Segmental and supraspinal actions on dorsal horn neurons responding to noxious and non-noxious skin stimuli

Pain, 1 (1.975) 147-165 © Elscvier/North-l-lolland, Amsterdam

147

SEGMENTAL AND SUPRASPINAL ACTIONS ON DORSAL HORN NEURONS RESPONDING TO NOXIOUS AND NON-NOXIOUS SKIN STIMULI

H. O. HANDWERKER, A. IGGO* At,q3 M. ZIMMERMANN

II. Physiologisches Institut der Universitiit, D-69 Heidelberg (G.F.R.) (Accepted November 7th, 1974)

SUMMARY

(1) Singi'e dorsal horn neurons have been recorded extracellularly in 8 anesthetized (pentobarbital-Na) cats and in I unanesthctized decerebrated cat. The animals were either spiaalized by transection of the cold at L1 (6 experiments) or a cold block was used for reversible spinalization at L:t (3 experiments). (2) Sixty-five units were recorded in the, dorsal horn and in the dorsolateral funiculus whicl~ could be excited by electrical stimulation of the ipsilateral plantar nerves and by natural stimulation of the sk in in the foot region. Th~ recording position of the microelectrode was verified histologically. (3) According to their excitability by electrical stimulation of the afferent nerve fibers and by natural stimulation of the receptive fields, 2 major classes of units cotdd be distinguished in the spinalized cat. Class I cells were excited by electrical stimtilation of myelinated axons (group If) in the plantar nerves. Four out of 9 could be excited by low threshold cutaneous mechanoreceptors; 5 l~d input probably from deep receptors. Class 2 cells, which were more than twice as common as class 1 calls, could, like the latter, be excited by electrical stimulation of group II myelinated afferent fibers in the plantar nerves, but in addition, were excited by electrical stimulation of Cfibers. (4) When stimulated naturally, virtually all of the class 2 cells received an excitatory input from low threshold cutaneous mechanoreceptors and also from receptors excited by noxious radiant heat stimulation in their receptive fields. They responded to noxious heating in a quantitatively similar manner as the primar3 C-heat nociceptors. * On leave of absence from the University of Edinburgh.

148 (5) The di,;caarges of the class 2 cells evoked by heating the skin could be suppressed by electrical s~:imulation of cutaneous myelinated afferent nerve fibers, or by electrical stimulation of the col!aterals of myelinated afferent fibers in the dorsal columns. (6) As was revealed by experiments using reversible cold b!ock of the ~pinai cord, the excita.tory influence of electrical C-liber stimulation and of noxious radiant heat stimula~,ion was diminished in all class 2 cells, and in some of them it was completely s :ppressed when the spinal cord ,vas intact. This tonic descending inhibition did not depend on the presence or absence, of activity in large myelinated fibers.

INTRODUCTION

El:ctrical sti:aulation of cutaneous nerves has revealed that activity from large afferent myelinated (group II, or Aft) and noa-myelinated (group IV, or C) fibers converge onto dorsal horn neurons of the cat2~,4a,47 and the monkey sS. The majority of dorsal horn neurons exhibit this convergence2L On the other hand, electrical Cfiber stimttlation is known to evoke s~gnificant motor and autonomic reflexeslg,as,ag, St, which mis:ht be mediated by those dorsal horn neurons. Non-myelinated/myelinated convergence has also been observed in neu, ons originating in spinal ascending tractst0 z0 41,43, suggesting its role in sensory events. The question of the functional significance of the non-myelinated input however canttot be answered solely on the basis of electrical st~.mulation. Therefore, in the e~periments reported in the present paper, we have been i~tvestigating the function ~f dorsal horn neur,gns with par:icular emphasis on the actions of an input from 'thermal noc:~ceptors', contrasted wiih the it',put from sensitive mechanoreceptors. Both electrical and natural stimuli were used. It i~ t~ow well established, that the large myelinatcd axons in cutaneous nerves (group II~ innervate sensitive mechanoreceptorsg.lX,X2, 36. Electrical stimulation of cutaneous nerves can therefore be used to provide a selective (though not complete) afferent input from ;ensitive mechanoreceptors. On the other hand, the great majority of 1,2,25,3°,z5, though not allt, a0, skin receptors excited by high temperatures ( > 43 °C) in the cat have non..myelinated axons. Radiant heat, in the absence of physica', contact, which raises the skin temperature to 45 °C or higher thus preduces almcst exclusively ~, thermal nociceptor input. Preliminary accounts of these results have been publisted23, :'~. METHODS

