I came across an interesting article the other day while reading a paper by Alvin Plantinga on the nature of the mind. He footnoted an article entitled Molecular Turnover by John McCrone. He delves into the subject of molecular death/regeneration within our neurological systems. This is directly relevant to a subject that I’ve dealt with here on the blog before. Namely, the problem of persistence of self.
How can our identity and memories persist when our neurological chemistry is in a constant state of death and regeneration? Well, according to McCrone, the problem is much worse than the simple regeneration cycle that goes on in the rest of the body:
The issue of molecular turnover is starting to hit home in neuroscience, especially now that the latest research techniques such as fluorescent tagging are revealing a far more frantic pace of activity than ever suspected. For instance, the actin filaments in dendrites can need replacing within 40 seconds, making microtubules look like positive greybeards (Star et al, 2002).
A turnover time of five days for NMDA receptors seemed pretty steep when it was reported a few years back. (Shimizu et al, 2000). But recently Michael Ehlers at Duke University Medical Center in Durham, North Carolina, reported that the entire post-synaptic density (PSD) – the protein-packed zone that powers synaptic activity – is replaced, molecule for molecule, almost by the hour. Ehlers had expected the turnover to take days and when he found no labelled protein on his first 24 hour assay, he thought he must have mucked up the experiment
Myelin and RNA molecules seem to last months. And DNA is of course fairly hardy, though it still needs continual repair. But on the kinds of figures that are coming out now, it seems like the whole brain must get recycled about every other month. And certainly everything points to the synapses as being about the most dynamic part of the whole system.
So, the question remains as to how, on the physicalist model, an ego is maintained in light of the chemical “boil” that goes on within our neurochemistry. If Dave is identical to Dave’s brain, how does Dave persist when Dave’s brain is dying and being reborn in it’s entirety on a bi-monthly basis? Good luck with that one.