Refining the sensory and motor ratunculus of the rodent upper extremity: evaluation of the C7 nerve root using fMRI and direct nerve stimulation.

BACKGROUND Since the 1980s, the C7 nerve root has gained clinical relevance as a donor nerve in severe brachial plexus root avulsion injuries. Despite success with the cross-chest C7 nerve transfer, inducing injury on an otherwise normal side hinders global acceptance. By sacrificing the C7 nerve root, a predictable pattern of transient sequelae is seen, including extensor weakness and index and middle finger anesthesia. The purpose of this study is to observe cortical activity during direct stimulation of the C7 nerve root using blood oxygen level dependent functional magnetic resonance imaging (fMRI) in a rat model. METHODS A total of 12 male Sprague-Dawley rats, weighing 200-250 g, were used in this study. Following an acclimation period of 1 week, 12 rats underwent exposure and dissection of the brachial plexus. Seven rats underwent placement of an implantable electrode (AISI 304, Plastics1, Roanoke, VA, USA) on the C7 nerve root, while five rats underwent electrode placement on the radial nerve. All animals then underwent fMRI during direct nerve stimulation. Ten consecutive coronal images were obtained during nerve stimulation, using a 9.4-T small-animal MRI scanner. RESULTS Cortical activation is seen within a very specific area of the primary sensory region of the forelimb during C7 nerve root stimulation. The cortical activation seen during radial nerve stimulation includes that seen during C7 stimulation but extends several slices caudally. CONCLUSIONS The sensory representation of the C7 nerve root is seen in only a small area in the S1FL region compared to that seen in the terminal branches of the brachial plexus. However, this area shows a significant overlap with the S1FL area of activation seen during radial nerve stimulation. This makes sense as the C7 nerve root contributes some, but not all, sensory axons to the radial nerve. Mapping of the C7 cortical representation in the rat brain not only adds to the ongoing development of the motor and sensory ratunculus but also provides an important foundation to study subsequent C7 donor nerve models.