electrical responses of maia nerve to single and repeated stimuli

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ELECTRICAL RESPONSES OF MAIA NERVE TOSINGLE AND REPEATED STIMULI

By J. YULE BOGUE AND H. ROSENBERG(From the Department of Physiology and Biochemistry, UniversityCollege, London, and the Marine Biological Laboratory, Plymouth)

(Received February 29, 1936)

THE numerous investigations of the peculiar properties of the neuro-muscular system in Crustacea mainly concern the direct and indirect-excitability and the different types of muscular contraction and inhibition.In spite of the significant role of the nervous action in transmission andsummation, relatively few attempts have been made to analyse closelythe action-potential waves of the nerve response to a single or repeatedstimulus, as is usually applied for indirect stimulation.

The theoretical importance of a comparison between the activitiesof non-medullated and medullated nerves is evident. An ingeniousexplanation for the superiority of conduction in myelinated nerves is theassumption, based on L i ll i e's model experiments, that in these nervesthe excitation is propagated discontinuously from node to node, whereasin myelin-free nerves it passes continuously from point to point [G era rd,1931]. This structural conception disregards possible differences inphysiological characteristics. Do the time relations, for instance, of theaction-potential waves in crustacean nerve furnish any support for afunctional interpretation of the facts?

From such considerations we studied, firstly, form and velocity ofthe composite (statistical) wave evoked in the nerve trunk by a singlemaximal stimulus. These observations were completed by experimentswith graded stimulation of thin fibre bundles in which we intended todetermine the shape of the axon-potential wave and the velocity of thesensory components of the trunk. In addition, the responses of the nerveto periodic stimulation with constant current pulses and inductionshocks of lower and higher frequencies were investigated with specialregard to the after-potentials. Eventually we hoped to secure a basis

ELECTRICAL RESPONSES OF MAIA NERVE

for comparative studies on other non-medullated nerves [B o gu e andRosenberg, 1934; Bogue, Rosenberg and Young, 1936]. Theresults of other workers will be discussed in connexion with the descrip-tion of the present experiments.

I. METHODSIn view of the slow components of the action potentials of non-

medullated nerve we used a direct-coupled amplifier similar to thatdescribed by Matthews [1935], details of which were kindly placed atour disposal prior to publication. For records from the whole nervetrunk four stages of amplification were usually used, while five stageswere used for the thin bundles.

The amplifier was worked in conjunction with a Matthews oscillo-graph and a drum camera at 3 m. distance with an average paper speedof 1-75 m./sec. In order to avoid distortion we operated the oscillographwithin the range of its linearity which at 3 m. gave a deflection of theorder of 3 cm. The natural frequency of the instrument was nearly6000 cycles/sec. After each record (series of 2-4 curves) a calibrationvoltage was recorded.

The moist chamber was fitted with three pairs of platinum electrodesfor induction shocks, and seven calomel half-cells so arranged that it waspossible to lead off at different distances and stimulate when necessarythrough non-polarizable electrodes.

Single shocks and constant current pulses were applied by hand-operated keys. Periodic stimulation by constant current pulses and in-duction shocks was produced by means of a commutator, the time ofapplication being regulated by hand. The electrode chamber was in-sulated against mechanical vibration.

The upper part of the limb nerve of Maja squinado was dissected bythe usual L e vin technique and kept in aerated sea water for 1 hour. Theexperiments were done during August and September 1934, while thoseon the thin fibre bundles were carried out in September 1935 in thefollowing manner. The nerve was cut between the mero- and carpo-podite, and the joint between the proto- and dactylopodite carefullyexarticulated. The nerve was then allowed to slide out of the carpo- andprotopodite, since it was not fixed by the connective tissue. After cuttingthe nerve between a ligature and the dactylopodite, it was transferredto a shallow dish of sea water. The thinnest bundles of the floating nervewere selected and tied at both ends, and after soaking in sea watermounted in a moist chamber containing two pairs of calomel and two

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pairs of platinum electrodes. The bundle was burnt with a hot glass rodat its central end while resting on the distal leading-off electrode. Thisprocedure localized the injury, secured a monophasic response, andminimized the danger of destroying the minute bundle. KCI as used forinjuring the upper part of the trunk was not so satisfactory in nerves ofloose texture. The data presented in this paper were derived from over600 curves.

II. SINGLE INDUCTION SHOCKS

(a) Monophasic conditionsThe first notable feature of the monophasic response elicited by a

single maximal induction shock was the long duration of the negativity.With a conducting length of 20-25 mm. the duration of the falling part isof the order of 40 msec. at room temperature (200 C.), whereas a residueof after-negativity ("retention of the action current" [Levin, 1927]) incrustacean nerves lasts for a few seconds. For longer distances andlower temperatures the decline is still slower. These observations are inagreement with the results obtained by Lullies [1933] and by Bayliss,Cowan and Scott [1935]; while Monnier and Dubuisson's [1931] andAuger and Fessard's [1934a] figures are considerably smaller (seebelow). Within the time limits of our records (about 100 msec.) there wasno sign of after-positivity (P1. I, fig. 1).

The second striking fact was the low negativity of the action potentialwhich usually amounted to about 2-0-2-5 mV., i.e. it scarcely attained atenth of the maximum injury potential (about 30 mV., Cowan). Thisresult confirmed the statements of the above workers. This was not dueto the stimulus being submaximal and presented a remarkable contrastto the temporary total depolarization by faradic stimulation in galvano-metric measurement, i.e. the complete abolition of the injury current bythe negative variation as described by Furusawa [1929]. In his experi-ments, however, the injury potentials ranged from 1-14 to 12-8 mV. withan average of 6-3 mV. only.

The rise of the action potential in addition to the usual S shape oftenshowed a small distinct step at the start which might be due to a smallnumber of the quickest fibres preceding the majority of this fibre group.About 1-3 msec. after the beginning of the action-potential wave thecurve rose abruptly and was essentially a straight line for about 1 msec.,after which time the rate of rise decreased. The rapid component of therise began when the negativity had attained almost 20 p.c. of its finalvalue, and embraced two-thirds of the maximum negativity. The

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gradient was of the order of 1.0 V./sec.; we shall see that this figure iserroneous. The total duration of the rising phase was 3-4 msec. Thesefigures were obtained with 20 mm. distance between stimulating andrecording points and room temperature 2100C. At 40 mm. distance, how-ever, all the durations were prolonged and the gradient correspondinglydecreased. The height of the action potential and the area enclosed by thecurve and the base line were frequently also reduced, suggesting thedevelopment of a real decrement of negativity in the isolated nerve, inagreement with L ulli e s' results.

