The broken vertebrae controlling the reflex arc of the leg movement can be temporarily blocked by the interneurons. The reflexes are controlled by the central nervous system, and interneurons are a part of it (the brain and the spinal cord). The pathway of the reflex arc of the leg movement from the sensory neurons and then back to the motor neurons will be blocked by the interneurons as the vertebrae is broken.
Hence, the answer is 'interneurons'.