The reptilian brain (also called R-Complex or Archipallium) is responsible for self preservation. It is there that the mechanisms of aggression, survival and reproduction reside. It also controls some involuntary actions and the control of certain visceral functions (cardiac, pulmonary, intestinal, etc), indispensable to the preservation of life.
How does it effect me?
When on our spiritual paths, we will have to question our own beliefs constantly, it is all part of the process, however....
What happens in our brains when someone disagrees with a belief we have taken on as truth?
The small nut sized amygdala (there are two, one on each side of the brain) among other things is the centre for identification of danger, (fear headquarters) and is fundamental for self preservation. When triggered, it gives rise to fear and anxiety (through the R-complex) which leads an animal or human into a stage of alertness, preparing for “FIGHT or FLIGHT.”
This is triggered when someone says something to upset a belief system we have, we get angry and aggressive and often start to hate the person for ever even mentioning it. So we either attack them trying to bring them down and belittle them, or we run away.
How can I avoid reacting this way?
Becoming aware of these responses is important to our enlightenment, as fear can only damage our hearts. When you are aware of these reactions, you can learn how to control them, and not let it have a negative effect on you and others.
So when someone says something that you don't agree with, don't take it personally, they are not attacking you and they are entitled to their own views about things. Calmly listen to their side, then put your side across, that way you might both learn something from each other, instead of becoming enemies.