With maximal shocks there was generally a series of extra-wavessuperimposed along the falling part of the main wave M. It was possibleto differentiate three such waves E1, E2 and E3. The first extra-wavestarted soon after the peak of the main wave, the second usually towardsthe end of the quick part of the falling slope, and the third occurred ratherlate on the decline. Occasionally one or the other of the extra-waves wasindistinct or even missing. E1 and E2 which more or less broadened anddistorted the top and fall ofM were not sufficiently distinct for a detailedmeasurement of their characteristics; their shapes, however, resembledthat of M, whereas E3 appeared in a different form as a separate widesummit. Obviously they dispersed with distance. With the means ofstimulation at our disposal we were unable to select the single extra-waves by their refractory periods, and to eliminate the possibility ofrepetitive response.

As far as we can gather from our records the duration of a singlemaximal break shock of our induction coil is not longer than 2 msec., i.e.it does not exceed noticeably the absolutely refractory period of thequick fibres which is of the order of 2 msec. (1.5-2.5 msec. according toLullies). The electrotonic potential developed in the nerve, however,will subside with a certain time lag.

Admitting that the first extra-wave may be elicited by a repeatedimmediate action of the stimulus, we can dismiss this possibility whenreferring to the second and third extra-waves, which, if they hadapproximately the same speed as the main wave, would begin about 5-9and 10-15 msec. after the start of the stimulus.

From electrograms of crustacean muscles which were indirectlystimulated Wiersma [1933] deduced that a single induction shock ofsufficient strength was followed by a multiple after-discharge. Actuallywe observed irregular small oscillations of comparatively long durationsafter periodic stimulation with frequent induction shocks and afterapplication of constant pulses of either direction (see Sections IV and V).

PH. LXXXVII. 11

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In contrast with this apparently asynchronous activity of various smallbundles, the extra-waves had to be produced by an almost simultaneousonset of a large part of the trunk in equal or lengthening intervals deter-mined by the refractory period. Our measurements did not reveal sucha rhythm, the intervals ranging from about 2-5 to 8 msec. without aprogressive increase; these figures, however, might be vitiated to someextent by an incorrect determination of the very beginning of the extra-waves.

Wiersma even supposed that the slow second wave observed byMonnier and Dubuisson in different crustacean nerves was due to arepetitive response which was propagated at a reduced velocity, since thiswave was obtained only in connexion with the quick first wave. In Maianerve the dependency of E3 and possibly of E2 on the main wave wasapparently not absolute. Occasionally, with long conducting distances atlow temperature this wave E3 was higher than the reduced wave M. Bydouble stimulation Lullies determined a proper absolutely refractoryperiod of 4-7 msec. for his first slow wave which had the same velocity asour E3. In the non-medullated stellar nerve of Sepia the only slow wavemaintained its height independently of the variations of the quick spikeand sometimes was provoked alone [Bogue and Rosenberg, 1934].

All these facts support the assumption that E3 is a genuine sign for theactivity of a particular group of fibres different from those generating themain wave, M. We suggest, therefore, the following conception: E1 ispossibly a repetitive response produced during the persistence or at thedisappearance of the primary electrical process set up in the nerve by amaximal induction shock, it may even become multiple with very strongand long shocks (see below). Its gradual augmentation or even repetitionwith increase of the stimulus explains the growing efficiency of indirectsingle shocks on the crustacean muscle which does not respond to a singleimpulse [Wi e rsm a], and the variation of the muscular reaction in gradedfaradic stimulation [Pa ntin, 1934]. E2 (probably) and E3 (most certainly)are typical waves of separate fibre groups.

(b) Diphasic conditionsIn diphasic conditions, even with comparatively long leading-off

distance (usually 18 mm.), the positive phase was much smaller than thenegative and in many experiments hardly visible. It is impossible tocompute the form of the diphasic wave from the monophasic curve re-corded at the same distance between stimulating pole and first leading-offelectrode. The measured time interval between the negative and positive

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maximum is considerably greater than the value calculated from thevelocity and the duration of rise of the monophasic wave. Because of thetemporal dispersion, and possibly the real decrement in height of thewave during its passage between the leading-off electrodes, a properreconstruction of the second phase can only be made by deriving thispositive phase from a monophasic curve which has passed through adistance equivalent to that between the stimulating pole and the secondleading-off electrode.

It is misleading, therefore, to calculate the speed of propagation fromthe distance between the negative and positive maximum even for inter-electrode distances which are longer than the wave front [see Hill, 1934].From our experience we cannot support this method recently recom-mended by Auger and Fessard [1934 b], which may, however, beuseful when applied to single axons or uniform bundles. Taking 3-5 msec.as a duration of the rise and 5 m./sec. as an average speed at roomtemperature, the wave front embraces 17-5 mm. length of the nerve. Withthis leading-off distance the rising part of the negative phase should notbe distorted. We proved this assumption in shortening the leading-offdistance from 18 to 8 and 2-5 mm. The records obtained are shown inP1. I, fig. 2. While the duration of the negative phase is reduced from11 to 8 and 6 msec. respectively, the total duration of the rise, the heightand other characteristics of the rising part of the negative phase areunaltered when changing from 18 to 8 mm. leading-off distance, only at2-5 mm. are the height and time relations of that part diminished. Itseems, therefore, safe to make comparisons of the rising part of thenegativity between monophasic and diphasic responses if the leading-offdistance is greater than the critical length.

The height of the negative phase is 2-5-3-5 mV. With regard to themonophasic value this difference suggests that KCI not only injures thetreated end of the nerve, but apparently affects a greater length of thenerve than that intended. In addition the straight part of the negativerise is somewhat steeper, lasting about 0O8 msec., hence the gradientapproaches the higher value of 2-5 V./sec. at 210 C. and 25 mm. distance(P1. I, fig. 3). The other characteristics of the development of thenegativity correspond to those under monophasic conditions. At 150 C.and 22 mm. distance all the durations were prolonged while the heightwas unaffected, the gradient of the straight rise of negativity was onlyhalf that at 210 C., namely 1-25 V./sec. Between 15 and 210 C. thetemperature coefficient Qlo for the total duration of rise was approxi-mately 2. With increase of the distance to 42 mm. at 150 C. the wave

11-2

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spread out, the time relations prolonged, the height diminished and thegradient reduced to a third, namely 0-36 V./sec.

These observations and those following with one specified exceptionconcerned the upper part of the trunk of the Maia limb nerve. In orderto obtain a shape of curve as near as possible to an axon-potential wave,we employed thin bundles of the lower part of this nerve as alreadydescribed. The reduction of the diameter was evident from the increasein resistance to a weak direct current. While about 8 mm. length of theupper part of the trunk offered a resistance of 4000-8000O, the resistanceof the isolated bundles might amount to 400,000c for the same length.Single stimuli, ranging from scarcely supraminimal to inframaximalstrength, were provided by break shocks of a coreless induction coil, bymore or less rapid condenser discharges and by infrequent weak constantcurrent pulses, the two latter applied through non-polarizable electrodes.The leading-off distance was 8-5 mm., monophasic recording.

