This is illustrated in two cases. (1) Recovery from carpal tunnel surgery. (2) Phantom limb syndrome.
(1) Dead nerves do not cause pain. Live nerves, nerves trying to regenerate cause the sensation of pain. Pain is a mental reaction. When the nerves become alive, like the gradual reduction of numbness in the arm after sleeping on it for a night, one feels the onset of “pain” or at least discomfort after one shakes off the numbness.
A live nerve inside a rotting tooth cavity makes it extremely painful and sensitive to cold water. After temporarily numbing the nerve with over the counter pain killers for toothaches, or with garlic, one does not feel much toothache for a while. Such extreme pain from a toothache returns after the effects of the pain killers and the garlic wear off.
(2) In phantom limb cases, the pain is still felt even though the limb had already been amputated. The nerves that used to connect to that missing limb is still active in the brain where the signal of pain still fires.
In cases where a soldier still stands even though one of his legs had been blown off or shot through, the brain does not immediately react and the soldier does not realize that he is bleeding and one of his legs is missing. He will feel the pain when he looks down and the mind suddenly realizes the condition.