections in any network will lead to increasingly erratic behavior of various sorts, ending in the inability to do anything at all.
The tangles in the networks that constitute Alzheimer's are quite different from the ordinary death of neurons. We have seen that we are constantly losing neurons, but that the loss of a few of them does not substantially change the functioning of any given network. In normal aging many neurons die, and this may be one reason why it becomes harder to learn new things as we get older. But as long as the remaining neurons retain their proper connections, we can still do all the things we could do before, even if it may take a little longer.
The way Alzheimer's differs from normal aging is that the neurons don't just die and disappear - instead, the axons and dendrites that connect them become tangled up. In normal aging, some of the connections that have activated the pattern yielding the name "Judith" when I see my friend may be lost, and so it may take a little longer than usual for me to recall her name. The remaining connections will still be intact, however, so I will not mistake her for someone else. But if John, say, has Alzheimer's, then the axon branches coming from the human-face network that are supposed to activate some dendrites in the human-name network to allow John to say "Jerry" when he sees his friend may become tangled up with other axon branches in the face network. This may lead to a situation in which several different patterns in the name network are slightly activated at the same time, with no one of them being activated sufficiently to allow the neurons to form one stable pattern for one particular name. John will thus be unable to recall Jerry's name.
As the tangling of the dendrites and the axon branches spreads to other areas of John's brain, he may no longer recognize Jerry as his friend at all. Later, even though he may still be able to talk, he may not recognize his own children when they come to visit him. They may seem familiar, but he will not remember how they are related to him. Later still he will not be able to perform even the simplest functions.
The loss of function in patients with Alzheimer's does not always occur in the same order, as the random tangles may begin in different areas of the brain and spread in different ways, but in the end the entire brain is affected.
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