Neuroticism or emotionality is characterized by high levels of negative affect such as depression and anxiety
. Neuroticism, according to Eysenck's theory, is based on activation thresholds in the sympathetic nervous system or visceral brain. This is the part of the brain that is responsible for the fight-or-flight response in the face of danger. Activation can be measured by heart rate, blood pressure, cold hands, sweating and muscular tension (especially in the forehead). Neurotic people, who have low activation thresholds, and unable to inhibit or control their emotional reactions, experience negative affect (fight-or-flight) in the face of very minor stressors - they are easily nervous or upset.
Emotionally stable people, who have high activation thresholds and good emotional control, experience negative affect only in the face of very major stressors - they are calm and collected under pressure.