Analysis of motor fibers in the communicating branch between the cervical nerves and the spinal accessory nerve to innervate trapezius in the rat.

The communicating branch between the ventral rami of cervical nerves and the spinal accessory nerve (SAN) has been reported to also send motor fibers to supply the trapezius. However, the motor fiber type of the communicating branch and its peripheral distribution are still unclear. To determine the fiber elements within the branch and its peripheral distribution of the motor fibers in the trapezius, the anterograde tracing method was used in this study. The results show that a few a motor end plates from the communicating branch were observed on the extrafusal fibers, while in the muscle spindle the motor elements from the communicating branch were distributed to the polar portions of the intrafusal fibers. These results indicated that the motor fibers passing through the communicating branch to supply the trapezius are mainly y motor fibers, with some a motor fibers. Moreover, the a and y motor fibers from the communicating branch were observed in the clavotrapezius, acromiotrapezius and the rostral part of spinotrapezius. These findings also correlate with the clinical observation indicating that even when the spinal accessory nerve is injured, the trapezius is still capable of slight movement.