If you are reading and you unexpectedly hear a loud noise, your head generally snaps up. which portion of the midbrain is most likely to be responsible for this reflexive motion?

The inferior colliculus would be the most likely responsible for making you snap your head. The inferior colliculus consists of three parts, the first is the central nucleus a second one is the dorsal cortex and the third one is external cortex. This part of the midbrain is located just slightly below the superior colliculus