What you are talking about here is a fight or flight response. This is a biological response in which your body's autonomic nervous system is engaged in order to release a quick strong burst of energy. When the autonomic nervous system is engaged it is likely that Tommy's heart rate and breathing will increase. He will feel a rush of adrenaline as the increased blood flow brings more oxygen to his muscles. It is also likely that his pupils will dilate.
When the bear occurs all of these body functions will occur simultaneously. At this time Tommy will need to decide if it is best to face the bear, a fight response; or leave the area, a flight response. As he is having a stress response with his autonomic nervous system his body will have the energy to respond to this threat for a short time but at a unattainable level.