Electrical stimulation of peripheral nerve in man has been shown to produce cerebral responses (Dawson, 1947), and analysis of these responses has made clear the need for some means of investigating the nature of the sensory afferent volleys concerned. Experiments carried out in 1945 suggested that it was sometimes possible to record through the skin action potentials in the ulnar nerve of one of us (G.D.D.) when the nerve was stimulated electrically at the wrist. These observations were later repeated independently for the median nerve by Dr. Merton, but on both occasions the action potentials were small and could not be applied to the problems in hand; the experiments were therefore not carried further. However it later seemed worth while to repeat these experiments to see if better results could be obtained. The only previous reports of the possibility of recording through skin nerve action potentials in man which have been found are those ofRusinov and Chugunov (1939, 1940) and Rusinov (1943, 1947). They report recording nerve action potentials from over the median and ulnar nerves above the elbow whenl they stimulated proprioceptive sensory endings by hanging weights on the fingers, and when they pricked the hand or brushed the skin of the fingers. We have repeated the experiments of these authors with tactile and painful stimuli, but we have been unable to obtain any records resembling theirs so long as the muscles near to the recording electrodes were well relaxed. We have not repeated the experiments in which muscles were stretched, as it is difficult to avoid contraction of muscles with the stimuli they used: up to 500 g. hung on one finger. They do not seem to have carried out adequate control experiments to exclude the possibility that they were recording muscle action potentials. From the experiments to be described it now seems clear that, after electrical stimulation at the wrist, action potentials of useful size may be recorded from the skin over the more superficial parts of the median and ulnar nerves near the elbow. Since it is not generally known that this is the case, and since the technique appears to have application to the study of peripheral nerve injury and disease, it is the purpose of this paper to report the methods used and to describe the characters of the action potentials which may be recorded.
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