For much of the past century, scientists believed that the number of neurons in the human brain was fixed at birth. With this theory, complex processes such as learning and memory were thought to occur through changes in the existing neuron networks. While there is extensive research to show that learning and memory are facilitated by structural changes at the synapse, scientists are just now unraveling the mechanism by which new neurons in the hippocampus contribute to learning and memory.
Neurogenesis, or the birth of new neurons occurs in areas of the brain that contain neural stem cells. These cells, like other types of stem cells, possess the unique potential to divide indefinitely, replenish themselves, and mature into a variety of daughter cells. Neural stem cells develop into most if not all of the different types of neurons in the brain. Though these cells exist in all brain regions, they are normally prevented from dividing. However, under normal conditions, neurogenesis continuously occurs in two distinct areas of the brain including a region involved in the processing of smells as well as the hippocampus.
In the brain, it is estimated that several thousand new cells are produced each day, most of which develop into functional neurons in the subsequent weeks. The lifespan of adult-generated neurons ranges from a few days to the lifetime of the organism. With time, these neurons incorporate into the existing circuitry to help execute the normal functions of the hippocampus in learning and memory. Therefore, scientists speculate that the added neurons allow the hippocampus to process more complex information in forming new memories. Not surprisingly, the disruption of hippocampal neurogenesis results in learning and memory deficits.