The way we lose nerve cells from birth How many neurons (nerve cells) exist in the human brain? We have roughly 85 billion of them. By comparison, a jellyfish has only 800, a cockroach only a million, and an octopus 300 million. Most people believe that nerve cells die only in old age, but the bulk of them are lost to us in childhood, when in the child's head occurs the process of natural selection. As in the jungle, among the neurons, the most efficient and adaptable ones survive.