A developmental process, independent of morphogenetic (shape) change, that is required for the accessory nerve to attain its fully functional state. 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. [ GOC:cls http://openisbn.com/search.php?q=0838580343&isbn=1/ GOC:jid GOC:dgh http://purl.obolibrary.org/obo/go/references/0000021 GOC:dph ]