Palmar cutaneous nerve recording and clarification of median premotor potential generators

In this study of the median nerve, serial bipolar and referential recordings from the thenar eminence show that the median premotor potential actually consists of two distinct negative waves, an early (N-I) and a late premotor potential (N-II). Anesthetic block of the palmar cutaneous branch of the median nerve eliminated the early premotor potential in all subjects. This suggests that the early premotor potential (N-I) is the sensory nerve action potential of the palmar cutaneous branch of the median nerve. Local anesthesia of the palmar cutaneous branch of the median nerve also defined its area of innervation as circumscribed in our subjects. No normative data concerning the sensory nerve action potential of the palmar cutaneous branch of the median nerve is currently available, because previous studies apparently recorded the late premotor potential. The late premotor potential (N-II) is a negative far field potential seen only on referential recordings. It seems unlikely that the late premotor potential (N-II) can be completely explained as a junctional potential from the thumb as some have proposed. This study demonstrates a positive far field potential (P-I) at the palm-thumb junction, having a latency inconsistent with that of the late premotor potential. The late premotor potential can also be recorded with the reference electrode at an electrically neutral site, questioning previous explanations for its generator. A second traveling wave (N-III) was recorded distal to the area of the palmar cutaneous branch of the median nerve innervation continuing into the thumb. N-III is most likely the median digital sensory nerve action potential.