Trapezius muscle innervation from the spinal accessory nerve and branches of the cervical plexus.

The aim of this study was to demonstrate the extent of motor innervation of the trapezius muscle from the accessory nerve and branches of the cervical plexus using intraoperative electroneurography and histochemical staining. In 34 patients during radical neck dissection the accessory nerve and C2-C4 branches running to the trapezius were identified and stimulated. Potentials were registered under three conditions: intact accessory nerve, section of superior part of communication between the nerve and the cervical branches, and complete section of the nerve. Projections that did not elicit responses were analyzed for acetylcholinesterase activity. Before cutting the accessory nerve, its stimulation led to a recordable contraction in all parts of the trapezius muscle in all patients. C2 contributions were seen in 15, C3 in 21 and C4 in 20 patients. After sectioning of the upper half of the nerve, the results were similar. After the nerve was completely cut, C2 contributions were seen in only 2 patients, but C3 were seen in 20 patients and C4 in 19 patients. Histochemical staining revealed that the branches with no responses contained both motor and sensory axons. The accessory nerve provides the main motor input to the trapezius muscle, but preservation of the C2-C4 branches to the muscle during modified neck dissection should improve outcomes.