Nine c:tts, of botL sexes, weighing 2,7-3.5 kg were used and, with the exception ci: one an.;nLal whi.;h was decerebrated, were anesthetized with pen tob~rbital-Na (initial dose: 40 mg/kg, further doses if nec:essary). The trachea was cann~ lated arid on cemplelio:a of the su~'gic~l prep~traticn, the animal paralyzed with gallamine triethiodide (Flaxedil, 7 mg/kg with 3upplements as required) and ventilated af t~fic~ally;

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Fig. 1. Recording and stimulating methods, cold block of the spinal cord. A: sites of electrical and natural stimuli, of recording electrodes on and in the spiral cord, and of the:mode for local cooling of the spinal cord; further details in text. B: surface records in L6/L7 following st!mulad.~n (~ T) of dorsal columns at the site rostral to the thermode (see A). Negativity of the ball tip electrode is downwards; iabelings a--d refer tc, C. C: time course of amplitude of N-wave (i.e. maximum deflection of potentials shown in B), before, during and after cooling the thermode at L~ to 0 C .

the end tidal CO2 was kept at 3.5 ~ . The right common carotid artery was cannulated for continuous monitoring of arterial pressure, which was normally greater than 80 mm Hg. Rectal temperature was maintained between 37-38 °C by heating of the ventral body surface.

Surgical preparation The lumbar spinal cord was exposed by laminectomy from L1 to $1. The left hind foot was fixed to a holder, pad upwards, t.y embedding it in paraffin wax. The medial and lateral plantar nerves (MP, LP) were exposed and arranged for bipolar electrical stimulation, as were the L6 and L7 dorsal roots; the nerves and roots were left in continuity (Fig. 1A). The spinal cord and nerves were covered with paraffin oii. In 5 experiments the spinal cord was transe~:ted at L1. In 3 experiments the spinal cord was intact and a metal thermode, 15 mm long and shaped to closely contact th~~, dorsal half of the cord was placed at L1/L2 (Fi~. 1A). By irrigating with methanol at about 0 °C, this thermode could be used to lower the cord temperature lecally and thus reversibly block axonal conduction. The extent of the spinal block was checked by measuring the ~arface N-wave following stimulation of the dorsal columns rostral to the thermode (Fig. IB and C).

Electrical recording Cord dorsum potentials at L6/L7 were recorded with a monopolar surface electrode (Fig. IA); the size of tlhe N-wave upon nerve stimulation was used to locate the segmental level of maximum input from the plantar nerves 21. Unit activity in the. dorsal born and superficial white matter at L~/L7 was recorded extracellularly with

150 glass micropipettes filled with 3 M KCI ~'DC resistance was 15-30 M~). The electrical r~cording apparatus has been described previou:;ly9-t. An FM_"magnetic tape recorder was used for data-acquisition, storing nerve spike,;, temwrature of the foot-pad, electrical stimuli and trigger pulses on separate tracks. Sub.~quent analysis was carried out 'off-line' with a Biomf c 1000 compt~ter.

St,'rnulation procedures Afferent input confined almost exclusively to the thermal nociceptors of the foot-pad region was produced by radiant heat. Exactly controlled high skin temperatures (40-55 °C) were obtained with a quartz-halogen lamp (focused by a ;arge condensor lens), the current through which was controlled by feedback from a thermocouple on the skin at the focus of the lamp. BotI.. the temperature and duration of this heat stimulus z~uld be pre-set. The details of the me:hod have been described previou':ly~. In addition to the radiant hea~: the f~llowing 'natural' stimuli were used to i:lcntify the afferent input oi" a partictdar unit: (a) mecbanical, by stroking er brushing t he pad surface and hairs of the ventral surface of the foot, and by exerting mild pressure onto the pads; (b) thermal (cold) b3 pouring wate: at different temperatures over th- foot, or by spraying the foot with ethylchloride, e,-aporation of which cooled the skin very rapidly (Fig. 3D), The electrical stim,di wer.~ stlu~e ~':.!:,c: delivered from a .Digitimer sti~nulator via isolation units (up to 1.5 V, 0.1 msec duration for myelinatea fibers and up to 30 V, 0.5 msec duration tbr tb.e non-myelinated fibers). The stimulus intensities will be stated in multiples of the threshold of the most exc:'taole fibe::s (i.e. n × T); thus the intensities mentioned abuve were about 8 T and 250 T, respectively.

Synaptic :tela.v The central latency was measured following 4 r stimulation of dorsal roots. Action potential~ in large myelinated Frimary fibers, which were often found by the microelectrode, had latencies of about 0.2 msec. By subtracting this presynaptic conduction time, the synaptic delay of dorsal horn neurons was determined. A synaptic delay up to 1.2 msec was taken to denote monosynaptic affe-ent input from large myelinated fiber~, greater values signalled polysynaptic connections 21.

