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The blink reflex may be evoked by auditory click or light stimulation, the latter being constantly present in all healthy subjects. The latency and amplitude of the light-stimulus-evoked blink response (L-BR) is influenced by the light intensity, distance between the light stimulus and the eye, attention, and background facilitation. The L-BR was abnormal in 75% of definite and 61% of probable multiple sclerosis (MS) patients. When abnormal, the L-BR was more commonly absent than delayed. Optic nerve lesions could be identified by the pattern of abnormalities in the L-BR in some patients. The L-BR has proved to be a sensitive detector of abnormalities in the visual connections and, in combination with the Vth nerve stimulation-evoked blink response, a valuable detector of brainstem lesions.

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SUMMARY 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 following nerve transection. The importance of the observation lies in the more precise localisation of the 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.The median nerve most commonly innervates the abductor pollicis brevis, opponens, part of the flexor pollicis brevis and the first two lumbrical muscles (Li and L2) in the hand.1 It has been claimed that the respective motor fascicles to the thenar and lumbrical muscle groups are located close together in the ventral half of the median nerve.1 Others, however, observed that partial transections of the median nerve could totally denervate the thenar muscles, leaving the lumbrical muscles intact.2 This observation was interpreted to mean that the lumbrical fascicle occupied a more dorsal position in the median nerve. The innervation of the lumbrical muscles and relative positions of the lumbrical and thenar motor fascicles in the median nerve are important anatomical points to recall in the examination of patients with median nerve entrapments beneath the flexor retinaculum or other neurogenic disorders involving the innervation of the hand muscles. In median nerve transections total denervation of the thenar muscle group is not evidence, by itself, of complete transection of all the motor fibres in the median nerve.

SUMMARY:In human entrapment neuropathies the characteristic abnormalities in conduction are frequently limited to a short segment of the nerve. Recognition and precise localization of these discrete conduction abnormalities may require measurement of conduction over shorter lengths of the nerves than those lengths commonly employed in the clinical laboratory. Techniques are described for the more precise location of the primary conduction abnormalities in median, ulnar and peroneal nerve entrapments. Distinctive or atypical locations of the major conduction abnormalities may point towards different mechanisms in the pathogenesis of these localized neuropathies.