N10 component in median nerve somatosensory evoked potentials (SEPs) is not an antidromic motor potential

There is a hypothesis that the N10 far-field potential in median nerve somatosensory evoked potentials (SEPs) is generated by the antidromic volley of the motor axons. To test this hypothesis, we examined five patients with amyotrophic lateral sclerosis (ALS) showing pronounced or complete denervation of median-innervated small hand muscles. We evaluated N10 over scalp and proximal plexus volleys (PPVs) at lateral or anterior cervical electrode. As a result, N10 and PPVs were definitely preserved for every ALS subject. N10 amplitudes of ALS subjects were even significantly larger than control subjects. In one ALS patient completely lacking motor axons, N10 was larger than the largest one among control subjects. These results clearly indicate that N10 is not predominantly generated by motor axons but by the whole median nerve dominated by sensory axons. We propose a hypothesis that N10 is a junctional potential generated by the entrance of the median nerve into bone at the intervertebral foramen, producing a positive pole at the non-cephalic reference electrode. Significantly larger N10 in ALS subjects may be due to the lack of cancellation by slower motor axons.