In a certain number of preparations with an appropriate intensity ofstimulus, particularly with break shocks of medium strength, smoothcurves were recorded without manifest signs of phase difference orrepetitive action (conduction length 15 mm.) (P1. I, fig. 4). These curvesbegin with a sharp bend without a perceptible preceding step or slope andrise abruptly with only a slightly marked S-shaped curvature to a pointedpeak. The duration of the total rise is about 1 msec. The negativity nowfalls rapidly for about 2 msec. and then changes to a gradual declinewhich after another 1 msec. continues as an after-negativity for about30-60 msec. (as far as detectable in our records), i.e. the spike negativitypersists at least for 4 msec. The completely monophasic spike wasassumed to cease where its gradual decline bent more or less suddenly andjoined the flatly curved after-negativity. The real duration might outlastthe apparent end of the spike. The assumption that the negative tail doesnot belong entirely to the spike is based on the appearance of the after-potential during and after periodic stimulation. According to Monnierand D ubuis son, in the claw nerve of the American crab, Callinectessapidus, when stimulated with inframaximal shocks, the rise required1-35 msec. and the total negativity of the quick wave lasted for 3-6 msec.Similar values might be deduced from a record published by B a r n e s[1930] of a monophasic response descending probably in motor fibresfrom the cut end of the limb nerve of Cancer pagurus.

Although the resistances of the bundles occasionally were of the orderof the input resistance of the amplifier, 0 5 megohm, the submaximalpeak potentials were usually considerably higher than the maximum

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action negativity of the whole trunk, they might exceed 10 and evenreach 15 mV.; maximum potentials approached 20 mV. This increase inpotential is probably due to the diminution of the shunting fluid whichsurrounds the bundles of the trunk. A comparable result is brought aboutin the frog's sciatic by replacing the conducting content of the interspacesby a non-electrolyte [Tanaka, 1925]. At the apparent start of the after-potential its height generally ranged from 10 to 20 p.c. of the spikenegativity, higher values were exceptional. The size and the duration ofthe after-negativity seemed to depend partly on the magnitude of thespike response and partly on the condition of the nerve (see Section V).We missed again any trace ofan after-positivityin monophasic conditions.

With supraminimal stimuli, condenser discharges of approximately5 x 10-9 coul. and 20 msec. discharge time or with constant currentpulses of about 5 x 10-7 A. and 50 x 10-3 sec. duration, all the time rela-tions were abbreviated, the rise lasted scarcely 0 5 msec., the steep partof the fall for 1 msec. and the gradual decline for 0*5 msec., i.e. the spikenegativity persisted for 2 msec. In some experiments the rise and steepfall formed the almost straight sides of an acute angle, resembling thesupposed shape of the axon potential. The after-negativity, with aninitial height of about 15 p.c. of the spike negativity, was traceable for10-30 msec. In the quickest components, separated by conduction overa great length of Maia nerve, Auger and Fessard [1934 a] found0-55 msec. for the rise and 2 msec. for the total duration of the spike.

Even with comparatively weak induction shocks the curve oftenpresented irregularities. Frequently the rise is discontinuous owing to aphase difference between the active fibre groups. Sometimes the courseof the main wave is interrupted at short intervals by small peaks, occa-sionally forming a rapid sequence of oscillations. This phenomenon isparticularly marked after conduction of the impulse over some length ofnerve. In such a serrated curve the peaks were separated by the intervals0-68, 0-68, 0-62, 0-86, 1-60 and 1-23 msec. Since these intervals are smallerthan the absolutely refractory period, the second and also the thirdwavelets cannot be due to repetitive responses of the same fibres. Evenif we regard each fourth wavelet as a repetition of the correspondingpreceding impulse, the intervals in this case being 1-98, 2-16, 3 09 and3 70 msec., at least three different fibre groups have to be active in turn.Similar types of composite waves occurred with condenser discharges of atime constant far below the absolutely refractory period (P1. I, fig. 5).

Multiple responses to supermaximal induction shocks of an intensitytwice or thrice that of the submaximal shocks of medium strength show

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different characteristics. They appear as more or less separate waves withintervals, measured from peak distances, which are at least equal to theabsolutely refractory period. For instance, in two series of this type thefollowing intervals were observed: 2-50, 2-50, 2-50 and 2-96 msec. and2-66, 2-96 and 3-52 msec. res'pectively. It is very unlikely that theduration of the stimulus is as long as 10 msec., on the contrary, it isprobably shorter than 2-5 msec. Presumably responses originating afterthis period, therefore, are not immediate effects of the electrical process ofstimulation, but after-discharges generated by changes which outlast thestimulus proper (P1. I, fig. 6).

III. VELOCITIES OF THE MAIN AND EXTRA-WAVES

Measurement of speed was done by stimulating with single maximalinduction shocks at two different points which were about 25 and 45 mm.from the proximal leading-off electrode. If the speed for the two distancesis the same, then (a) the velocity of the propagation of the impulse suffersno decrement, and (b) the latency between the stimulus and the start ofthe impulse is negligible for the distances and durations involved. Incertain circumstances, as in a cooled nerve, the conduction time for ashort distance might be relatively greater, indicating a delay in theinitiation of the impulse. In such cases the difference of the conductiontimes still yields the true interval for the passage of the impulse through alength corresponding to the difference of the two distances. This usualmethod, however, is only valid if the velocity is uniform.

The change in shape due to temporal dispersion of the statistical wavewith the accompanying reduction in steepness and height necessitatesspecial care in the measurement, when the wave is propagated over along distance. It can only be done if the base line is absolutely straight.There is increased difficulty in ascertaining the beginning of the relativelylow extra-waves superimposed on a more or less curved line. (Therecording system did not permit of an increase of the deflections.)

In a number of experiments the speed of the wave M is identical fordifferent distances within the limits of error. Occasionally, however, theconduction time for the long distance is relatively greater, suggesting acertain decrement in velocity. The average values of six experiments at21.10 C. under diphasic conditions were 5-29 and 5-15 m./sec. for distancesof 26-5 and 47 mm. respectively (see Table I). All the records in which theextra-waves were measurable showed no decrease, but even an apparentincrease in the velocities, the differences being quite distinct in individual

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experiments. This effect had to be expected if the extra-waves were pro-duced by repetitive responses or after-discharges in the same fibres, inwhich the main wave was set up by the stimulus. The result, however,might be due to a systematic error in measurement, caused by the im-proving separation of the waves during propagation, i.e. the speed deter-mined for short distances was presumably too slow because the firstvisible deviation did not coincide with the actual commencement of thesewaves.

