Whatever a student hears in class or reads in a book travels these pathways as he masters yet another iota of understanding. Indeed, everything that happens to us in life, all the details that we will remember, depend on the hippocampus to stay with us. The continual retention of memories demands a frenzy of neuronal activity. In fact, the vast majority of neurogenesis--the brain's production of new neurons and laying down of connections to others--takes place in the hippocampus. The hippocampus is especially vulnerable to ongoing emotional distress, because of the damaging effects of cortisol. Under prolonged stress, cortisol attacks the neurons of the hippocampus, slowing the rate at which neurons are added or even reducing the total number, with a disastrous impact on learning. The actual killing off of hippocampal neurons occurs during sustained cortisol floods induced, for example, by severe depression or intense trauma. (However, with recovery, the hippocampus regains neurons and enlarges again.)20 Even when the stress is less extreme, extended periods of high cortisol seem to hamper these same neurons.