cranial nerve development accessory nerve development spinal accessory nerve development CN XI development cranial nerve 11 development GO:0021565 biological_process The process whose specific outcome is the progression of the accessory nerve over time, from its formation to the mature structure. In mice, the spinal branch of this motor nerve innervates the trapezius and the sternocleidomastoid muscles. The cranial branch joins the vagus nerve and innervates the same targets as the vagus nerve. cranial nerve XI development