~iswlogical metrzods The location of units recorded was established from a "econstruction of the ,:lectrode tracks in frozen sections, combined with depth measurements from the cord surface made during the experiment. ~.ESJLTS

A total o:' 65 dorsal horn neurons were examined in 9 cats. No systematic attempt was mate to discriminate axonal from soma spikes. Second and higher order :ells were distinguished from ::3rimary afferent collaterals by: (a) following frequency ,n response :: elect.:~'al stimulation of the plantar nerves or of the dorsal roots; (b)

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Fig. 2. Location of recorded units. Superimposit,on of the location of un ts as reconstructec, from the coordinates of the microelectrodes in frozen s~x:tions.Data are from .~;experiments. Filled circles (O): units having both A- and C-fiber input, i.e. class 2 units. Open circles (©): unit~ having only A-fiber input, i.e. class 1 units. Half-flied circles ( I)): units whose response to input in C-fibers was uncertain. I-VI indicate approximately the lami~ae 3f the dorsal horn according to R.exed49. latency on dorsal root stimulation; (c) characteristics of the response to natural (mechanical) skin stimuli. The usual search stimulus used during penetration of the spinal cord by the microelectrode was electrical stimulationt of both the medial (MP) and lateral (LP) plantar nerves at an intensity of 8 T. The units examined were thos,: responding within 50 msec of the stimulus, in one experiment an intensity of 200 T was used to search for units excited by non-myelinat,:d (C) fibers exclusively. No units were found that were excited only by volleys in C-fiters, as has been reported occasiorLalty'-'~,2~,~'f'. Thirty-six units, excited from the MP- and/or LP-nerves, were held for sufficient time to allow collection of the following data: excitability by electrical stimulation of MP/LP group II and group IV (C) afferent fibers; natural mechanical stimulation (receptive fields, kind of effective natural stimulus); controlled noxious heat; spontaneous activity; spike discharge latency upon the dorsal root stimulus. These 36 units are included in the sample of 39 units, of which the location of record;ng could be established (Fig. 2).

Location o f units in spinal cord Fig. 2 shows the topographic reconstr lction, with superimposition of the resul* s from all experiments (39 units). For orientation, Rexed's ~-9 cytoarchitectonic lamination is indicated in the figure: although the cell bodies may reveal a larr'inar organization, the orientation and extent of the dendrites is not so well defined ~s,~'°. For this reason the grey matter location indicated in Fig. 2 need not represent the iocation of cell bodies, since the recordings could have been made from the dendrites or the

152 axons. As can be seen, 23 units were recorded in the dorsal horn, with 2 major foci, :n lamina IV (9 units) and at the cornu of the horn (6 units) where it is difficult to distinguish the lamination. Five of the units were in laminae I and II in a medial position. The remaining units were either more deeply placed in the grey matter (2 units below lamina ¥I) or were in the dorsolateral quadrant of the white matter (15 units). These various locations were established after the experiment and the only limit imposed during an experiment was that the recording electrode was not inserted to depths greater than 2.5 ram, in order to restrict the search to cells in the dorsal quadrant of the cord. Two kit~ds of unlit were found and examined: (a) those excited only by group II afferen! volleys and by light mechanical stimuli (©), and (b) those excited in addition by group IV (C) afferent volleys and by noxious heating of the skin (®). It is evident from Fig. 2 that there was no spatial separation of the 2 major classes of unit, and in particular, that both kinds are present in or near lamina I. l[n 4 units (~) either ti,,; ,.,,,.;~ability by electrical C-stimuli or by noxious heat, or both, could not be established with certainty, because of either" too high spontaneous activity or irregular responsiveness to consecutive stimuli. Classification o f unit responses

