Morphologic study of the connections between the accessory nerve and the posterior root of the first cervical nerve

The purpose of this study was to evaluate the anastomotic relationships between the accessory nerve and the posterior root of the first cervical nerve and to determine the course of the posterior root nerve fibers after anastomosis. The relationships between these two nerves were studied in 100 sides of the spinal cord and then classified into four types. In the most common type of anastomosis (38% of cases), either a branch from the posterior C1 root was seen to course cranially and join the accessory nerve, or the posterior root and accessory nerve fused as they coursed orthogonal to one other. In the second most common type (36% of cases), the accessory nerve anastomosed with a posterior C1 root that had no direct connection with the spinal cord; the nerve fibers of the posterior root were observed in stained samples to course caudally along the accessory nerve and join the posterior C2 root. In the least common type (6% of cases), no connection was observed between the accessory nerve and the posterior C1 root. In the next least common type (20% of cases), the posterior C1 root was absent and a connecting branch was sometimes observed between the accessory nerve and the anterior C1 root.