j. neurol. neurosurg. psychiat., 1949, 12, 259. - bmjstimulated electrically at the wrist. these...

J. Neurol. Neurosurg. Psychiat., 1949, 12, 259.

THE RECORDING OF NERVE ACTION POTENTIALSTHROUGH SKIN IN MAN

BY

G. D. DAWSON and J. W. SCOTTFrom the National Hospital, Queen Square, London

IntroductionElectrical stimulation of peripheral nerve in man

has been shown to produce cerebral responses(Dawson, 1947), and analysis of these responseshas made clear the need for some means of in-vestigating the nature of the sensory afferent volleysconcerned. Experiments carried out in 1945suggested that it was sometimes possible to recordthrough the skin action potentials in the ulnarnerve of one of us (G.D.D.) when the nerve wasstimulated electrically at the wrist. These observa-tions were later repeated independently for themedian nerve by Dr. Merton, but on both occasionsthe action potentials were small and could not beapplied to the problems in hand; the experimentswere therefore not carried further. However itlater seemed worth while to repeat these experimentsto see if better results could be obtained. The onlyprevious reports of the possibility of recordingthrough skin nerve action potentials in man whichhave been found are those ofRusinov and Chugunov(1939, 1940) and Rusinov (1943, 1947). Theyreport recording nerve action potentials from overthe median and ulnar nerves above the elbow whenlthey stimulated proprioceptive sensory endings byhanging weights on the fingers, and when theypricked the hand or brushed the skin of the fingers.We have repeated the experiments of these authorswith tactile and painful stimuli, but we have beenunable to obtain any records resembling theirs solong as the muscles near to the recording electrodeswere well relaxed. We have not repeated theexperiments in which muscles were stretched, as itis difficult to avoid contraction of muscles with thestimuli they used: up to 500 g. hung on onefinger. They do not seem to have carried outadequate control experiments to exclude thepossibility that they were recording muscle actionpotentials. From the experiments to be describedit now seems clear that, after electrical stimulationat the wrist, action potentials of useful size maybe recorded from the skin over the more superficial

parts of the median and ulnar nerves near theelbow. Since it is not generally known that thisis the case, and since the technique appears to haveapplication to the study of peripheral nerve injuryand disease, it is the purpose of this paper to reportthe methods used and to describe the charactersof the action potentials which may be recorded.

MethodsThe course above the elbow of the superficial part of

the median or ulnar nerve to be examined was plotted,using an electrical stimulus, and was marked on the skin.A pad electrode 1P5 cm. in diameter on a handle wasused as a cathode to search for the places at which thenerve was most easily stimulated, and the stimulatoranode was attached to a plate electrode 3-5 cm. by6 cm. placed in any convenient position on the arm. Toreduce the spread of the stimulus, and to give moreprecise location of the nerve, a small shock was used,just great enough to cause visible twitching of theforearm and hand muscles. The length of nerve whichwas relatively superficial varied considerably from onesubject to another. Sometimes a length of more than6 cm. was found over which the threshold for stimulationaltered little; sometimes the length was less than 3 cm.In the same way the most superficial part of the nervewas plotted at the wrist. The skin over the nervetrunk was then rubbed with Cambridge electrode jellyto produce an erythema and afterwards washed withsoap and water. This is important, as the productionof an erythema reduces the skin resistance and thetrouble from stimulus escape, but this gain may be lostif an excess of electrode jelly is left on the arm. Therecording electrodes, with a small quantity of electrodejelly on each, were then applied to the skin over thenerve above the elbow and the stimulating electrodesat the wrist. In order to obtain good results the inter-electrode resistance, measured with direct current atnot more then 1 volt, should be less than 10,000 ohms,but it is advisable not to prick or abrade the skin underthe stimulating electrodes in order to reduce the resist-ance, as this may make the stimulus painful at the siteofstimulation and prevent muscular relaxation. Recordsfree from muscle action potentials were obtained onlywhen the muscles near to the recording electrodes wererelaxed. For the same reason the stimulus was always

