Brain stem

The brain stem, or brainstem, was a part of the brain necessary for life. (TV: The Magician's Apprentice) Its neurological function involved memory storage, (PROSE: Piecemeal) and in the final stages of mental shutdown, when physical shutdown would commence, brainsteam death would be followed by complete respiratory collapse. (PROSE: Anachrophobia)

Missy suggested that Time Lords only had one brain stem, and that two snipers to it, as well as three for each heart, could take her out before she could initiate regeneration. (TV: The Magician's Apprentice) According to Rafando, however, Time Lords had three; necessary steps to killing a Time Lord were stopping both hearts, all three brain stems, and delivering a cellular shockwave which would permanently disable regeneration. (TV: Extremis)