TABLE I. Velocity of the main wave "M" and the extra waves E2 and E. (averages).Distance M E2 Es

°C. mm. m./sec. m./sec. m./sec.21.1 26-5 5.29 2*09 1*3914-75 22 4*22 1-48 1-04 Diphasic204 46 5*12 2*26 1-8120-3 23 5.35 2*11 1*73 Monophasic

In about half of our experiments the four waves were present whereasLullies observed three and Bayliss, Cowan and Scott as well asMonnier and Dubuisson described only two waves. While the formerauthors used an inert instrument or a slow paper speed, both unsuited fora detailed recording, this explanation would not hold for the cathode-rayoutfit used by the latter. Actually Monnier and D ubuisson found onseveral occasions three or four waves in the claw nerve of Callinectessapidus (as shown in three of five records ofthe two reported experiments).They regarded the supernumerary waves as artefacts caused either by ananodic break excitation of strong induction shocks or by leading-offcross-sections of accidentally cut fibres. As we have explained, we alsosuppose that E1 is due to a repeated stimulation of the fibre groupresponsible for the m-ain wave, and therefore have omitted to tabulatethe corresponding velocities. 'Since a stimulus of sufficient strength andduration evokes a sequence of responses it is not necessary to referexclusively to a break excitation which is obtained only with difficulty incrustacean nerves (see next section). For E2, however, the consistency ofthe results does not seem to comply with an occasional factor such asmight be provided by injuries in dissection.

A comparison, relating the results ofthe workers quoted to our scheme(Table II), suggests that the slow waves obtained by Monnier andDubuisson and by Bayliss, Cowan and Scott are not the same, butcorrespond to our extra-waves E3 and E2 respectively. This explains theincongruity already perceived by Bayliss, Cowan and Scott. Lulliesapparently observed E3, and during the first stage of urethane narcosis

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TABLE II. Velocities of the main wave M and of the supposed type of extra-waves E2and E., in the experiments of Monnier and Dubuisson; Lullies; Bayliss, Cowanand Scott; Auger and Fessard.

M E2 E3° C. m./sec. m./sec. m./see.

22-23 4*75 1*60 Callinectes sapidus (M and D)23-25 3-7 1-75 Maia 8quinado, Naples (1i)20-21 4-22-3-9 ? Maia squinado, Mediterranean (A and F)12 2-5 1.0 Maia squinado, Plymouth (B, C and S)

another wave of 0-5 m./sec. velocity. The M wave speed he measuredwas exceptionally low, probably on account of the inertia of the stringgalvanometer. Auger and Fessard [1934 b] also demonstrated a slowsecond wave in diphasic records of crustacean nerves in which periodicactivity was induced by contact with a crystal of sodium hyposulphite;from their records it might be identified as E2.

L ulli es attempted to correlate the different waves with fibre groupsof different diameter. A comparison of the figures he obtained in Maiawith the data relating velocity and fibre size in nerves of vertebrates[Bishop and Heinbecker, 1930] showed that the fibres of the samevelocity are considerably thicker in Maia than in vertebrates. Thesefibres in vertebrates are medullated. From osmic acid preparationsLullies concluded that the fibres in Maia were also myelinated to acertain extent. Young [1935] proved that although the manifold layersof connective tissue which form the sheaths of the big Maia axonscontain some fatty substances, a true myelin envelope was absent, therewas also no trace of nodes. In a similar controversy about the allegedmyelin in the pike's olfactory nerve, Garten [1903] obtained exactly thesame results as Young. The different waves, therefore, presumablyoriginate in different groups of non-medullated fibres. Monn i e r andDubuisson's assumption that the slow wave belonged to motor fibreswhich innervated the slow portion of the muscle was rejected byWiersma [1933] and Jasper [1935], who threw some doubt on thehypothesis of Keith Lucas [1917] concerning the duality of functionin the crustacean neuro-muscular system. On the contrary, it seems thatactivity of thin sensory fibres is accompanied by slow waves [B a rn es,1932; Jasper, 1935].

In contrast with the mixed upper part of the limb nerve in Maia, thelower part consists mainly or even entirely of sensory fibres of smalldiameter [Young, private communication]. In thin bundles of this part,isolated as already described, we found a noticeably smaller maximumspeed. In sixteen different bundles the velocities ranged from 2-52 to

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ELECTRICAL RESPONSES OF MAIN NERVE

4*12 m./sec. with an average of 3*54 m./sec. at 17-70 C. The slowness ofthe impulse was very probably not due to any injury inflicted on thebundle during preparation, since the wave was propagated withoutdecrement in speed and the rate of conduction remained unchanged for1 hour. The activity of the quickest groups of these fibres obviously isincluded in the temporal development of the main wave generated by theproximal trunk, and contributes to the delay of the rise and of the summitof the main wave. Apparently the differences in duration of rise of theaxon potentials in non-medullated crab's and in medullated frog's nerve(0.5 and 0 4 msec. respectively) are insignificant regarding the greatdifferences of velocities in fibres of the same diameter. The differences inrate of rise, gradients about 40 and 70 V./sec. respectively, are also com-paratively small and, therefore, cannot constitute the only essentialfactors. The significance of these differences has to be reconsidered whenother characteristics of non-medullated nerve, required for such anestimate, have been sufficiently established.

IV. CONSTANT CURRENT PULSES, HAND-OPERATED AND MECHANICALLYINTERRUPTED

Applying a descending direct current of sufficient strength, directionof current relative to the position of the leading-off electrodes, Bieder-mann [1886] observed a permanent excitation originating at the cathodein the non-medullated visceral connective of Anodonta. With this directionof the current an excitation at break rarely occurred, whereas a similarascending current usually yielded a break excitation. In the pike'solfactory nerve only extremely strong currents evoke an excitation atbreak [Cremer, 1907]. During the flow of a current just above thresholdstrength the fibres of crustacean nerves produce a regular series of dis-charges, the frequency of which is augmented with increase of the currentstrength to a maximum limited by the refractory period [Jasper andMonnier, 1933]. Apparently this continuous rhythmic response is notan indispensable attribute of the myelin-free axon, since it seems to bealmost absent in the pike's olfactory nerve [Garten, 1903] and missingin non-medullated snail's nerves [Lap i c q u e, 1935], and also missing inthe fin nerve of Sepia [Bogue, Rosenberg and Young, 1936]. It is asign of the very slow accommodation of crustacean nerves, observed bySolandt [1935]. The accommodation of medullated nerves is con-siderably delayed at low temperature. In accordance with the supposedrelation, a periodicity, lasting for several seconds during the current flow,

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is developed in the medullated nerves of cooled frogs, as indicated bythe make tetanus [v. Frey, 1883] and proved directly by neurograms[Garten, 1908; Fessard, 1935].