Al}art from these 4 (labeled ~ in Fig. 2), the bulk of units (32) examined in detail it, :.pinal animals fell into 2 clear classes: class 1 B excited only by afferent volleys in group II afferent fibers (9 units); and clvss 2 ~ excited as in class l by myelina e t afferent fibers and, in :tddition, by afferent volleys in C-fibers (23 units). ClTss 1 units (9 units in the zample of 32). In response to a single afferent volley ir group II afferent fibers in the plantar nerves the typical response was a brief burst of impulses. The synaptic de ays of discharge to volleys in the Ln or L7 dorsal roots ranged from 0.7 to 12 msec. Delays up to 1.2 msec signalled a monosynaptic, those above 1.2 msec a polysynap~ic activation. The class 1 units therefore include both thl'. MA and PA groups of 3regor and Zimmermann 21. Per definitionem the class l chits were excited to dischalge impulses only by an aXerent input in the group II fibers of the planh~ nerves, anc therefore by impulses that normally arrive from sensitive cutaneous and joint mechanoreceptors. None of them were excited by noxious he~tti ng. Four were excited by low threshold receptors of the hairy and the glabrous skin, th.~ remainder (5) had no clmr-cut cutaneous receptive fields for mechanical stimuli. They might have afferent input tYom deep receptors (e.g. toe joints). Class 2 units (23 units in t~ e sample of 32). The other main class of units, witlh which we are pripcipal!y con "erned in this paper, could be excited by a volley of imptlses i'a gioup I_~ zxtanev,,.s afferent fibers (Fig. 3A), by mechanical stimulation of ~:he skin or deeper tissues and, in addition, by C-afferent fibers and no:doua heating of the skin (Fig. :A and B). Thirteen units were excited by blowing oa the hairs surrounding the foot.-pads (Fig. 3B), 8 gave a tonic discharge to light pre,ssure on the foot- or toe-pads, and 3 gave ral,idly adapting responses to displacemcnt of the central pad and/or the toe. pads. Finally, some units received an excitatory

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Fig. 3. Respons~ of a class 2 neuron to various kinds of stimuli. A: upper two traces: electrical stimulation at 2 T of both plantar nerves. Lowermost trace: electrical stimula:ion at 250 T of the plantar nerves, which was above the threshold of C-fibers. B: hair movement (indicated by the bar below) on the plantar surface of the foot. C: skin cooling by ethylchloride evaporation; the skin s ur face temperature recorded by a thermocouple is shown below. D: heating the s~:in of the foot-pad by radiation to 40, 44 and 50°C, respectively; the skin surface temperatures measured with a '.hermocouple are shown below each record. E time course of discharge frequencies plotted as histo!grams, during and after heating the foot-pad to various temperatures. Duration of heat stimulus indicated by the bar below. Spinalized animal. Unit was 1750 ibm from cord dorsum and had polysynap':ic connection to group II afferents.

drive from subcutaneous receptors which, so far as could be judged, were :in the joints or ligaments of the toes. In several units there was a response to 2 types of these mechanical stimuli. In 4 units the type of effective mechanical stimulus was uncertain. These effective stimuli establish that type G and possibly type T hair follicle receptors (which have group II afferent fibers) have a powerful excitatory a:tion (Fig. 3A and B), but do not exclude the possibility that type D kair follicle affe rents (group III) also converge on these neurons. The responses to indenta,:ions or the glabrous skin of the pads are probably mediated by pad RA and SA afferent units 36. which have group II axons. Some of the tonic responses prebably come from deep receptors, which are not yet classified. As judged from the synaptic latency upon stimulation of large myelinated dorsa, root afferents, class 2 fell in the 2 categories of monosynaptic or polysynaptic. Since these units were also excked by affi~rent C-fibers, they comprise the 2 classes, M C (monosynaptic with C input) and I)C (polysynaptic with C input), of Gregor a:.d Zimmermann 2z. Three-quarters (17 L:f 23) of the neurons were MC and the remainder were PC. When the skin was cooled, by the evaporation of ethylchloride sprayed on the

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Fig. 4. Intensity function of a class 2 neuron. The total number of spikes discharged in a period t)f 20 sec during and after radiant heat stimuli to the foot-pad (10 sac in duration) is plotted against the intensity of the.ae stimuli. Measuren~nls £,erformed when the cord was blocked reversibly at Lx by ~:ooling (O), and before and after the cold block ( 0 ) . The dotted lines indicate the spontaneous activity a,~exaged frc,m 10 sec periods before each stimulus. Un;,t was 1470 Ina below cord dorsum and had n ~onosynaptic input from group Ii afferents. Same unit as in Fig. 7 A

skin, ti~..r ~ was a brief vigorous discharge of irr,pulses (Fig. 3C) which occurred with a short latency. Simt4taneous recording of the temperature of the skin surface revealed that this kind of "cold' stimulus provoked a very rapid, sudden and relatively short low tempe;ature (e.g. 12 °C, Fig. 3C). Slow cooling, produced by dropping ice-cold water on the skin, did not excite these dorsal horn neurons. Obviom ly the specific cold receptors which are )?owerfully excited by a minor fall in ~kin teraperature (e.g. from 30 to 25 °C; ref. 32) do not mediate these responses of the class 2 cells tested. It is most iikely that this excitation by cooling the skin with eth)lchloride spray is ~nediaTe,a by mechanoreceDtors with myelinated aflierent fibers which are known to be activated by such steep falls in temperatureZt,3x,aL

