Shiners were humans converted by the Myloki into zombie-like slaves. They did not feel pain and had no self-will. A few were left with part of their original personalities to allow them to infiltrate military installations.
When the Second Doctor was accidentally shot in the head, he was injected with Shiner DNA to attempt to stabilise his condition. This prevented him from regenerating as he normally would have from a fatal wound. Instead, his body healed itself while in a coma, from which he revived six months later. (PROSE: The Indestructible Man)