Axonal excitability in the forearm: Normal data and differences along the median nerve

ObjectiveThis study was designed to validate excitability studies of motor axons in the median nerve at the elbow, innervating forearm muscles (flexor carpi radialis, FCR) and collect normal data for this stimulation site. The differences in measures of excitability due to different sites of stimulation or a different test muscle group was also determined.MethodsThe median nerve was stimulated at the elbow and excitability studies were recorded from FCR and abductor policis brevis, APB. Further studies stimulated the median nerve at the wrist and recorded from APB. The study was performed using the TRONDCM protocol of QTRACS.ResultsStimulus-response curves for FCR were significantly less steep than those for APB. There was “fanning in” of threshold electrotonus and less accommodation to hyperpolarising pulses for FCR axons. FCR studies showed significantly less supernormality.ConclusionThere are differences in axonal excitability along a single nerve in the human upper limb, even when the nerve is stimulated at one site and recordings are made from different muscles.SignificanceNormal values for one muscle group cannot be assumed to be the same for a different muscle group. The findings for FCR axons will be of value in studies on patients with central nervous system disorders.