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G. D. DAWSON AND J. W. SCOTT

FIG. 1.-The diagram shows the form of the recordingelectrodes before padding with lint. The distancebetween the holes in the bakelite supporting blockis 1 cm. Blocks of different lengths were used fordifferent numbers of electrodes.

applied at the wrist and the record made from over thenerve above the elbow. In this way the only musclesexcited by the shock were those in the hand, which wereas far as possible from the recording electrodes.The recording electrodes which were used were of the

form shown in Fig. 1, narrow rectangles of silver sheet1 mm. thick, 0 5 cm. wide, 2-5 cm. long, and curved intheir long axes to locate the nerve under them. Theywere mounted at their centres on threaded silver tubesand covered with lint and gauze pads soaked in brine.The silver tube stems were screwed into a bakelite blockin which there was a line of holes at intervals of 1 cm.;for recording nerve action potentials two or threeelectrodes were used at intervals of 3 cm. in a blocklonger than that shown in Fig. 1. The electrodes werearranged with their long axes parallel, and the blockswere pressed on to the skin by rubber or webbing strapswith the length of the electrodes as nearly as possible atright angles across the course of the nerve. The formof the electrodes is important as they must be small inthe direction along the length of the nerve if the site ofpick-up is to be even approximately known. The effectof movement of the electrodes in a direction across thelength of the nerve is minimized if they are made largein this direction; a movement ofa small circular electrodeaway from the nerve by only a centimetre may lose theaction potentials. For recording action potentials inthe thenar or hypothenar muscles, surface electrodeswere used, placed over the bellies of the muscles. Theelectrodes were either of the strip form used for recordingthe nerve action potentials, or were silver cups, 1 cm.in diameter, held on with collodion.

Several arrangements of stimulating electrodes havebeen used. In early experiments a large anode wasplaced on the hand or arm and for the cathode a smallpad electrode was placed over the nerve; the electrodeswere of the same type as those used for plotting thecourse of the nerve. Later it was found that the stimulusescape could be greatly reduced if both electrodes weremade small and of equal size and were placed closetogether. When only the nerve action potentials proximalto the point of stimulation were being recorded, a pair

of electrodes of the same type as those shown in Fig. 1,with the more proximal one the cathode and the moredistal one the anode, was found to be suitable. How-ever, this arrangement of the stimulating electrodes,with the anode over the nerve distal to the cathode, maylead to blocking of some of the fibres conducting im-pulses to the periphery, and the volleys travelling awayfrom the electrodes in the two directions will not becomparable. To avoid this difficulty, when actionpotentials in muscles distal to the point of stimulationwere being recorded, as well as nerve action potentialsproximal to it, small silver cup electrodes were used.One of these over the nerve was made the cathode; andthe other, placed several centimetres medially or laterally,was made the anode. The stimulating voltage used hada steep front rising to its peak in not more than 4 1' sec.,and an exponential delay, usually with a time constantof 50 p sec. The stimulating electrodes were both un-earthed and were coupled to the stimulator through aMuirhead Wide-Range transformer. When workingbetween input and output circuits of 2,000 ohms im-pedance this transformer has a flat frequency responseup to 150 kilocycles per second. Since the stimulatorhas a low output impedance the form of the stimulatingcurrent in the tissues did not follow the form of thepotential difference between the stimulating electrodes.However, the results obtained with this stimulator wereconsistent, and it has been pointed out (Walterand Ritchie, 1945) that a stimulator with a low outputimpedance tends to produce less discomfort at the siteof the stimulating electrode, for any given degree ofnerve stimulation, than one of high output impedance,and for this reason the low impedance type was preferred.This is important not only when examining patients butalso when doing laboratory experiments on volunteers,since the subject must be as relaxed and comfortable aspossible if the records are to be free from muscle actionpotentials. Occasionally when a search was being madefor action potentials from fibres with higher thresholdsa shock of the same form but with a time constant of0-4 msec. was used.The recording amplifiers, which have balanced input

