Internal, physical reactions to emotion occur when your autonomic nervous system is called into action. Depending on the emotion and its intensity, the body's energy distribution either increases or decreases. When anger or fear is experienced, the pulse quickens and shortness of breath and trembling may take place, while emotions such as heartache, grief, and regret may cause bodily functions to slow down significantly. When this takes place, you fall into a depression, resulting in excessive sleeping and physical weakness.

The James-Lange Theory of Emotions

How you are able to differentiate emotions is the subject of much debate. William James and Carl Lange (1834–1900) theorized that it is the physical actions and reactions that determine the subjective experience of an emotion. Common sense tells you that you run from danger because you're afraid. In complete opposition, James's idea focused on the physiological aspect of emotion, holding the position that you are afraid because you run and you are excited because you shout. Lange's ideas were similar, only he took it further and included autonomic responses as well. He suggested that not only are you afraid because you run, you are afraid because your heart is beating rapidly. So, for example, imagine that you are walking through the woods and you come upon a mountain lion. Your heartbeat races, your breathing quickens, and you start to tremble. Based on James's and Lange's ideas, your brain looks at these physical reactions and concludes that you are frightened, leading to your emotion of fear. Arriving at these conclusions at about the same time, the two positions were combined and the James Lange theory was born.

Arguments Against the James-Lange Theory

Walter Cannon (1871–1945) strongly disagreed with the James-Lange theory for several reasons. First, he argued that due to the body's lack of nerve-sensitive organs, visceral changes are too slow to take place and therefore are not contributing factors in felt emotions. Second, emotions that are experienced after epinephrine injections are not true emotions as much as they are physical affectations (accelerated pulsations, sweaty palms). Cannon's final argument acknowledged that the autonomic patterns of emotions are often too similar to be a source of distinction — for example, fear, anger and love all incite a faster heartbeat. This suggests that other factors are at work in differentiating emotions.