Stress can be either good for brains or bad for brains, the effect depends on the level of stress that is experienced. What is true for physical stress (exercise) is true of psychological stress: none is bad, some is good, but too much is bad. Stress results in the release of a chemical called cortisol into the blood. Although not a neurotransmitter itself, cortisol does influence the release of the neurotransmitter norepinephrine, which causes the fight-or-flight response with its characteristic increase heart rate and more focused attention. In moderate levels, cortisol is associated with normal and healthy building of new connections between neurons and the brain functions necessary for social interaction. Too much cortisol can interfere with the normal building of connections and brain function. Exposure to excessive levels of cortisol for an extended time (for example when individuals experience chronic stress) is associated with degraded brain function.