Ivan is experiencing phantom pains, which can be defined as the pain sensations that seem to be related to the part of the body which is no longer a part of the person's body. The cervical part of the spine (C1 to C8) receives these phantom pain signals from the amputated part of Ivan's arm. The cervical part of the spine is associated with controlling the back part of the head, arm, neck, hands, shoulders, and diaphragm.
Hence, the answer is 'cervical'.