Professor Ronald McKay explains that neurons in the hippocampus and olfactory bulb are unique in that they are produced throughout life. This is known as neurogenesis.

So, in the nervous system, in most regions of the nervous system the structure of the nervous system is established during development and neurons are made in a very precise sequence during development. It is like the way people now manufacture modern motorcars, that you don’t have all the parts lying around and just go and get them when you need them, the parts are provided in a very specific timeline. In most regions of the brain, you don’t produce anything more once you’re born, all the neurons are there. But in the hippocampus and in the olfactory bulb there are two regions where new neurons are made throughout life and the answer to the question, "why?" is not completely clear. But it looks like, in the olfactory bulb, it’s because the olfactory stimuli are constantly stressing the cells, and so you have to have a mechanism for replacing them. But, in the hippocampus, the answer is less clear, but it has to do with the acquisition of new memories.

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