In our experiments the applied potentials, ranging from 0X18 to1-33 V., about 80-90 p.c. of which dropped between the points of contactover 10 mm. length of nerve, elicited an irregular series of responses dueto the interference of waves in different fibres. The true latency variedbetween 5 and 7 msec. for the lowest potentials. These oscillations con-tinued during the flow of the current, the duration of which was of theorder of 30-70 msec., and generally outlasted the break of the current fora considerable fraction of this time. A distinct break excitation, however,was absent after the passage of a descending current with a few doubtfulexceptions, and only occasionally present after comparatively strongascending current, i.e. due to the disappearance of anelectrotonic condi-tions. With the lower potentials the mean negativity rose slowly to afraction of 1 mV. during the passage of the current. Medium potentialsevoked a swell of negativity in short steps, the quick fibre groups ap-parently responding to the currents in a sequence according to theirdifferent latencies. With higher potentials, if the current strength wassufficient to stimulate the majority of the quick fibres simultaneously,the negativity increased rapidly in form of the rise of the maximumaction-potential wave. After a certain time a level of more or less steadynegativity was attained which obviously was composed of superimposedwaves. There was no genuine summation even in strong stimulation,since the maximum height of the level at the most only slightly surpassedthe maximum of the single action potential.

These common features showed certain modifications dependent uponthe direction of the current. With comparatively strong descendingcurrent (catelectrotonic condition) after an initial rapid rise to a firstmaximum, the curve gradually reaches a second maximum which ismaintained for the duration of the current pulse. With the same ascendingcurrent the initial rise is slower and longer; after the first maximum,which is attained with considerable delay, the negativity declines. Thismaximum of action negativity in anelectrotonus exceeds distinctly thefirst and nearly equals the second maximum in catelectrotonus (P1. I,fig. 7 A, B). These changes,' which were observed with interelectrodedistances of 5-5 and 8-0 mm., were obviously caused by the interposedanode which delayed, broadened and enlarged the action-potential wavein the beginning and gradually blocked its passage during the course ofthe current flow-in accordance with observations on medullated nerves

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[Bishop and Erlanger, 1926; Schmitz and Schaefer, 1933]. Withan interelectrode distance of 8 mm. there was no noticeable electrotonicdisplacement of the base line, whereas with a distance of 2-5 mm. thebase line was raised in cat- and lowered in anelectrotonus. In theseconditions the differences between the initial parts of the curves wereenhanced. In catelectrotonus a slightly reduced first maximum of actionnegativity was added to the electrotonic negative potential shift. Inanelectrotonus the action potential rose with the described shape from thenew positive level to the absolute value of its original negative maximum,i.e. it was enlarged by the amount of the electrotonic positive potentialshift. The additional positive charge, therefore, was almost completelyabolished by depolarization of the membranes in excitation, while thepassive reduction in positivity of the membranes results in a slightdiminution of the initial active potential change. Subsequently theresponses were subject to secondary alterations (P1. I, fig. 7 C and D).

In di- or monophasic conditions with distances of about 30 mm.between the leading-off electrodes the differences were comparativelysmall during the passage of the stimulating current. When the action-potential wave reached the second intact leading-off electrode indiphasic recording there was no marked fall of the negativity, with weakcurrents the mean negativity even continued to increase. After break indiphasic conditions, a positive phase appeared, the duration and amplitudeof which increased with the strength of the stimulating current. But evenin full development the positivity did not attain the value of themaximumnegativity. The pronounced positive phase reached its summit in about10 msec. and approached the base line gradually in about 50 msec. Thisphase was not due to any polarization, since it followed the application ofascending as well of descending currents. It disappeared entirely afterinjury of the distal end of the nerve, it was therefore caused by a pro-longed negativity in the region of the second leading-off electrode.

This general description is illustrated by an analysed record (P1. II,fig. 8) obtained with a descending current ofmedium strength. A potentialof 0-67 V. was applied to electrodes 1 and 2 at the peripheral region of thenerve, the proximal leading-off electrode 3 was placed 5-5 m. fromelectrode 2, and the distal leading-off electrode 4 was connected with theintact central region. The true latency and the velocity of the action-potential wave were calculated from two records with different positionsof electrode 3, i.e. with different conducting lengths. In the record thestimulus, make of current, commences at 0; after a latency of 1-36 msec.the first action potential starts at A, reaches the first leading-off electrode

171


3 at B after a conduction time of 1-81 msec., and the second leading-offelectrode 4 at C after a conduction time of 9*76 msec. The stimulusceases at D when the current is broken after a flow of 27-4 msec. Theaction potential originating at this instant reaches electrode 3 at E aftera conduction time of 1-81 msec,., equal to that of the first impulse. Nowelectrode 3 loses its negativity when the spike of this wave has passedand now approaches electrode 4, which is reached shortly after themaximum positivity at F and after a conduction time of 9-76 msec. Somerhythmic activity of the nerve persists after E for at least 12 msec. Thepositivity now gradually declines, and the curve reaches the base lineafter another 53 msec. It is obvious that none of the submaximal singlewaves produce the large positive phase with its marked summit sincewith this leading-off distance even a maximal diphasic action potentialshows no comparable second phase. In monophasic records, the otherconditions being identical, the negativity declines very gradually afterbreak of the current (P1. I, fig. 7). This negative retention probablyresults in the positive phase when the retention is decaying at the intactdistal part, with a delay approximately corresponding with the conduc-tion time between the leading-off electrodes. It will be shown that thepositive phase has a considerable decrement in height during conduction.A combination of these factors might give a fair explanation of thediphasic curve.

A shift of the mean negativity and a positive phase were also obtainedby interrupted stimulation with frequent constant current pulses, theduration of flow and interval being almost equal. With comparatively lowpotentials (e.g. 0 4 V., 140/sec., duration of each make being 3-57 msec.or 0-67 V., 350/sec., each 1-43 msec.) after the first shocks of the series afew fibres only respond, the number of active fibres increased considerablyduring stimulation until a fairly steady state was reached (P1. II, fig. 9).The mean deflection of these oscillations, diphasic recording, was about1 mV. above the base line. In these circumstances therefore the numberof excited fibres was fairly constant only after a certain time. Thisphenomenon might affect the results in indirect muscle stimulation withconstant current pulses of apparent threshold strength, in addition to thepossibility of repetitive nerve responses with pulses beyond a certain dura-tion [Jasp e r], and complicate the mechanism of summation. This is notcontrary to Katz's [1936] statement that facilitation of muscle responsedoes not depend on local summation in nerve (limb nerve of Carcinus).

With stronger stimulation, e.g. 1 V., 200/sec., 2f5 msec. each, de-scending current, a plateau was formed with superimposed waves in the

172


rhythm of the pulses. At this frequency the first (submaximal) wave hadnot returned to the original base line when the second excitation occurred,thus each following wave rose from a higher level of negativity, namelythat left behind by the preceding waves. While the level of maximumnegativity ("crest" Gas s e r) increased, the height of the waves (spikes)decreased until a certain equilibrium was established, finally due to therapid sequence of the stimuli, a part of the fibres failed alternately. Therewas no definite excitation at break of the stimulation. The positive phaseafter break which was present in diphasic conditions was abolished whenthe nerve was killed at the distal leading-off region, the negativitysmoothly and slowly falling to zero (P1. II, fig. 10). Within the timelimits recorded, about 100 msec., no other after-positivity was observed.

