Where does pain start or end?

Below the level of the neck, if a finger is severed then pain is felt. The rest of the body remains intact. If an arm is cut off this too is acknowledged by the brain. The receiver of the information. The brain senses the injury and essentially dispenses what we feel as pain. Above the neck, if an ear is lost this is recognised.

At neck level, if this is severed does the brain sense the injury, albeit for a very short time.

Until the oxygen in the blood has been exhausted, the brain is intact as it hasn't itself been injured. It simply cannot be supplied with everything the blood provides.

It's a macabre scenario.

Brain activity should be examined at a beheading. It is the only way this question could be answered.