Beachguy's right. What cannabis does is assist with a natural cell-pruning process called apoptosis. That's basically the process through which cells kill themselves off when their business is finished. So while the anti-cannabis propagandists like to interpret that negatively to mean "Cannabis kills brain cells," what most research in animal studies and in vitro studies on neurons has shown is that the active compounds in cannabis aid apoptosis in a beneficial way, helping with the natural pruning process and sparing (and even helping proliferate) the cells that aren't pre-programmed for natural destruction.

You can read some abstracts about cannabis and brain function here in this sub-forum. Look for the sticky containing health-related abstracts from Storm Crow. Or you can also do a Google on "cannabis apoptosis" and read what you find there. Just be aware that abstracts are only brief summaries and that the vast majority of the research that's been done has been about the effects of THC compounds on rat or mouse brains.

Here's some more info:
Cannabis Research Library - Apoptosis and Proliferation