4.5 The ageing brain
The cellular components of the human brain are remarkably heterogeneous, both morphologically and functionally. The brain is a very complex structure, composed of around one hundred thousand million (i.e. 1011) neurons. These nerve cells share the biochemical components and intracellular ‘machinery’ of all other living cells. Unlike most other cells, however, they are ‘post-mitotic’; that is, they do not divide and must therefore ‘last’ for a lifetime. They also have particular features that are not possessed by other types of cells and which relate to their specialised function as nerve cells: the ability to transmit nerve impulses and to synthesise and release different chemical messengers, known as neurotransmitters. Each neuron in the brain makes specialised contacts (synapses) with, and thereby ‘signals’ to, many other neurons; this circuitry is essential to ‘higher’ neural function. Moreover, while other body organs are able to utilise a variety of alternative biological fuel molecules, such as sugars, fats and amino acids, except under conditions of starvation the brain can utilise only blood glucose. So, although the cells of the brain are in most ways similar to other cells, changes to their structure or function that may occur during ageing can have profound effects on the function of the nervous system as a whole, and in consequence on the functioning of other body systems that depend upon normal brain activity.
Recent evidence has shown that – in contrast to what was previously believed – only relatively few neurons die as we grow older. While this is clearly ‘good news’, it also means that nerve cells can accumulate a lifelong ‘catalogue of insults’. We are only now, with the advent of more sophisticated technologies for studying cells, beginning to characterise and understand the nature of these changes.