http://qz.com/450517/us-navy-seals-conquer-fear-using-four-simple-steps/
The mechanism of fear
The frontal lobes (part of the neocortex) are responsible with conscious and rational decision making processes.
The amygdala is twice as fast in response than the frontal lobes and this sometimes can be misleading. The purpose of the amygdala is to protect, no matter what.
There are some fears that are pre-programmed in the human brain. This means that we are born with them. Fear of choking, drowning, heights, and even the fear of public speaking are examples of such.
Whenever in peril, the amygdala kicks in as the first commander; it sends signals to the hippocampus (another part of the limbic system) which in turn releases stress hormones, such as cortisol and adrenaline. They prepare our body for the fight or flight response.
Thus, all the energy available is hijacked and directed to the feet for running from, or to the hands for fighting against imminent danger.