At late stages of median nerve entrapment in the carpal tunnel there may be total denervation of the thenar muscles. Surface electrodes over the thenar endplate zone may record an initial and predominantly positive "M" response to supramaximal median nerve stimulation. By a combination of techniques, it has been established that this surface positive response originates from surviving lumbrical muscles innervated by the median nerve. Their relative preservation may be related to their location in the median nerve. Intraoperative investigations have shown that the thenar motor fibres are primarily located in the ventral-lateral part of the median nerve whereas lumbrical motor fibres are in a more dorsal location and hence are probably better protected against the flexor retinaculum. This relative preservation of lumbrical motor fibres has been observed in other peripheral neuropathies and motor neuron disease, but not in median nerve regeneration of the following nerve transection. The importance of the observation lies in the more precise localisation voltage source for "M" response in terminal or near-terminal carpal tunnel median nerve entrapments and avoidance thereby of possible misinterpretation of electrophysiological observations in this most common nerve entrapment syndrome.
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