circuits, were used without grid resistances in the inputstages. An earth connexion was made to the subject'sforearm, between the electrodes used for stimulating andthose used for recording' the nerve action potentials.Differences in the contact resistance at the recordingelectrodes may produce a serious imbalance in the inputcircuits if grid resistors are used, much greater than theimbalance of the amplifiers themselves. This imbalancemay increase seriously the stimulus artefact in therecords, and it is easier to reduce this effect if the gridresistors are omitted. The frequency response of theamplifiers was set to be uniform within 10 per cent.from 20 c/s. to 700 c/s., and to attentuate by half fre-quencies of 5 c/s. and 2,800 c/s. The response of oneamplifier and recorder to a rectangular pulse with aduration of 2 msec. and an amplitude of 20 gV., from asource of 500 ohms impedance, is shown in Fig. 2c.Fifty records were superimposed, and the peak-to-peakamplitude of the inherent irregularity of the apparatus(" noise ") is about 4 [±V. The output of the amplifiers

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NERVE ACTION POTENTIALS.

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FIG. 2 (A, B, C, left).-The records show the patential differences between a pair of electrodes on the skin over themedian nerve above the elbow. In (A) electrical stimuli were applied to the nerve at the wrist at the point in-dicated by the black dot below the time scale. In (B) the conditions were identical except that no stimuli wereapplied. Record (c) shows the response of the recording system to a rectangular pulse of 20 p.V. amplitudedelivered from a source of 500 ohms impedance. The time scales show intervals of 1 msec. In theseand in all succeeding records fifty traces were superimposed.

FIG. 3 (A, B, C, right).-The records show the effects of movements of the stimulating or recording electrodes.Record (A) shows the responses recorded when the stimulating cathode was over the median nerve at the wristand the recording electrodes over the nerve above the elbow. (B) shows the result of moving the stimulatingcathode from over the nerve to the radial styloid, keeping the other conditions as in (A). (c) shows theresult of leaving the cathode over the nerve at the wrist but moving the recording electrodes medially off thenerve on to the belly of the biceps muscle. The time scales show 0 1 and 1 msec. intervals.

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FIG. 4.-The records show the change in the latency of the responses with change in distance from the stimulatingpoint. Traces 1 and 3 were recorded simultaneously from the distal and central electrodes, traces 2 and 4from the central and proximal ones. The response from the proximal pair of electrodes is delayed by 0-5msec. In the diagram the letters indicate: An., anode and Ca., cathode of the stimulator, E, earthconnexion, Rec., recording electrodes separated by 3 cm. The time scales in (A) show interv. Is of 1 and 5msec. and in (B) of 01 msec. The calibration marks in (A) indicate 20 V.V.

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FIG. 5.-The records show the relation between the action potential volley in the median nerve and the numberof motor fibres being stimulated. In trace 1 are recorded the action potentials of the muscles in the thenareminence, and in trace 3 the action potentials from the median nerve above the elbow. In (A) the stimulusto the nerve at the wrist was supramaximal for the motor fibres, and in (B), (C), (D), (E), and (F), the strengthof the stimulus was progressively reduced. Records (D) and (E) show that the greater part of the nerve actionpotential in (A) is due to fibres with a lower threshold than motor fibres. Trace 2 shows the time scale fortrace 1 in 1, 5, and 20 msec. intervals, and the pedestal shows the relative time and duration of trace 3, thetime scale for which is shown in intervals of 0-1 and 1 msec. in trace 4. The calibration mark in (E) at the endof trace 1 shows 8mV. for records (A) to (E), and that in (F) 400 ,uV. The mark at the end of trace 3 in (F)shows 20 [±V. for this trace in all the records.

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was taken to cathode ray tubes from which photographicrecords were made of either single responses or up to fiftyresponses superimposed.

