At the beginning, we mentioned that little stress sharpens the memory. The explanation for this from the evolutionary point of view is really simple. Faced with a predator, it makes sense for you to remember all the things you did previously that saved your life. It also helps to memorize hiding places, special dangers and all other information that will help you escape the predator in future. There are two ways in which the memory is improved during short stressors.
First, as blood flow increases with the onset of a stressor, more energy is available to the brain and that helps in improving the performance of the neurons. The potentiation of neurons1 is an expensive operation and the extra energy helps in that process. The second way is through the effects of the glucocorticoids on the hippocampus in a localized effect. The hippocampus has numerous receptors for the glucocorticoids and they reduce the threshold for potentiation—the number of Ahas!’ required for the neuron to learn something.
When the stress goes on for too long, bad things start to happen. Instead of helping the potentiation, the glucocorticoids now disrupt it. Worse, there can be memory loss in a process called long-term depression. The mechanism for this process is not clear but the effect is obvious—you start to forget things easily. Further, the glucose delivery (energy) is disrupted. The neurons in the hippocampus now take up 25 per cent less energy and the result is that your memory and concentration suffer.
With chronic stress, something truly insidious begins to happen—the neurons get damaged. The branches of the neural network are retracted. It is not that the memory is lost—we just need more associations to retrieve it. There is some research in animals that shows that prolonged exposure to glucocorticoids kills neurons in the hippocampus. This has not been proven in humans but it clearly indicates the dangerous possibilities.
It is amazing to note how expensive thinking is (in terms of energy requirements). The brain accounts for only 3 per cent of our body weight but hogs about 20 per cent of the energy.