Noxious heating A striking ar:d consistent I~ature of the class 2 neurons was their excitation, in the spinal animal, by noxious heating of the skin (Fig. 3D and E). A typical response, from a class 2 neuron in lamina IV is shown in Fig. 3D and E. The skin was heated for 5 sec a~_d the maximal surface temperatures reached were between 40 and 50 °C, as indicated at each trial. A definite response from the dorsal horn neuron is evident at 4_:.5 ~C. Threshold measurements of this kind were made for 9 of the class 2 neurons" they ranged from 42 to 47 °C. Under identical conditions corresponding thresholcl values were reported for the thermal nociceptors of the foet 1, and comparable values were found in other published workZ,Zs,30. qZhe relation of discharge frequency to temperature was examined in several units. Aa example is shown in F~g. 4 (©). Between 44 and 50 °C there was a progressive inc,:ease in cell discharge which was correlated with skin temperature. Beyond 52 °C there was ai~ :ndication of a :aturation of the discharge frequency. This saturatior~ i~as not been observed in the therma!, nociceptors ~. W: ha~ e r-ot investigated this phenomenon systematically; however, it should

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be noted that there seemed to be no leveling off of the response in this unit, when the cold block of the co "d was terminated ( e in Fig. 4). The effect of reversible blocking of the cord on the i:ltensity function will be considered later in this paper. Time course o f respense to noxious heat

In all class 2 neurons the response to heating the skin began after a delay which usually was the shorter, the higher the temperature level of the stimulus was (Fig. 3D and E). This shift in latency corresponds with that observed in the thermal nociceptors (C-heat receptors) fi'om the foot-pad examined under identical conditions 1. The dine course of the clzss 2 neurons differed, however, from that of the primary afferent fibers in the. much greater persistence of the discharge (for l0 sec or Iongr:r) after the thermal stimulus w~s removed (Figs• 3D, E and 5). The 23 class 2 neurons all showed some degree of continuous but irregular background disehalge: in the spinal preparation (Figs. 4 and 6-8). lit is not known whether the spontarteous discharges of primary afferents (e.g. SAII re:cet~torsz4,~.G,3:3) contribute to this background activity. When subdividing arbitrarily the class 2 neurons accord!.'ag ~o the level of spontaneous activity, it was established that the time courses of ~he average discharge rates differed ir: the subsampl,~s having either

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!ow ( < I0 Hz) or high ( > 10 Ez) levels of background activity, respectively(Fig. 5B and C). 1"~the group with the high spontaneous activity (Fig. 5C) the discharge during heating closely paralleled that in the afferent fibers (Fig. 5A) except for the greater pen;istence on withdrawal of the stimulus. In the low spontaneous activity group the onset of discharge was delayed for 4--6 see (Fig. 5B), indicating that more temporal summation of (:',-input is required to drive these units. A further difference was the lower average r~Ltes of firing in the neurons with low spontaneous activity (Fig. 5B). The persistence of the responses of the class 2 neurons after removal of the sCmulus may be caused by input from skin receptors cther tkan the C-heat receptors: e.g. the SA receptors with grotq;~ II afferents have a depressed excitability during

157 heatingl,14,1~,3a.6'~; many of them exhibit a rebound discharge after the removal of the radiant heat stimulus.

Segmental interaction of myelinated and non-myelinatedfiber input The t;owerful excitatory central action of an input in the C-heat receptors in spinal cats gave an opportunity to test interactions between myelinated mechanoreceptor and C-noeiceptor afferent inputs at a segmental level. The convergence, as exemplified in Fig. 3, of different types of afferent fibers with an excitatory action on dorsal horn cells of our class 2 is well known 21,27,28,43,46,47,54,56,57, as well as inhibitory or mixed excitatory/inhibitory actionsS,10,21,27,2s. We were particularly interested in the interaction of the sensitive mechanoreceptors and the non-myelinated nociceptors because of the possible significance of such interactions for 'pain'. To study these interactions, discharges in class 2 dorsal horn neurons in spinal cats were evoked by sustained (20-30 sec) heating of the foot-pad to temperatures of 50 or 52 "C (Fig. 6). During such a prolonged heating the mean frequency of discharge became more or less constant, although there was 3 high degree of variability in the intersplke irtervals. Two methods were used to excite mechanoreceptor afferent fibers, either (a) electrical stimulation of the plantar nerves at intensities t2 T) that excite only part of the group II (Fig. 6B), or (b) electrical stimulation at * T of the dorsal columns at L1 with a superficial electrode to excite collaterals of the group II cutaneous afferent fibers.