Using the methods described, fifteen subjects havebeen examined. Of these subjects, thirteen couldgive no history of any neurological disorder; inthe fourteenth the ulnar nerve on one side had beencompletely severed above the elbow and had beensutured, with good recovery of function. On thesame side as the ulnar nerve which had been cut,the median nerve also had been damaged by atourniquet at the operation for suture, but hadrecovered in three months. Otherwise this subjectalso was healthy, and in this paper only recordsfrom the uninjured side will be considered. In thefifteenth subject no history of neurological disorderwas obtained before the examination, but theresults in his case were so different from those inthe other subjects, which were uniform, that afurther inquiry was made. This disclosed a historyof mild disorder which will be discussed later.

ResultsFig. 2A shows an oscillogram in which are super-

imposed fifty records of the changes of potential atelectrodes on the skin above the elbow, producedby stimulation of the median nerve at the wrist.The stimuli were applied at the rate of one persecond and were strong enough to give approx-imately maximal stimulation of the motor nervefibres to the small muscles of the hand. On thelower trace in the record are shown time intervalsof 1 msec., and the instant at which the stimulus wasapplied is indicated by the black dot below thistrace. Synchronously with the stimulus the shockartefact appears in the upper trace, which wasrecorded from the electrodes above the elbow.This artefact lasts for 1-5 to 2 msec. and with anyone electrode arrangement its size is proportionalto the stimulus strength, but it may vary greatlyin size, duration, and form from one experimentto the next and with the electrode contact resistanceor position. With the precautions described theartefact could usually be reduced to such an extentthat the baseline became flat again within 2 msec.of the stimulus, even when this was of fifty volts ormore and when the record was being made with asensitivity of 1 mm. deflection per microvolt betweenthe recording electrodes. Next there is a triphasicwave which begins 3 to 4 msec. after the stimulusand which is complete in a further 3 msec. Whenthe stimulus was increased slowly from zero thestimulus artefact appeared first, then at a thresholdlevel, which was accurately repeatable, the triphasicwave appeared. This wave grew rapidly up to apoint where an increase of stimulus strength of

20 or 30 per cent. produced no significant increasein its size, although the stimulus artefact continuedto get larger. The stimulus strength at which thewave first ceased to increase was approximatelythat which gave maximal stimulation of the motorfibres in the nerve. The example in Fig. 2A iStypical of the records which were obtained fromover the median or ulnar nerves of fourteen subjects,though the relative sizes of the three phases of thewave varied from subject to subject. Recordsfrom the fifteenth subject at first showed to similardeflections at all, apart from the usual stimulusartefact. At a later examination, and after thecourses of the nerves had been plotted with particu-lar care, regular deflections were detected in therecords between 4 and 10 msec. after the stimuli.When stimulating the median nerve on the rightor left side these deflections were only just detectableabove the irregularities in the records, and whenstimulating the ulnar nerves they were less than halfthe size of the deflections in any of the otherfourteen subjects. The small size of the responsesand the limited area over which they could bedetected probably accounts for the failure to recordthem at the first examination. In the other fourteensubjects the responses were recorded withoutdifficulty at the first examination. The sizes of theresponses were measured from the centre of thedarkest part of the baseline in the superimposedrecord to the centre of the darkest part of the traceat the peak of the upward deflection, when thepotential difference between the electrodes wasgreatest. The measurements were all made fromrecords taken with a standard inter-electrodedistance of 3 cm. and with a stimulus which wasmaximal to the extent that a 20 per cent. increasein its size would produce no significant increase inthe size of the response. The Table gives thenumber of subjects in whom the sizes of theresponses fell into the ranges shown, when theywere recorded under the conditions described.The connexion of the amplifiers to the electrodes

over the nerve was such that an upwarddeflection was recorded when the more distalelectrode of a pair became negative with respect tothe more proximal one. The deflections between3 and 7 msec. after the stimuli in the records inFig. 2A, therefore, show that the distal electrodebecame first positive, then negative, and thenpositive again with respect to the proximal one.This triphasic form appeared constantly when theelectrodes were at places where the nerve could bestimulated equally easily, and where it was probablyequally superficial. Occasionally, when a pair ofelectrodes over the median nerve was more distalthan usual, a predominantly diphasic wave was

