Judith Lee, of Tapanui, asks :-

I understand that most of our body cells are replaced often during our lifespan. So how do our memories transfer to new brain cells?

David Green, a structural biologist at the Otago Medical School, responded.

We have our full complement of nerve cells before we are born and they are not subsequently replaced at the cellular level during our life time.

However, we are able to replace individual components of nerve cells (eg membrane lipids, ion channels, receptors, etc.) without replacing the whole cell. There are changes that occur in the brain at a grosser level over time but these do not reflect changes in nerve cell number. As our head grows after birth, nerve cells individually grow larger and glial cells (which support nerve cells) are free to increase in number. As we age, glial cells proliferate further, giving the impression that our nerve cells are dwindling in number with age. In normal circumstances, that is not the case, although it may occur in some diseases.

Brain nerve cell numbers are tightly regulated. One reason may be because they are expensive to service. They account for about 2 per cent of body weight but consume about 18 per cent of the oxygen we inhale.