Hello.
Here is your answer:
Your answer will be nervous system.
Reason: That's because when someone pinch's your arm and it hurts its your nervous system reacting to the pain which is the reason why we say the terms: "ow" and we try to pull the arm away from its harm. In this case when Brian focus something hot his nervous system reacts and he will say "ow" and pull away so his hand will not get harmed from the heat!
Your answer is nervous system.
Hope this helps.