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TABLE

RANGE OF SIZES OF RESPONSES RECORDED

No. of Amplitude of responses:Subjects microvolts

I less than 10

1 from 10 to less than 20




3 from 50 up to 60

0 more than 60

recorded. This may have been due to the distalelectrode being over the nerve where it was be-coming less superficial as it entered the antecubitalfossa. In these circumstances a typical triphasicwave was still recorded from a pair of electrodesfarther up the arm. Before it may be acceptedthat these or similar changes are due to nerve actionpotentials certain sources of artefact must beexcluded.From the fifty superimposed records in Fig. 2A

it seems that the triphasic wave follows each stimulusby an interval, which varies by not more than plusor minus 0-1 msec. Disturbances not related tothe stimulus, such as action potentials associatedwith steady or random contractions of musclesnear to the recording electrodes, produce onlyirregular deflections at different parts of the trace.In Fig. 2B are shown fifty superimposed recordsmade from the same electrodes and under the sameconditions as the records in Fig. 2A, except that nostimulus was applied to the nerve. No deflectionsappear like those in the records in Fig. 2A. Thissuggests that the disturbance was provoked by thestimulus and that the time at which it occurred wasaccurately related to the time of the stimulus. Therecords in Fig. 3 show the effects of movements ofthe stimulating or recording electrodes away fromthe nerve. Fig. 3A shows the responses recordedwhen the stimulating cathode was over the mediannerve at the wrist and the recording electrodeswere over the nerve above the elbow. In Fig. 3Bthe records show the result of moving the stimu-lating cathode away from the nerve to a point overthe styloid process of the radius. The stimulusartefact in the record was almost unaltered but thelater deflections disappeared completely. Thissuggests that these later deflections were not dueto spread of the stimulating current, or to any ofits effects on tissues other than the nerve. When

the stimulus was applied to the nerve, the smallmuscles of the hand which were innervated by theparticular nerve being stimulated, twitched with,each stimulus and their action potentials were about8 mV. in amplitude from the baseline to the firstpeak. The latency between a stimulus to the mediannerve at the wrist and the beginning of the actionpotentials in the thenar muscles was between 4 and5 msec., which was very similar to the latencybetween the stimulus and the beginning of theresponse at the electrodes above the elbow. Thedisturbance above the elbow, which was never morethan 60 ,V. in size, might therefore have been anelectrical spread of the action potentials from thetwitching muscles in the hand. If the recordingelectrodes were moved away from the nerve, on tothe belly of the biceps muscle, the size of therecorded deflections was reduced or they wereabolished. The records in Fig. 3c show this effect.There is greater irregularity in the baseline becauseof incomplete relaxation of the biceps muscle, andthe deflections 4 msec. after the stimulus are muchsmaller than in the records in Fig. 3A. They wereprobably not abolished in this experiment becausethe recording electrodes were of the rectangularstrip type and to abolish the deflections it wasnecessary to move them two and a half to threetimes as far from the nerve as when circularelectrodes, 1 cm. in diameter, were used.The type of experiment described showed that the

potential changes were greatest over the plottedcourse of the nerve; if they were due to electricalconduction of the action potentials from the handmuscles there was no reason why this should havebeen so. In addition the action potentials from thethenar muscles were some five times longer induration than the potential changes recorded fromthe electrodes above the elbow. The reduction insize of the response as the electrodes were movedaway from the nerve and on to the biceps musclealso suggests that it was not due to a reflextwitch of the biceps muscle caused by the shock,and this is supported by the short latency of only3 to 4 msec. between the stimulus and the response.Additional and confirmatory evidence that thedisturbance was being conducted in nerve wasfound in measurements of the latency ofthe responseat different distances from the stimulating electrodes.Fig. 4 shows two records made simultaneously fromthree electrodes at intervals of 3 cm. with thearrangement indicated in the diagram. In Fig. 4the response from the distal electrode and thecentral one, traces 1 and 3, occurred earlier thanthat from the central electrode and the proximalone, traces 2 and 4. The latencies of the negativepeaks of the waves differed by 0-5 msec. This is

