Even though the cells in your body are constantly renewed—much like let’s say a car that gets all its parts replaced over time—there’s a mystery: why does the “you” that exists today feel exactly the same as the “you” from years ago? What is it that holds your identity together when every individual part is swapped out?
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You kind of answered it yourself. The brain cells are generally not replaced.
Because braincells dont do that.Neurons in the brains cortex stays the same from birth to death
Basically your brain rewrites the pathways that encode your memories, using different cells, as you go.
Imagine you’ve got a book, and every year the first ten pages wear out and fall out, so you add ten new pages and copy the information into the back of the book.
If you’re reaching for the Ship of Theseus question, then there is no physical, literal distinction between you from one moment to the next, even as what makes up you changes. You have every right to define your current perception of self as the correct and proper one, of course, but it will change over time as bits wear out and are replaced. To exsist is to change, so enjoy it and don’t worry too much about the fact that there’s very little of your body that hasn’t been replaced several times.*
*If you want a bit that hasn’t been replaced, the mineralized parts of your skeleton get added onto, but they are the same for life and don’t change much once they fuse. Your skull is, with added mass, much the same skull that developed in your mother’s womb.
The brain creates a sense of self/agency. [It’s the product of the medial prefrontal cortex (mPFC), posterior cingulate cortex (PCC), and anterior precuneus (aPCu).] It can be turned on and off with, for example, general anesthesia. The sense of self doesn’t actually refer to any one thing that is you, but the sense certainly has survival benefits. It’s an illusion, but a useful one.
I fear there is no available answer for your question yet. The consciousness – the thing that makes you know that you are you – is still a mysterious area.
Because neurons aren’t replaced.
They can die, and if they die in large enough numbers that can give rise to neurodegenerative diseases like Alzheimer’s or Parkinson’s Disease; however, after they mature (i.e. the “brain development” that is said to continue into early adulthood), neurons don’t undergo mitosis and divide any more. The way that they connect and communicate with other neurons can change over time (which enables, for example, the formation of new memories and the forgetting of old information), but the neurons themselves are not replaced.
The question is a restatement of the Theseus’ Ship paradox. Good information about it here: https://en.wikipedia.org/wiki/Ship_of_Theseus
But the difference here is not all your cells get replaced. Neurons you have for life.
On the other hand “you” are not the same person you were last year or even a minute ago, you are always changing, getting new memories and experiences, aging, etc.
Think about it like a car, or a computer, or some other piece of equipment that performs a function but is comprised of multiple replaceable parts.
If your computer keyboard dies, you buy a new keyboard, and it works fine. If your car’s tire goes flat, you replace the tire and it runs fine. Cells, ultimately, are very much like a replaceable part. What cell performs a function isn’t really important so long as cells do perform that function. This is true for just about every cell in your body.
However, it is worth noting that people do change over time. Unlike computers, we learn new things, and adopt new behaviors. You very literally are not the person you were when you were a toddler, but you do have some of the memories of that person. That continuity of memory is part of what forms our image of ourselves and gives us the illusion of being the same person as the person we used to be. Certain dissociative disorders can actually cause people to feel disconnected from their memories, which can cause issues for people suffering from it to have issues with their sense of self.
It’s outside the scope of ELI5, but if you feel ambitious and really want to know how your brain works, and what drives your behavior and sense of self, I’d start with Robert Sapolsky’s “Behave”. It does discuss some pretty advanced topics however, well above a 5 year old level.
Here’s a fun thought experiment: Let’s pretend everyone has a single “key cell” somewhere in their body. It’s impossible to predict which cell is the key cell, but if that cell dies, you stop being you and start being a different person just carrying the memories of who you were.
How would you know when your key cell died? You’d be a different consciousness, but with otherwise the same brain structure and memories and body. If your key cell died last night as you slept, what sign would there be?
Maybe you see what I’m getting at. On a more practical note, do you actually feel the same as you did years ago? Or do you feel like you’ve changed?
Because yohr neurons don’t. One thing the few parts of the body that don’t reslly change is the brain and heart cells. Both of these places replace cells very very slowly. For the brain irs in the hundreds ro few thousands of neurons per day which sounds like slot but for hundreds of billions of cells it’s nothing. This means many neurons may live with you from your birth entirely to your death entirely unchanged. This is also why neurodegenerative diseases are ao incredibly bad. Your brain can’t heal properly so if it begins getting degraded it stays that way even if the disease goes away your brain will be crippled until you die or if you somehow live many more decades some of it might heal. The heart is the same way which is why heart attacks are so long lasting but that’s besides the point.
Everyone has answered correctly about neurons not getting replaced.
I just wanted to add that it wouldn’t change much if neurons were correctly replaced. Our self, our memories, are just how neurons are connected one to the other. The connections between neurons are basically our information storage. So if you replaced a neuron, as long as you replaced it exactly how it is, you would have a new you with the same memories, but with different actual molecules.
When you are born, you are you, and you are 4 kg. When you are an adult, you are 80 kg, so there are still 4 kg of you + 76 kg of not you.
Many neurons in the brain are replaced over time as we age, different rates in different areas, some remain the same. This is newer science, so it’s understandable so many comments state that they are never replaced at all ever.
The simplest understanding, however, is that we are in ways not the same people we once were, but we also very much are. because only parts are being replaced at a time, and those parts are being built with the exact same (or effecitvely exactly the same) ‘code’ they are functionally the same. When it comes to neurons, I expect that ‘new’ neurons replacing old ones are…. integrated, trained, learn their function within the network that is “you” to replace the part that was there before so closely that it’s the same thing, and if it is different it’s different due to changes in you from when the first one existed.
AKA, it could be considered similar to learning, or developing changes over time based on what has happened before. Not a new thing, but a developed, changed, version of what was there before.
Also focusing overly hard on the brain is, I think, a mistake. Our larger nervous system and physiology has very meaningful impacts on us, to the point of I recall a story of an organ transplant recipient developing a new favourite food – the favorite food of the organ donor.