Central-peripheral transitional zone of the spinal accessory nerve in the rat.

The spinal accessory nerve rootlets emerge from the lateral aspect of the upper five segments of the cervical spinal cord underlying the nerve trunk. They cross the lateral funiculus of the cord with a slight rostral inclination. Here some pursue a relatively straight course while others have a dorsal convexity. The transitional zones may be classified into three distinct types, related to their orientation as they traverse the glia limitans to emerge as free rootlets. The fibres in Type 1 rootlets bend sharply rostrally on reaching the glia limitans. Type 2 rootlets turn ventrally to run in the glia limitans in the transverse plane of the cord before emerging. Type 3 rootlets are found only at C1. Their fibres initially turn caudally in the glia limitans and then loop rostrally. The morphology of the central-peripheral transitional zones of the spinal accessory rootlets closely resembles that of cervical ventral rootlets, and is therefore correlated with the motor function of these rootlets rather than with their intermediate location between the ventral and dorsal cervical rootlets.