How Does This Happen? (Another Question That Keeps Me Up Nights)

Nerve cells don't divide very often, if at all. It's one reason why brain damage is such a catastrophic thing. The nerve cells don't divide much (if ever) because their growth doesn't affect internal cellular function the way most other cells do; the external surface area can take in enough materials (and remove enough waste) that the phases of cellular division never start. When a person suffers brain damage, as I understand it (I only took high school biology) the restoration of function is by the still-intact cells attaching to each-other and rerouting connections around the damaged cells. As for memories, if I remember, one hypothesis is that they were sequences of potassium chlorate ions. Or else that's the transmission medium for neurological signals throughout the nervous systems. I forget.

Pure speculation; perhaps it's the state of relations of nerves to each other? A kind of gestalt? This (and the preceding) is built on shaky knowledge that I haven't touched in almost five years, so take it for what it's worth.

In some tissue, a single cell of a certain type can divide to give rise to two daughter cells of the same type. For example, a liver cell can divide to generate two liver cells. Similarly the cells which line the inside of our blood vessels duplicate by dividing into two cells.

However, most tissues in the human body contain differentiated cells that perform the function of the tissue and a population of undifferentiated cells (stem cells). When a stem cell divides some of the daughter cells remain stem cell and others become the tissue to be replaced. This, I understand, is the most common means of cell replacement in the human body.

Many adult tissues lack both the ability to divide and a population of undifferentiated cells and so do not, as a rule, regenerate. These include heart muscle, nerve tissue, and some kidneys cells. So cells that are not generally replaced are cardiac muscle cells, neurons, and the cells that make up the "filters" in the kidneys.

The non replacement of the neurons partially explains the persistence of memory. Another part of the explanation is how memory is stored and redundancy is generated in a healthy brain.

Memories aren't "stored" in single cells, they are stored in pathways and networks of cells. When you remember, it's not a single datum that pops up, it's all the associations and connections. Some of those connections may "fit" in the given circumstances and some of them don't, but they all pop up in order of how useful (read emotional impact) each association has been to us in the past. Each association is constantly readjusted according to it's "history."

In a very real way, what we call our consciousness is just the filter set we use to focus attention on our sensory impressions, our memories, and our dreams.

If you really want to blow your mind, remember that on an atomic level, electrons are being exchanged all the time. That's in addition to the stuff that cells absorb and excrete. So there is a significant portion of you that was part of your loved ones yesterday, your house last week, or the sunshine that hit your face last month.

Boondocks is down. :-(

I thought it was every seven years that your (non brain) cells supposedly replaced themselves, but it seems it's more complicated...

Here's a fun facts site

New evidence suggests that, contrary to long-held belief, adult mice and humans can also generate new brain cells (Kempermann & Gage, 1999; Van Praag and others, 2002). Moreover, monkey brains have recently been discovered to form thousands of new neurons each day. These baby neurons originate deep in the brain and then migrate to the thinking frontal lobe and form connections with neighboring neurons (Gould & others, 1999). Such are the brain’s ways of partially compensating for the gradual loss of neurons with age.

Source: Psychology, 7th ed., by David Myers, p.83.

I have also read in a developmental psych text (not at hand, so I can’t quote from it, sorry) that human brains undergo two noteworthy growth spurts—one right after birth, not surprisingly, and one in the teenage years. So, I think it’s pretty clear that thinking of the human brain as a rigid physical structure is not very accurate.

And while I appreciate the distinction that memories aren’t stored within neurons [although didn’t some recent research counter that idea too?], the fact remains that our neurons must somehow transfer something to their successors to keep the neural pathways operating as smoothly as they had been. Is it in mRNA or tRNA? Doubt it, for the same reasons as I doubt it’s DNA.

Astoria, sorry to hear that about the Boondocks! Also sorry I’m unable to browse the site you pointed us to at the moment. I will check it out, though.