(a) Combination of skin heating and electrical stimu;ation of peripheral nerves The discharge evoked by heating the skin (50 °C, Fig. 6A) was suppressed by electrical stimulation at 5 Hz of the MP and LP nerves (Fig. 6B). The residual activity during nerve stimulation was mainly due to an excitatory action of group II volleys on the class 2 cell, as is evident from the original records (not illustrated). The heatevoked discharge recovered within 0.5 see of the end of stimulation of the fast group II fibers. At a higher intensity of the electrical nerve stimulation (4 T), when slower group II and some group III fibers were recruited, the suppressing effects were the same.

(b) Skin heating and dorsal column stimulation Since the dorsal columns contain collaterals of gr,~ul: II cutaneous afferent fibers but not of the smaller afferent fibers ~, it is possible by electrical stimulation of the dorsal columns to send synchronized volleys of cutaneous group II impulses antidromically into the caudal dorsal horn. At low frequencies of electrical stimvlation (5 Hz) these dorsal column volleys diminished the heat-evoked discharge in the cla~s 2 neurons (Fig. 6C), and almost completely suppressed it at higher frequencies (50 Hz) in some units (Fig. 6D). The suppression was abrupt in onset and termination. The nociceptor induced discharge was suppressed even when the group It response of the class 2 neurons ceased in the course of the dorsal column stimulation, at higher fiequencies. It should be noted that the suppression shown in Fig. 6 is ,t segmental

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effect, since the activation of a supraspinal loop was precluded by blocking of the spinal cord proximal to the dorsal column stimulus. The suppressive effects were tested in 5 class 2 neurons in spinal animals, with similar results in 4 of them; in 1 unit there was a strong excitatory drive by the repetitive dorsal column and nerve stimulations, thus concealing a possible suppressing effect on the heat response. In 1 unit (that oi" Fig. 6C and D), the suppression of the heat response was also tested with the spinal ccrd being intact; the effect was the same as in the spinalized state. Descending :'i;![tuences on dorsal hor,~ neurons

There are powezful descending influences that are known to modify central transmissability through spinal reflex pathwayslT,18,z2,26,5s, 5a and through the spit, ocervical tract4, 'r, t0,61. In experiments using natural stimulation of cutaneous receptors, it was reported that spiaocervical tract units had an altered peripheral receptive field in spinal and normal cats4, 61, with a greater precision and sharpening of modalities in 1;.ae intact animals, ,:hus establishing a selective action of the descending influences. The effect of descer:caing mechanisms on the dorsal horn neurons was tested in 3 cats in which a hollow metal thermode, circulated with methanol at controll :d ~emperatures down to 0 °C, was plac~d on the spinal cord at L1. Ten of the 36 units were examined in spinal cord blocking experiments, and all were tested in bo::h conditions of the cord so that it was possible in the same unit to compare the excil.ability of the cells in the absence and presence of descending influences.

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unit 2313-11

Fig. 8. Comparison of the effects of cord block by cooling and of cc,rd transection, n,: response of a class 2 neuron to radiant heat stimulation (54 °C, indi~ated by the bar .,rye,low) during cold block of the cord at L1. B: response of the same unit when the cord was intact, recorded ~:tween A and C. C: response of the unit when the cord was transected at L~. Unit was 1500/zm bel3w cord dor~uLa and had monosynaptic group II input.

(a) Spontaneous activity In 7 class 2 units with varying degrees of spontaneous; aclivity with the cord intact, there was an increase in activity after the cord was blocked (Fi::,,s. 4, 7 and 8). The increased discharge was not caused by a stimulating effec~ of the low temperature at L1, since there was a similar enhancement of the spontaneous discharge when the: cord was cut (Fig. 8A, C). Three class 1 neurons were tested in the cord blocking series; they had no spontaneous discharge in either condition of the cord.

(b) Afferent evoked activity The effect of reversible spinal block was examined in 7 class 2 neuroas. The responses to weak mechanical stimuli, to group II afferent volleys, to nonivus heat and to group IV/C) volleys, were differentially affected by the block. There was an enhancement of the response to controlled skin-heating (Figs. 7 and 8), whereas the responses to mechanical stimuli such as a standard puff of air, were unaffected (e.g. we measured in unit 2373-4: 6.3, 5.3 and 5.2 impulses/puff, before, during and after cold block of the cord, respectively). The effect of the spl.nal block is shown for 2 neurons in Fig. 7. In both units there was an increase in spontaneous activity and an enhat~cement of the response to heating the pad. These phenomena can be interpreted by a release of supraspinal inhibition, acting via descending pathways, which were b,ocked by the cooling at L1. The unit in Fig. 7A is an example for a minor supraspinal inhibition; in contrast, on

160

A spintaIizedstate 't ..... l

.

.

.

.

B

intact stale

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.

