Object: There is conflicting and often anecdotal evidence regarding the potential motor innervation of the trapezius muscle by cervical nerves, with most authors attributing such fibers to proprioception. As knowledge of such potential motor innervations may prove useful to the neurosurgeon, the present study aimed to elucidate this anatomy further.
Methods: Fifteen adult cadavers (30 sides) underwent dissection of the posterior triangle of the neck and harvesting of cervical nerve fibers found to enter the trapezius muscle. Random fibers were evaluated histologically to determine fiber type (that is, motor vs sensory axons).
Results: In addition to an innervation from the spinal accessory nerve, the authors also identified cervical nerve innervations of all trapezius muscles. For these innervations, 3 sides were found to have fibers derived from C-2 to C-4, 2 sides had fibers derived from C-2 to C-3, and 25 sides had fibers derived from C-3 to C-4. Fibers derived from C-2 to C-4 were classified as a Type I innervation, those from C-2 to C-3 were classified as a Type II innervation, and those from C-3 to C-4 were classified as a Type III innervation. Immunohistochemical analysis of fibers from each of these types confirmed the presence of motor axons.
Conclusions: Based on the authors' study, cervical nerves innervate the trapezius muscle with motor fibers. These findings support surgical and clinical experiences in which partial or complete trapezius function is maintained after injury to the spinal accessory nerve. The degree to which these nerves innervate this muscle, however, necessitates further study. Such information may be useful following nerve transfer procedures, denervation techniques for cervical dystonia, or sacrifice of the spinal accessory nerve due to pathological entities.