As Gas s er [1935 b] has pointed out, for the ascertainment of thedevelopment of an after-potential during periodic stimulation and for theinterpretation of its influence on the shape of the curve, the intervalbetween the stimuli must reasonably exceed the absolutely refractoryperiod. In stimulation by constant current pulses the interval left to thenerve for recovery is only definite when the quickest fibres are excitedby threshold stimuli. With stronger stimuli, during the single pulsedifferent fibre groups are activated in succession and cause an extensionof the negativity. If, in addition, the frequency of stimulation is in-creased until the interval between the actual end of action of the previousand the start of the following pulse approximates the absolutelyrefractoryperiod of a part of the active fibres, the responses are lengthened due toaugmented latency and protracted conduction. At suitably high fre-quencies a number of fibres alternate and finally the majority of theaxons interfere in action, producing an almost permanent negativitywhich may be superimposed on the essentially different negative after-potential. With regard to the difficulties in mechanical generation of aregular series of rectangular shocks of very short duration, a stimulationby induction shocks or rapid condenser discharges, as used by Gasser,is preferable for this problem.

V. PERIODIC INDUCTION SHOCKS

(a) Lower frequenciesBecause of the long persistence of the action negativity in Maia nerve,

distinct effects are already obtained with the usual faradization. Ifbreak shocks only are effective with a frequency of 59/sec., the singleshocks are spaced by 17 msec. With the unfavourable assumption that

173


the excitation requires 2 msec. from the start of the stimulus, 15 msec.are available for recovery. This interval is 6-10 times the absolutelyrefractory period of the quick fibres.

With this frequency there may be no definite change in the base lineand height of the first few diphasic responses in a fresh nerve, indicatingthat the relatively refractory period is shorter than the interval of thestimuli (P1. II, fig. 11). After a few responses the base line graduallyshifts from zero to negativity, and the spike height, as measured frombase line to peak, may decrease correspondingly. In this way the level ofmaximum negativity is fairly maintained, slight variations are probablydue to occasional changes in the strength of the stimuli. When a transitorysteady state for the spike height is reached, with increasing after-negativity, the crest attains a negativity even slightly higher than that ofthe initial spike negativity. This evolution happens within 1 min., thenegative shift of the base line touching the order of 0-5 mV. After someshort periods of stimulation, of the order of 0.1 sec. with similar intervals,the second spike may already originate from an enhanced negative leveland be adequately reduced in height, both features eventually con-tinuing unaltered for a certain number of responses. It seems, therefore,that the part of the excitatory process which corresponds to the spikepotential, does not exhibit supernormality in non-medullated nerves ofinvertebrates as is also the case in medullated nerves of vertebrates[Graham, 1934; Gasser, 1935a]. These at present inconclusive deduc-tions from diphasic records are made possible by the decremental conduc-tion of the slow fraction of the negativity; unfortunately monophasicresponses were not recorded at this rate of stimulation.

With continual stimulation the spike responses progressively deteri-orate, after 2-3 min. they are diminished to one-third of their initial size,apparently without a further increase of the after-negativity; afteranother 2 min. only small responses, sometimes of irregular height, areevoked. Following break of an exhaustive stimulation no after-positivityoccurs in diphasic conditions, i.e. the two leading-off regions are de-polarized to the same extent. During an interval of 0.5-1 min. the nerverecovers sufficiently to respond with spike deflections of about two-thirdsof their original height (P1. II, fig. 11). Shocks of appropriate distanceand strength may excite initially at break only; after a few responses,however, a minute make impulse is also set up, which, in the course offurther stimulation, develops to its full height, e.g. about one-half that ofthe break response. Obviously the excitability of a section of the fibreshas been raised as a result of effective stimulation, suggesting the

174


appearance of a supernormal phase simultaneously with the gradualnegative shift of the base line. This correlation between supernormalityand after-negativity is an established fact in medullated nerve [Gasserand Erlanger, 1930].

(b) Higher frequenciesIf the intervals between the stimuli exceed the absolutely refractory

period sufficiently, in submaximal stimulation the curve rises in stepsaccording to the frequency of the break shocks. After the initial gain,the height increases slowly to a level which is either maintained or evengradually raised during each period of stimulation of 0-05-0 1 sec. dura-tion with similar pauses. Occasionally the crest negativity declines afterpassing through a maximum. Although with maximal shocks the curvestarts in the form of a maximum single wave, the rise frequently continuesfor some time, attaining an excess negativity, e.g. 20 p.c. of the spikeheight. Sometimes the second superimposed wave yields the maximum,after which the negativity slowly diminishes. At a frequency of about200 break shocks per sec. a large number of the fibres usually continue torespond regularly to each stimulus. This condition is distinct in mono-and diphasic recording (P1. II, figs. 12 A, 13 A) and particularly expres-sive in diphasic recording with short leading-off distance, 2-5 mm. Inthis procedure, however, the development of the full negativity is de-pressed, although the curve remains above the abscissa between thepeaks (P1. II, fig. 13 B). In monophasic recording at the same frequencythe curve soon merges into a steady plateau owing to the slow decline ofnegativity in the individual responses of the whole trunk (P1. II, fig. 13 C).At a frequency of about 400 break shocks per sec., after a few regularresponses the fibres cease to work in phase, their interference results in acrest of irregular small oscillations in diphasic recording, and generally ina smooth plateau in monophasic recording (P1. II, figs. 14 A, 12 B). Attimes an alternating rhythm is established. During frequent stimulation,therefore, the refractory period augments, but usually does not surpass4 msec. In spite of the high frequency, the negativity may maintain oreven increase its level for several seconds; after 0 5-1 min. of continualstimulation, however, it is reduced to a fraction, e.g. one-tenth of itsoriginal maximum (P1. II, fig. 14 C, D). When after the end of stimulationthe conduction time has expired, the diphasic curve turns into a largepositivity which often reaches 60-70 and occasionally nearly 100 p.c. ofthe preceding negativity. After sustained stimulation this effect is almostabolished (P1. II, fig. 14 A, D). The positive maximum is attained withinabout 10-15 msec. after which the positivity subsides within 30-70 msec.

175


(P1. II, fig. 15). This description refers to short conducting distances ofabout 20 mm. If this distance is doubled, the positive phase is enor-mously reduced in height, e.g. from 95 to 21 p.c. of the correspondingnegativity, and is also considerably reduced in duration. The negativecrest is often almost unimpaired or only slightly diminished in diphasicas well as in monophasic records (P1. II, fig. 14 A, B). This differencesuggests a great decrement in conduction of the after-potential. Inmonophasic records the approximate restitution of the membranepotential requires 40-90 msec. without any sign of after-positivity (withcomparatively low amplification). The smooth curvature of the declinecontradicts the assumption that an after-positivity is masked by theafter-negativity.

