by JOE HERBERT
Most of our organs can be treated as repairable machines. Why can’t we treat mental illness by simply fixing the brain?
Our understanding and treatment of mental disorders is primitive. Why is that? The burden on our society is huge. A quarter of women will have an episode of depression at some stage in their lives (it’s about half that for men). Most will never reach a doctor or be diagnosed. About 40 per cent of those who do won’t respond to the first antidepressant they are prescribed, and about 60 per cent of those won’t respond to the second. About half of schizophrenics will get better or manage to live reasonable lives: the other half will relapse or never recover in the first place. Anorexia nervosa claims the lives of more patients proportionately than any other mental disorder. But mental disorders are only one category – a rather artificial one – of brain disorders.
The treatment of multiple sclerosis, Parkinson’s disease, stroke and, above all, Alzheimer’s disease remains deeply unsatisfactory. Motor neuron disease is invariably fatal. Why should this be? Why can’t we treat brain dysfunction more effectively? There’s a simple answer: we don’t understand enough about how the brain works. But this leads to more complex questions. What do we mean by ‘understanding’? What are we actually looking for? What do we need to know?
Let’s start with the heart. Not because the ancients thought it was the seat of the emotions, or because we talk about breaking it for love; but because it can illustrate what is meant by ‘understanding’ how a part of the body works.
The essential discovery was that the heart is a pump: that is, it provides the driving force to maintain the circulation of the blood. You don’t need to know how it does that to replicate its action: a heart-lung machine pumps blood, but not in the way a heart does. So it’s an analogue, not a homologue. However, you do need to know how the circulation works to understand how the heart is built.
It was the 17th-century English physician William Harvey who discovered this, making the inspired prediction that there must be tiny capillaries that link the arteries and veins. He never saw these himself: they were observed only after the discovery of the microscope. But as soon as the design of the circulation is known, it is obvious that blood entering the right side of the heart from the body (low in oxygen) must go to the lungs, be oxygenated there, and return to the left side, before being pumped out to the body again. Blood must go only one way, which explains why there are valves in the heart that prevent reflux. This also explains why damage to those valves impairs the ability of the heart to send blood either to the lungs or the body, depending on which valve is damaged. So now very successful surgical procedures can be developed that replace damaged valves, in a form that replicates their function.
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Move to the brain and our clear-sighted view turns suddenly foggy. Like the heart, the brain is made up of specialised cells, the neurons. We understand quite well how each neuron is activated, and how it passes this activation onto the next neuron. But now comes a major problem: we know how a collection of heart muscles makes a pump, but not how a collection of neurons makes a thought, a memory, a decision, an emotion. We know it must be a network of neurons: and simulations show us that such assemblies take on properties that are very difficult to predict from the way individual neurons behave. But what is it about such an assembly that represents what we know a brain can do?
Neurons work in a particular way: the brain is an electrochemical machine. Each neuron is activated by a chemical released from another neuron: this then initiates an electrical signal that passes down the neuron’s fibre and, in turn, releases another (or the same) chemical onto the next neuron. But this isn’t a simple chain: each neuron can communicate with about 10,000 others, meaning that the permutations are unbelievably huge. There are around 100 billion neurons in the human brain, and around 1,000 trillion possible connections.
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