It does seem weird that we can't replace nerve cells that get damaged.The answer is basically that having nerve cells reproduce can do more harm than good. Each nerve cell has a specific place in our nervous system. Its job is all about taking a signal from one specific place to another one. Adding new nerve cells would mess up these very specific connections in a very complex system. So we trade off the ability to repair our nerves in order to avoid messing up connections.

We used to think that people never got new nerve cells, but it turns out that we get a small number of them around the teen years. This does seem to make life a bit more complicated for young people whose brains have to adjust to the new nerve cells. This is also a good example of how science is constantly discovering new information.

If we can't add new nerve cells, how do you think we learn? Hint:growing and reproducing are different.

What a great question! I am guessing based on your question that you know that most other kinds of cells do keep reproducing, at least for a while. This "reproducing" is division of one cell into two using a process called MITOSIS. As you might imagine, nerve cells (also called neuronal cells) are very specialized - they do a very specific, complicated job in the body. As a result, their structures are very specialized - they have a small "cell body" and then long processes that branch off the to connect with other neurons or other types of cells such as muscles. As they become specialized, the cells devote energy and structures to their "new" jobs as neuronal cells and they give up the ability to do other things, such as divide (reproduce, to use your word). This specialization has a technical term that we use - we call it "terminal differentiation." This means that once a cell commits (makes a vow) to become a neuron, it is going to do that specialized job and that job only until it dies off.The cellular "mini-machines" that are used in mitosis are no longer made, so the neuron cannot divide. Now here comes some cool parts - which is why I thought this was such a great question. First, neurons can live a pretty long time, but because they are terminally differentiated and cannot reproduce, it was thought for many years that as we aged and neurons died off or as neurons were damaged and killed by things such as alcohol or drugs, there was no way to replace them. It turns out, however, that there are a small number of very specialized cells in mammalian brains called NEURONAL STEM CELLS that can actually replace the dead neurons (it still is not a good idea to damage your cells with drugs or alcohol though!). Scientists are very interested in trying to isolate and study these cells so that they could maybe be used to repair damaged brains or spinal cords. Second, it is still a mystery HOW a neuron "turns off" its cell division machinery - what makes it stop dividing? Scientists are interested in this question because if we understood this, we might be able to trick cancer cells into doing the same thing - stop dividing!