In yesterday’s post, I wrote about the fact that different parts of our bodies keep regenerating themselves periodically. This fact alone should make nonsense of the belief of some religious people that our bodies become physically reconstituted after death in the afterlife, because if so, the resurrected body of a person who died at the age of 70 would be unrecognizably grotesque, consisting of around 70 livers and 7 full skeletons, all surrounded by hundreds, maybe thousands, of pounds of skin.

But leaving aside that, there is an interesting question raised by this constant regeneration of the body and that is how we retain a sense of having a single identity over our full life spans even as individual parts of us get replaced periodically. The average age of the molecules in my body is around 7 to 10 years and yet I have the strong sense of continuity, that I am in some fundamental sense the same person that I was as a child, even though almost none of those molecules have stayed with me over that time. How is it that we retain a strong sense of permanence in our identity while being so transient in our bodies?

The answer may lie in the fact that our brain seems to be the most permanent of our organs, undergoing little or no regeneration. In the same article in the New York Times that I referred to yesterday, Nicholas Wade says:

Dr. Frisen, a stem cell biologist at the Karolinska Institute in Stockholm, has also discovered a fact that explains why people behave their birth age, not the physical age of their cells: a few of the body’s cell types endure from birth to death without renewal, and this special minority includes some or all of the cells of the cerebral cortex.

The cerebral cortex is the thin sheet that forms the outer layer of the brain and is divided up into several zones that have different functional roles. If the cortex were removed and smoothed out to eliminate all the creases and folds, it would look like a dinner napkin. It is gray in color, the origin of its popular euphemism of ‘gray matter’. The network of nerve cells in the brain (called neurons) determines how the brain functions.

While the brain seems to be the most enduring part of the body, even here there is variation. The cerebellum seems to contain non-neuronal cells that are close to the birth age (within three years or so) while the cerebral cortex (which is responsible for our cognitive capabilities and is thus most closely identified with our sense of self) has a slightly greater turnover of non-neuronal cells. But the researchers do not turn up any evidence that there is neuronal generation after birth, at least in the region known as the occipital cortex.

It was long believed that the number of neuronal connections in the brain grew rapidly during the first year or two of life and then got pruned and this was how our lives shaped our brains without new neurons being created. In 1999, there was research that found that new neurons were being created in the cerebral cortex of adult monkeys, suggesting that it could happen in adult humans too. This would complicate things somewhat as to how we retain a permanent sense of self but also provide hope that brains could regenerate. But this summary of later research (much of it by the same Karolinka group that I referred to yesterday) that appeared in the Proceedings of the National Academy of Sciences says that this does not happen with the neurons in the human cerebral cortex. (The neocortex referred to in the paper is the most recently evolved part of the cortex that is defined as containing the ‘higher’ functions and are “arranged in six layers, within which different regions permit vision, hearing, touch, the sense of balance, movement, emotional responses and every other feat of cognition.”)

The results show that the average age of the neurons (with respect to the age of the individual) is age 0.0 ± 0.4 years, i.e., the same as the age of the individual. In contrast, the nonneuronal cells have an average birth date of 4.9 ± 1.1 years after the birth of the individual.
…
Both of the experiments of Bhardwaj et al. indicate that there are no new neurons, either long-lived or transient, produced in the adult human for the neocortex. Importantly, these experiments are quantitative and indicate a theoretical maximum limit of 1% on the proportion of new neurons made over a 50-year period.
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Bhardwaj et al. settle a hotly contested issue, unequivocally. The two-pronged experimental approach clearly establishes (i) that there is little or no continuous production of new neurons for long-term addition to the human neocortex and (ii) that there are few if any new neurons produced and existing transiently in the adult human neocortex. Importantly, the results are quantitatively presented, and a maximum limit to the amount of production of the new neurons can be established from the data presented. The data show that virtually all neurons (i.e., >99%) of the adult human neocortex are generated before the time of birth of the individual, exactly as suggested by Rakic, and the inescapable conclusion is that our neocortical neurons, the cell type that mediates much of our cognition, are produced prenatally and retained for our entire lifespan. [My italics]

So basically, even though every other part of us gets sloughed off and replaced at different points in time, for good or bad we are pretty much stuck with the brains that we have at birth. This may be crucial to our ability to retain a sense of a permanent identity that lasts all through our lives, although this is not yet established. Even if new research emerges that new neuronal cells could be generated over time replacing older ones, it may turn out to be able to do this seamlessly and provide cognitive continuity, just the way our other organs give us the illusion of being permanent even though they are not.

It seems like our brains are our essential selves with the rest of our bodies just superstructure. Rene Descartes famously said “I think, therefore I am.” We could also say, “My brain is who I am.”