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shown in Fig. 4B, where these peaks were displayedon an expanded time scale the duration of whichcorresponded to the pedestal in the lower timescale in Fig. 4A. There may be considerable errorsin estimations of conduction velocity made fromrecords of this type, where the sizes of the twowaves are not identical, but a time of 0 5 msec. fora corfduction distance of 3 cm. would indicate aconduction velocity of the same order as that inmotor nerve fibres in man. Certainly the occur-rence of an initial positive wave at the electrodenearer to the point of stimulation does not conflictwith the idea that the changes were due to a volleyof impulses ascending the nerve towards therecording electrodes. When the region of relativenegativity associated with a volley of nerve impulsespasses along a nerve which is in a large volume ofconducting medium, a similar triphasic variation ofpotential difference appears between an electrodenear to the nerve and one remote from it. Theelectrode nearer to the nerve becomes first positivewith respect to the other, then negative, and finallypositive again. A full analysis of this sequenceof events has been made (Lorente de No, 1947),which applies to the present recording arrangementseven though in these the two electrodes were bothrelatively close to the nerve and at the boundary ofthe conducting medium.

Since the findings described above indicated thatthe changes at the recording electrodes were due tonerve action potentials, and were not due to arte-fact, experiments were made to determine the typeof nerve fibres concerned. The strength of stimula-tion that was used suggested that the fibres beingstimulated had a similar excitability to that of themotor fibres. Stimulation of the motor fibres willproduce an ascending as well as a descending volleyof impulses, so part or all of the action potentialsrecorded at the elbow might have been due to theseantidromic impulses in motor fibres. The experi-ment illustrated in Fig. 5 was made to test thispoint. In it the median nerve was stimulated at thewrist, the action potentials in the muscles of thethenar eminence were recorded from surfaceelectrodes, and fifty records were superimposed.Trace 1, at the top of the records, shows the muscleaction potentials, and trace 2 shows time intervalsof 1, 5, and 20 msec. and the first big spike indicatesthe time of stimulation. Trace 3 is a record of theaction potentials in the nerve above the elbow, andtrace 4 shows the time scale for this record withintervals of 0 1 and 1 msec. The time relation oftraces 3 and 4 to that of traces 1 and 2 is shown bythe pedestal in trace 2. In Fig. 5A the stimulus usedwas maximal for the motor fibres to the thenarmuscles. In the records in (B), (c), (D), (E), and (F),

the strength of stimulus was progressively reduced.In record (D), where the stimulus was only just abovethreshold for the motor fibres, and when no musculartwitching could be seen elsewhere, the nerve actionpotential volley has only declined to 70 per cent.of its original size. This suggests that the greaterpart of the volley recorded in (A) was not in motorfibres bdt in other fibres which had a lower thresholdthan that of the motor fibres. In this experimentthe subject reported that, with the strengths ofstimulus used for records (D) and (E), considerablesensation was produced in the distribution of themedian nerve in the fingers. This indicates thatwhere the muscle action potential which wasproduced by the stimulus used in (A) had disappearedand no evidence of it could be obtained, even withan increased amplification, such as was used inrecord (F), sensory afferent fibres other than thosefrom muscle were being stimulated. In (F), wherethe nerve action potential also had virtually dis-appeared, some sensation was still reported by thesubject after each stimulus. Therefore, of the fibresin the median nerve which were stimulated and whichhad a lower threshold than the motor fibres, someat least were sensory afferent fibres from the fingers.The proportion of afferent fibres from muscles tothose from elsewhere was not determined. Ininterpreting observations such as these it is necessaryto consider the division of the nerve into funiculi,which has been investigated by Sunderland (1945),and which makes it possible with weak stimuli toexcite alone either motor responses from the thenarmuscles or sensations from the fingers, by varyingslightly the site of stimulation. In the experimentsdescribed the stimulating point was chosen to givethe greatest possible motor responses with weakstimuli, and therefore the interpretation of theresults is probably not invalidated by any effects ofdivision of the nerve into funiculi.