2001n's '--

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:

unit 2313-10

~ |'lr

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Fig. 9. Response of a class 2 neuron to C-fiber volleys. The plantar nerves were stimulated at a strength of 250 T, which was suprathreshold for C-fibers. Each dot indicates a stimulus. The responses of the unit to the group !I volleys are not resolved from the stimulus in these specimen recordings. A: responses during cold block of the spinal cord at L~ ; either single stimulus (upper two records) or repetitive stimulation (4 stimuli at 25 Hz, lower two records). B" response of same unit to repetitive stimulation (4 × 25 Hz and 5 × 30 Hz, upper and lower record, respectively). Unit was 1300 pm from cord dorsum and had polysynaptic group II input.

spinal block ~he unit in Fig. 7B exhibited a striking transition from unresponsiveness to a vigorous, though long latency, discharge to skin heating. In another unit (Fig. 8) it was shown that transection of the spinal c~rd at Lx (Fig. 8C) had principally the same consequence as had the cold block (8:\) on the heat response. In Fig. 4 the discharge rate of a class 2 unit versus intensity of the heat ,timu~us is shown before and after ( I ) , and during (©) a cold block of the spinal cord. The supraspinal influence was relatively weak in this unit (see Fig. 7A), when compared to others (e.g. Figs. 7B, 8 and 9). The intensity curve of the unit shows mainly a parallel shift, approximately by the amount of change of the average spontaneous activity, and a minor increase in slope. Hence, it is evident that the extrapolated threshold of this unit to heat stimuli was not altered appreciably by the cold block. The supp,'essive effect of supraspinal origin was also evident in the response of class 2 dorsal horn neurons to afferent volleys in C-fibers generated by electrical stimulation. The unit shown in Fi~. 9 was excited by afferent volleys in A-fibers when the cord was intact and unaffected even by iterative volleys in C-fibers in the plantar nerves (Fig. 9B). Blocking the cord dramatically altered the response (Fig. 9A), uncovering C-input. From Figs. 9 and 7B it appears that in some units the class 2 properties were concealed by the supraspinal inhibition. DISCUSSION

The resuks pre~.ented in this paper extend previous reports 6-8,zo,zl,26-z9'a~.,46,47, aa-zs,61 on the characteristics of eorsal horn neurons in two ways. Firstly, there is confirmation tl~at the superficial laminae o" the lumbar dorsal horn in the cat contain neurons that a:e strongly influenced by cutaneous afferent input in low threshold

161 (group II) fit~rs. We have divided them into 2 major classes (1 and 2) in spinal cats, which can be distinguished principally by their excitability by affer~mt C-volleys, either electrically evoked or induced by noxious heating of the skit~. Thus electrical nerve stimulation revealed that class 2 neurons were excited by both myelinated and non-mydinated cutaneous afferents, whereas class 1 had only myelinated fiber inpt;*. Secondly, the high level of activity in class 2 cells aroused by a;1 input in thermal nociceptor (C-heat receptor) afferent units could be suppressed by a concurrent input from sensitive cutaneous mechanoreceptors with thick myelinated axons, and by tonic st~praspinal inhibition. In this paper we are not concerned with neurons having a specific noci,.:eptive input found in the marginal zone of the dorsal horn15. 6°. The class 2 neurons were all excited by noxious radiant heating of the sk:.a. The actual afferent units which provided this drive were almost certainly C-therm;:.l nociceptors (C-heat receptors). In an investigation using identical stimulus con, litiorts 1 it was found that only this kind of afferent unit was excited in the foot regio~J of the cat in a quantitatively similar way as the class 2 ~eurons by such radiation, and the discharge of the class 2 dorsal horn neurons (Figs. 3 and 5) cannot be attributed :..-~ any incidental discharge of cutareous mechanoreceptors by such skin heating. Additional confirmation comes from experiments 34 using differential nerve block in combination with experimental procedures similar to those described in the present paper, which have shc wn that the class 2 neuror.s were still excited by heating tte skin when the peripheral nerve was differentially blocked and only the non-myelinated fibers were conducting. These results thus establish that the class 2 cells are converge d on by sensitive cutaneous mechanoreceptors and by nociceptors. The background acti,vity, as well as the peripheral receptivity of dorsal horn cells, is strongly influenced by supraspinal mechanisms. A comparison of the activity of the same cell in intact and spinal states often showed a substantially greater spontaneotl~ activity in the spinal state (Figs. 4, 7 and 8), especially in the class 2 cells. At the same time there was a striking enhancement of the response to noxious heating ,~.."*.he skin. This supraspinal influence can be assessed more quantitatively from the intensity function of the class 2 neurons (Fig. 4), i.e. their ability to encode information on the intensity level of the nexious heat stimulus. The increase in sensitivity by the spinal block appears in this plot as an upwards shift and as a slight increase in slope of the curve. In other units the increase in sensitivity was much more pronounced (e.g. Fig. 7). More experiments ,,f the type shown in Fig. 4 are nceded to evaluate the significance of supraspinal contl ol in terms of information trans..nission. So far as we could decide taere was no comparable enhancement of the response to an input from the sensitive mechanoreceptors. Our finding of a differential supraspinal influence on nociceptive input extends previous reports on differential s,upraspinal control on various kinds of mechanoreceptive input ~,~°. This selectNe effect may result from a difference il~ exc~tatory synaptic security of afferent A- and Cfibers, respectively. As could be judged from dorsal root stimulation many of the class 2 dorsal horn cells received a monosynaptic excitatory drive fiom group tI afferent fibers. On the other hand from available data nothing is known about the

