Snail neurones are an excellent source of large cells for studying cell biology. Snails are found in most back gardens. They can be left in cold boiled water overnight, when their bodies swell up and ultimately die but are easily dissected to find the brain. The snail neurones are surrounded by glial cells. As the snail gets older, the glial cells penetrate the nerve cell and send long processes into the nerve cell body and probably provide it with essential organic chemicals.
Glial cells can divide by mitosis and hence get rejuvenated.
Neuones can not divide by mitosis and hence probably run out of some essential chemicals over the animal's life time. We still do not fully understand the glial cell - neurone inter relationship.