A tired brain

The number of brain cells laid down in fetal life is in considerable excess of the number we use as adults.there is a progressive loss of there cells from birth and throughout life because they are essentially not renewed.there are a few stem cells in the brain but the evidence that they contribute to ongoing maintainance of brain function in humans is minimal.this is in constarst to some birds where brain cells are renewed throughout life,by death of old cells and their replacement using a well-regulated process of stem bell induction.

Experimentally the brains of animals which have been exposed to adverse intrauterine conditions show many alternations:there is a reduction in the number of cells in some regions,in the number of connections or synapses between them,and in the amount of nerve fibres in white matter.recent studies using new imaging techniques in the growth-retarded humans infant show that the cerebral hemisperes are smaller and the amount of grey matter is less:they appear not to catch up after birth.perhaps this is why growth-retarded infants are more likely to have later cognitive,attention,and learning deficits.

Does this implies that there has been a trade-off 'in utero'?does the fetus predict a dangerous and therefore shorter postnatal life and tits does not invest in a larger brain,with its greater flexibility and reserve capacity and higher metabolic demand?whereas once such growth-retarded infants had a markedly higher chance of dying in infancy,many more not survive.gr this a mismatch which originated through a trade-off in early life but is exposed by the improvements in child survival?

The argument can be extended further-although we have to admit that it is speculative.the number of brain cells we are born with was matched by evolution to a maximum lifespan of the order of 45-50 years.but while we are living longer we are not born with more spare brain capacity-is that why dementias appear once we exceed that age range? On the other hand there is evidence that keeping an active brain throughout life by stimulation through learning and activities such as crossword puzzles will slow lie loss of brain cells-perhaps because an active brain makes growth factor which inhibit the processes of cell death.this suggests that perhaps we have do have some capacity to override the cognitive impairments associated with the mismatch of ageing.

The major neurodegenerative disease are alzheimer's disease and parkinson's disease.we do not know what causes them although there is some evidence that viral or toxic agents,as well as genetic factors,might be involved.but diseases associated with ageing could be induced either because of a cumulative injury throughout life or because the inherent obsolescence of the brain becomes exposed when its reserve is lost through the normal process of ageing.there diseases are exceptionally serf in younger people and tits are a direct consequence of our living much longer,but we do not know which of the possible mechanisms are involved.

A Similar idea about failure of repair can be applied to virtually every other system in the body.ageing-related disease can be seen as the result of a trade-off between early life function and later life repair,coupled with the onslaughts of nopedro life taking their toll over manY decades.