Fear is a chain reaction in the brain that starts with a stressful stimulus and ends with the release of chemicals that cause a racing heart, fast breathing and energized muscles, among other things, also known as the fight-or-flight response. The stimulus could be a spider, a knife at your throat, an auditorium full of people waiting for you to speak or the sudden thud of your front door against the door frame.
The brain is a profoundly complex organ. More than 100 billion nerve cells comprise an intricate network of communications that is the starting point of everything we sense, think and do. Some of these communications lead to conscious thought and action, while others produce autonomic responses. The fear response is almost entirely autonomic: We don't consciously trigger it or even know what's going on until it has run its course.
Photographs by Jan Schjetne