Frequently on the falling part of the curve small after-discharges aresuperimposed for a considerable time, 20-30 msec.; occasionally they arelarger than the irregular crest waves, although a proper rhythm is sug-gested only here and there (e.g. P1. II, fig. 14 B). After make-and-breakshock application, polarization cannot possibly account for the after-discharges. Whether they are due to a transitory activation of backgroundexcitation at injured spots [Schaefer and Schmitz, 1933; Gasser,1935 b], or whether they are due to liberation of a substance which, untilit is dissipated or destroyed, irritates the scarcely adapting nerve, wecannot at present decide.

During periodic application of frequent stimuli, the transmission isdecelerated at the onset of the following burst, although during theinterval following the preceding burst the curve has almost reached thezero line. Apparently some sort of equilibrium is established, since thevelocity is almost constant for a few seconds of periodic stimulation andnearly identical in different records from the same nerve. The speed of thefirst wave is slightly in excess of 4 m./sec. at 21° C., it is about 20 p.c.slower than that obtained with single shocks in the same nerve. Usuallythere is no decrement in speed for distances up to 50 mm. In medullatednerves, frog's sciatic and dog's phrenic, following 15 min. faradizationwith 300 stimuli per sec., a steady state is reached during which thevelocity is reduced by about 30 p.c. [Gerard and Marshall, 1933]. Thedifference in resistance to fatigue is evident.

SUMMARYThe action-potential waves of the Maia limb nerve were investigated

by means of a direct-coupled amplifier and mechanical oscillograph,using single and repeated stimuli.

176


The waves obtained in the upper part of the mixed trunk with singleinduction shocks showed a low peak potential of about 3 mV., a relativelyslow rise, 3-4 msec., and a gradual decline of the order of 40 msec.

With maximal single stimuli three extra-waves were generally super-imposed on the main wave. The first is possibly a repetitive response dueto the immediate action of the stimulus. The second and third extra-waves are regarded as due to activity of separate fibre groups.

In the thin sensory bundles of the lower part a simple wave wasobtained with break shocks of medium strength. This wave rises in1 msec. to its peak value of 10-15 mV. The negativity declines rapidlyfor 2-3 msec. and continues as an after potential for 30-60 msec. Themaximum peak negativity reached 20 mV. With stimuli just overthreshold, the rise lasted 0-5 msec., the decline for 1-5 msec., and theafter-negativity for 10-30 msec. (monophasic conditions).

In the trunk the average speed of the main wave is 5-3 m./sec., that ofthe second and third extra-waves 2-1 and 14 m./sec. respectively at210 C. In thin bundles the average speed was 3-5 m./sec. at about 180 C.In general the velocity showed no decrement.

Constant current pulses of about 0-05 sec. duration producedrhythmic excitation, the features of which were analysed under variousconditions. The break of the current was followed by a large positivity indiphasic records, which in monophasic records was replaced by a longnegativity. Similar results were obtained with frequent constant currentpulses, which with appropriate strength produced local summation.

With periodic induction shocks of lower frequencies, 60/sec., a per-sistent after-negativity was gradually developed, accompanied by acertain reduction in spike height. Simultaneously a transitory state ofsupernormal excitability was established. After 2 or 3 min. of continualstimulation the nerve was exhausted, but partially recovered within

min. With higher frequencies the negativity rose to a more or lesssteady level. With frequencies up to 200/sec. regular waves were super-imposed following the rhythm of the stimuli, while with 400/sec. re-sponses became irregular. During stimulation the velocity of propagationwas reduced by about 20 p.c.

After break of the stimulation with pulses or shocks, in diphasic con-ditions, a marked positivity occurred which showed decremental conduc-tion with distance, and which was almost completely abolished in fatigue.As a consequence of strong stimulation the nerve often produced after-discharges which apparently were not an immediate effect of thestimulus.

PH. LXXXVII. 12

177

178 J. Y. BOGUE AND H. ROSENBERGWe are very grateful to Dr E. J. Allen and the staff ofthe Marine Biological Laboratory

for the facilities and help extended to us during our visits to Plymouth. The expenses ofthis research were defrayed partly by the Royal Veterinary College, London, and partly bya grant from the Thomas Smythe Hughes Medical Research Fund, University of London.

REFERENCES

Auger, D. and Fessard, A. (1934 a). C. R. Soc. Biol., Paris, 116, 98.Auger, D. and Fessard, A. (1934 b). Ibid. 116, 618.Barnes, T. C. (1930). J. Phy8iol. 69, 32 P.Barnes, T. C. (1932). Amer. J. Phy8iol. 100, 481.Bayliss, L. E., Cowan, S. L. and Scott, D. (1935). J. Physiol. 83, 439.Biedermann, W. (1886). S. B. A/kad. Wivv. Wien, 93, 56.Bishop, G. H. and Erlanger, J. (1926). Amer. J. Phy8iol. 78, 630.Bishop, G. H. and Heinbecker, P. (1930). Ibid. 94, 170.Bogue, J. Y. and Rosenberg, H. (1934). J. Phyviol. 83, 21 P.Bogue, J. Y., Rosenberg, H. and Young, J. Z. (1936). Ibid. 86, 6P.Cowan, S. L. (1934). Proc. Roy. Soc. B, 115, 216.Cremer, M. (1907). Z. Biol. 50, 355.Fessard, A. (1935). C. R. Soc. Biol., Paris, 119, 1355.Frey, M. von (1883). Arch. Anat. Phy8iol., Lpz. (Physiol. Abt.), p. 43.Furusawa, K. (1929). J. Physiol. 67, 325.Garten, S. (1903). Beitrdge zur Phyviologie der markloven Nerven. Jena.Garten, S. (1908). Ber. Sach8. Gev. Wivv., Lpz., 60, 85.Gasser, H. S. and Erlanger, J. (1930). Amer. J. Phy8iol. 94, 247.Gasser, H. S. (1935 a). Ibid. 111, 35.Gasser, H. S. (1935 b). J. Phyviol. 85, 15P.Gerard, R. W. (1931). Quart. Rev. Biol. 6, 59.Gerard, R. W. and Marshall, W. H. (1933). Amer. J. Physiol. 104, 575.Graham, H. T. (1934). Ibid. 110, 225.Hill, A. V. (1934). J. Phyviol. 81, 1P.Jasper, H. H. and Monnier, A. M. (1933). C. R. Soc. Biol., Paris, 112, 233.Jasper, H. H. (1935). Arch. int. Phy8iol. 41, 281.Katz, B. (1936). J. Phyviol. 86, 45P.Lapicque, L. and M. (1935). C. R. Soc. Biol., Paris, 119, 128.Levin, A. (1927). J. Physiol. 63, 113.Lucas, K. (1917). Ibid. 51, 1.Lullies, H. (1933). Pfluiger Arch. 233, 584.Matthews, B. H. C. (1935). J. sci. Instrum. 12, 209.Monnier, A. M. and Dubuisson, M. (1931). Arch. int. Phyviol. 35, 25.Pantin, C. F. A. (1934). J. exp. Biol. 11, 11.Schaefer, H. and Schmitz, W. (1933). Z. Sinnvphy8iol. 64, 161.Schmitz, W. and Schaefer, H. (1933). Pfligerv Arch. 232, 713.Solandt, D. Y. (1935). J. Phyviol. U.S.S.R. 9, 144.Tanaka, U. (1925). Z. Biol. 83, 399.Wiersma, C. A. G. (1933). Z. vergl. Phy8iol. 19, 350.Young, J. Z. (1935). J. Phy8iol. 85, 2P.

