Answer: Amygdala
Explanation: Amygdala is a major composition of the brains of complex animals including Man, Squirrel etc. It is the composition of the Brain that helps to react during fear condition. If the Amygdala is damaged it will be difficult or impossible for the Animal to react to fear,like the case of the Squirrel to signals from an impending shock after the damage of its Amygdala.