Our brains are made up of billions of cells (neurons). Each is in constant communication with others. In the tiny gaps between neurons (the synapses) biochemical activity transmits messages from one neuron to another, enabling us to think, act and feel. Several chemical messengers called neurotransmitters are activated in the synapses to pass the messages along. Scientists have so far discovered more than 40 neurotransmitters such as dopamine and serotonin, which affect our moods.

Neurons and neurotransmitters are both sturdy and delicate. Stroke, disease or accident can kill off whole chunks of brain, yet to some extent the brain can form new networks through the neurons that survive or 're-programme' other neurons to restore some of the original function.

We used to believe that brain cells couldn't be replaced but we now know that new cells may grow in the hippocampus, an area of the brain involved in memory and learning. We also know that our brains lose cells and shrink as we age. The implications of both of these discoveries are not yet clear.

How does chemistry affect the brain?

Things can go wrong in the interactions between the neurotransmitters too and even subtle changes in brain chemistry can have profound effects. For example, lack of dopamine causes the symptoms seen in Parkinson's disease, while low levels of serotonin are associated with depression.

Neurotransmitters depend on oxygen and glucose and there is much scientific debate about the role of oxygen levels in brain ageing. Although the brain accounts for only 2 per cent of body mass, it uses 25 per cent of blood oxygen and a massive 70 per cent of available glucose. Some scientists believe that the brain always takes the oxygen it needs, even if that means depriving other organs. But what is 'need'? Is it possible that the brain, if deprived of optimum levels of oxygen, will redefine its needs to manage on less? If that is the case, do we want our brains to tick over at less than full power?