The work on repetitive responses in axons by Erlanger, J. and Blair, E. A. (1936),Amer. J. Phyviol. 114, 328, came to our knowledge while this paper was in the Press.

THE JOURNAL OF PHYSIOLOGY, VOL. 87, No. 2

To face p. 178

iPLATE I

PLATE II THE JOIJRNAL OFl PHYSIOLOGEY, VOL. 87, NO. 2

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,. I

ELECTRICAL RESPONSES OF MAIA NERVE 179

EXPLANATION OF PLATES I AND 11

PLATE I

10 msec. time interval indicated on all records. Read from left to right unless otherwisestated.Fig. 1. 20° C. monophasic (KCI). Maximal induction shock, interelectrode distance 22 mm.;

leading-off distance 18 mm. Peaks of main wave and of first extra-wave almost mergedinto one summit. Negativity of main wave 2-04 mV. 13. ix. 34.

Fig. 2. 170 C. diphasic. Maximal induction shocks. Interelectrode distance 22 mm.;leading-off distance (A) 18, (B) 8, (C) 2-5 mm.; negativity of main wave (A) 3-32,(B) 3-48, (C) 2-44 mV. 25. ix. 34.

Fig. 3. 18-60 C. diphasic. Maximal induction shock. Interelectrode distance 32 mm.;leading-off distance 10 mm.; negativity of main wave 3-64 mV. 3. ix. 34.

Fig. 4. Thin bundle from lower part of limb nerve. 18.60 C. monophasic (heat). Sub-maximal break shock. Interelectrode distance 15 mm.; leading-off distance 8-5 mm.;resistance 173,000w; spike negativity 12-4 mV.; initial after-negativity 1-84 mV.(14-8 p.c. of spike height). 21. ix. 35. Read from right to left.

Fig. 5. Thin bundle from lower part. 18-50 C. monophasic (heat). Distinctly submaximalbreak shock. Interelectrode distance 35 mm.; leading-off distance 8-5 mm.; resistance93,000w; interfering wavelets. 25. ix. 35. Read from right to left.

Fig. 6. Thin bundle from lower part. 17-8° C. monophasic (heat). Strongly supramaximalbreak shock. Interelectrode distance 15 mm.; leading-off distance 8-5 mm.; resistance320,000w; repetitive responses. 25. ix. 35. Read from right to left.

Fig. 7. 180 C. monophasic (KCI). Constant current pulses, about 30-40 msec., break visibleon curves. Leading-to distance 10 mm., about 0-75 V. between points of contact.(A) and (C) descending, (B) and (D) ascending current. (A) and (B) interelectrodedistance 8 mm.; leading-off distance 28 mm. (C) and (D) interelectrode distance2-5 mm.; leading-off distance 33-5 mm. 24. ix. 34.

PLATE II

Fig. 8. 21.50 C. diphasic. Electrode scheme, p peripheral and c central end of nerve.Constant current pulse 0-67 V., 27-4 msec., 0 make, D break of stimulus. For furtherdescription see text. 3. ix. 34.

Fig. 9. 23.80 C. diphasic. 348 rectangular pulses per sec., shock (upwards) and interval(downwards) of almost equal duration. 0-67 V. descending current. Leading-to10-5 mm.; leading-off 35 mm.; interelectrode distance 2-4 mm. Initial deviationprobably due to slow catelectrotonic component. For further details see text.8. ix. 34.

Fig. 10. 18-80 C. monophasic (KCI). 207 rectangular pulses per sec. -0 V. descendingcurrent. Leading-to 10-5 mm.; leading-off 30-5 mm.; interelectrode distance 5-5 mm.Interference due to commutator shown along the decline. 23. ix. 34.

Fig. 11. 210 C. diphasic. Faradization, 59 break shocks per sec. Coil distance 10 cm.; paperspeed 28 cm./sec. Leading-off distance 15 mm.; interelectrode distance 25 mm.(A) Base line before stimulation, S f first stimulus, 1-8 first eight responses, {E lastresponse. (B) After another minute of continuous stimulation. (C) After 4-5 min.and end of stimulation. (D) Recovery 0-5 min. later. 4. ix. 34.

12-2

180 J. Y. BOGUE AND H. ROSENBERG

Fig. 12. 20.50 C. monophasic (KCl). Interelectrode distance 22 mm.; leading-off distance18 mm. (A) 207 make-and-break induction shocks per sec. Maximum negativity2-34 mV. (B) 430 shocks per sec.; maximum negativity 2-40 mV. 14. ix. 34.

Fig. 13. 17° C. 207 make-and-break induction shocks per sec. Interelectrode distance22 mm. (A) Diphasic, leading-off distance 18 mm., large positive phase. (B) Diphasic,leading-off distance 2-5 mm., small positive phase. (C) Monophasic (KCI), leading-offdistance 18 mm., no positive phase. 25. ix. 34. (Same nerve as in Fig. 2.)

Fig. 14. 17-2-17-8° C. diphasic. 432 make-and-break induction shocks per sec. Leading-off distance 18 mm. (A) Interelectrode distance 22 mm.; maximum negativity 2-18mV.;positivity 1-17 mV. (B) Interelectrode distance 42 mm.; maximum negativity2 48mV.;positivity about 0-1 mV. (C) Slower paper speed, interelectrode distance 22 mm.,continuous stimulation; beginning (for 2-5 sec.) and (D) end after 45 sec. Negativitygradually rising to 2-80 mV. and finally dropping to 0 23 mV. Positive phase 0-16 mV.12. ix. 34.

Fig. 15. 160 C. 207 make-and-break shocks per sec. Interelectrode distance 22 mm.;leading-off distance 18 mm. (A) Diphasic, maximum negativity 2-42 mV., positivity2-20 mV. (91 p.c.). (B) After ether. Base line with and without stimulation duringcontinuous running of commutator, start of stimulation and break shocks at regularintervals. Same amplification as (A) 1 mV. = 6X3 mm. 12. ix. 34.

Figs. 1 and 3 to 6, reduced to , Figs. 2 and 7 to 15 reduced to i.

electrical responses of maia nerve to single and repeated stimuli

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