Research on the double innervation of trapezius muscle and the mechanisim of the axons extension direction

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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, and the localization of motoneurons, which extend axons either directly through the SAN or through the ventral rami of cervical nerves to innervate the trapezius are still unclear. To determine the fiber elements within the communicating branch and its peripheral distribution of the motor fibers in the muscle the anterograde tracing method was used. Moreover, the localization of motoneurons, which extend axons either directly through the SAN or through the ventral rami of cervical nerves to innervate the muscle are also investigated using fluorescent dye labeling and 3D reconstruction method.The results show that a few a motor end plates from the communicating branch were observed on the extrafusal fibers, while in the muscl … More e spindle the motor elements from the communicating branch were distributed to the polar portions of the intrafusal fibers. The 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 γ motor fibers from the communicating branch were observed in the clavotrapezius, acromiotrapezius and the rostral part of spinotrapezius. On the other hand, the motoneurons whose axons pass through the ventral rami of cervical nerves and then enter the SAN, and those extending axons directly through the SAN are distributed within the same area in anterior horn. The neurons that extend axons through the SAN had a greater diameter than those axons that pass through the cervical nerves en route to the trapezius muscle. In addition, the axons that ultimately extend through the SAN exit the spinal cord dorsolaterally, while those that pass through the cervical nerves extend out the spinal cord through the ventral roots. We consider that the neurons that extend axons through the SAN are mainly α-motoneurons and that those projecting axons through the cervical nerves to the trapezius are mainly γ-motoneurons, because the distribution and fiber types of peripheral part are agreement with the central part. Taken together, these results could explain why patients in whom the SAN was used to treat brachial plexus injury retain some control of the trapezius muscle Less