DiscussionFrom the number of observations made, it is not

possible to draw any useful conclusions about therelation of the amount of subcutaneous fat in thesubjects examined to the size of the action potentialswhich could be recorded. However, the range ofbuild in these subjects was such that it seemsreasonable to expect that action potentials of 20 uV.amplitude or more may be recordable under theconditions described in any healthy person. Thesubject in whom all the action potentials were under10 ,V. and in whose median nerves they were barelydetectable had complained some years ago of apatchy anaesthesia and numbness of the left handwhich persisted for several months. At thattime he was the subject of experiments in which

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frequent readings of blood pressure were made andwhere in some instances cuffs inflated above systolicblood pressure were applied to either arm forconsiderable periods. At the time of the presentexamination there were no obvious residual defectsin power or sensation, and it seems that if the nerveswere damaged in the earlier experiments the recor-ding of nerve action potentials may be a delicatemeans of detecting minor degrees of damage. Theaction potentials recorded from the cut andsutured ulnar nerve and from the median nerveknown to have been paralysed by the tourniquet areto be described elsewhere but they also suggest thatthe method may have value in demonstrating smalldegrees of damage to nerve.One point of interest has been the apparent

homogeneity of the group of fibres being examinedby the methods described. The action potentialwas conducted over a distance of 25 to 30 cm., andmight have been expected to show more temporaldispersion than appears in the records. The factthat so little temporal dispersion appears is probablydue to the triphasic shape of the recorded wave,since this may have masked later deflections. Thisis suggested by the fact that, in the few recordstaken, at the expense of considerable discomfortto the subject, with shocks increased slowly up to50 and 100 per cent. stronger than th6se used in theother experiments, the first change in the recordswhich appeared was a reduction in the size of thethird phase of the wave and later the appearance ofquite clear additional deflections at that time.

Attempts to measure the conduction velocity inthe ulnar and median nerves with the methodsdescribed have proved on the whole unsatisfactory,on account of the difficulty of making pairs ofrecords which have the same shape from electrodesat different positions on the nerve. Using pairs ofrecords which were nearly superimposable, andtiming from the start of the stimulus escape to thefirst reversal of potential difference between eachpair of electrodes, gave differences in latency fromwhich the calculated conduction velocities were in

no case outside the range described by Helmholtzand Baxt (1870) for motor nerve fibres in the armin man. These writers also showed that temperaturewas a more important factor in producing variationin their results than errors in measurement ofeither time or distance. It seems, therefore, that nomore accurate results are likely to be obtained thanthose they give without accurate control of temper-ature, which was not made in these experiments.

Summary1. In fifteen healthy subjects action potentials

have been recorded from electrodes on the skinover the course of the median or ulnar nerve afterelectrical stimulation of the nerve at the wrist.

2. Sources of artefact are considered, and themethods used for stimulating the nerves and record-ing the action potentials are described.

3. The type of the fibres from which action poten-tials may be recorded is considered.

4. It is suggested that the method may have usein the study of minor degrees of nerve injury.The authors would like to express their gratitude to

Dr. E. A. Carmichael, in whose department this workwas done. The work was carried out whilst one of us(J.W.S.) was holding a Nuffield Foundation DominionMedical Travelling Fellowship.

REFERENCES

Dawson, G. D. (1947). J. Neurol. Neurosurg. Psychiat.,10, 134.

Helmholtz, H., and Baxt, N. (1870). Mber. Kgl. Preuss.Akad. Wissenschaften, Berlin. p. 184.

Lorente de No (1947). Studies of the RockefellerInstitute, vol. 132, chap. 161, p. 384.

Rusinov, V. S. (1943). C. R. (Doklady) Acad. Sci.U.R.S.S., 41, 36.

(1947). Amer. Rev. Soviet Med., 4, 436.and Chugunov, S. A. (1939). Bull. biol. med.

exper,. U.R.S.S., 8, 70., (1940). Soviet Psikhonevrol., 16, 53.

Sunderland, S. (1945). Brain, 68, 243.Walter, W. Grey, and Ritchie, A. (1945). Electronic

Engineering, 17, 585.

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