How do the cervical plexus and the spinal accessory nerve contribute to the innervation of the trapezius muscle? As seen from within using Sihler's stain.

OBJECTIVE To determine how the spinal accessory nerve and the trapezius branches of the cervical plexus contribute to the innervation of each of the 3 parts of the trapezius muscle. Special emphasis was placed on the nerve supply of the clinically most important descending part of the muscle. DESIGN Anatomical analysis of the distribution of the cervical plexus and spinal accessory nerve branches in the human trapezius muscle. MATERIALS Twenty-two trapezius muscles from 11 perfusion-fixed human cadavers ranging in age from 66 to 92 years (mean, 81.7 years). INTERVENTIONS The specimens were dissected free and macerated, decalcified, and stained according to Sihler's technique for about 6 weeks. The translucent, stained muscles were then backlit, and the findings were documented photographically and by schematic drawings. RESULTS In all 22 muscles, the innervation of each of the 3 parts of the trapezius muscle was seen. In all muscles investigated, the nerve supply to the descending part of the muscle consisted of a single fine branch of the spinal accessory nerve, whereas the transverse and ascending parts were innervated by both the spinal accessory nerve and the trapezius branches of the cervical plexus. CONCLUSION Our results, especially those involving the descending part of the trapezius muscle, may help to minimize the rate of unexpected trapezius muscle paresis after surgery of the neck.