162 synaptic connections between C-afferents and class 2 cells; possibly the C-afferent pathway has more synapses either in series or in parallel, which might result in a lower synap.:ic security and in a higher susceptibility to inhibition. The relative contributions of pre- and post',ynaptic inhibition to the suppression cannot readily be assessed. Presynaptic contro: of spinal afferents by supraspinal centers has been reported zs,4°. Thus far, however, there is no direct evidence of presyn~ptic inhibition of unmyelil, ated fibers aT. An important feature of the supraspinal selective suppression of the nociceptor input t.t: the class 2 dorsal horn cells is its tonic activity shown by the reversible spinal blocking. It is continuously active in the absence of any intentionally applied external stimulus. Thus the central pathways seem to be, by the supraspinal influences, more closed to nociceptor input irrespective of whether the large afferent fibers are active or silent. This central selectivity does not depend on the relative balance of peripheral input in large and small afferent fibers, as was envisaged by Melzack and Wall's 42 gate hypothesis. The dorsal horn neurons have in addition to the supraspinal inhibition segmental inh'bition acting on the nociceptive input. This has been shown previously by interacting the electrically evoked A- and C-responses2Z,4s, 5e. As in the case of supraspinal inhibition discussed above, pre- and postsynaptic mechanisms might contribute to this inhibitionla,Zg,27,2s,aT,4L Their relative involvement in the suppression of C-input is, however, not yet established directly. This inhibition of C-input by concurrent: A-volleys thus supports one type of interaction postulated by the gate control theory 4~ and by antecedent pain theories 4s. There are, however, strong arguments against a presynaptic facilitatory action of activity in small myelinated and unmyelinated fibers onto dorsal horn cells, the other postulate of the gate control theorya,Za,19,37,4z,02. The present results show more specifically that this segmental inhibition by A-fibers sul~presses powerfully the C-fiber input produced by noxious heat stimulation of the skin to dorsal horn neurons, defined by us as class 2. The inhibition can be evoked equally well by electrical stimulation of the dorsal columns where it acts by ant idromic discharge in the group II axon collaterals of the dorsal root fibers, and by electrical stimulation of group II axons in peripheral nerves (Fig. 6). These results are of considerable interest in the context of pain. There is, for example, the dramatic relief of human pain, reported by Wall and Sweet eg, in response to low-intensity electrical stimulation of peripheral nerves. In the patients described, the pain was due to traumatic injury or peripheral disease and was chronic. Brief repet!t!ve !c.:- intensity electrical stimulation of peripheral nerves induced pain relief during the stimulation in all patients and prolonged relief in some. The similarity with our results extends only to the short-term relief, since as Fig. 6 shows, the inhibition of class 2 cells was effective only during the group II stimulation. It is, of course, pre~nature to id~n~f~, *_beclass 2 cells as part of a central 'pain' mechanism, but it is ciear that these or similar neurons in the spinal cord and in the brain could be involved. The clear parallel lies in the inhibition of nociceptor induced neuronal discharge in the animal experiment, and the suppression of pain of peripheral origin in man, by

163 activation of large myelinated afferent fibers. More recently relief of h u m a n pain by dorsal column stimulation has been reported 44,5~, and o,acc again the mechanisms probably "nvalw, a segmental interaction between mecha~.oreceptor and nociceptor afferents i~ '~h,, spinal cord. ACKNOWLEDGEMENTS This work was supported by Die Deutsche Forschungsgemeinschaft, G r a n t Zi 110. We acknowledge the secretarial and technical help of Ms. K. Hempeler, Mrs. U. Nothoff and Mrs. J. Peddinghaus, and the participation of Mr. P. Beck and Dipl.-Ing. H. D i e k h a u s in some of the experiments. We wish to express our appreciation to Prof. Y. Z o t t e r m a n for critical reading of the manuscript.

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